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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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