COURSE ON MINERAL EXPLORATION

AT ZAWAR

GEOLOGICAL SURVEY OF INDIA

TRAINING INSTITUTE

ZAWAR CENTRE

JAIPUR

2010

Retorts used by ancient miners for Zinc smelting. Invention of Zinc smelting has taken place atZawar at least about 1000 years before present.

COURSE ON MINERAL EXPLORATION

AT ZAWAR

C O N T E N T S

Page

No.

1. ZAWAR CENTRE AND EXPLORATION MODULE 1

1.1 Zawar Centre 1

1.2 The Zawar Module of Mineral Exploration 2

2. PRECAMBRIAN GEOLOGY OF ARAVALLI MOUNTAIN

RANGE, WESTERN INDIA

3

2.1 Stratigraphic Framework 3

2.2 Archaean Basement 4

2.3 Basement – Cover Relationship 6

2.4 Aravalli Supergroup 7

2.5 Delhi Supergroup 10

2.6 Malani Igneuos Suite 11

3. METALLOGENY: DISTRIBUTION AND TECTONIC SETTING

OF ORE DEPOSITS

12

3.1 Spatial Distribution of Ore Deposits 12

3.2 Temporal Distribution of Ore deposits 13

3.3 Plate Tectonic Concept in relation to Ore Deposits 14

3.3.1 Rift-Related Ore Deposits 14

3.3.2 Arc-Related Ore Deposits 17

3.3.2.1 Principal Arc-Related Deposits 18

3.3.2.2 Arc-Related Rift Deposits 19

3.3.3.3 Kuroko-Type Massive Sulphide Deposits 19

3.3.4 Mineral Deposits Related to Divergent Plate Boundaries 20

4 ORE GENESIS AND DEPOSIT – TYPES 22

4.1 Ore Genesis 22

4.2 Deposit-Types 23

4.2.1 Volcanogenic Massive Sulphide (VMS) Deposits 23

4.2.2 Sedimentary Exhalative (SEDEX) Type Deposits 25

4.2.3 Mississippi Valley Type (MVT) Deposits 28

4.2.4 Porphyry-Type Deposits 28

4.2.5 Greenstone-Hosted Quartz-Carbonate Vein Deposits 30

4.2.6 Reduced Intrusion-Related Gold Deposits 31

4.2.7 Epithermal Deposits 31

4.2.8 Magmatic Ore Deposits 34

4.2.9 Weathering-Related Deposits 35

5. GEOLOGY OF ZAWAR LEAD-ZINC DEPOSITS 37

5.1 Regional Geological Set Up 37

5.2 Stratigraphy of Zawar Area 37

5.3 Structure and Mineralisation 39

5.4 Genesis of Zawar Pb-Zn Ores 40

6. STAGES OF EXPLORATION 45

6.1 Introduction 45

6.2 Reconnaissance Exploration 45

6.2.1 Regional Geological Set Up 45

6.2.2 Aero-geophysical Surveys 46

6.2.3 Application of Remote Sensing Techniques 48

6.2.4 Regional Geological Mapping 49

6.2.5 Regional Geochemical Surveys 49

6.2.6 Integration of Regional Data and Delineation of Prospect 50

6.3 Prospecting 51

6.3.1 Surveying 51

6.3.1.1 Topographic Surveys 51

6.3.1.2 Contouring 56

6.3.1.3 Surveying Methods and Instruments 58

6.3.2 Detailed Geological Mapping 65

6.3.3 Detailed Geochemical Surveys 66

6.3.3.1 Scale of Geochemical Exploration 67

6.3.2.2 Geochemical Anomaly 70

6.3.4 Geophysical Prospecting 75

6.4 Exploration 86

6.4.1 Borehole Planning 86

6.4.2 Core Logging 91

6.4.3 Borehole Geophysics 96

6.4.4 Core Sampling 97

7. RESERVE ESTIMATION OF MINERAL DEPOSITS:

PRINCIPLES AND METHODS

103

7.1 Principles and Assumptions 103

7.2 Cut off grade 109

7.3 Morphology of the Ore Body and Variability 120

7.4 Ore Reserve Calculation Methods 122

7.4.1 Cross Section Method 130

7.4.2 Level Plan 134

7.4.3 Statistics and Error Estimation 135

8 BLOCK-PANEL DIAGRAMS AS A MEANS OF 3-D

DEMONSTRATION : THE NEEDS AND THE

DISCRIMINATION

142

9. REFERENCES 147

1. ZAWAR CENTRE AND EXPLORATION MODULE

The village of Zawar in Rajasthan, western India is studded with several

impressive old working that were important source of zinc-lead-silver ores in

ancient India. It has a glorious history in mining and metallurgy of base metal ores

and silver. More importantly, Zawar holds the distinction of perfecting the art of

smelting Zn-Pb-Ag ore and in producing zinc metal several centuries earlier than

the western countries. Carbon dating of wooden artifacts collected from old

workings of Zawar area yielded ages of about 2500 years before present. Invention

of zinc smelting has taken place at Zawar about 1000 years before present.

1.1 Zawar Centre

Learning various aspects of mineral exploration requires detailed studies in

structure and economic geology. The famous Pb-Zn belt in Zawar is unique in

being associated with one of the most spectacularly preserved structures in

Palaeoproterozoic Aravalli rocks in southern Rajasthan. The Zawar Centre is

nestled amidst the well known Zn-Pb deposits of Balaria, Mochia, Zawar Mala

and Baroi around Zawar. Besides, there are several prospects that are under

various stages of exploration. High percentage of exposures, profuse surface

indications of mineralisation and a group of old workings and underground mines

provides perfect geological setting for developing Zawar as a centre for training in

mineral exploration. In addition, the location of Zawar Centre is advantageous as it

is in proximity to several important deposits such as the stromatolitic rock

phosphate deposit in Jhamarkotra (40km), Rajpura-Dariba and Sindesar Pb-Zn

deposit (135Km), volcanic-hosted massive sulphide deposit at Deri-Ambaji (200

Km), Bhukia gold deposit (150Km) and the world class Pb-Zn deposit at Agucha

(250 km).

Location and Approach

The Zawar Centre is located in the vicinity of Ramanath temple between

Zawar Mine’s township and the Tiri village in the Survey of India Toposheet no.

45H/11. The camp is about 45 Km south of Udaipur and 5 Km from Tiri which is

on the National Highway (NH) no. 8 connecting Udaipur to Ahmedabad. Zawar

Mines is the nearest Railway Station which is about 7Km from camp. Udaipur the

district headquarters is connected by train and air. The Dabok (Udaipur) Airport is

50 Km from the Zawar Centre and is approachable with the NH-8 that bypasses

Udaipur city. Air flights for Delhi, Mumbai and Jaipur operate from this airport.

Climate

The climate at Zawar Centre is dry and pleasant between November and

March. For remaining part of the year, climate remains dry and warm. The months

from April to June are fairly hot. Winter season prevails between middle of

November to February. The monsoon season commences between July to middle

of September. The temperature variation is between 40°- 45°C in summer and 5°-

10°C in winter. Woolen clothings are required during winter months.

1.2 The Zawar Module of Mineral Exploration

The Zawar Module mainly focuses on various aspects of mineral

exploration. The course content is flexible depending on the requirements of the

Geological Survey of India and various other organizations. The course material

presented here is only an outline of different themes and in way it should be

treated as a text on mineral exploration. The write up is structured into two parts.

The first part presents brief outline of Precambrian geology of the Aravalli

Mountain Range followed by a summarized accounts of time and space

relationship between ore deposits, their relationship with major tectonic structures

and deposit-types. A separate chapter on Zawar Pb-Zn deposits presents salient

features of the structure and pattern of mineralization in the Zawar group of mines

that surround the centre of the Training Institute. The second part covers all the

broad aspects of mineral exploration such as reconnaissance and the classical

methods that include introduction to survey techniques related to detailed mapping

(DM) and practices that are being followed for geochemical exploration and

geophysical surveys. The last two chapters are devoted to subsurface exploration

that includes ore reserve estimation by various methods. Emphasis is laid on

reserve estimation by cross section, level plan and other manual methods before

switching over to the use of computer softwares for such exercises.

2. PRECAMBRIAN GEOLOGY OF ARAVALLI MOUNTAIN RANGE,

WESTERN INDIA

The name Aravalli owes its origin from the words Ara meaning across and

Aval which denotes hills in local dialect. The Aravalli Mountain Range (AMR) cuts

across the state of Rajasthan (the erstwhile Rajputana) and divides it into two

physiographic regions represented by the Thar Desert in the west and Vindhyan

Plateau in the east. The 800 Km long AMR extends from near Delhi in north to

Jhabua in central India (Fig. 2.1). This impressive Proterozoic orogen trends NE-SW

for greater part of its length covering northern and central parts of Rajasthan and

swerves to NW-SE in the southeastern Rajasthan and northern parts of Gujarat and

Madhya Pradesh, conforming broadly to the outline of the Bundelkhand Craton of

Archaean age.

2.1 Stratigraphic Framework

Notwithstanding the earliest studies carried out by Hacket (1881), the basic

stratigraphic framework of the Aravalli region was given by Heron (1953) who is

credited with recognition of unconformable relationship between a dominantly

gneissic terrain and overlying metsedimentary rocks. The gneissic rocks that

formed basement for cover sequences were referred to as the Banded Gneissic

Complex by Heron (op. cit.). The overlying metasementary sequences include

carbonate and phyllite – mica schist-dominated Aravalli System, calcareous

facies-rich Raialo Series and quartzite, calc-silicate, biotite schist–epidiorite

bearing Delhi System. Subsequent studies by the Geological Survey of India and

various academic institutions grouped Raialo Series and quartzite and

conglomeratic outliers of the Delhi System with rocks of Aravallis and elevated its

status to the Aravalli Supergroup. Likewise, the Delhi System was given the status

of Delhi Supergroup. Based on the compilation and synthesis of regional

geological mapping Gupta et al (1980 and 1997) effected major changes in the

Precambrian stratigraphy of western India by grouping several metasedimentary

sequences with the gneissic rocks (BGC of Heron, 1953) and referring to them as

the Bhilwara Supergroup. This Supergroup included the metasedimentary belts

around Hindoli (earlier mapped as Aravalli rocks by Gupta, 1934), Bhinder,

Rajpura-Dariba, Jahajpur, Pur-Banera and Sawar. It may, however be mentioned

that interrelationship among various metasedimentary belts and older gneisses is

not yet clearly established and some workers do not support the rationale of

grouping these metasedimentary tracts as part of the Archaean Bhilwara

Supergroup (see Roy et al, 1988 and Deb, 2004). The stratigraphic scheme

proposed by the Geological Survey of India for Rajasthan and adjoining areas is

followed in the geological map of Rajasthan and Gujarat (Fig. 2.1).

2.2 Archaean Basement

Heron (1936 and 1953) considered the vast tracts of banded gneisses as

forming basement for the overlying Aravalli rocks. Lithological similarity,

presence of conglomerates and identification of angular and overlapping

relationships were the key factors in Heron’s formulation of stratigraphic

framework for Precambrian geology of Rajasthan and adjoining areas. He coined

the term Banded Gneissic Complex (BGC) that remained accepted in geological

parlance for about three decade with a note of dissent by Crookshank (1948) and

Naha and Choudhuri (1967) who considered the banded gneissic rocks in the north

of Nathdwara as parts of migmatised Aravalli sediments. Regional geological

mapping carried out by the officers of the Geological Survey of India after

publication of Heron’s (1953) memoir on Geology of Rajputana (Rajasthan)

brought out new additions in the regional geological set up which eventually led to

proposition of a new stratigraphic scheme (Gupta et al, 1980 and 1997). The

Banded Gneissic Complex (BGC) was redesignated as the Mangalwar Complex

and Sandmata Complex. The term Bhilwara Supergroup, originally proposed by

Raja Rao (1967) as Bhilwara Group, was adapted by Gupta and his coworkers to

include older gneissic and metasedimentary tracts of supposedly Archaean age.

The Bhilwara Supergroup includes most of the peneplained Banded Gneissic

Complex (BGC) that lies to the east of the Delhi-Aravalli orogens. The gneissic

rocks exposed in Mangalwar, Bandanwara and Sandmata areas constitute part of

the Bhilwara Supergroup. The Berach near Chittorgarh and Untala granites are the

major intrusion in the Archaean terrain. The metasedimentary tracts included in

the Supergroup by Gupta and his coworkers (op. cit.) are Hindolis, Rajpura-

Dariba, Pur-Banera, Bhinder and Jahajpur belts and the Sawar group of rocks (Fig.

2.1). Inclusion of these metasediments with the Bhilwara Supergroup was based

on the supposedly intrusive of late Archaean Berach granite into the

metasediments. Later studies by Roy (1988 and Deb, 2004) indicated that the

metasedimentary belts included in the Bhilwara Supergroup are equivalents of the

Palaeoproterozoic Aravalli Supergroup. Pb-Pb dating of sulphides in these richly

mineralized metasedimentary belts further supports their coevality with the

Aravalli rocks (Deb, 2004).

It may be mentioned that the geochronological framework in early eighties

was far from adequate. The mapping carried out Roy and his coworkers (1980 and

1985) brought out angular relationship between BGC (to which they renamed as

Mewar Gneiss Complex) and overlying basal quartzites of Aravalli Supergroup in

Jhamarkotra area. The geological mapping and field relationship by Roy et. al.

(1985) further indicated that the granitoids exposed around Udaisagar and in Ahar

river valley formed basement for the Aravalli rocks, contrary to their earlier

stratigraphic status as intrusives into the Aravalli rocks (see also Goswami et al,

1994).

Vinogradov et al (1964) dated detrital zircon separated from Aravalli

schists exposed near Udaipur by U-Pb method. It indicated 3.5 Ga age of the

provenance. Tonalitic to trondhemitic gneisses and amphibolite dyke forming

basement in the east of Udaipur have given a near contemporary ages ~3.3 Ga by

Sm-Nd method (Gopalan et al, 1990). Comparable age of 3281±3 Ma has been

obtained by Wiedenback and Goswami (1994) from zircon collected from

basement rocks that occur at about 9 km west of Jhamarkotra. Zircon overgrowth

from basement gneissic rocks yielded an age of about 2536±40 Ma (Wiedenback

and Goswami op.cit.). There are two different types of granitic intrusions that have

been recorded from the basement gneisses. The older granitoids exposed at Untala

and Gingla (Fig. 2.1) have given an age of about 2900 Ma by Rb-Sr method

(Choudhury, 1984 and Shastry, 1992). The potash feldspar bearing Berach granite

near Chittorgarh has yielded 2533 Ma age (Shastry, 1992). Emplacement of

Berach granite probably indicates cratonisation of the Archaean basement. Due to

disturbed nature of Rb-Sr systematics, the ages of Ahar river granite remained

controversial (Crawford1970 and Shastry, 1992) till Goswami et al, (1996)

determined 2615 Ma age by dating zircons.

2.3 Basement – Cover Relationship

The Archaean gneissic rocks constitute basement for several

metasedimentary basins of Proterozoic age in central and southeastern Rajasthan.

The basement – cover relationship has been well documented in the Udaipur

region where metasedimentary rocks belonging to the Palaeoproterozoic Aravalli

Supergroup lie unconformably over the basement gneisses. The nature of

Archaean basement and Proterozoic cover relationship observed in Rajasthan is

summarized below:

• The basement – cover contact in the east of Udaipur is represented over a

greater part by a gently convex zone of ductile shearing that shows

development of mylonites in both the gneisses and cover rocks (Naha and

Roy, 1983 and Roy et al, 1985). At places it also marks the western

extremity of the migmatitic front.

• Notwithstanding intense deformation at the contact at most of the places,

unconformable relationship between basement and cover rocks is

documented at a number of places around Udaipur. Heron (1953) reported

polymictic conglomerates at the base of Aravalli sequence in the east of

Udaipur. Looking at the areal extent of the basement-cover contact

occurrences of conglomerate have been remarkably low. Nevertheless it

does represent a period of erosion before onset of the Aravalli

sedimentation. In addition, conglomerates have been mapped around

Kanpur – Maton, Ahar river granite and Sarara inlier (Fig. 2.2).

• The angular relationship between basement gneisses and cover Aravalli

rocks is well documented at Jhamarkotra in the southeast and around

Nathdwara in the north of Udaipur. The E-W trending bedding planes in

quartzites that occur at the base of the Aravalli succession in Jhamarkotra

make high angle with gneissic banding in the south of rock phosphate mine.

At Nathuwas near Nathdwara, basal Aravalli quartzites make an angle with

preAravalli quartzites in banded gneisses, implying unconformable

relationship.

• A remarkable feature of the basement – cover boundary is the occurrence of

small discontinuous pockets of white mica schists that also contain kyanite

and fibrolite. These zones of high alumina rocks indeed trace out the

basement - cover contact. Field relations and petrochemical characters

indicate that these deposits (commercially sold as pyrophyllite) represent

paleosols (Roy et al. 1985; Banerjee, 1992 and Srinivasa et al, 2001).

• There is distinct drop in the grade of metamorphism across the basement-

cover contact. The basement gneisses and amphibolitic lenticles show

amphibolite grade against green schist facies of metamorphism in the

overlying Aravalli rocks.

• A swarm of basic dykes occurs in the basement gneisses that are exposed in

the Dhariawad region in southeastern Rajasthan. Absence of such dykes in

cover rocks and their confinement in gneissic basement close to the contact

region indicate unconformity represented by magmatic discordance.

2.4 Aravalli Supergroup

The Aravalli rocks and their equivalent metasediments form distinct belts

that rest unconformably over an Archaean basement of sialic composition (Fig.

2.1) The Aravalli succession is best developed between Nathdwara in north and

Sarara in south, and this tract with Udaipur in centre can be considered type area

for the Aravalli Supergroup (Fig. 2.2). The gneissic rocks forming basement lie to

the east while western part of the Aravallis is bordered by quartzites that belong to

the Gogunda Group of the Delhi Supergroup (Fig. 2.2). Heron (1953) gave

stratigraphic succession that starts with basal quartzites (comglomeratic at places)

followed by amygdaloidal basic volcanics and epidiorites, hornblende and chlorite

schists, dark quartzite (local), phyllite with limestone lenticles around Udaipur and

Zawar with local carbonaceous bands, limestone and calcareous sandstone in north

of Udaipur and lastly a thick sequence of well banded phyllite that contains thin

layers of quartzites. Based mainly on lithological similarity and overlapping

relationships, Heron (op. cit.) mapped outliers of Delhi ‘System’ and ‘Raialo’

Series’ in this type area of the Aravalli rocks. Later studies, unequivocally grouped

these supposedly outlier as integral parts of the Aravalli succession (Naha and

Halyburton, 1974; Roy et al, 1988; Gupta et al, 1980 and Sinha-Roy et al, 1998).

Geological and structural mapping of a large tract by Roy and his coworkers led to

recognition of three different types of carbonate rocks, one associated with basic

metavolcanics around Sarara, the second show development of phosphate bearing

stromatolites and the younger carbonates that host Pb-Zn mineralization in Zawar

and Katar area. There is no unanimity in stratigraphic succession of the Aravalli

Supergroup. Gupta et al. (1980, 1997) believed in consistently westward younging

of metasedimentary rocks from east of Udaipur to the west close to Gogunda;

while Roy et al. (1985) identified Ahar river granite as basement that lies to the

northwest of Udaipur. Based on cross stratification, graded bedding and facing of

stromatolitic columns and palaeogeographic studies Roy and Paliwal (1981)

brought out palaeoshoreline during the Palaeoproterozoic period around Udaipur

area. The distribution pattern of various lithological units is shown in the

geological map (Fig. 2.2).

Deb and Thorpe (2004) determined Pb-Pb model age of 2075 and 2150 Ma

for galena that was associated with barite veins in basic metavolcanics near

Delwara in the north of Udaipur. Since these metavolcanics occur at the base of

Aravalli succession, the lower age limit of the Aravalli Supergroup can be placed

at about 2.1 Ga, The upper age limit is constrained by dating of a prominent

granite intrusion at Dharwal that yielded a Rb-Sr age of 2.0 Ga (Shastry, 1992). It

may be mentioned here that the Pb-Pb model ages of the Zawar group of Pb-Zn

deposits fall in the range between 1694 and 1712 Ma (Deb and Thorpe, op. cit.).

The stratigraphic succession of Precambrian rocks from Rajasthan and

northeastern Gujarat given by officers Geological Survey of India (Gupta et. al.

1980) is given in the following table

MARWAR SUPERGROUP Malani volcanic suite Malani plutonic suite

VINDHYAN SUPERGROUP Erinpura granite and gneiss Godhra granite and gneiss

DELHI SUPERGROUP Punagarh Group Sindreth Group Sirohi Group Sendra – Ambaji synorogenic granite and granite gneiss and associated migmatite. Epidiorite, Kishangarh syenite, hornblende schists/amphibolite, pyroxene granulite and gabbro, ultramafics (Phulad ophiolite suite) Kumbhalgarh Group Ajabgarh Group Gogunda Group Alwar Group Unconformity in the northern portion with Bhilwara Supergroup whereas in south, structural discordance with Jharol Group.

ARAVALLI SUPERGROUP Champaner Group Lunavada Group Udaipur, Salumbar, Udaisagar and Darwal granites, serpentinite, talc carbonate rocks, talc-chlorite schists. Jharol Group Dovda Group Nathdwara Group Synsedimentational Basic volcanism Bari Lake Group Kankroli Group Udaipur Group Synsedimentational basic vocanism Debari Group UNCONFORMITY

BHILWARA SUPERGROUP Undifferentiated granites and dolerites Ranthambhor Group Berach granite and gneiss; Jahazpur granite

Synsedimentational Basic Volcaniscs

Rajpura-Dariba Pur-Banera Jahazpur Sawar Group Group Group Group

Mafic & Ultramafic bodies & synorogenic granites

Dolerite sills & (Untala & Gingla Dykes granites) Synsedimentational Synsedimentational Acidic, mafic and Basic volcanics basic volcanics ultramafic bodies

Hindoli Group Mangalwar Complex Sandmata Complex Base of the Bhilwara Supergroup is not exposed and is believed to have been concealed under Vindhyan Supergroup (?) 2.5 Delhi Supergroup

The typical NE-SW trend of the Aravalli Mountain Range is indeed the

trend of Delhi rocks that form core of the range. The Delhi Supergroup rests

uncomformably and /or with structural discordance over high grade banded

gneisses and granulites in the northeastern part; and the Aravalli metasediments in

the southeastern part of Rajasthan. In the northeast the Delhi rocks are exposed in

a number of isolated basins while in the central and southwestern part the expanse

of the Delhi Supergroup is more or less continuous upto northeastern Gujarat.

