Exposicion de Brechas
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Transcript of 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
organisation A + B + C
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
Descriptive Names for Breccias
5 + 4 + 3 + 2 +1Alteration Internal Components Grainsize Geometry
organisation A + B + C
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 +1Alteration Internal Components Grainsize Geometry
organisation A + B + C
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
Breccias in Magmatic-Hydrothermal Systems
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)
Breccias in Magmatic-Hydrothermal Systems
• 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
volcaniclasticsst/slt
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
volcaniclasticsst/slt
20 m
60 m
andesiticintrusion
volcaniclasticsst/slt
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
Brecciation: Implications for Ore Formation
• 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
Brecciation: Implications for Ore Formation
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