ENGLISH5 QUARTER4 MODULE1 WEEK1-3 How Visual and Multimedia Elements.pptx
01. Brechas epitermales - Cooke.pdf
1. David R. Cooke &
David R. Cooke &
Andrew G.S. Davies
Andrew G.S. Davies#
#
#
# Current Address:
Current Address:
TeckCominco,Vancouver
TeckCominco,Vancouver
Breccias in epithermal and porphyry deposits:
Breccias in epithermal and porphyry deposits:
The birth and death of magmatic
The birth and death of magmatic-
-
hydrothermal systems
hydrothermal systems
CODES, University
CODES, University
of Tasmania
of Tasmania
Sericite
Sericite-
-chlorite altered polymict
chlorite altered polymict
rock flour matrix breccia, Acupan
rock flour matrix breccia, Acupan
Gold Mine, Philippines
Gold Mine, Philippines
2. Talk Outline
Talk Outline
z Breccias - Descriptive
Methodology
z Genetic Classes
z Overview of Breccia
Types in Magmatic-
Hydrothermal Systems
z Case Study: Kelian
z Implications for Ore
Formation and
Exploration
3. Brecciation
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
4. 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?
12. Breccias in Magmatic-Hydrothermal Systems
1 - Magmatic-hydrothermal
breccias
Volatile-saturated intrusion
undergoes catastrophic brittle
failure due to hydrostatic pressure
exceeding lithostatic load and the
tensile strength of the wallrocks
•
• Containment and
Containment and
focussing of volatiles
focussing of volatiles ⇒
⇒
birth of a magmatic
birth of a magmatic-
-
hydrothermal ore deposit
hydrothermal ore deposit
•
• Permeability enhancement
Permeability enhancement
through the formation of a
through the formation of a
subsurface breccia body
subsurface breccia body
allows for focussed fluid flow
allows for focussed fluid flow
•
• Can precipitate abundant,
Can precipitate abundant,
well
well-
-mineralised cement which
mineralised cement which
contains hypersaline &
contains hypersaline &
vapour
vapour-
-rich fluid inclusions
rich fluid inclusions
•
• Rock flour matrix and clasts
Rock flour matrix and clasts
may be altered to high
may be altered to high
temperature mineral
temperature mineral
assemblages (e.g. biotite)
assemblages (e.g. biotite)
13. Magmatic-Hydrothermal Breccias
Biotite
Biotite-
-altered rock flour
altered rock flour
matrix breccia, Gaby, Chile
matrix breccia, Gaby, Chile
Chalcopyrite
Chalcopyrite-
-cemented
cemented
monzonite breccia, Mt
monzonite breccia, Mt
Polley, British Columbia
Polley, British Columbia
14. 32oS
33oS
70o W
71o W
0 50 100
34oS
N
km
Rio Blanco -
Los Bronces
El Teniente
Los
Pelambres
Santiago
Los
Andes
Pacific
Pacific
Ocean
Ocean
• Largest known
breccia-hosted
copper-molybdenum
porphyry system
• Located 70 km NE of
Santiago, Chile
Rio Blanco
15. Rio Blanco - Los Bronces
Rio Blanco
Rio Blanco Los
Los Bronces
Bronces
Sur Sur
La Union
La Union
South
South
16. • Ore at Rio Blanco is hosted in
biotite-cemented and biotite-
altered rock flour matrix
breccias (‘magmatic’ breccia)
Biotite breccia, Rio Blanco
Biotite breccia, Rio Blanco
Biotite Breccia
17. Tourm
Tourm.
. bx
bx
Sur
Sur-
-Sur
Sur
• Ore at Sur-Sur, La Union and
Los Bronces is hosted in
tourmaline-cemented
breccias
Tourmaline Breccia
Tourm
Tourm.
. Bx
Bx
Los
Los Bronces
Bronces
Tm
Tm-
-cp
cp-
-py
py-
-qz
qz-
-anh
anh
cement:
cement: Sur
Sur-
-Sur
Sur breccia
breccia
19. 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
~5 km paleodepth
paleodepth
~2 km paleodepth
20. Breccias in Magmatic-Hydrothermal Systems
Maar-diatreme
breccia complex
Late intrusion
into active
hydrothermal
system
2
-
5
km
paleodepth
2 - Phreatomagmatic
breccias
•
• Rock flour & milled clasts
Rock flour & milled clasts
abundant
abundant
•
• Surficial and subsurface
Surficial and subsurface
breccia deposits
breccia deposits
•
• Bedded and massive breccia
Bedded and massive breccia
facies
facies
•
• Venting of volatiles to the
Venting of volatiles to the
surface
surface
⇒
⇒ death of a porphyry deposit
death of a porphyry deposit
⇒
⇒ shortcut to the epithermal
shortcut to the epithermal
environment
environment
21. 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, U.S. Geological Survey / photo by D.
