Richards PCD Shortcourse5(Epithermal)
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Transcript of Richards PCD Shortcourse5(Epithermal)
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Porphyry Short Course Part 5
Richards (2011)1
Porphyry Cu-Mo-Au Systems
Part 5: Epithermal Deposits
Jeremy P. Richards Dept. Earth & Atmospheric Sciences
University of Alberta Edmonton, Alberta, T6G 2E3, Canada
The fumarolicepithermal environment
Hedenquist et al. (1996)
Active fumaroles on Volcn Lastarra, Chile
Richards (2011)
Mina Julia sulfur mine, NW Argentina
Richards (2011)
Sulfate fumaroles, Corrida de Cori, NW Argentina
Left: Gypsum fumarole spiressurrounded by volcanic bombs (black).
Right: Anydrite veins feeding fossilfumaroles.
Richards (2011)
Two main types ofepithermal deposits:
(a) High sulfidation(acid-sulfate)
(b) Low sulfidation(adularia-sericite)
Newly recognized class:
(c) Intermediate sulfidation(basemetal sulfides, illite).
Evans, A.M., 1993, Ore geologyand industrial minerals, anintroduction, 3rd edn: BlackwellScientific, 390 p.
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The epithermal environment
Hedenquist et al. (1996)
.
1 1 / / / .
Intermediate-sulfidationepithermalAu-Ag
High-sulfidation epithermaldisseminatedAu Ag Cu
High-sulfidation
lode Cu-Au AgCarbonate-replacementZn-Pb-AgAu (or Cu)
DistalAu/Zn-Pbskarn
Marblefront
ProximalCu-Au skarn
PorphyryCu Au Mo
Base oflithocap
1km
1km
Subepithermalvein Zn-Cu-Pb-
Ag Au
Sediment-hosted distal-disseminated
Au-As Sb Hg
Late-mineral porphyry Phreatic brecciaLITHOCAP
PORPHYRYSTOCK
PRECURSORPLUTON
HOSTROCKS
MAAR-DIATREMECOMPLEX
Dacite porphyry plug-dome
Lacustrine sediment
Late phreatomagmatic breccia
Early phreatomagmatic breccia
Late-mineral porphyry
Intermineral magmatic-hydrothermal breccia
Intermineral porphyry
Early porphyry
Equigranular intrusive rock
Dacite dome
Felsic tuff unit
Andesitic volcanic unit
Subvolcanic basement / carbonate horizon
V
V
V
V
V
V
V
V
V
V
. .,
,
. ,, .
, .. 1
, , , , .1 , 1 , .
Relationshipbetween
porphyry and
HSISepithermal
deposits
Sillitoe, R.H., 2010, Porphyrycopper systems: EconomicGeology, v. 105, p. 341.
Epithermal deposit characteristics:
Shallow environment: Typically "1 km depth offormation.
Ore fluid: 50300C, typically
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Richards (2011)3
HS fluids
Possible pathwaysfor magmatic fluids
to contract tomoderate salinityliquids, as seen inHS systems (0.2 to4.5 eq.wt.% NaCl
(Mancano andCampbell, 1995).
Mancano, D.P., andCampbell, A.R., 1995,Microthermometry ofenargite-hosted fluidinclusions from theLepanto, Philippines, high-sulfidation CuAu deposit:Geochimica etCosmochimica Acta, v. 59,p. 39093916.
Richards (2011)
Possible pathwaysfor magmatic fluids
to contract tomoderate salinity
liquids:(1) condensation,
(2) non-condensationpaths of Heinrich et
al. (2004) andHedenquist et al.
(1998), respectively.
Hedenquist, J.W., Arribas, A., Jr.,and Reynolds, J.R., 1998,
Evolution of an intrusion-centeredhydrothermal system: Far
SoutheastLepanto porphyry andepithermal Cu-Au deposits,Philippines: Economic Geology, v.
93, p. 373404.
Heinrich, C.A., Dreisner, T.,
Steffnson, A., and Seward, T.M.,2004, Magmatic vapor contraction
and the transport of gold from theporphyry environment toepithermal ore deposits: Geology,
v. 32, p. 761764.
