exposicion de brechas

David R. Cooke & Andrew G.S. Davies#

# Current Address: TeckCominco,Vancouver

Breccias in epithermal and porphyry deposits:The birth and death of magmatic-hydrothermal systems

CODES, University of Tasmania

Sericite-chlorite altered polymict rock flour matrix breccia, Acupan Gold Mine, Philippines

Talk Outline

Breccias - Descriptive Methodology

Genetic Classes

Overview of Breccia Types in Magmatic-Hydrothermal Systems

Case Study: Kelian

Implications for Ore Formation and Exploration

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Brecciation

Rocks break when they fall, cool, grind, explode, corrode, etc.

This means that breccias can form in many geological environments:

• Sedimentary

• Volcanic

• Tectonic

• Magmatic

• Hydrothermal

Igneous-cemented breccia: trachyandesite clasts set in a quartz monzonite porphyry cement, cut by

quartz-bornite veins with orthoclase alteration halos, E31 prospect, North Parkes, NSW

Breccia Description and Interpretation

• Breccias should be described in terms of:

• composition (matrix, cement, clasts)

• texture (clast-supported, jigsaw fit, etc)

• morphology (pipe, vein, bed, etc.)

• contact relationships

• Genetic nomenclature should only be applied with caution after a breccia has been fully described

Push-up, fall-down, or break-apart breccia?

Ideal combination:5 + 4 + 3 + 2 +1Alteration Internal Components Grainsize Geometry

organisation A + B + C

Minimum Combination: 4 + 3 + 2

Breccia Description

Bat Cave breccia pipe, Northern Arizona. (Wenrich, 1985)

1) Geometry• pipe, cone, dyke, vein, bed,

irregular, tabular...• Contact relationships: sharp,

gradational, faulted, irregular, planar, concordant, discordant

Descriptive Names for Breccias

5 + 4 + 3 + 2 +1Alteration Internal Components Grainsize Geometry


2) Grainsize• microbreccia (< 2mm) or breccia (> 2mm)...

3) ComponentsA: clasts

• monomict or polymict

• Composition: lithic, vein, breccia, juvenile magmatic, accretionary lapilli, mineralised, altered

• Morphology: angular, subangular, subround, round, faceted, tabular, equant




3) Components (cont.)B: matrix• rock flour, crystal fragments, lithic

fragments, vein fragments

• texture: banded, laminated, massive• grainsize - mud, silt, sand, gravel, pebble,

cobble

C: cement• texture: cockade, massive, drusy, etc.• Ore & gangue mineralogy, & grainsize

D: open space (vugs)




4) Internal Organisation• Clast abundance, clast, matrix or cement-

supported• Clast distribution: jigsaw-fit, rotated, chaotic• Massive (non-graded) or graded• Stratified or unstratified

5) Alteration• Clasts, matrix or cement• Alteration paragenesis

Sericite-altered polymictic rock flour matrix breccia, Braden Pipe, El Teniente

Breccia Facies Associations

Chlorite-altered, jigsaw-fit, in-situ, pyroxene-phyric andesite clast-supported monomictic chlorite-cemented breccia

Chlorite-altered, pyroxene-phyric andesite clast-rich, polymictic, clast-supported, massive, jigsaw-fit to rotated rock flour matrix breccia

Chlorite-sericite altered, matrix-supported, chaotic, polymict pyroxene-phyric andesite and mudstone-clast-rich rock flour matrix breccia

Hematite-carbonate-pyrite-chlorite-sericite cemented, polymict pyroxene-phyric andesite and diorite-clast breccia

Chlorite-hematite-carbonate-pyrite-altered, polymict pyroxene-phyric andesite and

diorite-clast massive to stratified rock flour breccia and microbreccia

Diorite breccia complex

brecciated diorite

rock flour zone,

increases inwards

increased permeability – cemented facies

facies with sub-vertical fabrics

Variations in clast types & matrix abundance

Fractured diorite

Diorite host rock

HydrothermalBreccias

Volcanic Breccias

Magmatic-hydrothermalbreccias

Tectonic Breccias

MagmaticBreccias

Igneous cementbreccias

Magma intrusion into magmatic-hydrothermal system

Fault breccias

Stockwork veins Structural control on

breccia location

Breccia Genesis

• More than one process can be involved in breccia formation

• This overlap means that genetic terminology is generally applied inconsistently