Dewhurst, 1990
1977
22. 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, 1990
U.S. Geological Survey / photo by C. Nye, 1994
23. 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
24. Mine Level #6 (2165m asl)
Bedded rock flour matrix polymict breccia
Bedded rock flour matrix polymict breccia
facies, Braden Breccia Pipe, El Teniente
facies, Braden Breccia Pipe, El Teniente
Dacite pipes (5.5 Ma)
Dacite dyke (5.3 Ma)
Sewell Diorite (8.9-7 Ma)
Teniente Host Sequence
500 m
El Teniente
El Teniente -
-
Braden Breccia
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
> 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)
25. Breccias in Magmatic-Hydrothermal Systems
•
• Phreatic steam
Phreatic steam
explosions caused by
explosions caused by
decompression of
decompression of
hydrothermal fluid
hydrothermal fluid
•
• No direct magmatic
No direct magmatic
involvement
involvement
⇒
⇒ epithermal gold
epithermal gold
deposition
deposition
3 – Phreatic, hydraulic
& fault breccias
•
• Fault breccias: grinding and
Fault breccias: grinding and
abrasion may produce gouge,
abrasion may produce gouge,
cataclasite, etc
cataclasite, etc
•
• Phreatic
Phreatic breccias: in
breccias: in-
-situ
situ
subsurface brecciation (jig
subsurface brecciation (jig-
-
saw fit to rotated textures)
saw fit to rotated textures)
•
• Hydraulic breccias
Hydraulic breccias -
- only
only
minor clast transport and
minor clast transport and
abrasion (angular clasts
abrasion (angular clasts
common)
common)
•
• Abundant hydrothermal
Abundant hydrothermal
cement
cement
26. 2 cm
2 cm
Fault breccia with clasts of quartz
Fault breccia with clasts of quartz-
-chalcopyrite
chalcopyrite
veins in a rock flour matrix, and with
veins in a rock flour matrix, and with
chalcopyrite smeared along the breccia
chalcopyrite smeared along the breccia
margin, Ridgeway Au
margin, Ridgeway Au-
-Cu porphyry, NSW
Cu porphyry, NSW
Fault Breccias
28. Phreatic Breccias
• 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)
29. 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)
30. Phreatic Breccias - Triggers
•
• Seismic rupture
Seismic rupture
•
• Overpressuring
Overpressuring and failure of
and failure of
hydrothermal seal
hydrothermal seal
•
• Instantaneous unloading
Instantaneous unloading
(landslip, draining of lake, etc.)
(landslip, draining of lake, etc.)
•
• Temperature increase (magma
Temperature increase (magma -
-
water interaction)
water interaction)
Hydrothermal explosion
triggered by draining of
glacial lake (Muffler et
al, 1971)
Hydrothermal eruption crater, Pocket Basin, Yellowstone.
Hydrothermal eruption crater, Pocket Basin, Yellowstone.
Fragments of lake sediments were deposited in a low
Fragments of lake sediments were deposited in a low
aspect ratio ejecta apron after draining of glacially
aspect ratio ejecta apron after draining of glacially-
-
dammed lake 20
dammed lake 20-
-25,000 years ago
25,000 years ago
31. 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)
32. A PhD study by Andrew
A PhD study by Andrew Dav
Davies
ies
Centre For Ore Deposit Research (CODES)
Centre For Ore Deposit Research (CODES)
University of Tasmania
University of Tasmania,
, Australia
Australia
Native gold disseminated in sphalerite, pyrite and carbonate
Native gold disseminated in sphalerite, pyrite and carbonate
The
The Kel
Kelian
ian Breccia Complex:
Breccia Complex:
host to a giant epithermal Au
host to a giant epithermal Au-
-Ag deposit,
Ag deposit,
East Kalimantan, Indonesia
East Kalimantan, Indonesia
1 cm
1 cm
Singapore
Singapore
KELIAN
KELIAN
Jakarta
Jakarta
33. Regional geology
•
• Located in uplifted
Located in uplifted
block of Cretaceous
block of Cretaceous
volcaniclastic rocks
volcaniclastic rocks
•
