turner brecciacomplexes sept2011

S.J. Turner

April, 2003

Updated: Sept, 2011

DEFINITIONS

* a ‘diatreme’ breccia is the vent zone of a maar-type or hydro-

magmatic volcanism, and results from a phreatomagmatic

eruption.

* a phreatomagmatic breccia MUST have evidence for a juvenile

magmatic component = juvenile clasts and matrix.

* a phreatic or hydrothermal breccia is formed by over-pressured

fluids but with no direct magmatic component.

* phreatomagmatic and phreatic breccias commonly occur within

the same breccia complex.

Relationship of Mineralization to Breccia Complexes

• the formation of breccia complexes is simply a highly effective

PROCESS for mineralization; as a fluid conduit, as a mechanism to

fracture and brecciate surrounding rocks, and as a focussed zone of

pressure and temperature gradients, and fluid mixing.

• the presence of a breccia complex (diatreme) is not necessarily an

indication of a mineralized system. Many, maybe even most,

phreatomagmatic breccias are not associated with any mineralization.

Similarly, many mineralized breccia complexes may not have

significant or economic mineralization.

• Vicuña (Chile/Argentina), La Carolina (Argentina) and (Bald

Mountain, Australia) are examples of weakly mineralized breccia

complexes. There are many others.

Major Deposits Associated with Breccia Complexes

Deposit Endowment Deposit Type

(MM oz Au)

Yanacocha Complex, Peru +41 high-sulfidation

Pierina, Peru 8 high-sulfidation

Pascua / Lama, Chile 22.9 high-sulfidation

Veladero, Argentina 20 high-sulfidation

Pueblo Viejo, Dominican Republic 36.6 high-sulfidation

Kelian, Indonesia 5.8 carbonate - base metal - Au

Rosia Montana, Romania 13 carbonate - base metal – Au

Penasquito, Mexico 27 carbonate - base metal – Au

Camino Rojo, Mexico

Sari Gunay, Iran +3 epithermal, disseminated

Cripple Creek, USA 28 alkalic

Rattlesnake Hills, Wyoming ~2 alkalic

NOTE : endowment includes past production / reserves and resource

Formation of Phreatomagmatic Breccias • phreatomagmatic breccias form where a rising magma intersects an aquifer at a

sufficiently shallow level to erupt.

• an eruption will occur ONLY when Pfluid > Plithostatic, which is at quite shallow levels (.

• the diatreme grows by ‘drilling’ downward with time, due to a decreased Plithostatic

directly over the breccia column (eg Valley of Ten Thousand Smokes, Alaska).

• the diatreme will continue to form as long as there is a continuing (periodic) supply of

magma and continued input of shallow meteoric water.

• if the magma supply shuts off then the diatreme becomes quiescent and water-

saturated until the next magma pulse.

• if the meteoric water supply is exhausted then the magma will continue to rise in the

breccia column or margins as dikes or domes, which explains the common association

of diatremes and flow dome fields.

The Setting of Breccia Pipes

from Corbett, 2003; The Ishihara Symposium: Granites and Associated Metallogenesis

from Corbett, The Ishihara Symposium: Granites and Associated Metallogenesis

The Setting of Breccia Pipes

Textures and Characteristics of Phreatomagmatic Breccias

* rock flour matrix

* presence of ‘basement’ clasts

* accretionary lapilli

* funnel-shape with upward / outward flaring margin

* polymictic; including multi-stage breccias

* altered / mineralized clasts eg. vuggy silica

* tuff ring (if preserved)

* slump blocks of tuff ring material

* bedded fallback breccias

* interbedded lacustrine sediments and eruption breccias

* matrix to clast-support

* milling and fluidized matrices

* reaction rims on juvenile clasts

* ‘hypogene exfoliation’ of juvenile clasts

* deformed / very irregular-shaped, ‘wispy-textured’ juvenile clasts

* clay overprint by magmatic volatiles

* intruded by endogenous / exogenous domes and dikes

* peripheral fracturing and crackle-type brecciation in competent units

* peripheral hydrothermal breccia and pebble dikes

La Zanja, PERU

Unmineralized quartz -

tourmaline altered

diatreme breccia with

slightly deformed juvenile

clasts and strong reaction

rims in clasts and in the

matrix.

Textures:

Juvenile Clasts

Deformed, basaltic juvenile clasts with

chilled, reaction margins in k-feldspar

altered phreatomagmatic breccia.

Sapucai,

PARAGUAY

alkalic gold

prospect

Textures:

Juvenile Clasts

Santa Barbara, Puno, PERU

Weakly clay altered phreatomagmatic

breccia with chilled margins on

basaltic juvenile clasts.

Textures:

Juvenile Clasts

wispy-textured dacitic clasts : juvenile component

of clay-altered phreatomagmatic breccia

wispy-textured juvenile quartz - porphyry clasts in

clay-altered, polymict phreatomagmatic

breccia

Martabe, INDONESIA

Kelian, INDONESIA

Textures:

Juvenile Clasts

Lienetz Pit

Juvenile

clasts ?

