Breccias in epithermal and porphyry deposits: The birth and death of magmatichydrothermal systems
David R. Cooke & Andrew G.S. Davies# CODES, University of Tasmania Sericite-chlorite altered polymict rock flour matrix breccia, Acupan Gold Mine, Philippines
# Current Address: TeckCominco,Vancouver
Talk Outline
Breccias - Descriptive Methodology Genetic Classes Overview of Breccia Types in MagmaticHydrothermal 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?
Breccia Description Ideal combination: 5
+4
+3
+2
+1
Alteration
Internal organisation
Components A+B+C
Grainsize
Geometry
Minimum Combination: 4 + 3 + 2
1) Geometry • pipe, cone, dyke, vein, bed, irregular, tabular... • Contact relationships: sharp, gradational, faulted, irregular, planar, concordant, discordant
Bat Cave breccia pipe, Northern Arizona. (Wenrich, 1985)
Descriptive Names for Breccias 5
+4
+3
+2
+1
Alteration
Internal organisation
Components A+B+C
Grainsize
Geometry
2) Grainsize • microbreccia (< 2mm) or breccia (> 2mm)...
3) Components A: clasts • monomict or polymict • Composition: lithic, vein, breccia, juvenile magmatic, accretionary lapilli, mineralised, altered • Morphology: angular, subangular, subround, round, faceted, tabular, equant
Descriptive Names for Breccias 5
+4
+3
+2
+1
Alteration
Internal organisation
Components A+B+C
Grainsize
Geometry
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)
Descriptive Names for Breccias 5
+4
+3
+2
+1
Alteration
Internal organisation
Components A+B+C
Grainsize
Geometry
4) Internal Organisation • Clast abundance, clast, matrix or cementsupported • 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, matrixsupported, chaotic, polymict pyroxene-phyric andesite and mudstone-clast-rich rock flour matrix breccia
Chlorite-hematite-carbonate-pyrite-altered, polymict pyroxene-phyric andesite and diorite-clast massive to stratified rock flour breccia and microbreccia
Hematite-carbonatepyrite-chlorite-sericite cemented, polymict pyroxene-phyric andesite and diorite-clast breccia
Diorite breccia complex increased permeability – cemented facies
facies with sub-vertical fabrics
Variations in clast types & matrix abundance
Diorite host rock Fractured diorite
brecciated diorite
rock flour zone, increases inwards
Volcanic Breccias
Breccia Genesis
Magma intrusion into magmatichydrothermal system
• More than one process can be involved in breccia formation Hydrothermal Breccias
Magmatic-hydrothermal breccias Stockwork veins
• This overlap means that genetic Phreatic breccias terminology is generally applied inconsistently
Tectonic Breccias
Magmatic Breccias Igneous cement breccias
Structural control on breccia location
Fault breccias
Breccias in Magmatic-Hydrothermal Systems 1 - Magmatic-hydrothermal breccias • Permeability enhancement through the formation of a subsurface breccia body allows for focussed fluid flow
Volatile-saturated intrusion undergoes catastrophic brittle failure due to hydrostatic pressure exceeding lithostatic load and the tensile strength of the wallrocks
• 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)
• Containment and focussing of volatiles birth of a magmatichydrothermal ore deposit
Magmatic-Hydrothermal Breccias Chalcopyrite-cemented monzonite breccia, Mt Polley, British Columbia
Biotite-altered rock flour matrix breccia, Gaby, Chile
Rio Blanco
71o W
70o W 0
50
Los Pelambres km
100 32o S
N
Pacific Ocean
• Largest known breccia-hosted copper-molybdenum porphyry system • Located 70 km NE of Santiago, Chile
Los Andes
Rio Blanco Los Bronces
33o S
Santiago
34o S
El Teniente
Rio Blanco - Los Bronces South
Sur Sur
La Union
Rio Blanco
Los Bronces
Biotite Breccia • Ore at Rio Blanco is hosted in biotite-cemented and biotitealtered rock flour matrix breccias (‘magmatic’ breccia)
Biotite breccia, Rio Blanco
Tourmaline Breccia • Ore at Sur-Sur, La Union and Los Bronces is hosted in tourmaline-cemented breccias
Tourm. Bx Los Bronces
Tm-cp-py-qz-anh cement: Sur-Sur breccia
Tourm. bx Sur-Sur
Sur-Sur XC50
Tourmaline breccia
Diorite wallrock
Late- Tourmaline stage breccia rock flour breccia
Biotite breccia Tm bx cut by RF bx, Rio Blanco
Rock Flour breccia
Breccia-Enhanced Permeability ~2 km paleodepth
Farellones Fm Drawdown of meteoric water?