Sinha-Roy et al (1998) divided the Delhi Fold Belt into two the North Delhi Fold

Belt which includes Khetri, Jaipur-Bayana-Alwar basins and the South Delhi Fold

Belt which occurs to the south of Ajmer. According to Sinha-Roy et al (op. cit.)

the two belts have different geotectonic evolution. The Delhi Supergroup is

divided into the Raialo, Alwar and Ajabgarh Groups in northeastern Rajasthan

(Das Gupta, 1968 and Datta and Ravindra, 1980). In the south of Ajmer it is

divided into Gogunda and Kumbhalgarh groups. Two more volcanosedimentary

sequences occur to the west of the main AMR. They have been referred to as the

Punagarh Group around Pali and Sindreth Group around Sirohi in southwestern

Rajasthan. According to Gupta et al (1980) they represent the youngest members

of the Delhi Supergroup.

The Delhi Supergroup, the SDFB in particular contains a substantial

amount of extrusive and intrusive magmatic activity. The western part of the

SDFB contains metabasic and felsic volcanics that host several polymetallic base

metal prospects. Zircon separated from felsic volcanics yielded ~ 1.0 Ga age by

U-Pb method (Deb et al, 2001).

730

740

750

760

770

730

740

750

760

770

230

240

250

260

270

280

230

240

250

260

270

280

FIG. 2.1 : GEOLOGICAL MAP OF RAJASTHAN AND

NORTHEASTERN GUJARAT

Compiled by Gupta et al., 1980 and Northeastern Rajasthan compiled by Banerjee and Ravindra, 1977)

Deccan trap

Marwar Supergroup

(<0.83 b.y)

Malani volcanic and plutonic suite

(0.75 b.y)

Erinpura granite (0.83 b.y)

Delhi Supergroup with granite

and syenite (1.7 b.y to 0.85 b.y)

Aravalli Supergroup with granite

(2.5 b.y to 2.0 b.y)

Bhilwara Supergroup/ Metasedimentary

Belts/granite (3.0 b.y to 2.5 b.y)

80 KM

Udaipur

ZawarAmbaji

Beawar Bandanwara

Ajm

er

JAIPUR

Sikar

Jhunjhunu

Dungarpur

Lunavada

AHMEDABAD

Baroda

Abu

Road

Sirohi

Pindwara

Chittorgarh

Jodhpur

C

Jhabua

DH

BU

U

G

M

B

DB

BH

BN

SW

H

AG

J

AG - Agucha

BN - Banera

BH - Bhilwara

BU - Bhukia

B - Bhinder

C - Champaner

DB - Dariba

DH - Dhariwad

G - Gigla

H - Hindoli

J - Jahajpur

M - Mangalwar

S - Sandmata

SR - Sarara

SW - Sawar

U - Untala

DELHI

S

SR

FIG. 2.2 : GEOLOGICAL MAP OF THE NATHDWARA - SARARA BELT (TYPE ARAVALLI

SUPERGROUP) SHOWING DISTRIBUTION OF MAJOR LITHOLOGIC UNITS

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

KT

N

G

D

I

M

KB

GOGUNDA

B

UDAIPUR

DB

U

K

J

ZAWAR

SARARA

JHAROL

0 6 KMS

N

B - BARI

D - DELWARA

DB - DEBARI

G - GORACH

I - ISWAL

J - JHAMARKOTRA

K - KANPUR

KB - KABITA

KT - KATAR

M - MADAR

N - NATHDWARA

U - UMRA

1. Banded gneisses 2. Granitoids 3. Carbonate rocks 4. Quartzite - conglomerate 5. Metabasic volcanics 6. Quartzite 7. Dolomite

8. Carbonaceous phyllite 9. Argillaceous phyllite 10. Greywacke - phyllite 11. Conglomerate - arkose 12. Zawar dolomite

13. Machhlamagra quartzite 14. Slaty phyllite 15. Quartzite 16. Mica schist 17. Ultramafic rocks 18. Rocks of Delhi Supergroup

(Based on the geological map of Heron, 1953 and Roy and coworkers, 1988)

DELHI

UDAIPUR

Volpe and MacDougall (1992) determined 1.3 to 1.4 Ga model Sm/Nd age of

metabasalts that occur in the central part of the SDFB. There are two phases of granitic

intrusions. One is represented by synorogenic granites that are dated 850 to 900 Ma

(Tobisch et al, 1992). The important granitic bodies of this phase include Sendra, Sadri-

Ranakpur, Sai and Ambaji granites. The post-orogenic granites are represented by

Erinpura phase which profusely intrudes the southwestern part of the SDFB. The

stratigraphic succession of the Delhi Supergroup by Gupta et. al.(1980) is given in the

following table:

Delhi Supergroup

South-western Rajasthan and north-eastern Gujarat

MALANI IGNEOUS SUITE (Volcanics and plutonic)

ERINPURA GRANITE

GODHRA GRANITE

INT

RU

SIV

ES

(P

ost –

Del

hi)

(exposed in Gujarat)

PUNAGARH GROUP Sojat, Bambolai,

Khambal and Sowania Foramtion

SINDRETH GROUP (Angor

and Goyali Formation)

SIROHI GROUP Jiyapura, Reodar, Ambeshwar and

Khiwadi Fms.

SENDRA-AMBAJI GRANITE and GNEISS

KISHANGARH SYENITE

PHULAD OPHIOLITE SUITE

KUMBHALGARH

GROUP

Todgarh, Beawar, Kotra, Sendra, Ras, Barr, Basantgarh and Kalakot Formation

DE

LHI S

UP

ER

GR

OU

P

GOGUNDA GROUP Richeri, Antalia and Kelwara Formation

2.6 Malani Igneous Suite

Late Proterozoic (ca 750 Ma) Malani Igneous Suite represents felsic

volcanic- dominated magmatic rocks that cover an area of over 50,000 sq km in

western Rajasthan. It mainly comprises of subaerially emplaced felsic volcanics

that are associated with post-Erinpura magmatic activity and pre Marwar

(equivalents of upper trans-Vindhyan) sedimentation. Rhyolites and rhyodacites

alone occupy about 30,000 sq km and probably constitute one of the most

voluminous acid flow deposits not only in India but in the entire world. The other

components in the Malani Igneous Suite include trachytes, dacites, pitchstone,

welded tuffs and ignimberites. According to Bhushan and Chandrashekhar (2002)

the cumulative thickness of volcanics exceeds 3.5 km at Siwana which is located

at about 35 km to the NW of Sirohi.

3. METALLOGENY : DISTRIBUTION AND TECTONIC SETTING OF

ORE DEPOSITS

Early studies on economic geology described various attributes of mineral

deposits and classified them mainly by taking cognizance of host rock and depth

and temperature of their formation (see Bateman, 1992 for a succinct summary).

Studies on mineral deposits occurring in Japanese archipelago, Andes and more

importantly the discoveries detailing sulphide deposits in formation at sea floor

and ocean floor bottoms led to beginning of understanding in spatial relationship

between plate tectonics and occurrence of mineral deposits. With swelling of the

geochronological database it was possible to punctuate timing of formation of

mineral deposits in relation to plate tectonics. With increased geological

knowledge, the applicability of plate tectonic theory is gaining acceptability in

Proterozoic period. Palaeomagnetic studies in particular have led to understanding

of assembly and break up of continental masses and movement of such plates even

in Palaeoproterozoic time. Plate tectonic considerations have thus become an

integral part of metallogenic studies and consideration of tectonic setting has

assumed important role in making investment decisions by exploration agencies.

The ambit of plate tectonic theory is not just confined to hotspots, centres of

spreading, subduction, transform faults and tectonics close to plate margins but it

does include major structural features that are distinctly intracratonic. It only

emphasizes that tectonic framework of regional geology is interpreted in terms of

plate tectonic models.

3.1 Spatial Distribution of Ore Deposits

Prior to the advent of Plate Tectonic Theory, the ore deposits were

considered to be inhomogenously distributed in space. It was however, known that

some areas in the Earth’s crust were richly endowed with specific ore deposits

than the adjacent ‘barren’ terrains. For example cupriferous tracts of Andes in

South America and coal bearing belts that define transcontinental distribution of

the Gondwana rocks, represented very large linear tracts that are richly studded

with several workable deposits of copper and coal respectively. The term

‘metallogenic province’ is used by economic geologists for such mineralized tracts

of regional aerial extent. Wright (1992) defined a metallogenic province as a

region of the crust generally more enriched with a variety of mineral deposits of

different ages than are the adjacent terranes. A more extended version requires

metallogenic provinces to constitute a specific geotectonic entity, show dominance

of specific type of mineralization to such a geotectonic setting and also display

intense nature of mineralization. Provinces may be monometallic or polymetallic

and deposits may be isochronous or polychronous in time, isogenetic or

polygenetic in origin and zoned or unzoned in space (Mookherjee, 1999).

The concept of spatial distribution of ore deposits took a definitive shape

when major tectonic domains related to generation and consumption of Earth’s

tectonic plates were identified and specific mineral deposits were known to

occupy such mega-tracts like the subduction-related porphyry copper deposits in

Andes or volcanogenic massive sulphide (VMS) deposits bearing volcanic arc of

Japanese archipelago.

3.2 Temporal Distribution of Ore Deposits

Studies on temporal distribution of ore deposits takes cognizance of the

known deposits since Precambrian and attempts to relate their tonnage proportions

through geologic ages. Certain metal deposits like gold and iron (BIF) are

overwhelmingly more abundant in Precambrian period than in the Phanerozoic.

The term metallogenic epoch is invoked to describe intense mineralisation of

specific metal deposits during particular geologic periods on global scale. Meyer

(1985) has given age versus tonnage proportion of metal deposits of all major

genetic types in various histograms for Cr, Ni, Fe, Ag, Au, Cu, Zn, Pb, U, Sn and

Mo deposits of the world. Such studies bring out global trends of metallogenic

epochs. For example gold metallogeny was more prolific during late Archaean to

Palaeoproterozoic than in any other period. Likewise, BIF type iron ore deposits

are generally restricted to the Early Palaeoproterozoic and unconformity-related U

deposits are particularly abundant in Mesoproterozoic. Distribution pattern of

metallic ores through geologic ages further reveal that some metals (Au, Cr, Ni,

Fe) form deposits from Early Achaean itself while deposits of others (Pb, Sn, W,

Mo, Hg, Sb) occur much later in geological history. Changing patterns in

geothermal gradients and Earths atmosphere (oxygen and geochemical

differentiation) are cited as some of the reasons in metal-specific metallogenic

epochs. With increasing differentiation, the Earth’s crust developed a thick sialic

component which is a prerequisite for operation of Plate Tectonic Theory and

development of various rift-related repository basins of some of the giant metal

deposits.

3.3 Plate Tectonic Concept in Relation to Mineral Deposits

There are two main repositories of metal deposits-the magmatic systems

and sedimentary basins and generation of hydrothermal systems being product of

the interaction between the two. The regional magmatic structures and

development of different orders of basins can be interpreted in terms of plate

tectonic models. Much of the understanding about mineral deposits and plate

tectonics comes from the Phanerozoic terrains where such a relationship can be

built with confidence. Important and well-studied terrains that have greatly

contributed to the understanding of plate tectonic and mineral deposits are the

great volcanic arc of Japan, ophiolitic belts of Greece and Oman, East African rift

system and subduction-related terrain in Andes Mountain Chain. They represent

well defined plate tectonic settings. Hutchinson (1990) has given relationship

between plate tectonic regimes and some important type of deposits in Fig. 3.1.

Important tectonic regimes and mineral deposits associated with them are

described in the following sections.

3.3.1 Rift-Related Ore Deposits

Continental rifts are common manifestation of regional zones of extensions

in stabilized cratonic terrains. Rifts occurring in oceanic and arc setting also host

important mineral deposits. The present section is, however, devoted mainly to

continental rift settings. Dewey and Burke 1974) have emphasized that the

continental rifting is a necessary initial step in operation of Wilson Cycle. Sawkins

(1990) has dealt with metal deposits that are associated with early and advance

stages of continental rifting. Geologic features in rift environments include:

• Rapid sedimentation of clastic sediments that are often thick

• Development of anoxic episodes in many advanced rifts

• Tendency of evaporite to form at some stage of basin development.

• High heat flow regimes are typical of advance stage of rifting.

• Active extensional faulting during basin filling.

Sawkins (op. cit.) has recognized early and advanced rift settings. The

early stages of rift systems are characterized by one or more of the characters like

updoming, common presence of alkaline rocks including carbonatites, presence of

breccia zones, presence of K-feldspar and albite-rich veins and development of

cauldron type structures. Metal deposits associated with early stages of rifting

include

• Breccia-filling Cu mineralisation such as at Messina copper deposit in

South Africa (Sawkins and Rye, 1979).

• Disseminated and stratiform copper deposits in central part of Africa (The

Zambian Copper Belt) and Kupferschiefer Cu deposits in Germany.

• Pb–Zn ores in general are insignificant in early-formed rift settings.

• Disseminations and veinlet – filling type Mo mineralisation such as in Oslo

rift (Geyti and Schoenwandt, 1979) and in East Greenland (Nielson, 1978)

In contrast to early stages of rifts, the advanced rifts constitute much larger

structures that are characterized by thick sequences of dominantly clastic rocks (5

to 15 km). The other general characters of advanced rifts are

• Basin filling is generally accompanied by contemporaneous faulting that

act as conduits for upwelling of ore bearing solutions in the basin.

• There may be development of second or third order basins, which are more

preferred repositories of metalliferous deposits.

• Later deformational episodes tend to obscure basinal faults. In some cases

such faults may act latter as locales of shearing.

• Volcanism in intracontinental advance rifts generally does not attain

significant proportions. On the other hand carbonaceous shales with

biogenic carbon may form part of the sequence.

Metallogenic characters of advance rifts include:

• The ore bodies have high aspect ratios (the ratio between lateral extent and

maximum stratigraphic width of ore body) and generally form tabular

bodies.

• The sulphide deposits in advance rift settings are Pb-Zn-Ag ores.

• Size of the deposits is one order more than other deposit types that makes

them attractive targets for exploration.

• Cu and Au lacking in such setting.

Some of the examples of advanced stages of rifts are parts of the Red Sea,

and basins that contain some of the giant Pb-Zn deposits like Sullivan, and Broken

Hill and Mississippi Valley Type deposits.

3.3.2 Arc-Related Ore Deposits

Sawkins (1990) defines principal arcs as linear, typically continuous belts

of batholiths, stocks and coeval volcanics generated above actively subducting

lithospheric slabs. Geometry of arc systems varies with change in tectonic

regimes. The tectonic regimes in turn are controlled by subducting plates. Based

on the inclination of subducting plates and accompanying rollback, Dewey (1980)

recognized three types of arcs (Fig. 3.2). Steeply dipping subducting plate leads to

development of extensional arcs, moderately dipping plates may result in neutral

or compressional regimes in arc, while the shallow dipping subducting plates

create active overthrusting of arc system on continental margins. The arc types

exert control on magma genesis and eventually the metallogeny associated with

arc sequences. For example extensional arcs are dominated by basaltic-andesite

and basalt-dacite bimodal volcanic rocks while compressional arcs may generate

andesite-dacite-rhyolite volcanic and associated tonalitic-granodioritic plutonic

igneous rocks (Sawkins, 1990).

3.3.2.1 Principal Arc Related Deposits

Prominent mineral deposits associated with compressional arcs are

porphyry type deposit particularly the one that straddle western margin of North

and South America (Udeya and Nishiwaki, 1980 and Sillitoe, 1980). Porphyry

copper deposits associated with calc-alkaline magmatism develop in this regime.

Such deposits are generally low grade-high tonnage deposits that are major

contributors of copper. In contrast to compressional arc systems involving

continental margin, the island arc porphyry deposits cluster around diorite to

quartz-diorite stocks. Copper deposits of Phillipine archipelago represent porphyry

type metallogeny in island arc setting (Sillitoe and Gappe, 1984). Less common

deposits associated with arc systems include cupriferous breccia pipe deposits,

magnetite and tungsten skarn deposits and precious metal bearing epithermal

deposits. In addition, some tin and tungsten deposits are associated with inner

sides of principal arc deposits.

3.3.2.2 Arc-Related Rift Deposits

Development of extensional tectonic regimes within the convergent plate

regimes may lead to rifting. The Lau Basin and Taupo Volcanic Zone (TVZ) in

New Zealand present the modern analogues of arc-related rift setting. The TVZ

contains huge amount of felsic volcanics (about 104 km3) that were erupted within

the last one million years. The TVZ sustains precious metal bearing sinters that

fall in the category of epithermal deposits. Indeed much of the understanding

about epithermal systems has come from detailed studies on this spectacular

volcanic zone (see Henley, 1985). In addition to the TVZ, the porphyry mineral

deposits of the Colorado Mineral Belt are believed to be related to rifts that are

spatially associated with back arcs in the western continental margin of United

States. Bookstrom (1981) associated development of back arc rift as a fall out of

ceased convergence at about 26 Ma in the Colorado Rocky Mountain region. Most

of the large, volcanogenic massive sulphide deposits in the Palaeozoic Iberian

Pyrite Belt in southern Europe were considered to be deposited in ensialic rifts

(Sawkins and Burke, 1980 and Sillitoe, 1982). Based on study of magmatism

Munha (1983) favoured an early back arc rifting environment for VMS deposit

bearing Iberian Pyrite Belt.

3.3.3.3 Kuroko-Type Massive Sulphide Deposits

The Kuroko type polymetallic massive sulphide deposits constitute an

important class of the volcanogenic massive sulphide deposits that are associated

with submarine emplaced volcanism. Sediment form an important part of the

deposit sequences. The Kuroko deposits form cluster in the northern part of the

Japanese archipelago, which is one of the most well studied arc system (Franklin

et al, 1981 and Ohmoto, 1996). The emplacement of dacite –andesite type

volcanism and also in some parts rhyolite – basalt bimodal volcanics, are linked to

rifting that effected subsidence and eventually formation of massive sulphide

deposits at volcanic intervals (see Martin and Piwinskii, 1972 and Silltoe, 1982).

The general characters of Kuroko type deposit are shown in Fig. 3.3.

3.3.4 Mineral Deposits Related to Divergent Plate Boundaries

Sea floor spreading is a major geodynamic process that drives tectonic

cycles including subduction and as such all major crustal level structures including

metalliferous basins are in some way or the other are related to it. This section

outlines salient features of mineral deposits that are associated with spreading

centers or divergent plate boundaries.

The present day oceans represent spreading center that generate new crust

(oceanic crust) along mid oceanic ridges. Much of the knowledge about petrologic

and geochemical features of oceanic crust comes from study of mafic-ultramafic

complexes (ophiolites) that appear as obducted masses in young orogenic sutures

(Coleman, 1977 and Constantinou, 1980); and also from direct study of mid

oceanic ridges (East Pacific Rise Study group, 1981). Discovery of black smokers

in proximity to mid oceanic ridges has emphasized the time and space relationship

between formation of massive sulphides and divergent plate boundaries.

Studies on ocean floor have brought out several locales of sulphide

accumulations. Well-studied sites include the East Pacific Rise (EPR), Juan Fuca

Ridge (Normark et al, 1983) and also the Ninety East Ridge in Indian Ocean. The

Red Sea is also considered to be zone of incipient spreading and is known to

possess metalliferous accumulations (Degens and Ross, 1969).

However, economic deposits that are mined for metals are from ‘inland’

ophiolite complexes that has basic volcanic-hosted massive sulphide deposits.

Important ophiolite-hosted massive sulphides include the Cyprus type deposits in

the Cretaceous Troodos Massif (Constantinou, op. cit.), Semail ophiolite in Oman

and late Proterozoic ophiolites in Bou Azzer in Morocco (Leblanc, 1981). The

massive sulphide associated with such oceanic crust are pyrite-dominated Cu-Zn

bearing sulphides that are conspicuously devoid of lead component. Lithosequence

in ophiolitic deposits is dominated by mafic-ultramafic rocks like harzburgite,

dunite, ultramfic cumulates, gabbro, sheared dykes and the overlying pillow lavas.

Thin veneers of siliceous sediments representing ocean bottom sediments form

part of the lithologic ensemble. In addition to Cu-Zn (±Au) deposits, the ophiolitic

complexes may contain chromite deposits such as the one that occurs in late

Precambrian ophiolite in Arabian – Nubian Shield. The rich cobalt deposits at Bou

Azzer in Morocco are associated with late Proterozoic ophiolite in Trans-Atlas

Mountains.

4. ORE GENESIS AND DEPOSIT TYPES

4.1 Ore Genesis

Ore genesis deals with various attributes that provide direct or indirect

evidence related to formation of ore deposits. It is outcome of simultaneous

consideration of detailed studies involving geological, geochemical, petrological,

isotopic and time-space relationship between ores and their repository set up. The

following attributes are important in understanding genesis of ore deposits:

• Size and shape of repository basin.

• Tectonic setting of the basin.

• Structural relations between ore body and the host rock.

Stratiform

Stratabound

Cross-cutting relationships

It is often combination of more than one type of relationship.

• Recognition of primary relict structures.

• Remobilation of ores.

• Isotopic studies. Sulphur isotopes are used in estimating role of magmatic

and seawater sulphates in the genesis of sulphides. Lead isotopic ratios 207Pb/204Pb are used for assessing source of metals.

• Fluid inclusion studies for determining salinity and temperature of

formation of ore fluids.

• Recognition of distinctive lithological units.

• Geochemical indicators

• Timing of ore formation

Syngenetic vs Epigenetic

Age of ore deposit formation vs age of host or causative rock.

Dating of ores-Rb-Sr, Pb-Pb and Re-Os.

Magmatic deposits are thought to be consanguineous with the host

rocks and their formation is linked with petrogenesis.

4.2 Deposit-Types

Based on various geological characteristics including genetic aspects most

of the ore deposit may belong to one of the following types:

4.2.1 Volcanogenic Massive Sulphides (VMS) Deposits

They are also referred to as the Volcanic-Hosted Massive Sulphide

(VHMS) deposits. The VHMS deposits are mainly hosted by volcanic rocks or are

developed in the direct proximity to the submarine-emplaced volcanic rocks. The

term, VMS deposits on the other hand is used for massive sulphide deposits that

form part of a volcanosedimentary sequence and may not be directly hosted in the

volcanic rocks or their hydrothermally altered products. At present the term VMS

type is being used in more liberal sense and a deposit may be called a VMS type

provided a magmatic source can be recognized in association with massive

sulphides and the cause and effect relationship can be demonstrated between the

two. An idealized cross section of a VHMS deposit is shown in Fig. 4.1. The

general attributes of the VMS deposits are summarized below:

• >50 % sulphides can be termed massive sulphides.

• Cause and effect relationship with volcanics, emplaced in submarine

conditions

• One or more massive sulphide lenses rooted in stockwork, when not

strongly deformed.

• Intense hydrothermal alteration in footwall.

• Generally occur at breaks in volcanicity.

• Variable size of deposits from a few thousand tones to >100 Mt.

• Generally deca-km size volcanic structure is required for generation of a

sizeable deposit.

