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Outline
• Describing breccias
• Overview of genetic classes for breccias
• Emphasis on breccias from epithermal and porphyry deposits
Magmatic-hydrothermal
Volcanic-hydrothermal
Hydrothermal (phreatic)
Definitions
• Hydrothermal breccia:
Clastic, coarse-grained aggregate generated by the
interaction of hydrothermal fluid with magma and/or
wallrocks
• Infill:
Material that has filled the space between clasts in
breccias
Breccias can have two infill components – crystalline
cement or clastic matrix
2 cm
Breccia Description and Interpretation
• First breccias should be described in
terms of their components, texture,
morphology and contact relationships
• The next step is genetic interpretation, which can be difficult and often leads to problems
Ideal combination: 5 + 4 + 3 + 2 +1 Alteration Internal Components Grainsize Geometry organisation A + B + C + D
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
5 + 4 + 3 + 2 +1 Alteration Internal Components Grainsize Geometry organisation A + B + C + D
2) Grainsize
• breccia (> 2mm), sandstone (1/16 – 2 mm) or mudstone (< 1/16 mm)
The term ‘breccia’ is derived from sedimentology, where it refers to clastic rocks composed of large angular clasts (granules, cobbles and boulders) with or without a sandy or muddy matrix
Monomictic sericite-altered diorite clast breccia with roscoelite-quartz cement, Porgera, PNG
Breccia Description
5 + 4 + 3 + 2 +1 Alteration Internal Components Grainsize Geometry organisation A + B + C + D
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
Polymictic trachyandesite clast-rich sand matrix breccia, Cowal, NSW
Breccia Description
5 + 4 + 3 + 2 +1 Alteration Internal Components Grainsize Geometry organisation A + B + C + D
3) Components: INFILL
B: matrix
• Mud to sand to breccia-sized particles
• Crystal fragments, lithic fragments, vein fragments
Textures:
• bedded
• laminated
• banded
• foliated
• massive Polymictic diorite clast breccia with pyrite-quartz-roscoelite cement and roscoelite-altered mud matrix, Porgera, PNG
Breccia Description
5 + 4 + 3 + 2 +1 Alteration Internal Components Grainsize Geometry organisation A + B + C + D
3) Components: INFILL
C: cement
• Ore & gangue mineralogy
• Grainsize
• Alteration
textures:
• cockade, massive, drusy, etc.
D: open space (vugs)
Rhodochrosite-kaolinite cemented mudstone-clast breccia Kelian, Indonesia
Breccia Description
5 + 4 + 3 + 2 +1 Alteration Internal Components Grainsize Geometry organisation A + B + C + D
4) Internal Organisation
• Clast, matrix or cement-supported
• Clast, matrix and cement abundances
• Massive, bedded, laminated or graded
Clast distribution:
• In-situ (jigsaw-fit)
• Rotated
• Chaotic
Sericite-altered polymictic sand-matrix breccia, Braden Pipe, El Teniente, Chile
Breccia Description
5 + 4 + 3 + 2 +1 Alteration Internal Components Grainsize Geometry organisation A + B + C + D
5) Alteration
• Clasts, matrix or cement
• Alteration paragenesis (pre-, syn- and post-brecciation)
Sericite-altered polymictic sand matrix breccia, Braden Pipe, El Teniente, Chile
Breccia Description
Hydrothermal Breccias
Volcanic Breccias
Magmatic-hydrothermal
breccias
Tectonic Breccias
Magmatic Breccias
Magma intrusion into hydrothermal
system
Fault breccias & brecciated veins
Sto
ck
wo
rk v
ein
s
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
Igneous- cemented 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 magmatic-hydrothermal ore formation
Breccias in Hydrothermal Systems
• Permeability enhancement through the formation of a subsurface breccia body allows for focussed fluid flow
Polymict tourmaline breccia, Sierra Gorda, Chile
• Angular clasts -implies limited clast transport & abrasion
• Juvenile clasts (?)
• Variable amounts of clastic matrix
• High temperature alteration rinds (clasts) and altered matrix
• Open space fill textures
Characteristic Features
Tourmaline-chalcopyrite cement, Rio Blanco
Chalcopyrite-cemented monzonite clast breccia, Mt Polley, British Columbia
Characteristic Features
• Locally abundant hydrothermal cement (biotite, tourmaline, quartz, sulfides, etc)
Magmatic-hydrothermal breccia
Tourmaline-quartz cemented, sericite-altered, diorite clast breccia
Sulfide Mineralisation Styles
Altered clasts
vein cement
Tourmaline breccia, Río Blanco, Chile
• Hydrothermal cement
• Alteration of rock flour
• Alteration of clasts
• Cross-cutting veins
Magmatic-hydrothermal breccia
tm bx
tm vein halo
Sierra Gorda tourmaline breccia, Chile
Vein Halo
tm vein halo
tourmaline breccia, Peru
Vein Halo
• Aspect ratios of clasts can attain 1:30
• In many cases, tabular shape does not relate to closely spaced jointing or bedding
• Orientations change from sub-vertical on pipe margins to sub-horizontal in the central region
Tabular clasts
Providencia cp-tourmaline breccia, Inca de Oro, Chile
Tourmaline-quartz breccia, La Zanja, Peru
Volcanic-hydrothermal
breccia complex
Late intrusion into active
hydrothermal system
2 - 5
km
p
ale
od
ep
th
2: Volcanic-hydrothermal breccias
• Clastic matrix & 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
Breccias in Hydrothermal Systems
Modified after Lorenz, 1973
0 m
> 2500 m
Water Table
depressed
Increasing eruption
depth
‘wet’ pyroclastic eruptions
Diatremes
Common association of ‘diatremes’ with magmatic-hydrothermal ore deposits
(e.g., Kelian, Martabe, Cripple Creek)
• Abundant fine grained altered clastic matrix (massive to stratified)
• Rounded to angular heterolithic clasts, typically matrix-supported
• Generally significant clast abrasion & transport (mixing of wallrock clasts – transport upwards and downwards)
• Surficial pyroclastic base surge deposits
Subsurface polymictic sand-matrix breccia, Braden Pipe, El Teniente
Characteristics of Volcanic-Hydrothermal Breccias
Braden Pipe – surficial? bedded facies (courtesy Francisco Camus)
• Juvenile clasts
• Mineralised and altered clasts
• Surficial-derived clasts (e.g., logs, charcoal, etc.)