Phreatic breccias

Volatile-saturated intrusion undergoes catastrophic brittle

failure due to hydrostatic pressure exceeding lithostatic load and the

tensile strength of the wallrocks

1 - Magmatic-hydrothermal breccias

• Containment and focussing of volatiles

birth of a magmatic-hydrothermal ore deposit

Breccias in Magmatic-Hydrothermal Systems

• Permeability enhancement through the formation of a subsurface breccia body allows for focussed fluid flow

• Can precipitate abundant, well-mineralised cement which contains hypersaline & vapour-rich fluid inclusions

• Rock flour matrix and clasts may be altered to high temperature mineral assemblages (e.g. biotite)

Biotite-altered rock flour matrix breccia, Gaby, Chile

Chalcopyrite-cemented monzonite breccia, Mt

Polley, British Columbia

Magmatic-Hydrothermal Breccias

32oS

33oS

70o W71o W

0 50 100

34oS

N

km

Rio Blanco -Los Bronces

El Teniente

Los Pelambres

Santiago

Los Andes

PacificOcean

• Largest known breccia-hosted copper-molybdenum porphyry system

• Located 70 km NE of Santiago, Chile

Rio Blanco

Rio Blanco - Los Bronces

Rio Blanco Los Bronces

Sur Sur

La Union

South

• Ore at Rio Blanco is hosted in biotite-cemented and biotite-altered rock flour matrix breccias (‘magmatic’ breccia)

Biotite breccia, Rio Blanco

Biotite Breccia

Tourm. bx Sur-Sur

• Ore at Sur-Sur, La Union and Los Bronces is hosted in tourmaline-cemented breccias

Tourmaline Breccia

Tourm. Bx Los Bronces

Tm-cp-py-qz-anh cement: Sur-Sur breccia

Tourmalinebreccia

Rock Flourbreccia

Tourmalinebreccia

Biotitebreccia

Late-stage rock flour

breccia

Diorite wallrock

Sur-Sur XC50

Tm bx cut by RF bx, Rio Blanco

Buoyant magmatic gas

streams up through bx

column

Drawdown of meteoric water?

Upwelling magmatic-hydrothermal brines

precipitate ore

Breccia-Enhanced Permeability

San Francisco Batholith

Farellones Fm

~5 km paleodepth

~2 km paleodepth

Maar-diatreme breccia complex

Late intrusion into active

hydrothermal system

2 -5 km

paleodepth


2 - Phreatomagmatic breccias

• Rock flour & milled clasts abundant

• Surficial and subsurface breccia deposits

• Bedded and massive breccia facies

• Venting of volatiles to the surface

death of a porphyry deposit

shortcut to the epithermal environment

Diatremes

Diatremes are downward-tapering, cone-shaped breccia bodies (paleovolcanic vents)

• phreatomagmatic and phreatic explosions• filled by volcaniclastic debris and collapsed wall rocks• subsurface conduits beneath maars

100 m

U.S. Geological Survey / photo by R Russell, 1977

U.S. Geological Survey / photo by D. Dewhurst, 1990

Maars

Maars are 100 m to greater than 3000 m diameter,

monogenetic volcanic craters• surrounded by low aspect ratio ‘tuff rings’• wet pyroclastic base surge, fallout and re-sedimented volcaniclastic

deposits

25 m

U.S. Geological Survey / photo by D. Dewhurst, 1990U.S. Geological Survey / photo by C. Nye, 1994

Modified after Lorenz, 1973

0m

> 2500m

Water Table depressed

Increasing eruption depth

‘wet’ pyroclastic eruptions

Diatremes - Volcanological Model

No direct link to mineralisation - this model fails to account for common association of diatremes and magmatic-hydrothermal

ore deposits

Bedded rock flour matrix polymict breccia facies, Braden Breccia Pipe, El Teniente

Dacite pipes (5.5 Ma)

Dacite dyke (5.3 Ma)

Sewell Diorite (8.9-7 Ma)

Mine Level #6 (2165m asl)

Teniente Host Sequence

500 m

El Teniente -Braden Breccia

< 0.5% Cu

Grey porphyry (5.7 Ma)

Hble-phyric dykes (3.8 Ma)

Late dacite dykes (4.7 Ma)

Marginal Breccia (4.7 Ma)

Braden Breccia (4.7 Ma)

< 0.5% Cu

> 0.5% Cu

> 0.5% Cu

• World’s largest PCD: 12.4 Gt resource @ 0.63% Cu, 0.02% Mo

• Part of the deposit has been destroyed by the late stage Braden Breccia Pipe (diatreme complex)