• Convection currents effected by seawater serve in leaching metals from the

volcanics bearing lithopile.

• Genarally polymetallic base metal (Pb-Zn-Cu) and may contain Au± Sn.

• Sulphur mainly from seawater.

• Mineralogy:

- Dominant mineral: pyrite

- Abundant mineral: Chalcopyrite + sphalerite + galena

- Frequent minerals: Pyrrhotite + magnetite + hematite + cassiterite +

sulphosalts.

- Gangue: Quartz, sericite, chlorite, barite, gypsum, carbonates, talc,

tremolite, cordierite etc.

Important VMS belts of the world are the late Archaean Abitibi Belt in

Canada, Palaeozoic Iberian Pyrite Belt in southern Europe, Tasmanian Belt in

Australia and mid Proterozoic Fennoscandinavian belt in northern Europe. The

Mid-Proterozoic South Delhi Fold Belt in western India and Betul Belt in central

India are the two promising belts that host several occurrences and small VMS

deposits.

4.2.2 Sedimentary Exhalative (SEDEX) Type Deposits

The term SEDEX is derived from “SEDIMENTARY EXHALATIVE”

deposits. They are defined as sediment-hosted sulphide deposits that formed from

the discharge of hydrothermal fluids onto the sea floor. They differ from VMS

type deposits in lacking a substantial volcanic component, absence of conspicuous

hydrothermal alteration in the footwall and in being bimetallic (Zn-Pb) in contrast

to the commonly observed polymetallic (Zn-Pb-Cu) nature of volcanogenic

deposit. An idealized section showing the principal attributes of most SEDEX

deposits is shown in Fig. 4.2. The general characters of SEDEX deposits are:

• The bulk of the ore contained in a stratiformed sulphide body has a high

aspect ratio i.e. the ratio of lateral extent of the body to its maximum

stratigraphic thickness. Most of the Sedex deposits – have an aspect

(length -width) ratio of 20 or more. Most common morphology is

represented by sheets or tabular lenses of stratiform sulphides up to a few

tens of metres in thickness and a more than a km in length.

• Most SEDEX deposits are hosted by organic-rich sedimentary rocks.

• Role of extensional faults is important in discharging hydrothermal fluids.

• The stratiform body composed of sulphides, other hydrothermal products

such as carbonate, chert, barite and apatite and non-hydrothermal clastic

rocks are sedimentary rocks.

• The dominant sulphide minerals in most of the deposits is pyrite although

in some deposits pyrrhotite is most dominant.

• The main economic minerals are sphalerite and galena, although

chalcopyrite is also present in substantial amounts in some of the deposits (

Mt. Isa Cu and Rammelsberg).

• SEDEX deposits are important resources for Zn and Pb and accounts for

more than 50% and 60% of the world’s reserves of these elements

respectively. The proportion of the world’s primary production of Zn and

Pb from SEDEX deposits, however, is significantly lower due to fine-

grained nature of ores that leads to poor recovery of metals.

• The main economic minerals of SEDEX deposits are sphalerite (ZnS) and

galena (PbS) and the most typical manifestation of the ores is as regularly

layered sulphides that are interbedded with other hydrothermal products,

such as chert or barite and host lithologies.

• The average size of Sedex deposits which is about an order of magnitude

greater than VHMS or MVT deposits also contributes to the disparity

between the ratio of “in ground reserves to production rates for Sedex

deposits.

• Sedex deposits are generally larger in size and higher in base metal grades

compared to VHMS deposits. Sedex deposits also commonly contain

relatively low iron sulphide sulphide, which makes them more attractive

from an environmental viewpont.

• The SEDEX deposits are generally categorized as Broken Hill type and

Irish type. The Broken Hill deposit is the largest and richest Pb-Zn deposit

with reserves standing at 180 Mt and metal contents placed at 11.3% Pb,

9.8% Zn and 0.2% Cu. They are characterised by presence of chemogenic

sediments rich in manganese and iron. Rajpura-Dariba and Agucha in

central Rajasthan are examples of SEDEX type of deposits.

• The Irish type SEDEX deposits are formed slightly below tha sea floor and

are hosted predominantly by carbonates. They may often show epigenetic

features (Goodfellow and Lydon, 2007).

4.2.3 Mississippi Valley Type (MVT) Deposits

The MVT deposits are considered to be a subtype of SEDEX deposits. The

salient features of MVT deposits are :

• MVT deposits are epigenetic, stratabound, carbonate-hosted ore bodies

composed predominantly of sphalerite, galena, iron sulphide and

carbonates.

• Deposits occur mainly in dolostones as open space fillings, collapse breccia

and /or replacement of carbonate host rock.

• The deposits are epigenetic, having been emplaced after lithification of

rocks.

• MVT deposits originate from saline basinal metalliferous fluids at

temperatures in the range of 75 to 200º C.

• Located in carbonate plateform rift-related settings, typically in undeformed

orogenic foreland rocks.

• Individual deposits generally less than two million tonnes, are zinc-

dominated and possess grades that rarely exceed 10% (Pb-Zn).

• The deposits characteristically occur in clusters, referred to as ‘districts’.

4.2.4 Porphyry-Type Deposits

Porphyry deposits are important source of copper and molybdenum in the

world. A generalized model of porphyry copper deposit is given in Fig. 4.3. The

main attributes of porphyry deposits are :

• Porphyry deposits are large, low- to medium grade deposits in which

primary ore minerals are dominantly structurally controlled and which are

spatially and genetically related to felsic to intermediate porphyritic

intrusions.

• They are world’s most important source of Cu and Mo, and are major

source of Au, Ag and Sn.

• Large size, structural control (veins, vein sets, stockworks, fractures and

breccias) distinguish porphyry deposits from skarn and epithermal type

deposits.

• Associated igneous rocks vary in composition from diorite-granodiorite to

high silica granites that are mesothermal.

• Porphyry deposits occur in a series of extensive, relatively narrow, linear

metallogenic provinces.

• Generally associated with Mesozoic to Cenozoic orogenic belts in western

parts of North and South Americas.

• Most favoured tectonic settings are principal arcs at active continental

margins.

4.2.5 Greenstone-Hosted Quartz-Carbonate Vein Deposits

The greenstone-hosted quartz-carbonate vein deposits are also referred to as a

class within the broad category of lode gold deposits that are essentially hydrothermal

deposits (Lydon, 2007). These deposits represent deeper part of the lode gold system,

the upper part is represented by the epithermal gold deposits. Reduced intrusion-related

deposits occupy the intermediate position between the two. Au is the principal

commodity in the lode gold deposits. The general attributes of the greenstone-hosted

quartz-carbonate vein deposits are outlined below:

• These deposits are formed at depths of 5 to 10 km, and are variably termed

as orogenic, mesothermal and shear zone-controlled gold deposits.

• They consist of structurally-controlled quartz-carbonate vein deposits that

typically occur in green schist facies metamorphic rocks.

• The majority of deposits are adjacent to major deep-seated reverse oblique

faults, particularly dilational zones of various tectonic settings.

• The gold deposits are associated with large-scale carbonate alteration,

particularly along major faults.

• The mineralisation generally took place during later stages of orogenic

crustal shortening post-dating peak metamorphism of the host rocks.

• The mineralisation shows affinity with felsic to intermediate intrusions of

the lode gold district.

• Most orogenic gold deposits were formed during the intervals 2800 to 2550

Ma, 2100 to 1800 Ma and 600 to 50 Ma.

• Golden Mile complex in Kalgoorlie, Australia is the largest deposit of this

category with more than 1800 tons of Au. The Kolar Gold deposits in India

are also included in this category of deposits.

• The average grade varies from 5 to 15 g/t and recovery rate is about 90% of

contained gold. Au/Ag ratio 5-10.

4.2.6 Reduced Intrusion Related Gold Deposits

The reduced intrusion related gold deposits fall in the broad category of

lode gold deposits. Important attributes of such type of deposits are :

• This term is restricted to low-grade deposits associated with reduced

granitoids that occur on the foreland side of Phanerozoic continental arcs.

• They are formed at 1 to 5 km depth, generally in the same way as porphyry

deposits.

• The deposits have direct genetic link with a cooling felsic intrusion during

their formation

• The most characteristic feature of the RIRG is intrusion-hosted, sheeted

array of thin, low- sulphide quartz veins with a Au-Bi-Te-W signature

which typically comprise bulk tonnage, low grade Au resources.

• The reduced state of intrusion is characterised by pyrrhotite, and quartz

veins that host methane-rich intrusions.

4.2.7 Epithermal Deposits

These deposits also constitute a type of hydrothermal deposits that are

generally associated with younger volcanic and sub-volcanic rocks commonly in

the island arc setting. It is one of the type of lode gold deposits that are most

intensely studied due to availability of such systems that are operative in the

present time. Salient features of epithermal deposits are:

• Epithermal Au (±Ag) deposits form in the near-surface environment, from

hydrothermal systems typically within 1.0 Km of Earth’s surface.

• They are commonly found associated with centres of magmatism and

volcanism.

• Hot spring deposits both liquid and vapour dominated geothermal systems

are commonly associated with epithermal deposits.

• The characteristically shallow environment is marked by rapid changes in

temperature and pressure of hydrothermal fluids that may be accompanied

by boiling and mixing with other fluids.

• Average Au grade varies between 1 and 10 ppm.

• Epithermal deposits may also be characterised by presence of other volatile

elements like Hg, Sb etc.

• Meteoric water is an important constituent of hydrothermal fluids that form

epithermal ore deposits. Low δ18O/δ16O isotope ratio (against +5‰ δ18O in

magmatic waters) provides good signature of role of meteoric water in ore

fluids.

• High sulphidation (containing S, C, Cl) type also called quartz (-kaolinite)-

alunite, alunite-kaolinite, Au, or high sulphur forms part of the epithermal

system.

• Ex. Taupo Volcanic Zone i New Zealand and Puga Valley in India.

• Low sulphidation type is characterized by occurrence of adularia (low

temperature K-feldspar of rhombohedral shape) –sericite in hydrothermal

assemblage.

• Deposits are generally linked with continental volcanism or magmatism

that occur mostly in principal arc terrains.

From exploration point of view it is important to recognize sinters and

hydrothermal alteration associated with zig-saw puzzle type breccias. Intensive or

subvolcanic magmatic structures that are younger in age are favourable targets for

exploration. The epithermal system, being close to the surface of the Earth, is

likely to be eroded in older volcano-plutonic magmatic terrains. Diagnostic

features of the epithermal systems are:

• Sinter deposits

• Presence of lamellar calcite.

• Quartz pseudomorphs

• Vug-fillings and botryoidal features.

• Zig-saw puzzle type breccias.

Sb, As and Hg act as pathfinder elements for epithermal type deposits

during geochemical exploration. An idealized model for epithermal type precious

metal deposits is shown in Fig. 4.4

4.2.8 Magmatic Ore Deposits

Except placer deposits most of the deposit types are indirectly related to

various magmatic sources. However, the magmatic deposits differ from various

other deposit types in the sense that ores are physically confined to the magmatic

rocks and as a corollary are generally ascribed to the processes of crystallization

from magma. The important attributes of magmatic deposits are outlined below:

• Ore deposits generally crystallize directly from the magma and the resultant

ore deposits are consanguineous with the magmatic rocks. The deposits are

also called orthomagmatic deposits.

• The deposits can be considered as part of the rock and its genesis can be

linked with petrogenesis.

• Formation of such deposits does not involve any significant hydrothermal

activity and therefore, wall rock alteration around ore bodies is generally

lacking.

• Enrichment of metals to ore grades in magmatic deposits necessitates

induction of some sulphur from surrounding sources. Likewise, mild

hydrothermal fluxes may be generated within the ambience of magmatic

deposits like in deposits of REE fluorcarbonates that are associated with

carbonotite magmatism.

• Deposits of magmatic origin include nickel, copper, platinum group of

elements, chromite, Fe-Ti-V , REE and diamonds that occur variously as

sulphides, oxides and fluorcarbonates in mafic – ultramfic rocks,

carbonatites and kimberlites.

• Nickle, copper and PGE are associated with sulphide concentrations in

mafic and ultramafic rocks that generally generate in upper mantle.

Meteorite-impact generated mafic sheets that contain Ni-Cu deposits in

Sudbury in Canada are also grouped as magmatic deposits. Grade of nickel

in Sudbury deposits varies from 0.7 to 3.0 % and Cu from 0.2 to 2.0 %.

• The world class deposits of platinum and associated elements occur in the

Merensky Reef that is associated with Bushveld Complex in South Africa.

The most remarkable feature of these deposits is the consistency of the

cumulate layers that form the deposit. The grade of PGE ranges from 4.9 to

7.3 g/t with Pt/Pd ratio in Merensky Reef being always greater than 1.

• Chromite deposits in Sukinda Ultramafic Complex in Naushai area, Orissa

are an example of orthomagmatic deposits.

4.2.9 Weathering-Related Deposits

Weathering-related ore deposits form due to surficial processes that lead to

concentration of ore minerals into a workable deposits. Any ore mineral that can

withstand chemical ‘corrosion’ may form placer-type deposits. Oxidation (and

hydration) is the most common process that decomposes various metallic sulphide

minerals. Selective removal of undesirable constituents may lead to enrichment of

useful components that eventually form ore deposits. Deposits affliated to

palaeoweathering surfaces may also be described as unconformity-related

deposits. Some of the attributes of weathering and unconformity ore deposits are

given below:

• Preservation of weathering-related deposits necessitates a favourable

topographic set up. They are generally blanket-type deposits that have

limited thickness.

• Weathering processes lead to removal of ‘undesirable’ elements and

enrichment of ‘resistate’ minerals that form deposits. ‘Bouldery’

manganese ores, kyanite-sillimanite, diamond, gold and cassiterite may

form this type of deposits.

• Secondary deposits formed due to hydration related to weathering include

Fe-Al, Mn-oxides, Ni-silicates, Ti and Au-bearing laterites etc.

They include neoformed authigenic products (Mookherjee, 1999). Bauxite

deposits of India notably the East Coast bauxite is one of the fine example

of such type of deposits that have huge reserves of aluminium ore.

Lumkrythang deposit over the Sung valley alkaline ultramafic-carbonatite

complex in Meghalaya is a high Ti bearing bauxite that is valued as ore for

Ti. Most of the Nb metal supplies are met from the weathering –related ore

deposits of the world.

5. GEOLOGY OF ZAWAR LEAD – ZINC BELT

The area around Zawar village in southern Rajasthan, western India is

studded with several impressive old workings that were important source of zinc-

lead-silver ores in ancient India. It has a glorious history in mining and metallurgy

of base metal ores and silver (Fig. 5.1). More importantly, the ancient inhabitants

of Zawar hold distinction of perfecting the art of smelting Zn-Pb-Ag ore and in

producing zinc metal several centuries earlier than the western countries. Presently

it has several underground mines that produced Pb – Zn ores.

5.1 Regional Geological Set up

The Palaeoproterozoic Aravalli Supergroup forms southern part of the

Aravalli Mountain Range in western India. The regional trend of the Aravalli

rocks is controlled by disposition of pattern of the banded gneissic rocks that form

basement in the east and southeast of Udaipur. The NE-SW trending rocks in

Nathdwara-Delwara sector gradually swerve to N-S in Udaipur and to its

immediate south, beyond which it shows a ‘syntaxial’-type bend around Zawar. In

Zawar area the regional trend of rocks is WSW-ESE that veers to NW-SE in the

southeastern Rajasthan. This sharp change in the regional trend of Zawar area is

attributed to reorientation of regional stress system due to presence of a rigid

Archaean basement inlier around Sarara (Fig. 2.1).

5.2 Stratigraphy of the Zawar Area

The rock sequence exposed in the Zawar forms southern extension of the

type Aravalli Supergroup around Udaipur. Subsequent to Heron’s mapping

Strackzek and Srikantan (1966) mapped the Zawar area in detail for understanding

the geology of the lead-zinc mineralization. They presented a detailed lithological

and stratigraphic account of the rocks. Strackzek and Srikantan (op.cit.) divided

Zawar sequence of rocks into two series, the lower Sisa Magra Series and the

upper Tiri Series.

Sisa Magra Series

This series occupies Siasa Magra (magra in local dialect means a prominent

hill) and Dantali Magra in southeastern part of the Zawar village Fig. 5.2). This

series is very proximal to the Sarara inlier that exposes schists and gneisses of

preAravalli age. The series comprises of two stages The Sisa Magra Stage and

Dantali Stage that are separated by an unconformity.

The Sisa Magra Stage comprises thickly bedded polymictic well sorted

conglomerate and grits. The conglomerates contain angular to well rounded

pebbles in quartz matrix. They occur as thick wedge-shaped deposit bordering

northern part of the Sarara inlier. The Dantali Stage mainly comprises well

bedded, medium-grained, white quartzite and brown ferruginous dolomite.

Tiri Series

The Tiri Series is predominantly argillaceous with substantial amount of

quartzite, greywacke and dolomite. The series is divided into five stages.

The Kanpur Stage mainly comprises of fine-grained well-bedded quartzite

with subordinate grayish brown dolomite. The two lithological units grade

laterally into each other. Several primary sedimentary structures are observed in

phyllite, siltstone and dolomite (Fig. 5.3, 5.4 and 5.5).

The Kathalia Stage has sharp contact with the Kanpur Stage. The phyllite

constitutes bulk of the stage with dolomite that form thin yet fairly persistent band.

The grayish green phyllite grades upwards to carbonaceous phyllites. The Mandli

Stage comprises dark grey conglomeratic greywackes which contain minor

intercalations of phyllite and impure quartzites. In Mochia and Balaria areas the

gritty grewacke are dolomitic while at other places they are siliceous. Several

primary sedimentary structures including the characteristic graded bedding and

flame and load cast structures are recorded in greywacke.

The Borai Magra Stage is subdivided into two members, the Mochia

Member and the Harn Quartzite Member. The dolomite and its variants exposed in

Mochia and Balaria hills constitute the most important rock types that host

economically significant lead-zinc mineralisation. Dolomite and its variants

include fine-grained bluish grey dolomite, arkosic dolomite and conglomeratic

dolomite (Fig. 5.6). The later variant occurs on Mochia Magra, north of Rava and

at Zawarmala hill. The Harn quartzite derives its name from Harn Magra. The fine

to medium grained Harn quartzite is conglomeratic at places. Its contact with the

rocks of the Mochia Member appears tectonic and presumably faulted.

The Zawar Stage rocks overly Harn quartzite. This stage comprises

phyllites and slates with minor interbands of quartzite, dolomite and its facies

variants. An important rock of the Zawar Stage is dolomite – quartzite breccia that

form an unusual lithounit in the sense that it is conformable to the enclosing

phyllites and is clast-supported with low matrix content. In all probability it

appears to be an autoclastic breccia that formed along longitudinal faults. The

Rava quartz-phyllite occurs to the northeast of Zawar village. It is a well-foliated

rock that contains quartz, sericite and feldspar.

Metadolerite occurs as dykes in Kathalia carbonaceous phyllite and

dolomite. Presence of visible sericite and chlorite indicates that it has undergone

some metamorphism unlike the relatively fresh dolerite dyke that cuts across the

mineralized body at Mochia Magra. The dyke of dolerite shows branching and

shows remarkable depthward persistence.

5.3 Structure and Mineralisation

The geometry and orientation of minor and major structures in the Zawar

area differ considerably from rest of the Aravalli rocks in southern Rajasthan.

High percentage of exposures and favourable lithological attributes like

presence of fine-grained laminated to thinly bedded quartzite, dolomite-phyllite

intercalations, phyllitic dolomite and excellent state of preservation of structural

features make Zawar a unique place for study of interrelationship between

structure and mineralisation. Besides, presence of extensive old workings and

ongoing underground mining operations allow understanding of zinc-lead

mineralisation in the Zawar belt.

In most of the lithological units the primary structures like bedding, thin

alternation of layers various composition, graded bedding and colour banding etc.

are well developed at most of the places. A very thinly spaced cleavage is

developed in most of the rocks at Zawar.

Presence of several set of cleavages, deformed lineations and refolded folds

are some of the evidence that indicate polyphase deformation in Zawar area. A

large scale first generation fold (F1) is mapped in the western part of the

Zawarmala Hill. It is an inclined fold that plunges moderately towards SW with

North-South axial trace (Fig. 5.7). The Zawar area records a major departure in the

orientation of axial trace of the large scale second generation folds. Mochia and

also its extension in west at Rava show E-W axial trace of large-scale second

generation fold (F2). Such a trend virtually marks a syntaxial bend in the otherwise

NE-SW to N-S regional trend of the Aravalli F2 folds. The swerving of the

regional Aravalli trend is attributed to the presence of a rigid basement at Sarara

that lies to the immediate south of Zawar. Modal plunges and axial traces of

different generation of folds are shown in Fig 5.7 Some of the structural features

developed in the Zawar and adjoining areas are shown in figures 5.8 to 5.17.

5.4 Genesis of Zawar Pb-Zn Ores

Zinc-lead mineralization was known to inhabitants of Zawar area about

2000 years ago. Presence of several old workings and heaps and mounds of retorts

bear testimony to the once flourishing metal industry at old Zawar village.

Most of the Zn-Pb (Ag) mineralization in Zawar area is confined to

dolomite and its variants that belong to the Baroi Magra Stage. Small occurrences

of sulphide mineralization are also reported from quartzite and carbonaceous

phyllite (Strackzek and Srikanatan, 1966 and Bhattacharya, 2004). However, the

economically significant mineralization occurs in dolomite and is thus

stratabound.

Fig. 5.2 : Geological Map of Zawar Area, Udaipur District, Rajasthan

0 1 2 Km

MOCHIA MAGRA

BALAR

IA

MA

GR

A

BO

RA

I M

AG

RA

DA

NTA

LI M

AG

RA

KATHALIAM

AGRA

ZA

WA

RM

AL

A

HARNM

AGRA

730

40' 730

45'

ZAWAR

240

19'

240

19'

SMS

SMS

Sisa Magra Stage gritty andconglomeratic quartzite

Dantali Stage massive to wellbedded quartzite

Kanpur Stage dolomiteTS

TS Kanpur Stage quartzite

Kathalia Stage phyllite

Kathalia Stage dolomite

TS

TS

TS

TS

TS

TS

TS

Mandli Stage grewacke and its variants

Borai Magra Stage grey dolomite

Borai Magra Stage arkosic dolomite

Borai Magra Stage dolomiticconglomerate

Borai Magra Stage fine grained quartzite

TS

TS

TS

TS

TS

TS

TS

TS

Borai Magra Stage carbonaceous anddolomitic phyllite

Borai Magra Stage Harn quartzite

Zawar Stage autoclastic breccia withdolomite and quartzite

Zawar Stage conglomerate

Zawar Stage quartzite

Zawar Stage dolomite

Zawar Stage phyllite

Zawar Stage Rawa quartz - phyllite

Meta dolerite

Dolerite

Map after Straczek and Srikantan, GSI,1966SMS = Sisa Magra Series TS = Tiri Series

240

22'

240

22'

D

MD

D

D

MD

Surface indications of mineralization include presence of oxidation zones

and limonitic veins. Well developed gossans are rare. At places the sulphide veins

can be

observed a few centimeters below the surface. Iron sulphides are generally less

abundant than the sphalerite and galena. Banded pyrite-sphalerite ores are reported

from Zawarmala area. Zone of oxidation approximating to weakly developed

gossan occur only over Zawarmala deposit. Sphalerite and galena are the major

ore minerals in Zawar ores. At places minor chalcopyrite is associated with

sphalerite. Pyrite, arsenopyrite and pyrrhotite occur as associated minerals.