• Surrounded by
Surrounded by
terrestrial and shallow
terrestrial and shallow
marine sedimentary
marine sedimentary
rocks of the Tertiary
rocks of the Tertiary
Kutai
Kutai Basin
Basin
•
• Largest epithermal Au
Largest epithermal Au
deposit in a NE
deposit in a NE-
-
trending belt of
trending belt of
Miocene low sulfidation
Miocene low sulfidation
epithermal gold
epithermal gold
deposits
deposits
Kelian
Kelian
Busang
Busang
Indo
Indo
Muro
Muro
Muyup
Muyup
Mirah
Mirah
Masupia
Masupia
Ria
Ria
34. Kelian Au deposit
•
• Alluvial Au discovered by indigenous
Alluvial Au discovered by indigenous
Dayaks
Dayaks in 1950
in 1950’
’s
s
•
• Bedrock Au discovered by Rio
Bedrock Au discovered by Rio Tinto
Tinto
in 1975
in 1975
•
• Main exploration 1986 to 1989
Main exploration 1986 to 1989
outlined 75 Mt @ 1.8
outlined 75 Mt @ 1.8 g/t
g/t Au
Au
•
• Mining commenced in 1991
Mining commenced in 1991
•
• Total resource: 92 Mt @ 2.61
Total resource: 92 Mt @ 2.61 g/t
g/t Au
Au
•
• Total contained Au ~240 Tonnes
Total contained Au ~240 Tonnes
(~8
(~8 Moz
Moz)
)
•
• Carbonate, base
Carbonate, base-
-metal
metal-
-rich, low
rich, low
sulfidation epithermal Au
sulfidation epithermal Au-
-Ag deposit
Ag deposit
35. Kelian geology
•
• U. Cretaceous felsic
U. Cretaceous felsic
volcaniclastic basement
volcaniclastic basement
faulted against Tertiary
faulted against Tertiary
sediments
sediments
•
• Andesite and
Andesite and rhyolite
rhyolite
intrusions ~ 22
intrusions ~ 22 –
– 19 Ma
19 Ma
•
• Emplacement controlled by
Emplacement controlled by
NE
NE-
- and NW
and NW-
-striking faults
striking faults
•
• Phreatomagmatic and
Phreatomagmatic and
phreatic
phreatic breccia formation
breccia formation
•
• Mineralisation and alteration
Mineralisation and alteration
•
• Pliocene unconformity
Pliocene unconformity
•
• Plio
Plio-
-Pleistocene mafic
Pleistocene mafic
volcanism
volcanism
Pit outline
Pit outline
36. 1 cm
1 cm
Kelian Volcanics
60 m
60 m
andesitic
andesitic
intrusion
intrusion
volcaniclastic
volcaniclastic
sst/slt
sst/slt
diatreme
diatreme
breccia
breccia
•
• Upper Cretaceous volcanic siltstone, sandstone & breccia
Upper Cretaceous volcanic siltstone, sandstone & breccia
•
• Pumice and crystal
Pumice and crystal-
-rich
rich subaqueous
subaqueous mass
mass
flow deposits (possible
flow deposits (possible subaerial
subaerial source)
source)
37. Mahakam Group Sedimentary Rocks
Mudstone and sandstone
Mudstone and sandstone
Scoria breccia,
Scoria breccia,
basalt lava flows
basalt lava flows
QFP intrusion
QFP intrusion
Pleistocene
Pleistocene unconformity
unconformity
30 m
30 m
•
• Eocene to Oligocene carbonaceous
Eocene to Oligocene carbonaceous
mudstone and sandstone
mudstone and sandstone
•
• Terrestrial and shallow submarine
Terrestrial and shallow submarine
depositional environment
depositional environment
38. Kelian Breccia Complex Formation
Structural Preparation:
Structural Preparation:
•
• Transpressional
Transpressional fault system
fault system
•
• Structurally bounded blocks
Structurally bounded blocks
of carbonaceous mudstone
of carbonaceous mudstone
juxtaposed against
juxtaposed against
volcaniclastic rocks
volcaniclastic rocks
•
• Miocene surface developed
Volcaniclastic
rocks
1000
500
m
0
1500
2000
Carbonaceous
sediments
Miocene surface developed
40. Pre-Diatreme Igneous Stage
•
• Intrusion of andesitic stocks
Intrusion of andesitic stocks
•
• Initiation of early
Initiation of early
hydrothermal system
hydrothermal system
•
• Qtz
Qtz -
- Ser
Ser -
- Pyr
Pyr /
/ Chl
Chl -
- Cal
Cal -
- Epi
Epi
alteration
alteration
•
• ? Early
? Early phreatic
phreatic breccias
breccias
facilitated ingress of meteoric
facilitated ingress of meteoric
water
Descending
meteoric
water
Phreatic
Eruptions?