Buoyant magmatic gas streams up through bx column
Upwelling magmatichydrothermal brines precipitate ore
San Francisco Batholith
~5 km paleodepth
Breccias in Magmatic-Hydrothermal Systems 2 - Phreatomagmatic breccias
Maar-diatreme breccia complex
• 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
Late intrusion into active hydrothermal system
2 - 5 km paleodepth
• Rock flour & milled clasts abundant
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 C. Nye, 1994
U.S. Geological Survey / photo by D. Dewhurst, 1990
Diatremes - Volcanological Model ‘wet’ pyroclastic eruptions
Modified after Lorenz, 1973
0m Water Table depressed
Increasing eruption depth > 2500m
No direct link to mineralisation - this model fails to account for common association of diatremes and magmatic-hydrothermal ore deposits
El Teniente Braden Breccia
Mine Level #6 (2165m asl) 500 m < 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)
> 0.5% Cu
< 0.5% Cu
> 0.5% Cu
Bedded rock flour matrix polymict breccia facies, Braden Breccia Pipe, El Teniente
Teniente Host Sequence
Marginal Breccia (4.7 Ma)
Sewell Diorite (8.9-7 Ma)
Braden Breccia (4.7 Ma)
Grey porphyry (5.7 Ma)
Late dacite dykes (4.7 Ma)
Dacite pipes (5.5 Ma)
Hble-phyric dykes (3.8 Ma)
Dacite dyke (5.3 Ma)
Breccias in Magmatic-Hydrothermal Systems 3 – Phreatic, hydraulic & fault breccias • Fault breccias: grinding and abrasion may produce gouge, cataclasite, etc • Phreatic breccias: in-situ subsurface brecciation (jigsaw fit to rotated textures) • Hydraulic breccias - only minor clast transport and abrasion (angular clasts common) • Abundant hydrothermal cement
• Phreatic steam explosions caused by decompression of hydrothermal fluid • No direct magmatic involvement epithermal gold deposition
Fault Breccias
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
Phreatic Breccias
Porkchop Geyser, posteruption, 1992, Yellowstone
Phreatic Breccias • Gases accumulate beneath a silica seal during upflow of boiling waters
Gas cap in self-sealed geothermal system (Hedenquist & Henley, 1985)
• P increase can rupture the hydrothermal seal, triggering a steam explosion & phreatic brecciation
Au-mineralised vein breccia, Acupan
Phreatic Breccias Depressurisation can affect a significant vertical column of rock (hundreds of metres) and can trigger ore deposition as H2S partitions to the vapour phase
Instantaneous P decrease changes the depth of first boiling (Hedenquist & Henley, 1985)
Phreatic Breccias - Triggers • Seismic rupture • Overpressuring and failure of hydrothermal seal • Instantaneous unloading (landslip, draining of lake, etc.) • Temperature increase (magma water interaction)
Hydrothermal eruption crater, Pocket Basin, Yellowstone. Fragments of lake sediments were deposited in a low aspect ratio ejecta apron after draining of glaciallydammed lake 20-25,000 years ago
Hydrothermal explosion triggered by draining of glacial lake (Muffler et al, 1971)
Phreatomagmatic vs. Phreatic Explosions Phreatic explosion • no direct magma - water contact at explosion site • flashing of water to steam • no juvenile magmatic component
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 Eruption of Waimungu Geyser, New Zealand, 1904 (Sillitoe, 1985)
The Kelian Breccia Complex: host to a giant epithermal Au-Ag deposit, East Kalimantan, Indonesia
Singapore
KELIAN
Jakarta
A PhD study by Andrew Davies Centre For Ore Deposit Research (CODES) University of Tasmania, Australia 1 cm
Native gold disseminated in sphalerite, pyrite and carbonate