Extensive data are available on size, shape and disposition pattern of zinc-

lead ore bodies of Zawar area. Detailed mapping of eastern part of Mochia Hill

indicates that longer axis of old workings make an angle with the primary

layering. Their depthward extension appears to be parallel to plunge of F2 folds. In

plan some of the old workings virtually demarcate zone of extension that are

geometrically related to extension accompanying dextral shears. The geological

plan at 240 mRL in Mochia mine clearly brings out discordant and anastomosing

nature of zinc-lead ore bodies (Fig. 5.18). Their stratabound nature is also obvious

in plan. A cross section through the Mochia mine also reveals that despite

anastomosing nature of ore bodies the mineralization is confined to dolomites

(Fig. 5.19). Barring banded sphalerite-pyrite ores that simulate primary layering at

Zawarmala, the zinc-lead mineralization, by and large is remobilized along

favourable structural locales. Sarkar and Mukherjee (2004) have reported

cataclastic fabric displayed by tensional fracturing of pyrite aggregates, cataclastic

flow in sphalerite and dislocation creep in galena. Banerjee et al (1998) believed

that bulk of the Zawarmala mineralization was translocated into en-echelon pattern

in Mochia - Balaria sector. They attributed such a distribution pattern to R-shears

and extensional fractures that were related to development of F3 folds. It may be

important to mention here that no breccia type ore has been reported from Zawar

area.

Initial studies on zinc-lead mineralization in Zawar invoked hydrothermal

origin (Ghosh, 1957 and Mookherjee, 1964). Granite intrusions were considered

responsible for generation of ore-bearing hydrothermal fluxes. However,

confinement of bulk of the economically significant Zn-Pb mineralisation in

dolomite and its variants and non-availability of granites in proximity to sulphide

mineralization on surface or in underground sections, are some of the factors that

do not support hydrothermal origin of mineralization.

Detailed metallogenic studies indicate that sulphide mineralization in

Zawar area occurs in carbonaceous phyllite and dolomite and its variants. In

carbonaceous phyllite it is mainly pyrite with minor amounts of sphalerite and

galena. In addition to sulphides Basu (1976) reported native sulphur and

gypsiferous and ferruginous shales (Phyllites) from eastern part of the Mochia.

The native sulphur alternated with carbonaceous matter. Several depositional and

diagenetic features have been observed in zinc-lead ores of Zawar (Chauhan, 1976

and Poddar, 1965). The depositional features recorded include rhythmites of

pyrite-sphalerite (±galena) laminated pyrite – carbonaceous phyllite and slump

structures. Important diagenetic features in ores are flame structures, framboidal

pyrite and pyrite replacing sphalerite forming atoll structure (Bhattacharya, 2004).

In contrast, dolomite, the main host rock shows vein type ore which often makes

an angle with the primary layering. Cognizance of the following geological

features is required before proposing genesis of the Zawar lead-zinc ores:

• There is no volcanic component in the lithosequence of Zawar mines area.

• There are no granitic intrusions mapped in Zawar area. Nor are they

reported from underground mine sections.

• Economically significant mineralisation is stratabound – hosted by

dolomites and its variants. The higher values of Zn and Pb in bedrock

dolomite are 380 ppm and 45 ppm respectively (Sarkar and Banerjee,

2004).

• There is general paucity of iron sulphides in the ore assemblages. As a

consequence there is no development of pronounced zones of oxidations.

Pyrite-rich base metal poor mineralisation occurs in carbonaceous phyllites

that occur as thin layers in dolomite.

• There is no report of solution breccias in carbonates and breccia-fill ores

are conspicuously lacking.

• The depositional environment deduced from the lithologic attributes

indicate that Zawar metasediments were deposited in a basin that fluctuated

from shallow to deep water conditions. Presence of gypsum and

ferruginous shales alongwith native sulphur and proximity of rocks to the

Sarara inlier indicate shallow depositional environment. Absence of

stromatolites and presence of greywacke in the sequence are, however,

suggestive of deep water conditions for deposition of a part of Zawar mine

sequence. Probably Zawar rocks were deposited in a basin that fluctuated

from shallow to deep water conditions.

• Sulphur and C-O isotope values in sulphides and carbonates of Zawar area

show considerable variations. These values indicate normal marine

signatures to the effect of biogenic activity as indicated by presence of light

carbon (see Sarkar and Banerjee, 2004 and references therein).

• There are strong evidence of remobilization of ores.

• Lead isotopes available on Zawar ores indicate that the lead is more

radiogenic (207 Pb/ 204 Pb ~15.68) and can be interpreted as derived from

sialic basement rocks.

The above evidence suggest that the Pb-Zn metals available in the host

dolomite and also in carbonaceous phyllite formed proto-ore that was later

remobilized in dilational or extensional fractures developed during deformation of

Zawar rocks. Dewatering of basinal fluids was probably responsible for

remobilization of ore and formation of economic grade of Pb-Zn mineralization.

Fig. 5.1 Retorts used by ancient miners for Zinc smelting, Invention of Zinc smelting has taken place

at Zawar. Location : Furnace site preserved by HZL, Zawar

Fig. 5.3 : Bedding in Zawar phyllite having meta

siltstone and quartizitic inter bands. Note the

presence of load cast structures. Location: Near

GSI Training Institute, Zawar.

Fig. 5.4 Load cast at the contact of dolostone

and metasiltstone, Zawar Mine Sequence.

Location : Rava, north of old Zawar Village,

Udaipur district

Fig. 5.5 : Channel fill in quartzite with interbands

of phylite. Bedding in channel fill conforms to the

shape of the channel. Location Nala Section, East of

Training Institute campus, Zawar

Fig. 5. 8 : Very tight isoclinal reclined folds (AF ) in

bedding of dolomite. Note thickening at hinges and

attenuated limbs. Manpur, SE of Zawar.

1 Fig. 5.9 : Tight S-shaped (AF ) in bedding of

dolomite. Note shearing parallel to axial plane.

Manpur, SE of Zawar.

1 folds

Fig. 5.10 : Bedding - Cleavage relationship. Cleavage

is represented by dark seams of insoluble material

(Pressure solution cleavage) Apparent shift in

cleavage is also visible. Location : Rava, Zawar.

Fig. 5.11 : Crenulation cleavage and microlithons

in phyllite with thin interbands of quartzite.

Location : East Mochia, Zawar Mines.

s

Fig. 5.6 : Polymictic conglomerate having clasts of

granite, quartzite and phyllite embedded in

carbonate matrix. The conglomerate occurs at the

contact of greywacke and the dolostone hosting

lead - zinc mineralisation. East Mochia, Zawar

Fig. 5.12 : Deformed mullions. Tidi river section,

near Ramanath Temple, Zawar

Fig. 5.13 : Mullions, both cross sectional and

longitudinal view, developed in quartizitic

interbands in phyllite. Deraphala School, Zawar.

Fig. 5.14 : Refraction of cleavage. Cleavage is at

higher angle to bedding in quartzitic band than in

phyllite. Deraphala School, Zawar.

Fig. 5.15 : Quartz veins forming ladder-like

structures in dolomite. They represent extensional

features in the rock. Rava area, Zawar.

Fig. 5.16 : Tension gashes related to two successive

phases of deformations. The bluish grey coloured

carbonate tension gashes, indicating sinistral

movements, are associated with F folding. These

carbonated tension gashes behaved as a competent body

during

1

F folding and quartz tension gashes developed as

a result of second deformation. Rava, Zawar.

2

Fig. 5.17 : Faulting along axial plane cleavage (S2).

Quartz-Carbonate veins along the fault plane. Note that

quartz veinlets are at an angle to the trend of the fault

plane. Rava, Zawar.

100 m

Fig. 5.18 : Geological plan at 240 mRL showing disposition of ore bodies, Mochia mine

Fig. 5.19 : Geological cross section across theeastern part of Mochia mine showinganastomosing pattern of the ore bodies. Note thestratabound nature of the ore body.

1

6. STAGES OF EXPLORATION

6.1 Introduction

With most of the surface indications of minerlisation including old workings

already explored, the task of exploration is aimed to locate hidden or concealed

deposits. It is a multifarious activity that applies all the known geological,

geophysical, geochemical and remote sensing techniques in delineating targets areas

for detailed exploration. The exploration involves various stages that are adopted in a

sequential mode. This chapter outlines various such stages in the following sections.

6.2 Reconnaissance Exploration

The exploration strategy today is more evolved than in the past and it has

resorted to a multidisciplinary approach. Mostly, large exploration companies launch

exploration programs using all the exploration tools in a sequential mode. Such

exploration launches require a lot of background preparation like collation of previous

work on geological, geochemical and geophysical, creation of GIS database and

launching all scales of work from regional geology to airborne geophysics and

regional geochemistry. The uses of various GIS softwares for exploration are a

necessity for the interpretation of data in order to guide the exploration efforts

scientifically. The purpose of reconnaissance exploration is to delineate prospecting

targets. Even after adopting a rigorous scientific approach, the success rates for

striking bonanza are remarkably low.

6.2.1 Regional Geological Set Up

A very important step required before initiating reconnaissance exploration is

procurement of regional geological maps as such maps alone can provide vital input in

deciding the type of mineral commodity that can be explored in a given terrain. Apart

from distribution pattern of geological formations a map useful for reconnaissance is

ought to show mineral occurrences. Some economic input from working deposits may

be useful in predicting grade-tonnage type model. A good regional map also gives a

fair idea about lithosequences that eventually help in understanding type of ore

2

deposits that can be expected in it. For example small, isolated, intracratonic

metasedimentary basins in the Bhilwara Supergroup (which includes the Banded

Gneissic Complex) in western India constitute well known tract of SEDEX type of

Pb-Zn deposits. Likewise, western tract of the Proterozoic South Delhi Fold Belt

exposes a volcanosedimentary lithosequence that hosts polymetallic (Pb-Zn-Cu)

volcanogenic massive sulphides (VMS) type deposits; and the regional geological

milieu of southeastern Rajasthan is akin to Iron Oxide Copper Gold (IOCG) type set

up and this belt is indeed studded with several Cu-Au prospects and deposits.

Overlays of mineral occurrences, working mines and major lineaments with geology

provide clues to regional controls on mineralization. Information on abandoned mines

is sought at this stage of reconnaissance to strengthen geological understanding of the

nature of mineralization. The area to be applied for reconnaissance exploration

(Reconnaissance Permit or more popularly referred to as the RP) is mainly decided on

the basis of the regional geology and structure of the prospective tract.

6.2.2 Aero-geophysical Surveys

Airborne geophysical surveys are very effective in very fast reconnaissance of

large area. Such surveys are particularly useful in inferring geological formations in

covered and inaccessible terrains. Aeromagnetic surveys can be undertaken from

higher altitudes but for EM signatures the flight of the aircraft is maintained below

120m. Heliborne systems can provide data taken from very close to the grounds. It is

particularly very useful in airborne gravity surveys. Flight lines are planned at high

angle to the trend of formations and spacing between two flights lines vary from 200m

to 500m. The Geological Survey of India own an aircraft which has onboard systems

that generate data on EM, Total (γ –ray) count, U, Th, K and aero-magnetics (Fig. 6.1

and 6.2). Principles of geophysical method are same whether the measurements are

on ground, or inside a borehole, from air or over the ocean. It is the design, size

accuracy and resolution of instruments that varies on the type of surveys. The

instrument sensors in airborne systems being away from mineralization must have

more accuracy and better resolution. The power requirements of such instruments are

larger. Similar is the situation for marine surveys.

3

Fig. 6.1 : “Twin Otter” aircraft of the Geological Survey of India

Fig. 6.2 : Airborne Instruments on board

4

Magnetic, E.M and radioactivity detecting instruments and recently airborne

gravitymeters are being used for airborne surveys offering increased speed and

efficiency. Compared to the groundwork, airborne measurements imply a decrease in

resolution which means that adjacent geophysical indications tend to merge into one

another, giving the impression of only one indication. Based on the objective, a

decision is taken to carry out airborne surveys by fixed wing aircraft or by helicopter-

borne instruments. Measurements in some airborne EM systems are in Frequency

Domain using 3 frequencies while in other systems the measurements are in Time

Domain.

In addition to direct application for locating mineralized bodies, the

aeromagnetic data is most utilized in mapping the structures favorable for occurrence

of mineralization. Airborne EM essentially maps the electrical conductivity. Airborne

EM and aeromagnetic data together are main guiding factors in identifying shear

zones and fault zones. Discrete EM and Magnetic anomalies at the same locale are

generally useful for inferring ore bodies. Four parameters viz. Total (γ -ray) count, U,

Th and K counts are measured in radiometric surveys. As a routine, six maps are

produced for multi-parametric airborne surveys i.e. Total Field Aeromagnetic map,

EM anomaly map, Total (γ -ray) Count, U, Th and K maps. Gravity maps are in

addition to these maps. If required, derivative maps, magnetic susceptibility map, RTP

and Analytical Signal maps, radiometric ratio maps and other maps are produced.

These maps are handy tools in very fast reconnaissance of vast areas.

6.2.3 Application of Remote Sensing Techniques

Remote sensing includes various mapping techniques that are carried out from

airborne systems. Such techniques have received a great deal of attention especially in

connection with space exploration projects. The sensors can be fitted either in aircraft

or on spacecrafts. In fact aero-geophysical methods are also a type of remote sensing

methods. However, it is mainly the aerial photography and imaging systems that are

useful in reconnaissance exploration. Aerial photographs provide a means of quick

reconnaissance and even in areas of poor exposures it is possible to infer formational

5

contacts with reasonable confidence. Folds and lineaments are also well picked up in

aerial photographs. Aerial photo-interpretation with some field checks remained

popular techniques for several decades. They are still used advantageously for

mapping on larger scales. Satellite photographs and imageries taken from spacecrafts

may be useful as an aid to regional mapping in broadly defining the favorable areas of

mineralization. The hyperspectral data are currently in great demand by various

mineral exploration agencies. These data nowadays occupy prime place in

reconnoitary exploration programs. The hyperspectral data are particularly useful in

arid terrains as they help in inferring chemical variations in zones of hydrothermal

alteration. However, such data are not readily available to all the exploring agencies

and their interpretation requires refinement.

6.2.4 Regional Geological Mapping

The national surveys and state organizations publish regional geological maps

on 1:250,000 or on 1: 100,000 scale. However, specific information required on

prospective areas may be lacking or missing in such maps. Regional geological

mapping may be carried out on 1:50,000 scale and special thematic mapping for

example in the Geological Survey India is carried out on 1:25,000 scale. The later

mapping programs are with specific objectives and are supposed to generate

additional details on lithosequences, structure and metallogenic aspects. Such

mapping programs have some geochemical components and in the end a shorter area

of interest is demarcated in the reconnaissance permit area.

6.2.5 Regional Geochemical Surveys

These days mineral exploration is mainly aimed to locate hidden deposits, be

they sub-outcropping beneath thin soil or truly deeply buried. Using direct techniques,

which essentially comprise two disciplines, geophysical surveys and geochemical

exploration, such surveys have been reasonably successful.

Although it is true that spectacular successes have been obtained in regional

reconnaissance using airborne geophysics, this technique is largely aimed at and

6

limited to certain specific types of deposit - notably iron ore, massive sulphide bodies

and mineralization due to radioactive minerals.

Geochemistry on the other hand, especially with its multi-element approach,

has a wider range of applications and offers new opportunities at the regional

exploration stage for exhaustively filtering out and collecting a broad spectrum of

mineralized occurrences, regardless of the elements or types of occurrence involved.

Nevertheless this method, as it is currently applied, tends to be restricted to the search

for relatively shallow targets and sub-outcropping mineral occurrences that are

concealed by the soil cover which is so typical of most of our climatic regions.

The advances made in the analytical field, especially with the development of

spectrometric techniques, have greatly contributed to the resurgence of geochemical

prospecting. For example the quantometric direct-reading emission spectrometer,

plasma emission spectrometers, permit simultaneous testing of some 20 to 30 trace

elements at relatively cheaper cost. The mass of information obtained by this means

has revitalized geochemistry, primarily in terms of its improved efficiency,

performance and date usage.

6.2.6 Integration of Regional Data and Delineation of Prospect

It is truly a desk study. The available database on regional geology, aero-

geophysical signatures, remote sensing and geological mapping are integrated for

delineating a prospect. It is not merely the prospecting target but possible grade-

tonnage models are kept in sight before initiating exploration activities on the selected

prospect. The various parameters or attributes that have bearing on prospective ore

deposit are taken into consideration. Taking cue from a working mine or proven

deposit, the database on various aspects are given different weightage. Some

exploring agencies like BRGM, France, use point system in grading various

prospective targets. Such an exercise ultimately leads to preparation of prognosticated

maps. The less favorable areas are surrendered at this stage and reconnaissance

exploration comes to an end. The exploration activity henceforth shifts to the selected

prospect.

7

6.3 Prospecting

Prospecting is a very important stage of exploration that includes surveying,

detailed geological mapping, geochemical and geophysical surveys and finally

detailed sub surface exploration.

6.3.1 Surveying

Surveying deals with the techniques of detecting the relative position of points

at, above or below the surface of the Earth; or establishing such points. It is the

device, of measuring relative points at above or below the surface by making

horizontal and vertical distances and angles. The object of surveying is to prepare a

map, a plan or section on a reduced scale as desired. Thus it gives the true projection

of the surveyed area of Earth’s surface at a glance when the map is oriented. The

surveying may be divided in two parts;

1) Plane Survey: In plane survey Earth’s spheroid shape is neglected and up to

250 Km is considered as plane.

2) Geodetic Survey: In this survey curvature and altitude corrections are

considered for most precision survey. The Survey of India is the only

organization in India that carries out such high precision surveys.

6.3.1.1 Topographic Surveys

Measurements and computations constitute core of surveying. It includes

measurements of horizontal and vertical angles and determination of coordinates. A

horizontal angle is the angle formed by intersection of two lines in a horizontal plane.

A vertical angle on the other hand is an angle between two intersecting lines in

vertical plane. In surveying it is commonly understood that one of these lines is

horizontal and a vertical angle to a point is understood to be the angle in a vertical

plane to that point and the horizontal plane.

8

Introduction to Coordinates:

Coordinates.

You are all familiar with the following way of identifying a point P

This is known as a RECTANGULAR Coordinate System.

+ P(x,y)

y

x

Another way of identifying the same point P is

This is known as a POLAR Coordinate System.

+P (r , )

y

x

r

θ

θ

FOR SURVEYING we use a slightly different form of notation ...instead of x,y we use E,N (Easting, Northing)

+P ( E ,N)

N

E

D

θ

s

9

N.B. Easting is always quoted first and then Northing .

υ is always measured in CLOCKWISE direction from North .

υ is known as the WHOLE CIRCLE BEARING (WCB).

Triangulation

The Triangulation Survey is the highest grade of survey. It is being adopted to

have a master control for the survey of large area and in hilly and mountainous areas,

where measurements are difficult. Geodetic survey, underground and aerial survey

requires Triangulation Survey for ground control points. The total area may be framed

with the series of triangles in which one linear side is only measured which is called

base line and all other sides are being computed. It is desirable to measure the distant

of another side in the middle and at the end of a triangle to have a checking of the

computed distance which is known as check-base. It is essential that sum of three

angle of each triangle becomes 180°. In case of triangles forming a polygon, the centre

point of the triangles will be equal to 360°. The formula for computation of the

triangle may be stated as:

Where a, b, c = sides of triangle and A, B, C, = Interior angles

10

Errors:

The errors in the measurement of angles are mainly of two types:(1) Manual

and (2) Instrumental, which makes the total of interior angles of a triangles either >

180°00’00” or <180°00’00”. These errors are distributed in the interior angles

proportionately, if the errors are higher than the measurement of angles is repeated.

A triangulation system consists of a series of triangles. At least one side of each

triangle is also a side of an adjacent triangle; two sides of a triangle may form sides of

adjacent triangles. By using the triangulation method of control, one need not measure

the length of every line. However, two lines are measured in each system-one line at

the beginning and one at the closing of the triangulation system. These lines are called

base lines and are used as a check against the computed lengths of the other lines in

the system. The recommended length of a base line is usually one sixth to one fourth

of that of the sides of the principal triangles.

Base Line Measurement: The accuracy of all directions and distances in a

system depends directly upon the accuracy with which the length of the base

line is measured. Therefore, base line measurement is vitally important. A transit

must be used to give precise alignment while measuring a base line. For third-order

triangulation measurement (which is at times used in GSI) with a steel tape, one is

required to incorporate all the tape corrections. For measurement over rough terrain,

end supports for the tape must be provided by posts driven in the ground or by

portable tripods. These supports are usually called chaining bucks. The slope

between bucks is determined by measuring the difference in elevation between the

tops of the bucks with a level and rod. On the top of each buck, a sheet of copper or

zinc is tacked down, which provides a surface on which tape lengths can be marked.

Bucks are setup along the base line at intervals of one-tape length. The tape is

stretched between the supports and brought to standard tension. The position of

the forward end is marked on the metal strip with a needle-pointed marker. The

length of the base line will, of course, amount to the sum of the horizontal distances.

In case the line is being measured forward, after the forward measurement, the line is

11

to be again measured in the backward direction. If the backward measurement varies

slightly from the forward measurement, the average is taken as the length of the base

line. A large discrepancy would, of course, indicate a mistake in one measurement

or the other.

Calculation of Latitude and Departure

In the process of triangulation or polygons (with Total Station Instrument) only

base line and the bearing of the baseline is measured in the field rest of the bearings

and coordinates (Latitude and departure) are calculated trigonometrically.

1. Calculating of bearing from interior angles:

Bearing of last line + Interior angle ±180° or 540° = Bearing of next forward

line.

The measurement is carried out in anti-clockwise direction only

1. From the whole circle bearing the Reduced bearing calculation will be as

follows:

If bearing is between 0° and 90°, no reduction is required.

If bearing is between 90° and 180°, no subtract it from 180°.

If bearing is between18 0° and 270°, subtract 180° from it.

If bearing is between 270° and 360° subtract it from 360°.

12

3. Computation of finding out of latitude and departure:

Latitude = Distant × Cos θ of reduced bearing.