Accretionary

lapilli

Vein clasts

1 cm

Breccia facies of the Ladolam alkalic

epithermal gold deposit,

Lihir Island, Papua New Guinea

Jacqueline Blackwell, Jocelyn McPhie,

David R. Cooke, John Robinson

2007 JCU Breccia Symposium

Textures: Clasts

Ladolam, Lihir Island, PNG

Martabe, Indonesia

Clast with accretionary lapilli in

clay-pyrite altered,

unmineralized diatreme breccia

Argillized phreatomagmatic breccia (barren) with basement

shale clasts in the Carachugo Norte pit.

Carachugo Sur, Yanacocha, PERU

Textures: Juvenile Clasts

Pascua, Chile

Mineralized diatreme breccia:

alunite-pyrite-enargite ore

Diatremes: Internal Textures

Yanacocha, Peru

Hypogene exfoliation of

large juvenile clast of argillized

dacitic porphyry in diatreme = rapid

depressurization.

Minaspata, PERU

Alunite – clay – silica

altered phreatomagmatic breccia

(barren)

Diatremes: Internal Textures

Fluidized matrix to diatreme breccia with

altered clasts and pyritic margin to clasts

Martabe, Indonesia

Pascua, Chile

‘APE’ Au-Ag mineralization in

crackle-brecciated granite

marginal to the Pascua diatreme

Diatremes: Marginal Features

Yanacocha, Peru

slump block of bedded breccias and

tuffaceous units on margin of diatreme

breccia.


Yanacocha, Peru

Crackle-brecciation in silicified

margin to a diatreme breccia.

Bald Mountain, Queensland,

Australia

Silica-hematite-altered crackle-

brecciated competent quartzite unit

on the diatreme margin.


IS epithermal veins in cone fractures

around the margin of the Santa Barbara

diatreme breccia, Peru (Wasteneys, 1990)

Diatremes: Upper Facies

Upper facies in a maar diatreme breccia at

Wau, PNG, including slide blocks along low-

angle detachment faults, deformed lacustrine

beds, hydrothermal eruption breccias, tuff

rings, shallow hotsprings-style alteration

with gold mineralization and endogenous

domes (Sillitoe, 1984).

Pyritic, carbonized wood fragment

in lacustrine beds over breccia.

Bedded hydrothermal eruption

breccias.

Diatreme Breccias: Upper Facies Bald Mountain, Queensland, Australia

Lake infilling diatreme crater

Marginal interleaved breccias and

altered PE schist. Breccia dikes marginal to a diatreme

Pascua, Chile

Unmineralized eruption breccia

with native S + cinnabar.

Diatremes: Eruption Breccias / Tuff Ring

Pascua, Chile

Unmineralized, steam-heated

silica alteration in bedded

eruption breccias above deposit.

Diatremes: Eruption Breccias / Tuff Ring

Minaspata, Peru

Bedded eruption breccias

Diatreme Breccias: gold (silver, base metal)

mineralization

Mineralization may occur within different trap-sites around and within diatreme breccias:

• within permeable facies within the breccia, typically where meteoric water influx was

insufficient to alter the breccia matrix to clays eg. Pascua.

• within crackle-brecciated competent units around the margins of the diatreme eg. Yanacocha

• in fractured competent units within the breccia columns such as dikes and domes eg. Rosia

Montana.

• in permeable breccia facies below the flared out margin of a flow dome eg. Yanacocha

• in breccia zones below and between slide blocks of relatively impermeable wallrocks eg.

Peñasquito and Rattlesnake Hills

• in well-defined epithermal veins radial or concentric outwards of the breccia margins eg. Santa

Barbera in Peru.

• mineralized clasts may signify the presence of deeper porphyry and skarn mineralization eg.

Lepanto / Far South East in the Philippines and Rinti in Indonesia.

Diatreme Breccias: gold (silver, base metal)

mineralization

Pascua, Chile

The highly acidic alunite – pyrite – enargite

ore within the Pascua diatreme indicates that

this mineralization was dominated by

magmatic fluids, with virtually no meteoric

component. Competent (granitic) wall rocks

are also fractured and mineralized.

The lack of meteoric water limited clay

formation which enhanced the permeability

of the diatreme breccia column.

Yanacocha, Peru

Most of the diatreme breccias at Yanacocha

are clay-altered due to the influx of large

volumes of meteoric water following the

cessation of magmatism, which caused the

breccias to become relatively

impermeability and therefore barren.

Gold mineralization is hosted within

intensely fractured, competent massive

silica around the margins of the diatreme.

Weaker mineralization is hosted in less

competent alteration such as alunite –

pyrophyllite – clay – silica.

Diatreme Breccias: gold (silver, base

metal) mineralization

Rosia Montana,

Romania

A second stage phreatic

breccia , termed the ‘Black

Breccia’, between two dacitic

domes, is thought to be

responsible for extensive

fracturing and mineralization

of the relatively competent

intrusive rocks within a larger

clay-altered, and mostly

barren diatreme breccia.