Regional geology • Located in uplifted block of Cretaceous volcaniclastic rocks • Surrounded by terrestrial and shallow marine sedimentary rocks of the Tertiary Kutai Basin
Masupia Ria
Indo Muyup Muro
Mirah
Busang
Kelian
• Largest epithermal Au deposit in a NEtrending belt of Miocene low sulfidation epithermal gold deposits
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 Pit outline
• Plio-Pleistocene mafic volcanism
Kelian Volcanics • Upper Cretaceous volcanic siltstone, sandstone & breccia
volcaniclastic sst/slt
diatreme breccia andesitic intrusion
60 m
• Pumice and crystal-rich subaqueous mass flow deposits (possible subaerial source)
1 cm
Mahakam Group Sedimentary Rocks Pleistocene unconformity Scoria breccia, basalt lava flows
30 m
Mudstone and sandstone
QFP intrusion
• 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
0
Carbonaceous sediments
500
1000
• Miocene surface developed 1500
Volcaniclastic rocks 2000 m
Andesitic intrusions • Late Miocene plagioclase-hornblende-phyric porphryies
volcaniclastic sst/slt
diatreme breccia Andesitic intrusion
60 m
1 cm
Pre-Diatreme Igneous Stage • Intrusion of andesitic stocks • Initiation of early hydrothermal system
Phreatic Eruptions?
Descending meteoric water 0
• Qtz - Ser - Pyr / Chl - Cal - Epi alteration
500
1000
• ? Early phreatic breccias facilitated ingress of meteoric water
1500
Early hydrothermal system 2000 m
Early Diatreme Stage Quartz-phyric rhyolitic intrusions - structural control Phreatomagmatic and phreatic eruptions Subsurface: phreatomagmatic &
0
500
phreatic breccias
1000
Surface: Wet pyroclastic base-
surge deposits
1500
2000 m
Surface phreatomagmatic breccias Phreatomagmatic base surge deposits – dune bed forms
volcaniclastic sst/slt
Phreatomagmatic fallout – accretionary lapilli
diatreme breccia 20 m
• Phreatomagmatic eruptions produced base surge deposits and co-surge fallout • ‘Early’ hydrothermal system was disrupted catastrophically • Triggered hybrid and large-scale phreatic brecciation
1 cm
Subsurface phreatomagmatic breccias Phreatomagmatic breccia – juvenile QP clasts diatreme breccia
Phreatomagmatic breccia
volcaniclastic sst/slt andesitic intrusion
60 m
• Subsurface and eruptive facies of a maar-diatreme complex • Juvenile magmatic clasts are preserved • Polyphase breccias
0.5 cm
1 cm
Main Diatreme Stage Diatreme deepened and widened by: Continued explosive
fragmentation
Brecciation, collapse and
Downward transport in pipes
Block subsidence 0
subsidence of diatreme walls 500
Mega-block formation and
disaggregation
Multiple crosscutting breccia pipes
1000
1500
2000 m
Block subsidence breccias
Late Diatreme - Early Hydrothermal Stage Late stage rhyolite dome emplacement
Early stage hydrothermal brecciation overlaps phreatomagmatic brecciation
Early auriferous hydrothermal breccias Overlapping ‘diatreme’ and ‘hydrothermal’ breccias
0
500
Auriferous hydrothermal system
1000
1500
2000 m
Rhyolitic intrusions brecciated mudstone QFP intrusion
brecciated mudstone
QFP intrusion
Volcaniclastic sst / slt 150 m
Late Miocene rhyolitic intrusions emplaced into active hydrothermal system Quartz – feldspar porphyries
QFP intrusion
10 m
Main Hydrothermal Stage • Main stage hydrothermal system carbonate - adularia - sericite
alteration
• Widespread hydrothermal brecciation
0
500
• Gold - silver mineralisation veins, hydrothermal breccias
1000
& disseminations
Hydrothermal Brecciation
1500
2000 m
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 1 cm