Departure = Distant × Sin θ of reduced bearing.

6.3.1.2 Contouring

The configuration of earth’s surface is shown by contour lines. These lines on

the surface pass through equal altitude with reference to some datum line. Thus, the

elevation and depressions of the ground are shown on map by means of contour lines.

Such a map is called a topographical map. The vertical distance between two

consecutive contour lines is known as contour interval.

• A contour is a line drawn on a plan joining all points of the same height above

or below a datum.

• Contours cannot cross, split or join other contours, except in the case of an

overhang. e.g. a cliff.

• The height between successive contours is called the vertical interval or the

contour interval. Its value depends on the variation in height of the area being

contoured. The contour interval is kept constant for a plan or map.

• The plan spacing between contour line indicates the steepness of slopes.

Closely spaced lines indicate a steep gradient. Widely spaced lines indicate a

flatter gradient.

Leveling (with Dumpy Level)

It is method of determining the relative difference of elevation on earth’s

surface at various points. Level deals with the measurement in vertical plane. Thus the

process of leveling prepares a topographical map. Contouring is also done through this

process of leveling. The trigometrical leveling by taking vertical angle by theodolite

or Telescopic Alidade has the great advantage of this method. Dumpy level is very

simple, compact and suitable method that is widely used for rapid and precise work in

13

a fairly level ground. But in a higher hilly area, Trigonometric Leveling is more

accurate and has the advantage for work with a theodolite.

50

100

150

A contoured Hill

200

250

50

100150

200 250

The methods of determining contours are (i) Direct (ii) Indirect and (iii)

graphical.

B Indirect contouring

from random spot heights

from a grid of spot heights

14

6.3.1.3 Surveying Methods and Instruments

The following type of surveys are based on the methods employed during

surveying

(a) Triangulation Survey : In this whole area is divided into well

conditioned triangles.

(b) Traverse Survey : Survey is carried in a simple line

(c) Correction Survey : It is adopted in transferring the direction and levels

from one level to another level in underground

mine.

(d) Trilateral Survey : In this length of triangle is measured. It is more

accurate than triangulation.

Survey Instruments

The following are the type of surveys that are based on the type of survey

instrument used:

(1) Compass Survey

(2) Plane Table Survey

(3) Levelling Survey and

(4) Theodolite Survey.

Prismatic Compass

It is an angular instrument designed to measure magnetic bearing of a line. The

include angle from the different of two bearing of lines can be computed. The

important parts are a magnetic needle, a graduated circle ring and a line of sight-vane

with a prism and an object vane. This is a very light and simple instrument for rapid

15

surveys. Various methods of survey can be implemented with the prismatic compass

such as Traversing, Intersection and Grid system like other angular instruments. It’s

dial ring is marked inverted from 00° to 360° with the least count of 30 min. Traverse

may be closed or open. Open traverse are very less in accuracy. Traverse survey is the

frame work which consists of a series of connected lines with lengths and directions.

Grid or Graphical methods are being done at right angles with square methods at

desired distances. In this angle is always measured from north and distance is

measured by measuring tape.

Plane Table Survey with Telescopic Alidade

Plane table survey is useful in detailed mapping where field work and plotting

are done simultaneously. Plane table with Telescopic Alidade is commonly used for

the preparation of topographical map with geological aspects in planimetric position.

The Telescopic Alidade has the advantage of measuring horizontal and relative

vertical height difference instantly even in undulating ground through its BEAMAN

area. A vertical circle is also provided to measure the horizontal and vertical

differences by Tachometric Formula. The methods of survey are alike the other

angular survey methods namely, (a) Grid or Graphical, (b)Traversing, (c) Intersection

and (d) triangulation are most common in use. The basic procedure in plane tabling,

consists of drawing rays on the plane table drawing sheet along the sight-lines and

these rays intersect at the instrument station with the object point with some meridian

direction.

Theodolite Survey

A Theodolite is essentially a high precision survey instrument of measuring

horizontal and vertical angles directly. Extensive Surveys are being carried out with

the methods of traversing, triangulation and graphical system to the optimum level of

accuracy. Plane and Geodetic survey can be achieved applying necessary corrections.

The telescope is internal focusing so the additive constant of focal length is zero and it

is capable of being rotated along its transverse axis.

16

Surveys based upon the Method Employed

Traverse Survey

It consists of the framework with a series of connected lines where the lengths

and directions are measured. The traverse may be closed or open. In close traverse

lengths and directions are more accurate than in open traverse as these have many

arithmetical checks.

(a) Check on closed Traverse : Sum of the interior angles of a polygon = (2n-4)

90° or in angular measurements (n-2) 180°. Where

‘n’ denotes number of sides of Traverse.

Sum of the exterior angles of the polygon =

(2n+4) 90° or (n+2) 180°.

b) Check on linear measurement can be made by measuring twice in opposite

direction. In a closed traverse total northing in latitude will be equal to total southing

and sum of the eastings will be equal to sum of the westings in the departure and this

indicates no discrepancy in the measurement of angles and distance.

In open traverse angular measurement can be checked by observing bearing

and cut off lines between some intermediate station and also through intersection of a

prominent signal mark, visible from the traverse stations.

17

Essential measurements of a traverse are (1) bearing of one line, (2) interior or

exterior angles and (3) the lengths of all sides. In a closed traverse, plotting is done by

co-ordinates and not by protractor after proper applications of checks. Closed traverse

computation may be made in a tabular from known as Gale’s Traverse Table.

Tacheometry methods in Survey

It is a branch of angular surveying in which both horizontal and vertical

positions of points are determined with a Theodolite or Tacheometer instrument. The

direct distant measurement operations are entirely eliminated. So in the field this

system is most rapid for preparing topographical map with other features. The primary

object of tacheometry is the preparation of maps and plans.

An ordinary ‘transit theodolite’ fitted with a stadia diaphragm and anallatic

telescope is generally used for tacheometric survey (Fig. 6.3 and 6.4). Distances and

elevation or depression can be deducted with the following formula:

Horizontal distance (When the measuring plane is horizontal) = KS cos² α

where K denotes multiplying constant 100 exactly and S denoted difference of two

staff reading. α is the vertical angle.

Horizontal distance (when the measuring plane is inclined) = KS cos2 α + C cos α

Where C is total of focal length of objective lens and half the size of instrument

(Fig. 6.4)

Vertical height difference = KS cos α Sin α ; or ½ KS sin 2α.(Fig. 6.4).

Closing error in a prismatic compass and plane table traverse can easily be

eliminated graphically. The total closing is distributed round the polygon by shifting

proportionately and parallel to the direction and distant of each station of traverse.

Extension of base line can be made by means of forming Triangulation as shown in

the sketch below:

18

LEVEL FIELD BOOK (Rise Fall method)

Station Back Inter Fore Rise Fall Reduced level Remarks

PLANE TABLE FIELD BOOK

Inst. Stn.

Obser Stn.

Ht. of

Inst

Initial Curve reading

Distance Curve reading

Vertical Ht. curve

reading

Vertical Curve value D

ista

nce

Diff of Ht.

R.I. Remarks

19

Some useful definitions

A level surface is any surface parallel to the mean spheroid surface of the

earth, i.e. the surface of a still lake. Hence a level surface is a curved surface, parallel

to the earth.

A Horizontal Plane is a plane tangent to a level surface.

A Horizontal angle is an angle formed by intersection of two lines in a

horizontal plane.

A vertical line is a line perpendicular to the horizon, e.g. a plumb line.

A vertical angle is an angle between two intersecting lines in a vertical plane;

in surveying one of the two lines is horizontal.

A bench mark (BM) is a fixed reference point of known elevation. Generally

it is fixed by Survey of India.

A line of collimation or line of sight is the line joining the intersection of

cross hairs to the optical centre of the object glass and its continuation.

The axis of the telescope is a line joining the optical centre of the object glass

to the centre of the eye piece.

The elevation of a point is its vertical distance above or below the datum. It is

also known as the elevation of the point.

Reduced level (RL). Datum line is arbitrarily assumed level line from which

vertical distances are measured.

Parallax is the apparent movement of the image relatively to the cross-hairs

when the image formed by the objective does not fall in the plane of the diaphragm.

20

Modern Survey Instruments

Distomat (D13 S)

It is Wild-make modern survey instrument that can measure distances up to 2

Km with the use of prism-reflectors. It works on the principle of propagation of light

and distance is calculated by the attached computing system. Infra-red ray is used as

travel wave. It measures horizontal, vertical and incline distances with the accuracy of

1mm, 1 ppm.

Gyro-Theodolite

This instrument is used for seeking North azimuth bearing at surface as well as

in underground. The main use in underground is correlation in which only one piano

wire is used. It is more accurate and takes less time in calculating bearings.

Total Survey Station (Electronic theodolite T 1000 with DIOR 3002 distomat and

GRE 4a data recorder )

This instrument is having LASER attachment which is useful in stoped out area

survey. This instrument is also equipped with range finder in which distance

measurement can be done up to 200 m at surface and 100 m in underground without

use of the reflecting system. So this facility is very useful for surveying of the

inaccessible survey points especially in stopes. The instrument is operated by 12 V

rechargeable dry cell battery which is available external as well as internal to attach

with the instrument. The charging time for battery is 14 hours. The latest models are

more compact where distomat is in-built with telescope (co-axial), light weight, handy

and attachments are reduced and the rechargeable time of battery is also few hours.

The instrument is accurate, fast time and manpower saver. The final output can

be obtained directly in the field if on line PC facility is available. It is more useful in

all type of surveys subsidence survey, open cast mine survey, underground survey and

stope survey.

21

TC 303 Electronic Theodolite (Total Survey Station)

As name indicates it is suited to all types of survey observations, calculations

and plottings. The features of the total station are as follows:

Observation Work

The setting of the instrument at survey station is same as required in ordinary

theodolites. This instrument is equipped with laser plummet for centering the

instrument at surface survey stations. Electronic bubble for leveling the instruments

and other facilities like auto-collimation etc. are part of the equipment. The bisection

of object is also similar. The display of readings is digital which eliminates human

errors in reading of horizontal and vertical angles. The distance between two survey

stations is measured up to (1.2 km with miniprism and 3.0 km with circular prism) the

accuracy of about10 mm. and 10 ppm by using reflecting system. For distance

measurement a distomat is coaxial (in-built) with the telescope. Infrared wave is used

for distance measurement, which works on the principle of velocity of light.

Three dimensional coordinates are obtained for any observed survey station.

The azimuth, latitude, departure and level of the instrument station is to be feed to

obtained the three dimensional coordinates of the observed survey station. It can find

the area, line distance and height of inaccessible points. The observed values either

can be noted in field book or can be stored in internal memory of the instrument. The

internal memory can store a maximum of 4000 survey points (256 KB). Instrument

can also be connected with PC on line. All the calculations for coordinates, distance

and level are done by the instrument automatically by in-built computing system.

6.3.2 Detailed Geological Mapping

The detailed geological mapping is a prerequisite for taking up subsurface

exploration. The prospect by now has been well delineated and is ready for detailed

geological mapping. The detailed mapping (DM) is carried out on 1:2000 or 1:1000

scale depending upon the intricacies of the prospect sequence. The objective is to map

small lithological units that are important from the detailed exploration point of view.

22

For example the gold deposits in southeastern Rajasthan contain tourmalinites,

albitites and some other exotic lithological units that have direct bearing on the gold

mineralization. Such units are mappable only on 1:1000 or 1:2000 scales and yet they

are important in planning of boreholes. Detailed mapping is generally accompanied by

detail geochemical sampling on grid pattern, sampling along proposed profile of the

boreholes and pitting and trenching for exposing the concealed litho contacts and or

oxidized zones. Important cultural features are also plotted on the detailed map. For

accuracy it is advisable to carry out DM by tranigulation surveying methods. Borehole

plans and profiles are made on the basis of the detail map. Detailed structural features

of local scale are also given importance while mapping the prospect.

6.3.3 Detailed Geochemical Surveys

Introduction

Geochemical exploration as compared to other close nomenclatures such as

geochemical mapping and geochemical survey is almost entirely a dedicated method

employed for the search and establishing an ore deposit. The Zawar module of mineral

exploration is particularly focused to base metal exploration. From exploration point

of view this write up is essentially restricted to a group of five elements viz. Cu, Pb,

Zn, Ni and Co. The base metal exploration also extends to gold and silver also in

broader sense.

The scale of work in a geochemical exploration is an important parameter

which decides on the methodology (particularly sample grid spacing) as well as the

medium of sampling. Usually we talk of two scales of work – the regional scale and

the local scale with a third intermediate scale often introduced in many special

situations, Fig.6.5.

Depending on the scale, the ‘geochemical anomaly’, which is the final output

of a geochemical exploration, is graded. The geochemical anomaly of an element, in

this case a base metal, is termed ‘regional’ or ‘local’ based on whether the survey was

conducted on a regional scale or a locale scale. The anomaly value is mostly decided

by univariate statistics, but variations to this are practised in many countries where

23

multielement analysis and multivariate statistics is in vogue, for example BRGM,

France.

Fig. 6.5 : Regional to local geochemical exploration programmes

Base metal Deposit Types

The SEDEX, VMS and porphyry type deposits are the main contributors of

base metal production in world. In addition, the sediment-hosted copper deposits in

the Congo-Katanga belt in Central Africa also constitute an important category of

deposits on global scale. The geochemical (map) pattern and association of elements

is usually governed by the type of deposit or metallotect especially in case of regional

geochemical exploration. For example, the geochemical association of copper will be

very different in a porphyry copper deposit from copper association in a VMS

(volcanogenic massive sulphide) set-up. Or, copper from a sediment-hosted deposit in

the Congo-Katanga belt, Central Africa will differ in geochemical signature from

copper in a ‘Kuruko set-up’.

6.3.3.1 Scale of Geochemical Exploration

The scale of Geochemical Exploration is matter of board room decision – that

the work is of regional scale involving an area of n x 100 sq km (where n = 1 to 10 or

GEOCHEMICAL MAPPING

GEOCHEMICAL SURVEY

GEOCHEMICAL PROSPECTING

D.J. Das GuptaGSI, WR, 2003

24

more) or of local nature measuring less than 20 sq km or less. Also important is the

time frame as well as budgetary provisions for the analysis of samples. Competence

levels of the sampling team and the data interpretation party is also of great relevance

since this information will be integrated with the Geological and Geophysical data

base by using dedicated GIS software. The regional geochemical exploration is

generally an important part of reconnaissance exploration programs.

Areas exceeding 5,000 sq km usually are considered ‘Regional’ in scale. Those

measuring less, but exceeding an area of more than 20 sq km are task on ‘semi-

regional’ scale. In both cases the presumption is that a potential mineralization is

enclosed in the area and the objective is to locate it without wasting time in the other

details, like details of host rock and structure of the ore body. For such geochemical

surveys fast and cheaper options of sampling as well as analysis are adopted. The

most effective methods are ‘stream sediment’ as a medium of sampling and ICP-AES

as a method of rapid analysis. Sampling in regional geochemical survey may vary

from terrain to terrain, depending on local morphology. It may vary from 1 to 2

samples per sq km. Multielemental interpretations are made through the use of various

geochemical softwares. The regional geochemical surveys are more effective in

delineating target areas in tracts that have poor exposures.

Once areas are identified through a Regional Geochemical Exploration further

geochemical work can be launched in conjunction with ground geophysical surveys in

smaller blocks of 10 to 20 sq km area.

Detailed Geochemical Exploration

Detailed geochemical survey or exploration is usually carried out where the

base metal block has been identified (as indicated above) and the current objective is

to know its potential. Such types of survey have a regular grid pattern of sampling and

the medium of sampling is either bed-rock or insitu soil (from ‘B’ or ‘C’ horizon),

Figure-6.6a. The bed rock sampling is preferred along the borehole profiles so that a

surface and subsurface correlation of the chemical parameters is established properly.

25

Block identified during Geochemical Mapping

26

Stream Sediment Geochemical Survey

Many exploration agencies rely on stream sediment geochemical survey for

fast and effective results. This is a time tested (with the variation stated below)

reliable method of geochemical exploration which is effective in all scales of work.

BRGM, France adopts such surveys for near surface deposits. It involves selective

concentration of the “useful” mineral fraction at the sample preparation stage

based on specific physical properties. In this respect densimetric (fractions with

specific gravities more than 2.9, or analysis of flotation residues) and

magnetometric (paramagnetic fractions preferentially consisting of the oxidization

products of sulphides) methods are those best developed at present.

6.3.3.2. Geochemical Anomaly

As stated locating the ‘geochemical anomaly’ of a targeted base metal

spatially is the final output of a geochemical exploration. Such anomalies in a map

are either decided by univariate statistics or by multivariate statistics (factor maps).

An anomaly is a value of the target element (i.e.,Cu, Pb, Zn or Ni, Co, As, Sb

etc) generally several orders above its crustal abundance or its statistical population

mean. The main objective is to locate this ‘anomaly’ spatially. The anomaly values are

of different intensities for the same element in the two scales of exploration, viz.,

regional and detailed or local (see Figure-6.7A & 6.7B).

Traditionally the anomaly of an element is defined by univariate statistical

methods of mean value and standard deviation (SD) and assigning the values above

(mean + 2 SD) for the element as anomalous. Such anomalous values are plotted in a

27

map (geochemical map) to show the spatial disposition. The shape of the anomaly and

other informations like area covered by the anomaly and the contrast or the intensity

of the anomaly are of great importance to prioritize the exploration blocks.

The recent trends of geochemical anomaly selection for base metals depend on

multielement dataset and multivariate statistics. The advantage of this method is that

the geological influence on mineralization is more clearly understood and selection of

anomalous blocks for detailed exploration is geologically sounder. The multivariate

statistics allows interpretation via geochemical associations (Table-1) that is

fingerprinted in the overlying weathered profile.

A dominantly granite-granite gneiss country will definitely relay a ‘granitic

geochemical imprint’ to the weathered profile. For example the elemental values of K,

Si, Al, Na, Ba, Li, B, Pb etc. will be comparatively higher. A basaltic country on the

contrary will have significantly elevated values of Fe, Mg, Ca, Ti, V, Ni, Cu etc., (Fig.

6.8), in the weathered profile. Pure ‘chalcophile’ signature (Table-2) from epigenetic

or remobilsed ores is generally lost in the geochemical noise of country rock. The uses

of multivariate staistics like ‘factor analysis’ with nonparametric correlation matrix

comes in handy in selecting such low intensity base metal anomalies. Several

softwares like the BRGM’s GDM, Syn-ARC etc., are able to spatially plot the

multivariate data including factor scores of metallogenic importance.

Apart from the use of uni- and multivariate statistics a valuable interpretative

tool of geochemical data in exploration is the geochemical map. Simple contour maps

created by using any geochemical contouring software like Surfer-8 of all the

elements and overlaying them on one another and/ or the geological map, brings out a

rich set of information on various aspects of geology, structure and mineralization. In

any geochemical exploration, regional or of local scale of work, this contouring

option should be used freely even before statistical approach is resorted to.

28

Fig. 6.7a : Illustration of regional geochemical anomaly

Mn (OH)orFe(OH)or Al(OH)orHumus

Cu, Pb, Zn, Ni, Co

Primary mineralisation

Soil

Host rock

Dispersion halo

Cu- anomaly profileTSA

Fig. 6.7b : Geochemical anomaly

74.035 74.040 74.045 74.050 74.055 74.060

24.155

24.160

24.165

24.170

24.175

Cu (ppm) Ni (ppm) CO (ppm)

29

Table-1 : Geochemical Association of Elements

Type of Deposit Major Components

Associated elements

MAGMATIC DEPOSIS

Chromite Ore (Bushveld) Cr Ni, Fe, Mg

Layered magnetite (-do-) Fe V, Ti, P

Immiscible Cu-Ni-S (Sudbury)

Cu-Ni-S Pt, Co, As, Au

Pt-Ni-Cu in layered intrusion (Bushveld)

Pt, Ni, Cu Cr, Co, S

Nb-Ta carbonatite (Oka) Nb, Ta Na2O,Zr, P

Rare metal pegmatite Be, Li, Cs, Rb B, U, Th, REE

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

105

110

115

Zn (ppm)

79.02 79.04 79.06 79.08 79.1 79.12 79.14 79.16 79.18 79.2 79.22 79.24

23.02

23.04

23.06

23.08

23.1

23.12

23.14

23.16

23.18

23.2

23.22

23.24

79.02 79.04 79.06 79.08 79.1 79.12 79.14 79.16 79.18 79.2 79.22 79.24

23.02

23.04

23.06

23.08

23.1

23.12

23.14

23.16

23.18

23.2

23.22

23.24

V

MK

DT

79 79.05 79.1 79.15 79.2 79.2523

23.05

23.1

23.15

23.2

23.25

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

Cu (ppm)

DT

MK

V

79 79.05 79.1 79.15 79.2 79.2523

23.05

23.1

23.15

23.2

23.25

15

20

25

30

35

40

45

50

55

60

65

70

75

80

Ni (ppm)

MK

DT

V

79.02 79.04 79.06 79.08 79.1 79.12 79.14 79.16 79.18 79.2 79.22 79.24

23.02

23.04

23.06

23.08

23.1

23.12

23.14

23.16

23.18

23.2

23.22

23.24

0.50.60.70.80.911.11.21.31.41.51.61.71.81.922.12.22.32.42.52.62.72.82.93

DT

MK

V

DT - Deccan TrapMK - Mahakoshals (Carbonates-volcanics)

V - Vindhyans

Fig. 6.8 : Stream sediment map of Cu, Zn, Ni & TiO2 of toposheet 55M/4

30

Type of Deposit Major Components

Associated elements

HYDROTHERMAL DEPOSITS

Porphyry copper (Bingham) Cu, S Mo, Au, Ag, Re, As, Pb, Zn, K2O

Porphyry molybdenum (Climax)

Mo, S W, Sn, F, Cu

Skarn- magnetite (Iron Spring)

Fe Cu, Co, S

Skarn-Cu (Yrington) Cu, Fe, S Au, Ag

Skarn-Pb-Zn (Hanover) Pb, Zn, S Cu, Co

Skarn-W-Mo-Sn (Bishop) W, Mo, Sn F, S, Cu, Be, Bi

HYDROTHERMAL DEPOSITS

Volcanogenic MSD Zn-Cu-Pb

Zn, Pb, Cu, S Ag, Ba, Au, As

Base metal vein Pb, Zn, Cu, S Ag, Au, As, Sb

Sandstone type U U Se, Mo, V, Cu. Pb

SEDIMENTARY TYPES

Copper shale (Kupferschiefer)

Cu, S Ag, Zn, Pb, Co, Ni, Cd, Hg

Copper sandstone Cu, S Ag, Co, Ni

Mississippi Valley Pb-Zn Zn, Pb, S Ba, F, Cd, Cu, Ni, Co, Hg

Red-bed Cu Cu, S Ag, Pb

Table-2 : Goldschmidt’s Classification of Elements

Group Elements

Siderophile Fe, Co, Ni, Pt, Au, Mo, Ge, Sn, C, P

Atmophile H, N, O

Chalcophile Cu, Ag, Zn, Cd, Hg, Pb, As, S, Te

Lithophile Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Al, REE,

31

Extended classification

Lithophile Chalcophile Siderophile Atmophile

C, O, P, H, F, Cl,

Br, I, Si, Al, Fe,

Mg, Ca, Na, K, Ti,

Sc, Cr, V, Mn, Th,

U, Nb, Ta, Sn, W,

Be, Li, Rb, Cs, Ba,

Sr, B, Y, Zr, Hf,

REE, Ga, (Cd),

(Zn), (Pb), (Cu),

(Ni), (Co), (Mo),

(Tl)

S, Se, Te, As, Sb,

Bi, Ag, In, Ge, Tl,

Hg, Cd, Zn, Pb, Cu,

Ni, Co, Mo, Re,

(Fe), (Sn), (Au)

Pt, Ir, Os, Ru, Rh,

Pd, Au, (Fe)

N, O, C (as CO2), H,

He, Rh, and other

noble gases (S as

oxides), (Hg)

6.3.4 Geophysical Prospecting

Geophysical prospecting is the art of searching concealed deposits by

measuring physical property of subsurface from the Earth’s surface. Though the

fundamental principles of physics for these measurements are quite simple, it is

generally difficult to apply these to the study of rock material because rocks and

minerals are rarely homogeneous and have complex and /or overlapping physical

properties. The measurements yield information on the physical properties of material

within the Earth. When properly interpreted, these lead to location of mineral deposits

of economic value. The reliability of geological picture thus derived depends on the

quality of data and the skill displayed in their interpretation. The success rate in

mineral exploration is becoming increasingly poor and currently it ranges between 1:

150 to 1:100 (one economic deposit for 100 prospects or to 150 mineral shows). Thus,

mineral exploration is a challenge to geoscientists world-wide and more so because

outcropping or near surface ores have mostly been explored, exploited and/or

32

exhausted. The search for new deposits is now to be focused on concealed tracts. It is

here that geophysics enters into picture.