• Phreatic steam explosions caused by

decompression of hydrothermal fluid

• No direct magmatic involvement

epithermal gold deposition

3 – Phreatic, hydraulic & fault breccias

• Fault breccias: grinding and abrasion may produce gouge, cataclasite, etc

• Phreatic breccias: in-situ subsurface brecciation (jig-saw fit to rotated textures)

• Hydraulic breccias - only minor clast transport and abrasion (angular clasts common)

• Abundant hydrothermal cement

2 cm

Fault breccia with clasts of quartz-chalcopyrite veins in a rock flour matrix, and with chalcopyrite smeared along the breccia margin, Ridgeway Au-Cu porphyry, NSW

Fault Breccias

Phreatic Breccias

Porkchop Geyser, post-eruption, 1992, Yellowstone

• Gases accumulate beneath a silica seal during upflow of boiling waters

• P increase can rupture the hydrothermal seal, triggering a steam explosion & phreatic brecciation Au-mineralised vein breccia, Acupan

Gas cap in self-sealed geothermal system (Hedenquist & Henley, 1985)

Phreatic Breccias

Instantaneous P decrease changes the depth of first boiling (Hedenquist & Henley, 1985)

Depressurisation can affect a significant vertical column of rock (hundreds of metres) and can trigger ore deposition as H2S partitions to the vapour phase

Phreatic Breccias

• Seismic rupture

• Overpressuring and failure of hydrothermal seal

• Instantaneous unloading (landslip, draining of lake, etc.)

• Temperature increase (magma -water interaction)

Phreatic Breccias - Triggers

Hydrothermal explosion triggered by draining of glacial lake (Muffler et

al, 1971)

Hydrothermal eruption crater, Pocket Basin, Yellowstone. Fragments of lake sediments were deposited in a low aspect ratio ejecta apron after draining of glacially-dammed lake 20-25,000 years ago

Phreatomagmatic vs. Phreatic Explosions

Phreatic explosion• no direct magma - water contact at explosion site• flashing of water to steam• no juvenile magmatic component

Eruption of Waimungu Geyser, New Zealand, 1904 (Sillitoe, 1985)

Phreatomagmatic explosion• magma - water interaction at

the explosion site• explosion driven by flashing of

water to steam• magmatic gas contribution is

minor• juvenile magmatic component

A PhD study by Andrew DaviesCentre For Ore Deposit Research (CODES)University of Tasmania, Australia

Native gold disseminated in sphalerite, pyrite and carbonate

The Kelian Breccia Complex:host to a giant epithermal Au-Ag deposit,East Kalimantan, Indonesia

1 cm

SingaporeKELIAN

Jakarta

Regional geology

• Located in uplifted block of Cretaceous volcaniclastic rocks

• Surrounded by terrestrial and shallow marine sedimentary rocks of the Tertiary Kutai Basin

• Largest epithermal Au deposit in a NE-trending belt of Miocene low sulfidation epithermal gold deposits

Kelian

BusangIndoMuro

Muyup

Mirah

MasupiaRia

Kelian Au deposit

• Alluvial Au discovered by indigenous Dayaks in 1950’s

• Bedrock Au discovered by Rio Tinto in 1975

• Main exploration 1986 to 1989 outlined 75 Mt @ 1.8 g/t Au

• Mining commenced in 1991

• Total resource: 92 Mt @ 2.61 g/t Au

• Total contained Au ~240 Tonnes (~8 Moz)

• Carbonate, base-metal-rich, low sulfidation epithermal Au-Ag deposit

Kelian geology

• U. Cretaceous felsic volcaniclastic basement faulted against Tertiary sediments

• Andesite and rhyolite intrusions ~ 22 – 19 Ma

• Emplacement controlled by NE- and NW-striking faults

• Phreatomagmatic and phreatic breccia formation

• Mineralisation and alteration

• Pliocene unconformity

• Plio-Pleistocene mafic volcanism

Pit outline

1 cm

Kelian Volcanics

60 m

andesiticintrusion

volcaniclasticsst/slt

diatremebreccia

• Upper Cretaceous volcanic siltstone, sandstone & breccia

• Pumice and crystal-rich subaqueous mass flow deposits (possible subaerial source)

Mahakam Group Sedimentary Rocks

Mudstone and sandstone

Scoria breccia,basalt lava flows

QFP intrusion

Pleistocene unconformity

30 m

• Eocene to Oligocene carbonaceous mudstone and sandstone

• Terrestrial and shallow submarine depositional environment

Kelian Breccia Complex Formation

Structural Preparation:

• Transpressional fault system

• Structurally bounded blocks of carbonaceous mudstone juxtaposed against volcaniclastic rocks

• Miocene surface developedVolcaniclastic

rocks

1000

500

m

0

1500

2000

Carbonaceous sediments

60 m

Andesiticintrusion


diatremebreccia

1 cm

Andesitic intrusions

• Late Miocene plagioclase-hornblende-phyric porphryies

Pre-Diatreme Igneous Stage

• Intrusion of andesitic stocks

• Initiation of early hydrothermal system• Qtz - Ser - Pyr / Chl - Cal - Epi

alteration

• ? Early phreatic breccias facilitated ingress of meteoric water

Descending meteoric

water

Phreatic Eruptions?