Rattlesnake Hills,

Wyoming Gold mineralization focused on the margin

of a diatreme breccia where slide blocks of

Precambrian schist have partially slid back

into the breccia and blocked upwards fluid

flow on the margins of the breccia column.





Corimayo, Yanacocha, Peru

High-grade gold mineralization (eg

70 m @ 16 g/t Au) hosted in

massive silica trapped below less

permeable, clay and alunite-silica-

clay altered flow dome rocks. This

mineralization was blind below

clay altered and unaltered barren

rocks.

3600m

3400m

70m 16 g/t

3200m

Kelian, Indonesia

Carbonate-base metal – gold

mineralization hosted in diatreme

breccia with fluid flow focussed

along low-angle ‘detachment’

faults which formed large-scale

slump blocks sliding back into the

breccia column.



low-angle faulting felsic dome

Rinti, Indonesia: mineralized porphyry clasts within

diatreme breccia. The porphyry remains undrilled.



0 M 300

0

500

100

200

300

400

600

m RL Zone of abundant

mineralized

B

Soil-Au ( ppb ) 100

200

300

Soil-Cu ( ppm )

V

Diatreme Breccia

Dacite Porphyry

Tonalite Porphyry

Andesite Volcanics

Unaltered

Illite Clay

Chlorite - Epidote

Pyrophyllite - Alunite

High Sulfidation Silica

Chlorite -magnetite

Biotite -magnetite

Fault

Clast of Chalcopyrite - Bornite

mineralized Tonalite

10

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B’

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500

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m RL

Proposed Drill Hole

Zone of abundant mineralized fragments fragments

Gold in Soils

PURNAMA

PELANGI

BASKARA

KEJORA

GERHANA

< 10 ppb

100 ppb

> 300 ppb

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0 1 km

Diatreme Breccias:

Geochemical Response

Gold-in-soils geochemistry

showing the principal gold

mineralization focused in

competent silica-altered rocks

around the SW margin of the

Purnama diatreme breccia. The

diatreme is clay-altered and

barren with a late-mineral,

weakly altered felsic dome.

12/7/2011

Diatreme Breccias: Geochemical Response

A’

Diatreme Breccia

Diatreme

Breccia

1 Km 1 Km 0 0

PRD-03

PRD-01

PRD-02

PRD-04

PRD-05

PRD-06

PRD-07

PRD-08

Proposed

Drill Hole

Core Holes Y2000

0 500 1000

0.4

0.8

ppb Au

West Rinti, Indonesia: gold-in-soil response within competent silicified

rocks around a diatreme

breccia.

Peñasquito, Mexico

The Peñasquito diatreme

breccias have are characterized

by distinct gravity lows within

a broader gravity and magnetic

high.

The magnetic and gravity data

were assessed as good quality,

the CSAMT conductivity data

are dubious.

Diatreme Breccias: Geophysical Response

Residual Bouguer Gravity

RTP ground

magnetics

CSAMT:1500 m

inverted resistivity E-W tensor IP for Breccia Azul

Important Factors (1)

• in many districts diatreme breccias were mapped as conglomerates or volcanic

breccias in the early stages of exploration eg. Veladero in Argentina, Rosia

Montana in Romania.

• in many cases diatreme breccias are barren due to the presence of relatively

impermeable clays. This clay alteration also causes recessive weathering.

• competent rock units on the margin of diatreme breccias are highly favorable targets.

• upper parts of diatremes (maar volcanoes) are very complex

• lower parts of diatreme breccia may be invaded by felsic intrusions (porphyries),

which may also be mineralized eg. Peñasquito in Mexico, Spring Valley in Nevada

and Yanacocha in Peru.

• the presence of pervasive crackle-type brecciation, arcute hydrothermal breccia dikes,

bedded eruption breccias and / or interbedded lacustrine sediments, mineralization

associated with circular patterns in aerial photography or satellite imagery may all

signify the presence of a diatreme breccia.

• some breccia complexes may have been mis-interpreted as diatreme breccias eg.

Olympic Dam, where new data indicate the breccias are tectonic, forming a local

sedimentary basin. The jury is still out on Pueblo Viejo, which may comprise a

series of smaller diatreme breccias within a larger shale basin.

Important Factors (2)

YANACOCHA Characteristics of Diatreme Breccias

* dominantly clay-altered with dacitic clasts as the juvenile component.

* gold mineralization in fractured and brecciated silica-altered wall rocks flaring out

away from the diatreme margins

* gold mineralization is post-diatreme but not hosted in most of the diatreme due to

the impermeability of the clays

* diatreme hosts dacitic to rhyodacitic domes, and at depth young porphyry

intrusions with associated late breccia phases.

* minor bedded, eruptive facies were recognized at the surface

* hypogene exfoliated dacitic clasts and fluidal textures are present

* marginal silicified rocks are strongly crackle-brecciated with hydrothermal

(phreatic) breccia and pebble dikes.

turner brecciacomplexes sept2011

Documents

basaltic juvenile

bedded eruption

juvenile component

rosia montana

slide blocks

upper facies

breccia complexes

breccia column