Early hydrothermal system
Early hydrothermal system
1000
500
m
0
1500
2000
water
41. Early Diatreme Stage
¾
¾ Surface: Wet pyroclastic base
Surface: Wet pyroclastic base-
-
surge deposits
surge deposits
1000
500
m
0
1500
2000
Phreatomagmatic and
Phreatomagmatic and
phreatic
phreatic eruptions
eruptions
Quartz
Quartz-
-phyric rhyolitic
phyric rhyolitic
intrusions
intrusions -
- structural control
structural control
¾
¾ Subsurface: phreatomagmatic &
Subsurface: phreatomagmatic &
phreatic
phreatic breccias
breccias
42. Surface phreatomagmatic breccias
1 cm
1 cm
Phreatomagmatic
Phreatomagmatic
fallout
fallout –
–
accretionary
accretionary lapilli
lapilli
Phreatomagmatic base surge deposits
Phreatomagmatic base surge deposits –
–
dune bed forms
dune bed forms
•
• Phreatomagmatic eruptions produced base surge
Phreatomagmatic eruptions produced base surge
deposits and co
deposits and co-
-surge fallout
surge fallout
•
• ‘
‘Early
Early’
’ hydrothermal system was disrupted
hydrothermal system was disrupted
catastrophically
catastrophically
•
• Triggered hybrid and large
Triggered hybrid and large-
-scale
scale phreatic
phreatic brecciation
brecciation
diatreme
diatreme
breccia
breccia
volcaniclastic
volcaniclastic
sst/slt
sst/slt
20 m
20 m
43. Subsurface phreatomagmatic breccias
60 m
60 m
andesitic
andesitic
intrusion
intrusion
volcaniclastic
volcaniclastic
sst/slt
sst/slt
diatreme
diatreme
breccia
breccia
0.5 cm
0.5 cm
Phreatomagmatic breccia
Phreatomagmatic breccia
–
– juvenile QP clasts
juvenile QP clasts
•
• Subsurface and eruptive facies of a
Subsurface and eruptive facies of a
maar
maar-
-diatreme
diatreme complex
complex
•
• Juvenile magmatic clasts are
Juvenile magmatic clasts are
preserved
preserved
•
• Polyphase
Polyphase breccias
breccias
Phreatomagmatic breccia
Phreatomagmatic breccia
1 cm
1 cm
44. Main Diatreme Stage
1000
500
m
0
1500
2000
Diatreme deepened and
Diatreme deepened and
widened by:
widened by:
¾
¾ Continued explosive
Continued explosive
fragmentation
fragmentation
¾
¾ Brecciation, collapse and
Brecciation, collapse and
subsidence of diatreme walls
subsidence of diatreme walls
¾
¾ Mega
Mega-
-block formation and
block formation and
disaggregation
disaggregation
Multiple crosscutting breccia
Multiple crosscutting breccia
pipes
pipes
Downward
transport in
pipes
Block
subsidence
46. Late Diatreme - Early Hydrothermal Stage
1000
500
m
0
1500
2000
Late stage rhyolite dome
Late stage rhyolite dome
emplacement
emplacement
Early stage hydrothermal
Early stage hydrothermal
brecciation overlaps
brecciation overlaps
phreatomagmatic brecciation
phreatomagmatic brecciation
Auriferous
Auriferous
hydrothermal
hydrothermal
system
system
Early auriferous hydrothermal
Early auriferous hydrothermal
breccias
breccias
Overlapping
Overlapping ‘
‘diatreme
diatreme’
’ and
and
‘
‘hydrothermal
hydrothermal’
’ breccias
breccias
47. Rhyolitic intrusions
10 m
10 m
QFP intrusion
QFP intrusion
brecciated
brecciated
mudstone
mudstone
Volcaniclastic
Volcaniclastic
sst
sst /
/ slt
slt
Late Miocene rhyolitic intrusions
Late Miocene rhyolitic intrusions
emplaced into active hydrothermal
emplaced into active hydrothermal
system
system
Quartz
Quartz –
– feldspar porphyries
feldspar porphyries
150 m
150 m
QFP intrusion
QFP intrusion
brecciated
brecciated
mudstone
mudstone
QFP intrusion
QFP intrusion
49. Vein & Breccia-Hosted Mineralisation
•
• Hydrothermal breccia bodies at
Hydrothermal breccia bodies at Kelian
Kelian have vein halos
have vein halos
that contain infill minerals identical to the breccia
that contain infill minerals identical to the breccia
cement
cement
•
• Base
Base-
-metal
metal-
-enriched, Au
enriched, Au-
-Ag (1:1) system
Ag (1:1) system
•