Early hydrothermal system

1000

500

m

0

1500

2000

Early Diatreme Stage

Surface: Wet pyroclastic base-surge deposits

1000

500

m

0

1500

2000

Phreatomagmatic and phreatic eruptions

Quartz-phyric rhyolitic intrusions - structural control

Subsurface: phreatomagmatic & phreatic breccias

Surface phreatomagmatic breccias

1 cm

Phreatomagmatic fallout –

accretionary lapilli

Phreatomagmatic base surge deposits –dune bed forms

• Phreatomagmatic eruptions produced base surge deposits and co-surge fallout

• ‘Early’ hydrothermal system was disrupted catastrophically

• Triggered hybrid and large-scale phreatic brecciation

diatremebreccia


20 m

60 m

andesiticintrusion


diatremebreccia

Phreatomagmatic breccia

1 cm0.5 cm

Phreatomagmatic breccia – juvenile QP clasts

• Subsurface and eruptive facies of a maar-diatreme complex

• Juvenile magmatic clasts are preserved

• Polyphase breccias

Subsurface phreatomagmatic breccias

Main Diatreme Stage

1000

500

m

0

1500

2000

Diatreme deepened and widened by:

Continued explosive fragmentation

Brecciation, collapse and subsidence of diatreme walls

Mega-block formation and disaggregation

Multiple crosscutting breccia pipes

Downward transport in

pipes

Block subsidence

Block subsidence breccias

Late Diatreme - Early Hydrothermal Stage

1000

500

m

0

1500

2000

Late stage rhyolite dome emplacement

Early stage hydrothermal brecciation overlaps

phreatomagmatic brecciation

Auriferous hydrothermal

system

Early auriferous hydrothermal breccias

Overlapping ‘diatreme’ and ‘hydrothermal’ breccias

Rhyolitic intrusions

10 mQFP intrusion

brecciated mudstone

Volcaniclasticsst / slt

Late Miocene rhyolitic intrusions emplaced into active hydrothermal system

Quartz – feldspar porphyries

150 m

QFP intrusionbrecciated mudstone

QFP intrusion

Main Hydrothermal Stage

• Main stage hydrothermal system carbonate - adularia - sericite

alteration

• Widespread hydrothermal brecciation

• Gold - silver mineralisation veins, hydrothermal breccias

& disseminations

Hydrothermal Brecciation

1000

500

m

0

1500

2000

Vein & Breccia-Hosted Mineralisation

• Hydrothermal breccia bodies at Kelian have vein halos that contain infill minerals identical to the breccia cement

• Base-metal-enriched, Au-Ag (1:1) system

• Vertically extensive (> 700 m preserved)

• Five main mineralisation stages

• Main gold deposition occurred during stages 2 – 4

• Quartz is only a minor infill component

Pyrite Base-metal-sulfides-pyrite Sulfosalts

Sericite -quartz

Quartz - adularia Rhodo-chrosite- quartz

Kutnahoritedolomite -

calcite

Supergene oxidesOre mineralogy

Gangue mineralogy

Generalised paragenesis

Kaolinite

STAGE 1A/B

STAGE 2A/B

STAGE 3A/B

STAGE4

STAGE5

STAGE 3C/D

1 cm

Hydrothermal breccias

2 cm 2 cm 2 cm 1 cm 2 cm

Early phreatic breccias:

(Explosive brecciation, transport and milling, abundant matrix)

Main stage to late-stage hydraulic breccias:(Non-explosive in-situ brecciation, minor transport and milling, abundant cement)