Physical Properties

Ore bodies frequently differ in physical properties (Magnetic Susceptibility,

Electrical Conductivity, Density etc) from the country / host rock. Thus, ore bodies

can be located on ground by suitably observing variations in physical properties and

carefully interpreting the anomaly maps. Geophysicists use sophisticated instruments

to measure variation of physical properties such as:

Density

Magnetic Susceptibility

Electrical Resistivity

Natural Electrical fields

Radioactivity of rocks

Velocity of seismic waves

Variation in gravity and magnetic fields of the Earth

Reflection of Electromagnetic Waves

Geophysicists use one or more of these measurements to find base metals,

potash, coal, iron ore, oil, natural gas and many other minerals. In addition, the

variation in physical properties is also used to identify environmental hazards and

evaluate sites for dam, tunnel or building construction. Only a small fraction of

activity in geophysical prospecting has gone into search for solid minerals. It is mainly

because, the physical properties of many ore bodies do not show substantial contrasts

with the corresponding properties of the country rock surrounding them. Thus, quite a

few ore deposits may not be suitable as geophysical targets. Apart from locating the

mineral occurrence, the geophysical exploration is also useful in separating promising

areas from barren tracts. The selection of a geophysical method for exploration

depends on nature of physical properties of the desired material.

33

Physical Properties and Methods of Geophysical Exploration

Sl. No. Physical Property Geophysical Method

1. Density Gravity

2. Magnetic Susceptibility Magnetic

3. Electrical Conductivity/Resistivity Resistivity /EM/GPR

4. Electrical Polarization SP / I P

5. Elastic Wave Velocity Seismic

6. Thermal Conductivity Geothermal

7. Radioactivity Radiometric

Variation of Physical Properties

Resistivity

The resistivity of crustal rocks normally varies in the range of 105 to 106 Ω m

and falls to 101 - 10-1 Ω m in case of ore deposits. This property has been used very

successfully in locating sulphides, clays, graphite, aquifers, brine, etc and for

structural discontinuities.

Induced Polarisation (IP)

It is normally in range of 2-5 mV/V (0-2 PFE) in crustal rocks and may

increase to several tens (10-20 PFE) in ore environment. IP method is a very powerful

tool in locating disseminated sulphide deposits in particular, and in exploration of

sulphide hosted Au and Ag deposits. It is also useful in locating graphite and clay

pockets etc.

Spontaneous Polarisation / Self Potential / SP

34

It is ± 5 to ±10mV as background over crustal rocks and predominantly

negative (20 to Hundreds of mV) over sulphide and graphite deposits.

Density (ρ)

Normally the changes in density of rocks and minerals are not as wide as other

physical parameters (2.1 to 3.2 gm/cc). In cases of volcanogenic sulphides, chromite,

manganese, barite etc. the density is 3.5 to 4 gm/cc and in case of coal, evaporites

(halite/ sylvite) and sand the density goes down from 1.8 gm/cc and less. The gravity

methods find use in exploring massive type sulphide mineralization which is generally

associated with volcanogenic massive sulphide (VMS) deposits and also at times with

SEDEX type deposits. The density contrast between massive ores and the surrounding

country rock is quite conspicuous and can be measured by close spaced gravity

surveys for locating ore bodies.

Magnetic Susceptibility (κ)

Magnetic susceptibility varies from 10-6 cgs units in sedimentary and some

metamorphic rocks to as high as 10-4 cgs units for basic and ultrabasic rocks.

Magnetic susceptibility is as high as 10-3 cgs units in metallogenic environments rich

in pyrrhotite, iron-ore, ilmenite, manganese etc. Au or diamond as placers, burried

channels etc are easy targets by magnetic method. A good example of magnetic

surveys is provided by VMS type Cu-Zn deposit at Danva. The sulphide

mineralization is hosted by metabasalt (amphibolites), actinolite-chlorite-biotite and

muscovite-sillimanite-quartz schists. Muscovite-sillimanite-quartz schist occurs in

small amounts. Study of drill cores shows that there exists gradation from relatively

fresh amphibolites to muscovite-sillimanite-quartz schist through carbonated

amphibolites and actinolite-chlorite biotite schist. In relatively unaltered zone,

magnetite occurs in considerable proportions, often up to 5% of rock. It occurs both as

disseminated crystals and as thin discrete layers. However, as sulphide content

increases the proportion of magnetite decreases and it is almost absent in massive

sulphide zone. This is well reflected from low magnetic anomaly of 200 γ to 400 γ

over sulphide zones as compared with the enclosing unaltered amphibolites where

35

magnetic ‘high’ reach up to 2000 γ. Such a drop in magnetic susceptibility is linked to

break down of magnetite and its conversion to sulphidic phase, pyrite in this case.

Seismic Wave Velocity (VP)

It varies from 500 m/S in loose sand, 1000 in alluvium to 6000m/S in upper

crustal rocks and 7000 m/S in lower crustal rocks (basalts). It is not used in

exploration of metallic minerals. It is however, very useful in coal/lignite exploration

and for mapping the structures favorable for oil and natural gas accumulation. It is

also useful in mapping shear zones, faults and other structural features that act as

favourable locales of ore deposits.

Electromagnetics

SP, resistivity and IP surveys are used for conductive bodies and a good contact

with ground is essential to get any response. When contact with ground is difficult, the

current is induced by generating EM field on ground. The field propagates through air

to ground and to the subsurface conductor. If a conductive body is present in the path

of the Primary Field, it induces small currents in the conductor and a small secondary

EM field is generated indicating presence of a good conductor like massive sulphides,

shear zones etc.

Radiometrics

Measuring the γ - radiation emanating from isotopes of U, K, Th etc suggest

their presence. These are often used in boreholes for variety of other purposes. In

addition, this is an excellent tool in airborne mapping. Changes in the concentration of

the three radio-elements U, Th, and K accompany most major changes in lithology

that may be potentially important for indicating mineral deposits. The radiometric

measurements assist in locating some intrusion- related mineral deposits. The data is

presented both in profile as well as in contour form in general. At times, it is presented

as Bar Diagram or as Fence Diagram. Quite a few software produce 3-D fishnet

diagrams or images.

36

Combination of Methods and Integrated Approach to Mineral Exploration -

Interpretation

Each method studies the variation of some or other physical property of the

rocks at depth. Geophysicists face a lot of ambiguities. For example, a smaller object

at shallow depth and a larger object at greater depth show same order of gravity

anomaly.

Ground water, base metal sulphides, clay or graphite all have low resistivity.

Pyrrhotite, magnetite as well as amphibolite are all magnetic.

Limestone and gneisses have same P-wave velocity.

A combination of methods is deployed as integrated geophysical surveys

reduce the ambiguity and interpretation is close to factual. Due to overlapping

properties and complexities in nature, the solution is never unique. It is

emphasized here that close interaction and integration of the geophysical,

geochemical and geological studies alone can lead us to new mineral discoveries

at larger depth. It may be noted that all the geophysical data are digital in nature.

The objective is to “Interpret the geophysical data and to convert the digital data into

the terms of Geology”. This is achieved in 2 stages; firstly by interpreting the behavior

of the geophysical data itself and later interpreting the geological meaning of such a

data base.

Mineral Discrimination and Locating Missed Lodes

The physical properties of several minerals are overlapping thus necessitating

mineral discrimination. It is still a world wide problem. Some attempts have been

made on international and Indian level. The laboratory and tank model studies are

very encouraging but field results are qualitative only and thus far from satisfactory.

Similarly, attempts for missed lode has reasonable success in laboratory, but hardly

successful in field.

37

Interpretation

Geophysical Methods in Exploration and Indicator Anomalies

Mineral Commodity Method Used Indicator Anomalies

Pyrite, pyrrhotite, and

associated economic base

metal sulphides

(chalcopyrite, galena,

sphalerite, arsenopyrite etc)

and graphite deposits

SP, Magnetic,

Resistivity

and IP.

Massive Sulphides TSC>50%

Strong -ve SP, Moderate Magnetic, Very

high or even –ve IP and Very Low

Apparent Resistivity.

Total Sulphide Content > 15%

Moderate to strong -ve SP, Moderate

Magnetic, High IP and Low Resistivity.

Total Sulphide Content < 15%

Low -ve or No SP, Moderate or No

Magnetic, Low IP and High Resistivity.

Gold associated with

sulphide mineralization

SP, Magnetic,

Resistivity

and IP.

Same as above

Porphyry copper deposits Predominantl

y IP

Moderate IP anomaly

Manganese, barite, chromite

and other heavy mineral

deposits

Gravity and

magnetic

(GM)

Residual Gravity High and moderate

magnetic for Mn & Chromite. Moderate

IP for Mn.

Kimberlite pipes for diamond GM &

Resistivity

Residual Gravity High and strong

magnetic and low resistivity.

Potash, coal and lignite

deposits

GM,

Resistivity

and Seismic

Low Gravity Residual, high resistivity

for coal and low resistivity for lignite and

sylvite.

Groundwater aquifers Resistivity

sounding

Potable water – resistivity ratio of at least

5 : 1 : 5

Mineralisation associated

with fault/shear zones /

quartz veins etc.

Resistivity

profiling/soun

ding

In general, the fault/shear zones are

shown as fall in resistivity and quartz /

pegmatite veins are indicated by rise of

resistivity.

38

Field Examples

Various field examples illustrating utility of geophysical methods in

exploration of various mineral deposits are given below:

39

40

Fig. 6.12 : Geophysics in coal exploration

41

]

-150 -100 -50 0 50-200

-150

-100

-50

0

50

100

150

Mise-a-la-masse Map, Borehole DBH-1,Dhikan Area, Pali District, Rajasthan.

FS 2008-09

DBH-1

Old Working

Charging Point

Anomaly axis

-20-100102030405060708090100110120130140150160170180190200210220

(Body charged at depth of 120m)

INDEX

Casing Geophysical anomalyLITHOLOGS

1. Amphibolite 2.Pegmatite with amphibolite 3. Amphibolite with Quartz vein.

SP (mV)

-200 -190 -180 -170 -160 -150 -140 -130 -120 -110 -100

-90 -80 -70 -60 -50 -40 -30 -20 -10

0 -700

DEPTH (m)

SPR (KOhm)

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

0 3.5

IP(mV/V)

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

0 10 40

Rest(Ohm-m)

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

00 2500 5000

Mag.Susc(x10 -6 )

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

0 0 300

0 600 0

1

23

1

Fig. 6.13 : Geophysical log of borehole DBH-1, Dhikan basemetal prospect, Pali district, Rajasthan

Fig. 6.14 :

42

6.4 Exploration

This section deals with borehole planning, core logging and core sampling.

6.4.1 Borehole Planning

General

Although mineral exploration is concerned with answering a complex range of

geological, mining and economic problems, it is based chiefly on geology. Hence a

detailed geological map is a pre-requisite for carrying out any mineral exploration

program. Based on the data marked on a detailed geological map depicting lithology,

structure, altitudes, mineralized zones, old workings etc, planning of bore hole is

made.

Drilling is an important stage of the mineral exploration process where the third

dimension of a prospect, viz. the subsurface geometry is defined. Drilling provides

most of the information for the final evaluation of a prospect and will ultimately

determine the fate of the prospect as a workable deposit.

Due to its high costs drilling is usually only undertaken in the advanced stages

of an exploration program, after the target site has been located by other cheaper

methods, such as, geophysical surveys and by careful analysis of samples collected by

teams of geochemists and geologists from the suspected target zone. Diamond drilling

is the most expensive of all the drilling techniques, but its versatility allows it to be

done at most surface and underground locations. Diamond core drilling is the only

method available that provides a core, a complete record of geological structure and

rock texture. It is also the only commonly used method that provides reliable samples

for accurate geochemical testing.

Auger Drills for Shallow Boreholes

Shallow boreholes and also trial pits are methods of surface mineral

exploration which obtain data on the depth, extent and quality of the mineral, the

make up of overburden, and hydrological data. Shallow boreholes use small capacity

43

of rigs. Trial pits are mostly used in assessing shallow deposits, in particular sand and

gravel. After the information is recorded the pits are backfilled and reinstated. In the

Geological Survey of India truck-mounted Auger drills are used to spud shallow

boreholes up to depth of 30m to 35m for getting exploration data from the soil

covered target areas.

Augers are drills, which have rods with 'spiral shaped flights' used to bring soft

material to the surface. These are particularly used to sample placer deposits. Power

augers are useful for deeper sampling in easily penetrated lithologic units in areas

where a technique known as 'trenching' is not viable. Trenching is simply removing

the top layer of earth and rock, usually used where ore bodies are at shallow levels in

the ground. The sampling of trench can then be geochemically tested for traces of ore.

In soft ground augering is rapid and sampling procedures need to be well

organized to cope with the material that is continuously being drawn to the surface by

the spiral action of the auger head. Therefore, a lot of care is needed to minimize the

mixing and contamination between different depth samples. Augers are light drills and

are incapable of penetrating either hard ground or boulders.

Borehole Planning in Simple Ore Deposits:

For the geologically simple ore deposits, which are homogenous in extent with

horizontal to sub-horizontal to low dipping in disposition and covering large area like

coal, limestone, bauxite etc., the borehole planning is fairly simple. Usually vertical

boreholes are drilled at regular grid pattern of 100m x 100m or 200m x 200m. The

grid pattern can be modified according to situations such as the type of data required,

heterogeneity of the rock formations and nature of deposits.

44

Borehole Planning in Deformed Ore Deposits (Base metals and Gold)

The planning of boreholes is complex in base metals and precious metals. The

planning of initial borehole is important. The fate of exploration project depends on

the initial boreholes.

The following guide lines are observed for planning initial boreholes.

(1) The point of intersection of the mineralized zone should be planned to intersect

the primary zone of the ore body below the oxidized zone/weathered zone. The

intersections are planned generally at a depth of 20-30 m from the oxidized

zone. The depth of oxidized zone is tentatively estimated from the study of

gossan, weathered zone and in well sections. In case of any old mine existing in

the area, the target point of intersection should be located at least 20-30 m

below the deepest level of old working. The depth of old working can be

tentatively fixed by old mine dump. At times one or two boreholes may indeed

pass through the oxidized zone.

45

Fig. 6.16 Borehole on a geological map

(2) Some amount of pitting and trenching may be carried out to expose the ore

body at few places when it is covered by soil and the borehole area located on

the basis of geological information available in such pits/trench.

(3) The intersection of first few boreholes may be planned at the most promising

locales and representative portion of the mineralized body.

730

680

710

900

46

(4) Steeply dipping ore bodies may show reversal of dip directions. Such a

behaviour needs to be recorded in planning subsequent boreholes.

General Practice to be Followed In Drilling

(1) The bearing or Azimuth of the borehole should be perpendicular to the plane of

the ore body to minimize the length of borehole and to get correct picture of the

width of the body.

(2) The first borehole should be short in length, but not too short that it does not

reach the ore body.

(3) The boreholes are located generally in the hanging wall side of the mineralized

body.

(4) For inclined boreholes, the angles with the horizontal are generally between

30°-90°. But it is advisable to restrict the boreholes between 40° -90° as

shallower angle create lot of drilling difficulties which include high deviation

and low core recoveries.

(5) Faults, fracture zones or mine dump, old workings etc. have to be avoided.

(6) Cutting of large thickness of very hard rock should be avoided.

(7) Environmental factors should be taken into account.

(8) Availability for water for drilling is to be assured.

Fixing of Borehole on Ground

Keeping in view of the above factors, the boreholes are planned on the map.

Profiles are drawn generally across the strike and anticipated sections are prepared.

The proposed borehole is then fixed on the ground from the triangulation survey point

with the help of the Theodolite or Telescopic alidade or suitable survey instruments.

Once the borehole point is given on the ground its bearing and alignment is fixed by

putting pegs along and across the borehole point for positioning of the drilling

47

machine. The rig is brought to site for it’s anchoring. The alignment or bearing is

checked and it is ensured that the rig is perfectly horizontal (by spirit levels). The

machine is finally anchored and the proposed angle of the borehole is fixed. Thus, the

machine is all set to start drilling.

Fig. 6.17 : Fixing of borehole on ground

6.4.2 Core Logging

The cores obtained by drilling are kept in core boxes, which are generally

wooden and have partitions according to the size of the core. The wooden boxes area

now a days replaced with G.I. core boxes to preservation of cores. The cores are kept

with arrow markings and showing top and bottom of the core. Steel or wooden pegs

are placed between each run and the depth of the borehole is written on them. There

are two ways of keeping the core.

48

These days most of the drilling projects in the Geological Survey India, book

pattern is adopted for keeping the core.

Examination of Core

The following guidelines are observed while examining the core.

1 The core should be cleaned and wetted to get a clear picture.

2. The tools for examination of core are streak plate, grain size, index card, steel

knife, pocket lens, steel tape, HCl acid etc.

3. Attitudes of structural features like bedding, foliation, fracture, cleavage etc.

should be recorded and intersection of these planar structures with core axis, is

to be determined.

4. Variation in the lithology, if any, in a run should be recorded taking into

consideration the core recovery.

5. If core recovery is less, then it should be adjusted.

49

6. Quartz veins, calcite veins and other veins should be recorded depth wise.

7. Nature of core eg. Broken, fractured, powdery etc. should be recorded. The

naturally broken core pieces of 10cm length and above are to be measured and

recorded for determination of Rock Quality Designation (RQD).

8. Sludge should be collected in sludge boxes/polythene bags.

9. Mineralised portion of the core should be studied in greater details as follows.

a. Nature of mineralisation eg. stringers, specks, disseminations and grain-size

etc. should be mentioned.

b. Ore minerals which can be identified should be mentioned eg:, galana,

sphalerite, chalcopyrite, pyrite, pyrrhotite, etc. The unidentified minerals

should be described giving their properties.

c. Visual estimate of the ore should be given. The percentage of metal in ore

minerals is as follows :

i. Sphalerite-ZnS 67% Approximately

ii. Galena PbS 96%

iii. Chalcopyrite CuFeS2 34%

iv. Bornite Cu5FeS4 63%

v. Covellite CuS 63%

vi. Chalcocite Cu2S 80%

vii. Malachite Cu2CO3(OH)2 57%

viii Smithsonite ZnCO3 52%

ix. Cerussite PbCO3 77%

d. Sampling zones should be identified.

10. Size of the bit and core should be invariably mentioned.

11. Water loss zones, gaps in drilling (large voids) cavities, nature of starta should

be mentioned.

50

12. Core boxes of sulphide mineralisation should not be kept in open. This results

in oxidation and tarnishing.

Core Angle

Core axis is the borehole line that passes through the center of core along the

direction of drilling. The angle between core axis and bedding / foliation plane of the

rock unit that had undergone drilling is known as core angle (β). This angle is very

important in core logging for calculation of true width of the strata as well as to know

about the dip of the beds at that depth.

For measurement purpose the impression of the planar structure on the core is

first marked by a marker pen / pencil. Then the pencil is placed along the imaginary

core axis. Finally with the help of rounded protractor the angle is measured. For better

accuracy the angle is measured in both up dip and down dip direction and mean of the

two is taken.

Recording of borehole data

The details of core should be recorded in a tabular form as given below

Borehole No. - X

Location - Angle and Azimuth (bearing) -

Latitude - RL of borehole collar -

Departure - RL of borehole bottom -

Date of commencement - Total depth drilled (m.) -

Date of completion -

The table for recording of borehole data is given for core logging

51

Table (Core logging)

Drill run (m) Core

recovery Rock quality designation Sr.

No. Box / Run

From To

Length of run (m.) Drilled

(m) Core

% M %

Lithology Structure Mineralisation Core angle

Size of borehole

and casing

Remarks

Where M is

cumulative

length of core

pieces more

than 10 cms.

Color,

texture,

mineral

composition

nature of

rock

Bedding,

foliation,

fractures and

other structural

features

Ore minerals and

their description,

nature of

mineralization,

visual estimate

(VE) and other

details

52

6.4.3 Borehole Geophysics

Borehole geophysical logging is the systematic study related to depth for

obtaining information on physical parameters in borehole drilled during

exploration, testing and exploitation of sub-surface natural resources. It all started

in 1928 when Schlumberger brothers first made electrical well measurements in

the Pechelborn oil field in France and thereafter geophysical well logging is a

regular input in petroleum exploration. The application of borehole geophysics has

since expanded manifold and currently it is an integral part of sub-surface

exploration of various mineral commodities. It plays an important role especially

when the core recovery is poor or nil. Electrical logging of boreholes drilled

during base metal exploration play a significant role in 1) finding the depth and

thickness of mineralized intersections, especially in zones of core loss or when

borehole diameter is small or drilling is by percussion, 2) correlation of

mineralized zones from borehole to borehole, 3) delineation of lithology and

interpretation of surface geophysical data and 4) planning borehole geophysical

surveys. Generally self-potential (S.P.) and resistance /resistivity (or conductivity)

are considered as the main parameters to infer conductive zones of metallic

mineralization in electrical logging. However, electrical polarization ability is also

important because the occurrence of base metal deposits in a disseminated form is

quite common.