Richards (2011)
Physico-chemical
conditions of
epithermal oreformation
HS
LS250C, !S = 0.02 m,
salinity = 1 m
Heald, P., Foley, N.K., and Hayba, D.O.,
1987, Comparative anatomy of volcanic-hosted epithermal deposits: acid-sulfateand adularia-sericite types: EconomicGeology, v. 82, p. 126.
Physico-chemical
conditions of
epithermal oreformation
HS
LS
250C, salinity= 1 m
Heald, P., Foley, N.K., and Hayba,D.O., 1987, Comparative anatomy
of volcanic-hosted epithermaldeposits: acid-sulfate andadularia-sericite types: EconomicGeology, v. 82, p. 126.
Gold dissolution asbisulfidecomplexes
(e.g., Au(HS)2):
[Au]measured in natural fluids# 1.5 ppb
Experimentally determined solubilities:
[Au(HS)2] % 11.1 ppb
[AuCl2
] %
1.2 x 10
7
ppb
Therefore, Au is mainly dissolved as
bisulfide rather than chloride complexes.
Gold transport and deposition
Richards (2011)
Precipitationof Au caused by destabilization of Au-
bisulfide complexes:
Reduction acidification:Au(HS)2
+ H++ 1/2H2O "Au+ 2H2S +1/4O2
Oxidation neutralization: Au(HS)2
+ 33/4O2+1/2H2O "Au+ 2SO4
2+ 3H+
Gold transport and deposition
Richards (2011)
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Gold solubility in relation to acidity and oxidation state Boilingis an effective ore forming process. Gases such as
CO2and H2Sare lost to the vapour phase:
Au(HS)2+ H++ 1/2H2O "Au+ 2H2S(g)#+
1/4O2
HCO3
+ H+"H2O + CO
2(g)#(pH increases with loss of CO
2
gas)
Zn2++ HS"ZnS+ H+(sulfides and calcite ppt with increasing pH)
Ca2++ HCO3"CaCO3+ H
+
Ca2++ 2HCO3"CaCO3+ H2O + CO2(g)#(bladed calcite)
Gold and Ag-telluridespseudomorphing
bladed calcite in Py,Porgera, PNG
Bladed calcite andvuggy Qz,
Pachapaqui, Peru Richards (2011)
Casts of bladed
calcite in quartz
Casposo LS epithermalAu deposit,
NW Argentina
PPL
XPL
Richards (2011)
Boiling pointdepth relationships for pure and saline waters,and CO2-bearing waters
CO2increases the depth(pressure) or lowers thetemperature of boiling (phaseseparation or effervescence)
Henley, R.W., Truesdell, A.H., Barton, P.D., Jr., and Whitney,J.A., 1984, Fluidmineral equilibria in hydrothermal systems:
Reviews in Economic Geology, v. 1, 267 p.
Bodnar, R.J., Reynolds, T.J., and Kuehn, C.A., 1985,Fluid inclusion systematics in epithermal systems:
Reviews in Economic Geology, v. 2, p. 7397.
Wallrock sulfidationis also effective:
FeO + 2H2S +1/2O2"FeS2+ 2H2O
2Au(HS)2+ FeO + 2H+"2Au+ FeS2+ 2H2S#+ H2O
Gold transport and deposition
Pyritized Mt,
Porgera Audeposit, PNG
Richards (2011)
Relationship between epithermal systems and
volcanism (Sillitoe, 1973)
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Lepanto HS epithermal system linked to FSE porphyry
Shinohara, H., and Hedenquist, J.W., 1997, Constraints on magma degassing beneath the FarSoutheast porphyry Cu-Au deposit, Philippines: Journal of Petrology, v. 38, p. 17411752.