• Vertically extensive (> 700 m preserved)
Vertically extensive (> 700 m preserved)
•
• Five main mineralisation stages
Five main mineralisation stages
•
• Main gold deposition occurred during stages 2
Main gold deposition occurred during stages 2 –
– 4
4
•
• Quartz is only a minor infill component
Quartz is only a minor infill component
Pyrite
Pyrite Base
Base-
-metal
metal-
-sulfides
sulfides-
-pyrite
pyrite Sulfosalts
Sulfosalts
Generalised
Generalised
Sericite
Sericite -
-
quartz
quartz
Quartz
Quartz -
- adularia
adularia Rhodo
Rhodo-
-
chrosite
chrosite
-
- quartz
quartz
Kutnahorite
Kutnahorite
dolomite
dolomite -
-
calcite
calcite
paragenesis
paragenesis
Supergene oxides
Supergene oxides
Ore mineralogy
Ore mineralogy
STAGE
STAGE
1A/B
1A/B
STAGE
STAGE
2A/B
2A/B
STAGE
STAGE
3A/B
3A/B
STAGE
STAGE
4
4
STAGE
STAGE
5
5
STAGE
STAGE
3C/D
3C/D
1 cm
Au
Au
Kaolinite
Kaolinite
Gangue mineralogy
Gangue mineralogy
50. Hydrothermal breccias
2 cm
2 cm 2 cm
2 cm 2 cm
2 cm
Stage 1 and 2
Stage 1 and 2
Pyrite cement
Pyrite cement
Stage 3A
Stage 3A
Base
Base-
-metal sulfide cement
metal sulfide cement
Stage 4
Stage 4
Sulfosalt
Sulfosalt –
–
rhodochrosite cement
rhodochrosite cement
Stage 3C
Stage 3C
Carbonate cement
Carbonate cement
1 cm
1 cm 2 cm
2 cm
Main stage to late
Main stage to late-
-stage hydraulic breccias:
stage hydraulic breccias:
(Non
(Non-
-explosive in
explosive in-
-situ brecciation, minor
situ brecciation, minor
transport and milling, abundant cement)
Early
Early phreatic
phreatic breccias:
breccias:
(Explosive brecciation, transport
(Explosive brecciation, transport
and milling, abundant matrix) transport and milling, abundant cement)
and milling, abundant matrix)
51. Veins
1 cm
1 cm
Stage 1A:
Stage 1A:
Sericite
Sericite -
- pyrite
pyrite
Stage 2B:
Stage 2B:
Adularia
Adularia-
-quartz
quartz
1 cm
1 cm
2 cm
2 cm
Stage 2A:
Stage 2A:
Pyrite
Pyrite -
- quartz
quartz 1 cm
1 cm
Stage 3C
Stage 3C
Carbonate infill
Carbonate infill
Stage 4
Stage 4
Sulfosalt
Sulfosalt –
–
rhodochrosite infill
Stages 1 and 2
Stages 1 and 2
Pyrite cement
Stage 3A
Stage 3A
Base
Base-
-metal sulfide infill rhodochrosite infill
Pyrite cement metal sulfide infill
52. Post - Hydrothermal Stage
1000
500
m
0
1500
2000
• Erosion to Plio-Pleistocene
surface: ~1000 m removed
• Burial by mafic volcanic rocks
• Maar and associated facies
only preserved in subsided
blocks
Location of
Location of
economic
economic
resource
resource
53. 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
54. Diatremes and ‘Giant’ Epithermal Deposits
0 200 400 600 800
Kelian
Waihi
Puchuca-Real
Hishikari
Mc Donald
Comstock Lode
El Indio
Round Mountain
Ladolam
Porgera
Pueblo Viejo
Baguio
Yanacocha
Cripple Creek
Au (t)
• Epithermal deposits
associated with
diatremes
• Epithermal deposits
without diatremes
Modified after Sillitoe, 1997
55. Brecciation: Implications for Ore Formation
1: Fluid flow in breccia and wall rock
Armoured
Lapilli
Fluid
mixing
2500
m
Yanacocha
Mineralisation
both pre- and
post-diatreme
57. 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
Fluid
mixing
2500
m
Post Diatreme -
Large scale hydrothermal
explosions and brecciation
Kelian
Structurally
controlled
mineralisation at
margins of breccia
3: Fluid flow focussed within wallrocks
58. Possible effects on fluid flow
2500
m
Late Stage Diatreme Formation
El Teniente
4: Venting of volatiles and death of a mineralising
system
59. Porphyry systems - Birth and Death
1. 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
60. Epithermal systems
3. Rebirth: Flow path created to connect
the porphyry and epithermal environments
2500
m
Fluid
mixing
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
61. 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