• Quartz is only a minor infill component
Generalised paragenesis
STAGE 1A/B Pyrite
Ore mineralogy Gangue mineralogy
Sericite quartz
STAGE 2A/B
STAGE 3A/B
STAGE 3C/D
Base-metal-sulfides-pyrite Quartz - adularia
Kutnahorite dolomite calcite
STAGE 4 Sulfosalts Rhodochrosite - quartz
STAGE 5 Supergene oxides Kaolinite
Hydrothermal breccias Stage 1 and 2 Pyrite cement
2 cm
Stage 3A Base-metal sulfide cement
2 cm
2 cm
Stage 3C Carbonate cement
Stage 4 Sulfosalt – rhodochrosite cement
1 cm
2 cm
Early phreatic breccias:
Main stage to late-stage hydraulic breccias:
(Explosive brecciation, transport and milling, abundant matrix)
(Non-explosive in-situ brecciation, minor transport and milling, abundant cement)
Veins
1 cm Stage 1A: Sericite - pyrite
Stage 2B: Adularia-quartz
1 cm Stage 2A: Pyrite - quartz
2 cm 1 cm Stage 3C Carbonate infill
Stages 1 and 2 Pyrite cement
Stage 3A Base-metal sulfide infill
Stage 4 Sulfosalt – rhodochrosite infill
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
0
500
1000
1500
Location of economic resource
2000 m
Magma Emplacement into Active Hydrothermal Systems Magma intrusion triggers hybrid phreatomagmatic and phreatic explosions 200 C 300 C Champagne pool, Waiotapu geothermal area, NZ
Abundant hot fluids in active hydrothermal system, at or near boiling point
Catastrophic disruption of and irreversible changes to chemical and physical conditions in the existing hydrothermal system
Diatremes and ‘Giant’ Epithermal Deposits Cripple Creek
• Epithermal deposits associated with diatremes
Yanacocha Baguio Pueblo Viejo
• Epithermal deposits without diatremes
Porgera Ladolam Round Mountain El Indio Comstock Lode Mc Donald Hishikari Puchuca-Real Waihi Kelian Au (t) 0
200
400
600
800
Modified after Sillitoe, 1997
Brecciation: Implications for Ore Formation 1: Fluid flow in breccia and wall rock
Armoured Lapilli
Yanacocha Mineralisation both pre- and post-diatreme
Brecciation: Implications for Ore Formation 2: Fluid flow focussed within breccia
Cripple Creek
Brecciation: Implications for Ore Formation 3: Fluid flow focussed within wallrocks Post Diatreme Large scale hydrothermal explosions and brecciation
• 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
Structurally controlled mineralisation at margins of breccia
Breccia pipe inhibits fluid flow
Kelian
Possible effects on fluid flow 4: Venting of volatiles and death of a mineralising system Late Stage Diatreme Formation
El Teniente
Porphyry systems - Birth and Death 1. Birth: Magma intrusion and early magmatic-hydrothermal brecciation
2. Death: Magma intrusion into wellestablished hydrothermal system
Hydrothermal system advance Hydrothermal brecciation
Catastrophic volatile loss / pressure reduction
Hydrothermal system collapse
Early intrusion insufficient fluids for explosion Intrusion into hydrothermal system
Epithermal systems 3. Rebirth: Flow path created to connect the porphyry and epithermal environments Large scale hydrothermal explosions and brecciation
Phreatomagmatic explosions through active system trigger syn and post diatreme hybrid phreatic explosions
Mineralisation in wallrocks
Breccia pipe inhibits fluid flow hydrothermal system enhanced in wallrocks
Structurally controlled mineralisation at margins of diatreme
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 - Biography
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."
Data Source: http://www.halloffame.mining.ca/halloffame/english/bios/lindsley.html