HS: Advanced argillic
alteration: alunite-silica
body surrounded byquartz-kaolinite
(Aras, Iran)
Richards (2011)
Vuggy silicalithocap
overlyingalunite-
cemented
breccia,
Richards (2005)
Co. Laguna Pedernal,NW Argentina
Richards (2011)
5000m
3500m
6409m
Silicified lithocap above high-sulfidation
epithermal Au mineralization, with marginal
intermediate sulfidation Pb-Zn-Ag veins(Volcn Antofalla, Argentina)
Lithocap
Alunite-clay
alteration
Pb-Zn-Ag veins
Richards (2011)
Distal Pb-Zn-Ag intermediatesulfidation epithermal veins in
underlying sedimentary rocksbeneath volcanic sequence
Quebrada de las Minas,Volcn Antofalla
Richards (2011)
Bonanza-type (low sulfidation) epithermal deposits
Panteleyev, A., 1988, A Canadian Cordilleran model for epithermal gold-silver deposits, inRoberts, R.G., and Sheahan,P.A., eds., 1988, Ore deposit models: Geoscience Canada Reprint Series 3, Geol. Assoc. Canada, p.3143.
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Low-sulfidation epithermal deposits
Examples: Tonapah (NV), Creede (CO),Pachuca-Real del Monte (Mexico),Hishikari (Japan).
Derived from near-neutral, bisulfide-bearing fluids.
Commonly associated with rhyolitic rocks.
Alteration is characterized by Qz-adularia-carbonate-sericite assemblages.
High Ag/Au ratios, variable concentrationsof Cu, and anomalous Mo, W, Mn, F, Se.
Ore minerals include base metal sulfides,sulfates, sulfosalts, selenides, Au,electrum.
www.jamstec.go.jp/jamstec-e/XBR/suger/en/therod21.html
Hishikari, JapanTypical boiling textures
(bladed calcite, vuggy quartz)
Pachapaqui Pb-Zn-Ag mine, Pru
Richards (2011)
Hedenquist et al. (1996)Distal low-
sulfidationepithermal Aumineralization
Hishikari, Japan
Sakurajima volcano
Richards (2011) Richards (2011)
Efemukuru LS Au deposit,
Western Turkey
High grades (up to 210 g/t Au over 1 m)in quartz-rhodonite-rhodochrosite veins,
with minor sphalerite and galena.
Alkalic-type low-sulfidation
epithermal Au
mineralization
Porgera, Papua New Guinea
Bonanza grades over
1000 g/t
Richards (2011)
A veins cuttingintrusion
Pyrite, galena,
arsenopyrite, andrare chalcopyrite
Au in pyrite
Tetrahedrite,arsenopyrite,
sphalerite, (Au)
Galena, sphalerite,arsenopyrite, (Au)
Porgeraearly (A)
veins(Stage I)
Au
Richards (2011)
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FIs in sphalerite(Th = 300350C)
Hypersaline FIs in earlyquartz with phyllic
alteration
Porgera A veins:Fluid inclusions
Richards (2011)
A vein with late D-
type vuggy cavity,
showingparagenetic
sequence (A "D)
(Note: This AD veinterminology is mine-specific,and not related to Gustafson& Hunt$s (1975) porphyry vein
terminology)
Richards (2011)
Breccia-type D veins with visible Au
Richards (2011)
Typical brecciatedD vein
Roscoelite withpyrite and Au
Au withpyrite,
tetrahedrite
Au-Ag tellurides withpyrite
Au-Ag telluridespseudomorphing
bladed calcite in Py
PorgeraD veins
(Stage II)
Richards (2011)
Typical layered D vein: Auoccurs near the fine-
grained margin
Rare vapor-rich FIs
FIs in growth zonesin vuggy quartz (Th %150C)
Rare CO2-rich FIs
Porgera D
veins:
Fluidinclusions
Richards (2011)
Summary
Epithermal deposits are divided into two main categories:Low sulfidation (a.k.a., adularia-sericite) and highsulfidation (a.k.a. acid-sulfate). Intermediate sulfidation
deposits feature more abundant base-metal-sulfides andillite, and may reflect higher salinity fluids.
HS deposits are closely related to magmatic act ivity; fluidsof direct magmatic origin. Early intense acidic alteration by
magmatic volatiles provides permeability, followed(sometimes) by later less-acidic mineralizing fluids.
LS deposits are commonly distal to magmatic activity, andpost-date it by ~1 m.y. or more. Fluids dominantly of
meteoric origin, although some magmatic fluid may be
present. Metals may be derived from country rocks ormagmas. Boiling is a characteristic ore depositional
mechanism. Richards (2011)