Although electromagnetic logging is increasingly employed I. P. logging

offers some possible advantages, even when the ore comprises massive sulphide

and is a good conductor. This is because the target size is frequently increased by

the surrounding zone of alteration and disseminated sulphides which are

responsive to the I. P. method. Moreover, the target is in effect brought closer to

the hole. In addition, experience has shown that some zones of a fairly high

percentage of sulphide, which one would presume to be conductors, do not in fact

respond to EM methods but appear clearly as I.P. targets. Thus, I. P. can provide a

53

useful detection log for larger electrode spacing to detect anomalous zones away

from the hole.

6.4.4 Core Sampling

The core is halved into two, one for sampling and analyses and the other for

record. Core splitting is done using a rock saw or an impact core splitter. There is

always the problem of obtaining a representative split of the core. Great care must

be taken to avoid this problem. Sometimes the entire core is analyzed to avoid this

problem, but usually only if logging is extremely thorough.

Sampling

Sampling is done to ascertain the grade of mineral and metal values that

vary in proportion from one place to another. One single sample taken from one

part of the ore body generally does not provide a representative picture of the

grade of the entire ore body. A large number of well-spaced samples are required

for ascertaining the average grade with an acceptable amount of accuracy.

Normally, no amount of sampling will give a truly representative picture of the ore

body. There is always some degree of error between the actual value and the value

computed from the samples. The aim of sampling is only to reduce the error to the

minimum possible level.

In addition to knowing the grade of the ore, sampling also reveals the

pattern of mineralisation within the ore richer and leaner ore portions. Similarly,

the limits of mineralisation towards both the hanging and footwall contacts can

also be precisely defined by careful sampling.

After examination of the core, the mineralized zones are demarcated and

sampling zones are fixed. The sample lengths may vary from 25 cm to 50 cm

54

depending on the nature of mineralisation, core size and core recovery. The core

loss should be adjusted in all the samples in a particular run. These samples are cut

into two halves in longitudinal fashion by core splitter/ core cutter. Subsequently

the samples are crushed and powdered up to -120 to 200 mesh. The size of the

sample may be reduced by conning and quartering process. Half of it is sent to

chemical laboratory and the other half is preserved as duplicate sample for further

studies and check analyses.

The table of recording of sampling data is as follows:

Depth along

borehole

Rock type

Chemical assay in %

Assay x width Box

no. Sample

No. From (m.)

To (m.)

Length of

sample (m.)

Actual sample length (m.)

% core recovery

Cu Pb Zn Cu Pb Zn

Sp. gravity

Remarks

50 gms to 100 gms of sample finally will be packed and sent to chemical

laboratory for analysis. Remaining sample will be packed in polythene bags,

numbered and kept as duplicate for future reference.

Coning and Quartering

55

1. Assay checks

Samples are subject to following tests

i. Quantitative assay for principle constituent.

ii. Semi-quantitative spectrum assay and quick procedure to determine all

elements in one.

iii. Assay for useful (by products) and harmful constituents.

2. Check samples

It is customary to send about 5-10% of samples drawn from the same zone

with different number for checking assay values are given by laboratory

(Recording manual errors etc.).

3. Duplicate samples

Out of the duplicate sample one set of duplicate samples can be sent to

another laboratory to determine of the analysis and random error, if any.

4. Composite sample

It is customary to prepare composite sample by combining a number of

samples for determination of other elements in addition to the normal constituents.

The cumbersome and costly chemical analytical methods forbid the termination of

all or large number of elements in all the samples collected. Care should be taken

here so that equal amount of sample should be taken to mix and make a composite

sample.

5. Sample Error

56

i. Random assay error.

Measurement error due to imperfect equipment, human eye on judgment.

ii. Systematic Error:

Error due to crude mistake, miscalculation, misprint, confusion in

numbering, shortcomings of sampling techniques and treatment.

Testing of samples

Samples collected during exploration are generally subjected to the

following tests:

1. Assay of useful constituents and harmful ingredients.

2. Mineralogical investigations to ascertain mineral composition, grain size,

texture and structure.

3. Semi-quantitative spectral analysis to determine all the elements present in

the ore.

4. Technological tests to establish the most efficient method for treating the

mineral.

5. Tests to determine certain physical properties of the mineral to establish the

grade and mining methods and to estimate the reserves.

Of these, in most cases, only the first test is done to establish the grade of

the ore. The others are done as and when a special necessity for such tests arises.

A few important tests which find application in both exploration and ore

beneficiation are given below.

Laboratory-scale examination

Certain laboratory investigations are conducted as a part of mineral

exploration and beneficiation. The laboratory scale investigations are listed below:

57

1. Testing physical characteristics

2. Petrological tests

3. Chemical analysis

4. X-ray and spectroscopic and other method.

Testing of physical characteristics

The aim of these tests is to establish the hardness, specific gravity,

brittleness and toughness of the sample. Determination of grain sizes also may be

involved in certain cases.

Petrological test

These may involve a quick examination of the minerals in a powdered from

for identifying the major minerals. Detailed petrological studies are done by

obtaining thin sections of samples and examining them under a petrological

microscope. The details studied are the mineral assemblage, ore and gangue

minerals, texture, grain size, types of bonds between the various ore minerals and

between ore and gangue minerals, etc. Besides, studies can be carried out for

establishing the possible sequence of mineralisation or paragenesis. In complex

cases, it may become necessary to carry out modal analyses, grain counting etc.

which can be carried out only on microscope with a mechanical stage. Correlation

on the basis of mineralogical composition and textural features can be done by

these studies. Petrological studies are followed by ore-microscopic studies where

the polished ore surfaces are examined in an ore microscopic study where the

polished ore surfaces are examined in an ore microscope. Ore microscopic studies

can establish ore texture, grain size, and shape and the relationship between the

various ore minerals and gangues. Minerals are recognised in this case by their

colour, brightness, anisotropism, hardness, internal reflection, etch effects,

cleavage, polishing, characteristic behavior under oil immersion etc.

58

Chemical analysis

Conventional chemical analysis aims at establishing the chemical

composition of the ore minerals and gangue. The valuable element is determined

by chemical analysis and its percentage availability with respect to the whole

minerals is expressed which helps in determining whether an ore can be

commercially mined or not.

X-ray, spectroscopic and other methods

X-ray analysis helps in determining the mineral composition of certain ores

which are otherwise difficult for determination. The mineralogy of bauxite for

example can be reliably determined only by x-ray studies. X-ray fluorescence tests

are helpful in identifying the rare elements. Modern equipment are able to provide

percentage of minerals in a given powdered sample.

Minerals and ores are known to possess certain special characteristics

which show out conspicuously in the presence of activators. This principal has

been made use of in the neutron activation analysis which is the most reliable and

sensitive method for trace element determination. This method involves high cost

though up to 55 elements can be detected by it with very high degree of precision.

From the data so obtained, the geochemical history and ore genesis can be inferred

which in turn may lead to the discovery of valuable mineral deposits in adjoining

areas. REE and trace elements geochemisty is being increasingly used as a

prospecting tool during on going exploration programmes.

7. RESERVE ESTIMATION OF MINERAL DEPOSITS:

PRINCIPLES AND METHODS

The main aim of geological survey is to size up rocks, minerals, oil. Natural

gases and underground water etc. for the use of men kind. This sizing up of the ore

body is known as reserve, which tells about the quantity, the quality and amenability

to commercial exploration of raw material (ore, rock, coal, oil etc.). The calculation of

reserve of prospects, deposit and mine is done at every stage right from preliminary to

last stage of exploration and mining.

Reserve are computed to determine the extent of exploration, development;

distribution of values; daily and annul output probable and possible, productive life of

the mine: method of extraction; plant design treatment, processing, requirements of

capital equipment, labour, power and to prepare raw material report of a project.

7.1 Principles and Assumptions

Ore reserves and grade estimation principle involve certain unavoidable

but un- provable (i.e. geological uncertainties) assumptions based upon some accepted

principles like.

1. The sampling followed is reliable and random enough for enabling the

sampling average judiciously arrived at the approximate deposit mean vise

population mean.

2. Basic parameters established in any ore body based upon points estimates such

as surface, drill holes etc. extent to the adjoining areas, only in consonance with

appropriate principle of interpretation.

3. The commonest principles of rule of gradual change and/or rule of nearest

points have capability of generating realistic estimate in the matter of ore

volume computations.

Rule of Gradual Change

According to rule of gradual change or law of linear function, all elements of a

mineral body can be expressed numerically, change gradually and continuously along

a straight line connection two adjoining stations (Fig.7.1). With the help of this

principle one can calculate the grade, thickness or reserve of any unknown point or

block falling between two known points or block values.

While mining this helps in prediction the grade or reserve of the sub-blocks for

future planning, the volume reserve grade or thickness of the unknown point falling

between two points can be determined by graphical and mathematical procedure.

Mathematical Procedure : The following formula are used to determine value,

reserve, grade or thickness:

Ga (d1) + Gb (d2) Gc = ---------------------- (d1) + (d2)

Ta (d1) + Tb (d2) Tc = ---------------------- (d1) + (d2)

Ra (d1) + Rb (d2) Rc = ---------------------- (d1) + (d2)

Where

Gc = Grade at point ‘C’ the unknown point

Ga = Grade at point ‘A’ the known point

Gb = Grade at point ‘B’ the known point

d2 = Distance of ‘C’ from ‘B’

d2 = Distance of ‘C’ from ‘A’

Ta, Tb and Tc are the thickness at point A, B, and C respectively. Similarly Ra,

Rb and Rc are the reserve at point A, B and C respectively.

Graphical Procedure: In Fig. 7.2 the measurement of vertical height at any

point between point A and B of reserve, volume, grade or thickness will be value at

the point. Similarly in Fig. 7.3 the required distance from point. A or B can be

determine from the known value.

Rule of Nearest Points: This is also known as rule of equal influence and

according to this the value of any point between two station is considered constant,

equal to the value of the nearest station. In a general case of borehole A and B with

thickness t1 and t2, the value of each one extent to midpoint ‘M’ between holes (Fig.

7.4). Any point on line AB or in the area of t1 or t2, except M; is inside the “linear or

area of influence” of a station A or B and near to it than to the adjoining one. Thus this

property gives the rule its name of nearest point.

The rule of nearest point is widely used for construction of equal area of

influence for areas and volumes of individual intersection. In most of the cases the

rule of nearest point is used for reserve calculation.

Geological, Mining and Economic Constrain

The rule of nearest point and gradual change is governed by geological,

mining and economic considerations. In a simple case of two drill holes with

corresponding thickness t1 and t2 of ore and a prominent fault between them, the area of

influence may be assigned on the basis of geological interpretation as shown in Fig 7.5

and 7.6 (depending on the strike and dip of the fault).

The geological interference include natural geological boundaries due to

structural features (folds, faults, change in strike and dip and other discontinuity)

change in characters of mineralization, thinning out or pinching of ore shoots,

zoning, weathering, different physical properties, heterogeneous composition varied

alteration and presence of detrimental constituents.

Blocks

Common technological, physiographic and economic grounds for inference in

construction of blocks are topography, thickness of overburden or ratio of overburden

to thickness of ore body, depth of water level, mining method, processing methods and

cost of extraction; also property section, township and state boundaries.

Beside this administrative problem also play an important role in demarcation

of the blocks. Fig. 7.7 shows an example of construction of block on the basis of

geologic structures.

Pre-Assumption

a) Physical continuity of the ore body within the point of testing and also beyond

but disrupted by geological discontinuities.

b) Success in replicating any irregular natural shape and size to possible

geometrical configuration, capable of measurements.

c) Nature of ore necessarily changes smoothly from point to point in any

general pattern sampling, geological, mathematical or like.

d) Applicability of mensuration and integration techniques for ore volume

estimation in all probable shape and sizes.

e) Characters of samples recovered to be representative in spite of the

known recovery of only 52.2% in case of NV size hole (by volume ), under test

condition and assured 100% recovery on the basis of length parameter

measurement.

The ultimate object of an exploration at any stage is to compute the reserve of

the mineral body and that include the following:

1. Determination of the quantity of minerals and all its valuable contents.

2. Determination of quality and grade of mineral.

3. Assertaining spatial distribution of the mineral in the deposit as a whole and

in its separate blocks and

4. Reliability of the estimates of reserve (Categories).

By the exploration technique normally insitu ore reserves (or geological

reserve) are computed without allowing the loss or dilution. Other classes of reserves

are calculated at the mining stage. The insitu reserve can be subdivided in two major

groups.

1 Industrial reserves: Economically important with present mining techniques,

beneficiation and smelting under present economic conditions.

2 Non-Industrial reserves: Industrially important in future when new mining

beneficiation and smelting methods are developed and / or there is change in

general economic condition of the area.

Parameters for Reserve Estimation

Reserve is determined by multiplying the volume of the ore body by the

bulk density. For the calculation of reserve the following parameters are to be

defined and determined.

1. Cut off grade

2. Stopping width

3. Weighted average and average grade

4. Tonnage factors

5. Core recovery

6. Thickness

7. Strike influence or strike length

8. Dip length or width influence

9. Correlation of lode

7.2 Cut off grade

Cut off grade mainly depend upon many factors such as economic scenario of

the area, country, international market, value, strategic minerals, nature of deposit,

nature of occurrence, concentration techniques, method of mining and requirement

of industries.

The cut off which is taken directly from mine without admixing is known as

natural cut off. On the basis of economic consideration the cut off is determined by the

following formula

Pc x 100 Ct = ----------- Vm

Ct = Cut off grade Pc = Production cost Vm = Value of mineral content

Statistical Determination of Cut off Grade

The cut off grade of a prospect can be determined by preparing the histogram

of all samples analysed. The sharp fall and flattening point may be taken as the natural

cut off (Fig 7.8 & 7.9)

To determine the optimum reserve and average grade of a deposit the reserve

should be calculated at different cut off say for copper the calculation may be done for

0.2%, 0.4%, 0.6% and 0.8% or if required for more cut off. Similarly for Pb- Zn cut

off grade may be taken as 2%, 3%, 4%, 5% and 6% or as per requirement. Looking

at the present economic condition of the market about 10% to 12% average grade is

required to exploit the lead + zinc mine in a new area.

Stopping width

In general the minimum stopping width is taken as 2.0 m for underground

mining methods. However, for open cast mining the bench thickness is taken as

stopping width, which varies from 2m to 5m. In some cases like noble metal the

minimum stopping width may be taken as 1.0 m but that is in rare case.

Weighted Average and Average Grade

The mineralised zone may be termed as the zone, which contains the ore

mineral in traces or in visible quantity. However, the zones that contain the ore

mineral or element equivalent to or more than the cut off grade are defined as lode or

ore zones. The weighted average of lode may be determine by taking the average of

samples value if the samples are equal in volume, but this is in rare case. The sample

width depends upon run length, percentage of core recovery, lithological change and

grade. Therefore, all the samples may not be of equal length or volume. In such cases

the weighted average of the lode is determined by the following formula.

Vta = Σls x AsΣ1s

Vta = Weighted average or average grade

Is = Length of sample

As = Assay value of sample

Is = Length of samples

For the determination of lode only those sample will be considered which have

value equal to or more than cut off grade and the thickness of the continuous sample is

equal to or more than stopping width i.e. 2 m or more. However, while determining

the lode thickness, samples with lean values (i.e. less than cut off grade) with less than

stopping width (i.e. less than 2 m) falling between high values (more than cut off

grade; have to be considered provided that overall grade of lode do not fall below cut

off grade. Suppose in one opening or drill hole three lode are intersected than in that

case overall average grade of the opening have to be determined by the following

formula:

Wta1 x t1 + Wta2 x t2 + Wta3 x t3 Agr = -----------------------------------------

t1 + t2 + t3

Wta = Weighted average of lode No. 1

t1 = Thickness of the lode No. 1

Agr = Average grade

Similarly the reserve and average grade calculated for different block

have to be further averaged to determine the overall reserve and average grade

of the deposit with the help of following formula

Ag = ΣR x Agr/ΣR

R = Reserve of the block

Ag = Average grade of deposit

Agr = Average grade of deposit or block

Tonnage Factor

Tonnage factor or bulk density is a multiplier to the volume for the

determination of reserve. The bulk density is determined by the following methods.

The difference in density and bulk density is the volume of voids. The bulk density

is determined for large volume in which the opening in the rocks (joint, fracture,

brecciation, gouge material etc.) are also included where as density is determined of a

small piece of rock or mineral. The bulk density or density may be determined by

the following method.

Cubical opening method

Dig up a pit of lm x lm x lm size and weight all the material (rock, mineral

etc.). This weight is the tonnage factor. In general (in exploration stage) in case of

sulphide ore deposit at initial stage of exploration it is not possible to determine the

bulk density by this method. However, in the case of limestone, iron ore, rock

phosphates etc. exposed on the surface it is easy to determine tonnage factor by

this method. In the mining state normally bulk density is determined by this method.

Conventional Density Measurement Method

The density of samples (small or hand specimen) is determined by measuring the

weight and volume of the samples by traditional method. This may also be determined

by weighing the sample in the air and in the water by steel yard balance by the

following formula:

D = W/V

D = Density

W = Weight

V = Volume

D = W1/W1-W2

W1 = Weight in air

W2 = Weight in water

This method will give the actual density where as the bulk density is

found slightly less than the density.

Determination of Bulk Density Using Drill Cores

The bulk density can be determined by measuring the length of the core

or half split core. Since diameter of the NX, BX, AX, NUT, AUT etc. bit size

are known, therefore, the volume of the core can be computed by the following

formula.

V = πR2 1 (if the core is not split)

V = 1/2πR21 if the core is half split (Fig. 7.10)

V = Volume, R=Radius of core sample, l=length of core sample

Weight the measured core and determine the bulk density by DB=M/V

where DB is the bulk density and M is the mass of the measured core. The

average on very sizable number of determination may be taken to represent the

insitu tonnage factor. The samples should be taken from all the variations of

the grade, which are taken for lode computation for true representative of the

lode or deposit.

Core Recovery

Core recovery plays an important role in computation of ore reserves.

Therefore the core recovery should be very high at least in the mineralized

zone. In NX core the actual core-recovery by volume is 52.2% and BX size is

40%. However, here for all practical purpose the core recovery is measure

lengthwise i.e. by recovery = (Lr/L) x 100 where ‘Lr’ is core recovered and

‘L’ is run length.

If core recovery in the lode is more than 95% then for the reserve

calculation purpose it may be taken as 100%. However, it will depend upon the

nature of deposit, its occurrence and mineral content. If core recovery is less

than 95% than correction factor have to be applied while calculation of

thickness of the lode. Mainly there are three options.

Dilution Method

In this method the assay value of recovered core is distributed in the

whole run assuming that the part of core which is not recovered is barren; By

this method the grade will go down the assay value.

Gr = A* x L1/L Where Gr = Grade

A = Assay value of sample

*Only in case where core recovery is more than 90%.

Reduced Width Method

In this case the core loss is considered as voids and the lode width is

taken as the length of the core recovered. Thus the thickness of the lode will

reduce; however, the grade will be as per assay value.

Equal grade method

This method is adopted where core recovery is more than 90%

or 95%. In this method the grade of recovered length is taken as the grade of run with the

assumption that the uncovered portion also contains the same assay value. Thus the run

length is the thickness of the lodes. In the loss of the core, the sludge samples may give

some idea but that cannot be considered for the following reasons.

i) The sludge represents fully the uncased column in borehole rather than the bit

end.

ii) The sludge collection contraption could only hold middling, with the slim

falling running off and the heavier particles going down in the rock crevasses.

iii) The sludge extraction is the function of return water, which were minimized

(water loss) near the fault or shear zone.

Thickness

After fixing up the cut off grade the thickness of the body is determined by

computing the thickness of the lode in individual hole, after giving the angle

correction to determine the true thickness and then taking the average of lode thickness

of all the boreholes. The openings or boreholes may not cut ore body perpendicular,

therefore it will give the apparent thickness (Fig-7.11). The corrections of apparent

thickness are required to be determined in zenith and azimuth deflection of the path of

borehole. Thus, there are three possibilities and the true thickness can be determined

with the help of following formula:

Azimuth and Zenith Perpendicular to Strike and Dip Plane

In this case the intersection of the boreholes will give true thickness and

there is no need of any correction (Fig. 7.12 A & 7.12B).

Azimuth perpendicular to the strike and zenith oblique to the dip plane

In this the borehole falls in the vertical plane perpendicular to strike but not

perpendicular to the dip plane (Fig. 7.13 A & B and 7.14 A & B)

a) If borehole is vertical (Fig 7.13 A and B)

Tr = Tv x Cos α = Th x sin α

Where Tr = True thickness

Tv= Thickness along vertical hole

α= Dip angle of ore body with core axis

Th= Horizontal thickness

b) If borehole is inclined (Fig. 7.14 A and 7.14B)

Tr= Ti Sin β where Ti= thickness along inclined borehole

β = Angle between core axis and bedding plane (acute angle)

Azimuth and Zenith Oblique to the Strike and Dip Plane

In general case when the dips of the body and hole inclination are

unconformable (hole crossing the body at sharp angle to the strike and dip), the

thickness is found by following formula (Fig 7.15 A and 7.15 B).

Tr = Tap x cos β cos θ (cos α tan β + tan θ) or

= Tap (cos α sin β cos θ + cos β sin θ)

Th = Tap (cos α cos θ + contan β sin θ)

Tv = Tap cos θ (cos α tan β + tan θ)

Where α = angle between the plane of the dip and the plane of the hole direction

β = Dip of the ore body

θ = Angle of the hole intersected in the borehole

Strike influence (strike length)

The strike length is determined on the basis of openings along the strike of the

ore body, the strike influence of each opening is determined on the basis of nearest

point and gradual variation. In the case of correlate able lode the strike influence is

taken as half the distance between two openings (Fig-7.16). In non-correlate able lode

also the distance along strike is taken as half the distance between two points or less

than that depending upon the nature and variation of the ore body. More details are

discussed under reserve calculation method.

Dip length influence

As strike influence the dip length influence, for each opening is also taken as

half the distance between the adjacent openings in the case of inclined ore body (Fig.

7.17), if the ore body have been intersected at different levels.

For the computation of the reserve of each opening the volume is determined

by multiplying the strike influence with dip length influence and thickness intersected.

On the basis of strike length, dip length and thickness the geometry or

morphology of the ore body can be determined and that is classifed as follows:

7.3 Morphology of the Ore Body and Variability

To determine the geometric configuration of the ore body the exploration by

opening carried out in all the spatial direction.