Stage 1 and 2Pyrite cement

Stage 3ABase-metal sulfide cement

Stage 4Sulfosalt –

rhodochrosite cement

Stage 3CCarbonate cement

Veins

1 cm

1 cm

1 cm

2 cm

Stages 1 and 2Pyrite cement

Stage 3ABase-metal sulfide infill

Stage 4Sulfosalt –

rhodochrosite infill

Stage 3CCarbonate infill

Stage 1A:Sericite - pyrite

Stage 2A:Pyrite - quartz

Stage 2B:Adularia-quartz

Post - Hydrothermal Stage

• Erosion to Plio-Pleistocene surface: ~1000 m removed

• Burial by mafic volcanic rocks

• Maar and associated facies only preserved in subsided blocks

1000

500

m

0

1500

2000

Location of economic resource

Magma Emplacement into Active Hydrothermal Systems

Abundant hot fluids in active hydrothermal system, at or near boiling point

Magma intrusion triggers hybrid phreatomagmatic and phreatic explosions

Catastrophic disruption of and irreversible changes to chemical and physical conditions in the existing hydrothermal system

300 C

200 C

Champagne pool, Waiotapu geothermal area, NZ

Diatremes and ‘Giant’ Epithermal Deposits

• Epithermal deposits associated withdiatremes

• Epithermal deposits without diatremes

Modified after Sillitoe, 19970 200 400 600 800

Kelian

Waihi

Puchuca-Real

Hishikari

Mc Donald

Comstock Lode

El Indio

Round Mountain

Ladolam

Porgera

Pueblo Viejo

Baguio

Yanacocha

Cripple Creek

Au (t)

Brecciation: Implications for Ore Formation

Armoured Lapilli

Yanacocha

Mineralisation both pre- and post-diatreme

1: Fluid flow in breccia and wall rock

Cripple Creek

2: Fluid flow focussed within breccia


• Majority of mineralisation in wall rocks

• Diatreme breccias act as aquitards

• Hydrothermal brecciation and fluid flow focussed into wall rocks

• Phreatomagmatic explosions enhanced hydrothermal system and triggered gold deposition processes

Breccia pipe

inhibits fluid flow

Post Diatreme -Large scale hydrothermal explosions and brecciation

KelianStructurally controlled

mineralisation at margins of breccia

3: Fluid flow focussed within wallrocks


Late Stage Diatreme Formation

El Teniente

Possible effects on fluid flow

4: Venting of volatiles and death of a mineralising system

Porphyry systems - Birth and Death1. Birth: Magma intrusion and early

magmatic-hydrothermal brecciation

Hydrothermal brecciation

Early intrusion -insufficient fluids for explosion

Hydrothermal system advance

Catastrophic volatile loss /

pressure reduction

Hydrothermal system collapse

2. Death: Magma intrusion into well-established hydrothermal system

Intrusion into hydrothermal

system

Epithermal systems

Large scale hydrothermal explosions and brecciation

Structurally controlled mineralisation at margins of diatreme

Phreatomagmatic explosions through active system trigger syn and post diatreme hybrid phreatic explosions

Breccia pipe inhibits fluid flow -hydrothermal system enhanced in wallrocks

Mineralisation in wallrocks

3. Rebirth: Flow path created to connect the porphyry and epithermal environments

Conclusions

• Careful documentation of breccia facies and their interrelationships is essential prior to attempting genetic interpretations

• Brecciation can occur in response to a combination of phenomena, making genetic pigeonholing difficult

• Fluid flow will be affected profoundly by a major brecciation event

• Changes to the fluid flow regime will be dependent on the nature of the breccia and the wallrocks

Thayer Lindsley, described as ‘the greatest mine finder of all time’, was born in Yokohama, Japan

He took a civil engineering degree at Harvard, and moved to Canada in 1924 with a $30,000 stake from an iron mine in Oregon.

In 1928, Lindsley and a group of associates founded Ventures Ltd., as a holding company for various properties. Falconbridge Nickel Mines Limited was incorporated as a Ventures subsidiary in the same year.

Thayer Lindsley also founded Frobisher, and either found or was involved in the development of Sherritt Gordon, Giant Yellowknife, Canadian Malartic, United Keno Hill, Lake Dufault and Opemiska Copper, Connemara in Southern Rhodesia and Whim Creek in Australia.

"To be a successful mine finder, one must have determination, knowledge, tenacity, a rugged constitution to withstand the rigors of outdoor life, and enjoy overcoming obstacles of every description. Also, a little dash of imagination and enthusiasm is helpful."

Thayer Lindsley - Biography

Data Source: http://www.halloffame.mining.ca/halloffame/english/bios/lindsley.html

exposicion de brechas

Documents

fluid flow focussed

active hydrothermal system

structurally controlled mineralisation

phyric andesite clast

phyric andesite

los bronces

phreatic breccias

magma intrusion