• Complex facies relationships
• Limited open space little or no hydrothermal cement
Characteristic features
0.5 cm Chalcopyrite clasts, Balatoc diatreme, Acupan Au mine, Philippines
Phreatomagmatic breccia – juvenile quartz-phyric rhyolite
clasts, Kelian, Indonesia
Volcaniclastic sst / slt
150 m
QFP intrusion Diatreme breccia
Base surge deposits
Kelian, Indonesia
• Phreatic steam explosions caused by
decompression of hydrothermal fluid
• No direct magmatic involvement
epithermal gold deposition
3: Hydrothermal breccias – phreatic
• Phreatic breccias: in-situ subsurface and surficial brecciation – matrix can be abundant (jig-saw fit to rotated to chaotic textures)
Breccias in Hydrothermal Systems
Eruption of Waimungu Geyser, 1904 (Sillitoe, 1985)
• Hydrothermal steam explosions that breach the surface will generate pyroclastic ejecta, but lack a juvenile magmatic component
• The resultant hydrothermal eruption deposits are bedded and have low aspect ratios • The deposits have a poor preservation potential
Phreatic Breccias
Porkchop Geyser, post-eruption, 1992, Yellowstone
Phreatic Breccias
Waiotapu Geothermal Area, New Zealand
Phreatic Eruption Breccias
Champagne pool, Waiotapu, New Zealand
Altered & mineralised andesite clasts, with sulfide and sulfosalt cockade banding, Mt Muro, Indonesia
Hydrothermal Breccias: Mineralised
• High to low temperature hydrothermal fluids
• Structural complexity
• Open space fill
• Multiple generations
• Gangue and ore minerals
Hydrothermal Breccias Hydrothermal breccia, Peru
Hydrothermal Breccias
20 cm
2 cm
Lihir, Papua New Guinea Kelian, Indonesia
Hydrothermal Breccias
, Peru
• Structural opening and hydrothermal
fluid pressure
• No direct magmatic involvement
epithermal deposition
3: Vein breccias
• Vein breccias: clasts within veins, from wallrocks or existing parts of vein
Breccias in Hydrothermal Systems
Hydrothermal Breccias Vein breccia,, Peru
Kencana, Indonesia
Vein Breccias
What do these
textures mean?
Why are they
important?
(Gemmell et al., 1988)
Stage I breccia – cockade texture
Stage 1b
ore
30 cm
FW
HW
Stage Ia
ore Stage Ib
ore
(Gemmell et al., 1988)
Stage II breccia – cockade texture
Stage II
non-ore Stage IV
non-ore
30 cm
20 cm
20 cm
FW
HW
Stage II
non-ore Stage II
non-ore
(Gemmell et al., 1988)
Stage III
ore FW
HW
Stage III
ore
Stage III banding – crustiform texture
(Gemmell et al., 1988)
Stage IV
non-ore
5 cm 10 cm
FW
HW
Stage IV
non-ore
Stage IV – massive infill with vugs
Santo Nino vein
(Gemmell,1986 & Gemmell et al., 1988)
Stage I ore
Stage II non-ore
Stage III ore
Stage IV non-ore
30 cm 20 cm 20 cm
Long Section
Anhydrite-cemented vein breccia, Acupan gold mine, Philippines
Conclusions
• Magmatic-hydrothermal breccias have high temperature cements and alteration minerals
• Volcanic-hydrothermal breccia complexes have bedded facies and juvenile magmatic clasts
• Phreatic breccia complexes may contain bedded facies, but will always lack juvenile clasts
• Vein breccias result from structural opening and hydrothermal fluid pressure
Pyrite-roscoelite-gold cemented heterolithic breccia, Porgera Gold Mine, Papua New Guinea (Sample courtesy of Standing, 2005)
Conclusions
• Facies and structure control fluid flow and are the keys to understanding grade distribution in
hydrothermal breccias
• Hydrothermal brecciation typically involves several fragmentation processes
• Genetic pigeonholing of breccias can be difficult, and may not be particularly helpful
Fragmentation Processes
Non-explosive Explosive
Magma • Magma intrusion
Stoping
• Autoclastic Autobrecciation
• Gravitational collapse Dissolution
Magma withdrawal
Magma + External Water • Autoclastic
Quench fragmentation
Hydraulic fracture
Tectonic comminution, wear, abrasion,
dilation, implosion
Magma + Internal Water • magmatic
magma exsolves steam ± CO2
• magmatic-hydrothermal
magma exsolves steam + brine
Magma + External Water • phreatomagmatic
magma encounters external water
Water + External Heat • Hydrothermal (phreatic)
Flashing of water to steam due to seal failure, seismic rupture, heat input and/or mass wasting