Say X, Y and Z where

X= Strike length

Y= Depth or dip length

Z= Width or thickness

Geometrically mineral bodies fall into three main morphological type:

Sheet like bodies

The horizontal, low to steeply dipping beds, sheet like bodies lenses and other

flat bodies will have two long and one short dimensions (Fig 7.18). Horizontal,

low dipping bed, X and Y > Z to steeply dipping beds.

Pipe or Lensoid Body

In this ore body will have one long and two short dimensions, either X>Y and

Z or Y >X and Z (Fig 7.19).

Isometric Bodies

Isometric bodies such as great stock works, pockets and other small bodies

have more or less all dimension equal i.e. X= Y = Z

In simple homogeneous ore bodies ‘morphological features’ like thickness etc.,

vary gradually and continuously, excepting, in cases of abrupt tectonic disturbances. In

complex ore bodies' variation are irregular in thickness and quality. Sometimes the

mineral bodies suddenly pinches out and thickness tends to be zero. Sometime

deposits consists a series of parallel en-echelon ore bodies or a group of small lenses

and pockets separated by barren interval (Fig. 7.20). Depending upon the shape, size

and quality of the mineral bodies the different methods are used to compute the ore

reserves.

7.4 Ore Reserve Calculation Methods

The exploration delineates the geometry and quality of the ore body and with

the help of interpolation and extrapolation the volume of the ore body can be

determined. This volume estimation is a geological exercise rather than mathematical

exercises as lots of interpretation, interpolation and extrapolation are required on the

basis of geological variations. The volume of the ore can be determine by using

various method depending upon the type of exploration, opening space, complex or

simple nature of deposit and quality of deposit. Therefore, two or three methods

should be used to check the volume or reserve.

The tonnage and grade is calculated by conventional methods based on

geometrical models such as square, rectangular, polygonal and triangular blocks and

method of sections such as cross, longitudinal vertical and plans or horizontal

(structural contours) sections. Successful application of a particular method

depends upon the shape, attitude, complexity of a deposit and pattern of sampling.

The conventional methods have the following shortcomings:

a) The area of influence assigned to individual openings / drill holes usually

not actual.

b) The methods do not provide confidence in reserve and

c) They do not relate with amount of drilling and hence hamper optimization

of drilling cost.

To overcome the above shortcomings classical statistical techniques using

probability concept, moving average, variogram, krigging etc. are in practice by the

exploiting agencies.

In conventional methods the different methods are used for different types

of deposits as detailed below:

Reserve computation of homogeneous bedded horizontal or low dipping deposits

In general this type of deposit consists limestone, iron ore, coal, gypsum,

potash, evaporites bauxite etc. In such type of deposit the different methods are used

depending upon the type of opening on grid pattern, irregular pattern and alternate

pattern (Fig. 7.21) etc. and these arc described in details in following page:

Grid pattern and included area methods

This method is adopted where the opening samplings or borehole drilling is done

along a rectangular or square grid pattern. In this method one rectangle or square of the

grade is taken (Fig. 7.22) and the grade thickness is computed of the center point of the

grid by taking the average of thickness of all the four openings (at the corner of the

rectangular of square) as detailed below.

T1+T2+T3+T4 Thickness T= ---------------- 4

This thickness of center point or average thickness of the four openings T1. T2,

T3 and T4 are thickness intersected in BH1, BH2. BH3 and BH4 respectively (Fig.23).

T1C1 + T2C2 + T3C3 + T4C4 Average grade Gav = --------------------------------------- T1 + T2 + T3 + T4

C1, C2, C3 and C4 are the assay value of lode intersected in each borehole

Volume V = X x Y x T

Reserve of one rectangle or square = R= V x Bd

X - Length along strike or length along X grid

Y = Length along Y grid

Bd=Bulk density

Extended area and grid pattern method

In this method, the area of influence is taken around each opening, thus

constructing a rectangular keeping opening at the center of the rectangle (Fig. 7.23).

Thus the thickness intersected in the borehole becomes the thickness of rectangle and

reserve of each rectangle is as follow and grade remain as intersected in the borehole.

Volume = V = ST

Where T = Thickness of borehole

S = Area of recharge

Reserve of one block of rectangular = R = V x Bd = S x T x Bd

Where Bd = Bulk density

R = Reserve of one rectangle

Reserve of total deposit =

RE = R1 + R2 + R3 ………………Rn

R1G1 + R2G2 + R3G3 + ...........................Rn Gn Grade of deposit, Gd = ------------------------------------------------------

R1 + R2 + R3 + ...........................Rn

G1, G2, G3……………. Gn is the grade of each rectangle or block

R1 + R2 + R3……………. Rn is the reserve of each rectangle or block.

Irregular grid pattern included area methods or Triangle Method

When the sampling or opening drilling is done at irregular pattern,

triangles are drawn to determine the area using included area method.

In this the triangles are constructed keeping each open at the apex of

triangle (Fig. 7.24). The average thickness and grid of the triangle is determined

along with the surface area as detailed below :

T1 + T2 + T3 Average thickness = Ta ---------------

3

T1, T2, and T3 are the thickness of lode intersected in three boreholes.

T1 C1 + T2C2 Average Grid = ---------------------

T1 + T2 + T3

C1, C2 and C3 are the grade of lode intersected in each of the three boreholes.

Surface area triangle S = ½ perpendicular x base.

Volume of triangle = V1 = S x Ta

Irregular grid pattern and area influence method or polygon method.

The procedure is to determine the area of influence of exploratory point

or opening (pit or borehole) and construct polygon blocks by perpendicular

bisectrix method of the triangle (Fig.7.25) as described above the apex of

triangle are located at the point of opening. The polygon can also be

constructed by joining the angle of bisectrix of each triangle (Fig. 7.26) but

sometime this may give false influence area therefore, perpendicular bisectrix

should be used for the computation of the surface area. The area may be

measured by the graphical method or by planimeter or by computation dividing

into simple triangles. The height of each polygon is the thickness of the ore

body with a polygonal base. The entire outlined ore body is divided into

number of polygonal prism of different height i.e. thickness. The grade

intersected in the opening is the grade of entire polygonal prism.

Volume of polygon= S x T

Where S = Surface of polygon

T= Thickness of polygon or ore body.

Method of Isolines

The method of isolines is used in geological exploration to represent the

variability of shape and properties of mineral bodies. This method depends

upon the rule of gradual change from one opening to other. In this technique

the equal value (thickness or grade) points are joined by a simple line just like

drawing the topographic contours. This is too laborious especially when

applied to multi elemental deposits where the isolines are to be drawn separate

for each element. However, with the help of computer ii has become easy. The

determination of reserves and grade by this method is very helpful in open cast

mining especially when one has to decide the bench .wise reserve and grade.

When contours are drawn using different values the different terminology are

used as below.

Isopach Maps Method

The isopach maps are drawn through points at which the formation or

lode are of equal true thickness (Fig-7.27). In open cast mining for benching

purpose the contour interval may be taken equal to bench height and that will

give the bench wise reserve and total reserve. Suppose the bench height is kept

2m then contour interval should be taken as 2m. Besides this-the benches are

kept horizontal, therefore the contours are drawn with the RL of the thickness

i.e. just like the topographic contours.

In Fig. 7.27 isopach plan is prepared on the basis of extrapolation

contours at 2m intervals for actual thickness. The area falling between each

contour is to be determined for example in S1 from 0-2m, S2 from 2m -4m, S3

from 4m -6m, S4 from 6m - 8m and S5 from 8m -9m (9m is the maximum

thickness). In such case the volume is calculated as follows.

V=S x T, V,= S1 x (0+2)/2

Therefore, V=S1 (0+2)/2+S2(2+4)/2+S3(4+6)/2+S46+8/2+S5 (8+9)/2

The isopach can also be transferred on the vertical plane considering the

horizontal thickness; this may be named as longitudinal vertical isopach map. In this

case the horizontal thickness is considered with 2 m contour interval (Fig. 7.28). The

method of calculation of volume is same as described above.

Isochore Map

The map is prepared just l ike the isopach map but here instead of true

thickness the thickness intersected in the opening is considered and contours are

drawn.

Isograde Map

In isopach map the contours of different thickness are drawn which gives the

quantity of the deposit. To understand the quality of the deposit the grade of

respective thickness is plotted and isograde lines are drawn which gives an idea about

the quality of the deposit and its variation (Fig. 7.29 and 7.30).

The isograde map of particular slice or bench can also be drawn to understand

the variation of the quality in that particular slice or bench (Fig. 31). In this way

different isograde maps are drawn for different slice or bench.

Reserve computation of moderately to steeply dipping tabular ore body

(more or less homogeneous)

In general base metals and noble metals, especially in India, occur in the

tectonically disturbed area. In such area the ore body shows complex geometry because

of folding and faulting. To understand the geometry of the ore body in three dimension

different section and plans are prepared which give an idea of variation in the shape and

quality of the ore body in three dimensions. These sections are also used for reserve

calculations as detailed below.

i) Cross section method ii) Longitudinal section method

iii) Level plan method.

7.4.1 Cross Section Method

The cross section or transverse section prepared across the ore body represent

the actual geological features in shape and quality. Cross section is prepared

perpendicular to the strike along which openings or borehole have been drilled and in

such case azimuthal corrections are not required. The preparation of cross section is as

simple as preparing the geological cross section of any body, however, in this case the

data collected by openings or boreholes are also considered (Fig. 7.32).

For the calculation of the reserve by this method the area of influence and

quality is considered on the basis of the rule of nearest point. In estimates made by this

method the ore bodies are divided into various segments by transverse cross section

lines spaced at equal or in some cases at unequal intervals. In the cross section area

method the reserve is calculated for individual opening and the area of influence of

that opening is measured on the cross section (Fig. 7.33) or calculated by measuring

actual thickness and dip length (Table-1). In this way the reserve of individual lode

intersected in individual opening is estimated and from that reserve and grade of

cross section with its strike influence is estimated. Thus reserve of each section is

estimated and there by whole of the block. With the help of cross section the areas

of different influence distance with degree of confidence and reliability are

measured which can be defined to different category (Fig. 34).

Longitudinal vertical projected section method

This method is very helpful in correlating the ore body along the strike

which is very important factor. This method is useful in determining the reserve

of complex ore body like lensoid, vein like tabular and ore shoot body. !n

longitudinal vertical project section the R.L. of the intersection of the ore body

is projected on any vertical plane parallel to the strike of the ore body (Fig.

7.35) and lodes are correlated. In this the area of influence is taken half the

distance between openings and that is measured on the section or computed by

multiplying the X and Y.

If the ore body is inclined in the dip correction is applied to determine

the actual area or the dip correction is given to the dip length to determine the

actual dip length or actual dip length is measured by the cross section. In this

method the reserve of individual lode in individual intersection is determined just

like above described case, the difference is that in this case the area determined

by the longitudinal vertical section (Fig. 7.35) is multiplied by the true thickness

to know the volume of the body.

The volume can also be determined by constructing the panel. Panel

length for each intersection falls between half the distance between two

intersections on either side. Width is taken the difference between upper RL and

lower RL. The thickness is taken as the ore body intersected by a horizontal plane

i.e. horizontal thickness.

After determining the reserve and grade of the individual lode the

reserve and grade of whole of the deposit is determined.

7.4.2 Level Plan

Level plan is prepared by plotting of the lode intersection on a

horizontal plan passing through the level of intersection or at particular R.L.

(Fig. 7.36). It represents the lode at that particular level. The lode can be

joined along strike on the level plane and the strike length can be determined.

The level plan gives the horizontal thickness of the lode, which can be

converted into true thickness. Level plane is very helpful in deciphering the

correlation of the lode, geometry of the ore body and strike length. The dip

length of the ore body can be determined by the two level plane prepared at

different R.L (The dip length between two level plane with different R.L.

can be determined by the following formula). Since strike length, dip length and

the thickness is known, therefore, the volume of the ore body can be determined.

Dip L= Difference in R.L. / Sin Q

Q-is the dip angle of the ore body

Average method

In this method the complex body is transferred to a simple average

body (Fig. 7.37) by taking the average of thickness (true thickness intersected

in the each openings and dip length. The strike length is determined by level

plane or by L.V. Section. Volume of the ore body equivalent to average thickness

x average dip length x strike length.

7.4.3 Statistics and Error Estimation

While computing the reserves by conventional methods there are ample

chances of committing errors at various stages because of limitation of classical

methods. To overcome the limitations of conventional methods the classical statistical

techniques are employed as an efficient and handy tool for the exploration geologist

to check the results of conventional methods. The errors can be committed at the

following stages.

• Error in sampling

• Error in analysis

• Error in estimation of average grade.

• Error in computed average grade and analytical assay of composite samples.

• Error in determinations of bulk density.

• Error in estimation of thickness

• Confidence limit of tonnage

Error in Sampling

It is stated earlier that population mean is the deposit mean as the sampling

is reliable and random enough to represent the deposit. Any error in samples

mean will reflect the error in deposit mean. By plotting the histogram of the

sample value it can be predicted that the values show normal distribution or log

normal distribution and accordingly the statistical treatment is applied. The method

provides arithmetic mean, variance, confidence limit etc. This can be cross checked

by probability plot of cumulative distribution of the metal. This limits of error or

error of mean or confidence interval of population mean can be determine with

the help of following formula, if it shows normal distribution. For log normal

distribution the formula is used.

M - t5% D/√<M<M+5% D/√<M

Where M = mean of sample.

D= Standard deviation

N=Number of samples

T5% = ‘t’ value form table at 95% confidence it can be calculated for 90%, 95%

97.5% and 99% confidence.

M-t5% D/N indicate the lower limit of error at 95% confidence

M+t5%D/N indicate the upper limit of error at 95% confidence

Error in Analysis

The error can occur due to manual and method of analysis and this can be

cross checked by the check sampling. The analyses of primary samples and check

samples may be cross checked by the following statistical studies.

Statistical Parameter

Number of samples

Mean of check assay

Standard deviation of principal

Standard deviation of check

‘F’ value

Correlation coefficient

Pooled ‘t’

Paired ‘t’

Average random error

Average systematic error

Difference x 100 Average relative deviation of principal assay = ----------------------------------------- No. of sample x mean of Principal

In this the correlation coefficient should be equivalent to 1.0 if there is

no error at any palace and departure to it will give the error, which can be

determined on the percentage basis.

Error in Estimation of Average Grade

The error or standard error of average grade is determined just like as it

is determined in case of assay values of samples. However, here only those

values are considered which are used in estimation of average grade or in

determining the lode thickness.

Error in computed average grade and analytical assay of composite

samples

The average grade of the lode is computed at the particular cut off. To

check the computed average grade the analysis of the composite samples is

carried out and comparative statistical studies as detailed above (under error in

analysis) may be done to determine the error and acceptance of the results.

Error in determining the bulk density

If bulk density of a number of core sample of all grade are determined

then the standard error of the bulk density can be computed using the formula

as stated above under error in sampling.

Error in estimation of thickness

In a complex geometrical body the thickness of the deposit varies very

much and this variation deviates from the mean. Therefore, the error of mean

is determined to compute over all error of the deposit.

Confidence limits of tonnage

The reserves or tonnage of a deposit is computed by determining the

reserves of individual lode in individual opening and further computing it for

whole of deposits. Thus the reserves of individual lode show variation from

mean reserve and percentage of that deviation can be calculated as the

standard error of reserve estimation using the formula. Besides this reserves

and all above described errors can also be determined using Blias and Caslier

theorem if the distribution is log normal or population is log normal. For log

normal population first we have to convert standard deviation by the following

formula.

Ed2 = SD2/A2+I

Where d - standard deviation for log normal population

SD = standard deviation (arithmetic)

A = Average or mean value

e = exponential constant

Formula to determine the standard error of logarithmic mean

Eln - d/√N

Where Eln = standard error of logarithmic mean

N = number of samples

Formula for determination of confidence of interval of the lognormal population

M x e–tEln <M<M x etEln

Where M=Mean of reserve

t = Value at particular Fiducian level it is 1 at 68.6% and 95% Fiducian

level.

Besides, the percentage of error can also be determined in the case of

reserve by the following formula. In the formula the standard error of all those

variables are included which play important role in reserve calculation as

average grade, thickness bulk density, dip length and strike length.

% Error of reserve = √(EG2 + ET2 + EDb2)

Where EG = Standard error of average grade

ET = Standard error of thickness

EDB= Standard error of bulk density.

Cut off Grade, Average Grade and Reserve Relation

The cut off grade show direct relation with the average grade whereas

with reserves it shows inverse relation. The average grade also shows inverse

relation with reserve (Fig 7.38). By calculating the reserve at different cut off

the optimum cut off, average and reserve can be determined of a deposit. The

average grade and reserve are plotted against the cut off and the intersection

point of two curves gives the optimum value of all the three components (Fig.

7.38). While plotting case should be taken that on the graph the maximum

average grade falls in the line of maximum reserve and minimum falls in the

line of minimum average grid (Fig. 7.38) or the length of the graph should be

equal in the case of average grade and reserves.

“Anyone who stops learning is old, whether at

twenty or at eighty, anyone who keeps learning

stays young perennially for the greatest thing in

life is to keep your mind young”.

Henry Ford

Table-1 : Details of lode, grade, thickness, dip length, strike length, tonnage and category of the reserve at 0.2% Cu cut

off with 2 m stopping width determined by CS method, Block – E_______ district.

Depth of lode in m

Dip length Intersection Borehole No.

From To

Length along hole

True thickness

Core recovery

Grade Cu%

up RL Lr. RL up RL Lr. RL

Dip length

Strike length

Tonnage (factor 2.5)

Category of

reserve

Table-2 : Details of lode, grade, thickness, dip length, strike length, tonnage and category of the reserve at 2% Pb+Zn cut

off with 2 m stopping width determined by CS method, Block – E_______ district.

Depth of lode in m

Dip length Borehole No.

From To

Length along hole

Core angle

True thickness

Core recovery

up RL Lr. RL

Grade Cu%

Dip of lode

Dip length

Strike length

Tonnage (factor 2.5)

Category of

reserve

8. BLOCK-PANEL DIAGRAMS AS A MEANS OF 3-D DEMONSTRATION

THE NEEDS AND THE DISCRIMINATION

The three dimensional construction is intended to picture the three

dimensional aspects in an unified and scaled manner, the plane of the drawing,

naturally endowed with only to dimensions. The methods are often, two fold:

1. Block diagrams: The 3-D effect is brought about by distorting in

the surface contours and correspondingly the geological outcrops.

2. Block-Panel diagrams: The former can exhibit only the outer parts of a

block and therefore, the block interiors, decided after detailed mineral

exploration have no chance of figuring out Block-panel diagrams are innovated

to meet these shortcomings and devise the pictorial representation of transverse,

longitudinal and oblique sections through a target area, based upon

exploration openings and judicious interpretations. Each section is a panel and

the composite effect of placing these panels in their respective places to scale

and then viewing discreetly from a distance with a perspective vision;

demonstrates the prospect inside out without disproportionate distortion.

The concept: The Law of Perspective Vision Station, that the best effect

of watching the three aspects of a solid body is to choose such a solid angle of

vision that all the three axes are normally get uniformly tilted. The 90°-90°-

90° intersection is thus dilated into a 120°-90°-60° intersection about the path

of vision, aspected on the 2-D paper as a point of collimation ray intersecting

the place of construction.

41

The horizontal axes are realigned in the perspective of 30° dilated to

120° basis and the vertical is maintained still vertically. For ease of

comprehension, the vertical axis is kept parallel to the lateral edges, of the

construction plan, as far as practicable. This style of projection is known as

isometric 60°-30° projection.

If one of the horizontal axis and the vertical axis are maintained in the

original intersection angle of 90° and the second horizontal axis is projected

backwards at 135° either dextrally or sinistrally like common sketches of the

artists; the perspective is called as a cubic 90°-45° projection.

42

(iii) Experience with 3-D modelling has shown that a 60°-30° projection

distorts homogeneously and hence an uniformity is possible to attain so as to

synoptically infuse a scalar drawing. A 90°-45° projection tends to distribute

this distortion somewhat heterogeneously and hence the total effect may be a

sum total of variable scales infused at varying parts of drawing. Hence mineral

exploration sector prefers 60°-30° projection while the regional geological

representations may use 90°-45° mode with enhanced boldness of far away

structures.

The Methodology

For reason of direct application in mineral geometry depiction, the

procedural details of the 60°-30° isometric projection are outlined below.

1. Choose a large sized drawing base to accommodate an untimingly

growth of the picture in uneven directions.

2. Grid out the geological map at regular square mesh to scale where in,

cardinal grids are preferable though not necessarily an only requisite.

Number the eastings and northings suitably for direct transference to a

control1ed distortion late.

3. Depending upon the exact structure which is meant to be exhibit if any;

the direction of vision is predetermined, then the square griddings have

to be specifically aligned symmetrically with respect to this vector of

perspective.

4. Eastings and northings may satisfy the most common mineral

exploration girds, particularly if the maps etc. are tied up with Survey of

India co-ordinates; as is the modern tendency; for obvious advantage.

5. The square intersections are now transformed to gridded diamonds of

120°-60° internal angles; oriented symmetrically with respect to the

43

depth dimension viz. vertical going parallel to the edges of the drawing

media.

6. The horizontality of verticality of the diamond spread are bounded by

the desired and direction of distortion.

7. The X-sections or L-sections meant to be fitted into the Block-Panel

network are drawn with a datum line reduced to the lowest desirable

R.L. uniformly, since the drawing media now assumes this R.L. and

allows only for upward growth of each section thence.

Warning: Downward growth of the sections of emplacement is fraught

with the danger of disproportionate fitment.

8. Since the vertical of the sections and the vertical of the intended

drawing are kept identical, the section lines on the rhombus grid are

suitably inserted and plotted over for ground profiles, vertical boreholes,

pits and adits etc.

Warning: Horizontal gaps are now per the rhombus grid replacing the

squares but vertical ones are the properly vertical representations.

9. While the profile or geological boundaries result from smooth joining of

these vertically calibrated points, a point to note would be the

distortions infused into inclined lines, planes and line, controlled

imaginatively.

44

Warning: The ones dipping towards the vision vector get enhanced

angles while the ones away reduce proportionately based upon the

principle of seeing the maximum angle for the true dip in a section

perpendicular to the strike and seeing it as horizontal parallel to this

direction and intermediately in any apparent dip.

10. The overlapping areas of the sections may be suitably wiped off with

major lines of interest still kept dotted like see-through glass panes. The

side panels are completed by joining naturally horizontal stripes parallel

to the datum line as character induces an overall block-panel outfit to

the image.

The Output

Each panel thus fitted in place and the block laterals trimmed off with

suitable hatchings, present in totality a bread-slice composite appearance with

many a slices just taken off and the rest left over as it is without collapse or

compaction.

Sometimes many prefer to only insert the panels and .leave the

interpanel gaps unconnected, such as preferred in lode deposits and

discontinuous entities.

Continuous bodies like iron ores, limestone, coal or bauxite panels have

a preferential interpanel connection and subtle hatching or stipling for the

overview in entirely.

45

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