epithermal and porfiri gold-geological model

8/9/2019 Epithermal and Porfiri Gold-geological Model

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G Corbett 1

STYLES OF MINERALISATION

Magmatic Arc Cu-Au-Ag deposits are distinguishedmainly on

the basis of ore and ganguemineralogy n order to characterise

varying deposit styles which are derived from dramatically

different fluids, at varying crustal levels. Deposits are initially

divided between porphyry Cu-Au and skams, developed at

crustal depths of I

-

2 km, and epithermal Au-Ag :f: Cu which

form at shallowercrustal evels, and are categorised s high and

low sulfidation styles (Figure I). In the latter class t is possible

to further distinguish between he adularia-sericite tyle banded

epithermalAu-Ag quartz veins, and a group of more sulfide-rich

depositswith common associationswith intrusive source ocks,

which vary with often continuous progressions rom deeper o

shallower crustal levels as: quartz-sulfide Au :f: Cu,

carbonate-basemetal Au, (including the Andean polymetallic

Au-Ag veins), and epithermalquartz Au-Ag deposits.Sediment

hostedAu (Carlin-style) depositsdevelop rom the interactionof

quartz-sulfide style fluids with reactive calcareous ocks. The

class of intermediate sulfidation deposits defined recently

(Sillitoe and Hedenquist, 004) n part correspondso the earlier

documentedcarbonate-basemetal Au association (Leach and

Corbett, 1993, 1994, 1995; Corbett and Leach, 1998; Corbett,

2002a). High sulfidation Au deposits develop from a

characteristic volvedstrongly acidic magmatically-derivedluid

described elow n detail.

ABSTRACT

Geological models for the exploration and evaluation of epithermal Au

and porphyry Cu-Au deposits rely upon a classification of different

deposit styles and an understanding of the evolutionary processes of

deposit formation. These models guide the interpretation of subsurface

deposit anatomy in order to target drill testing and assist the

explorationist to focus on the more prospective projects. It is important

for the explorationist to distinguish between different styles of magmatic

arc Au deposits as each style displays distinctive characteristics with

exploration implications.

Magmatic arc Au deposits are distinguished as porphyry Cu-Au

formed at depths of I - 2 km at the apophysis to larger, buried, magmatic

sources which are also the ultimate metal sources for the varying styles of

shallower level epithermal Au deposits. Low sulfidation epithermal Au

deposits characteristically develop from dilute circulating meteoric waters

and are distinguished as the group with a closer relationship to intrusive

source rocks, and slightly higher sulfide contents (still generally


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G CORBElT

ffiGH ~rn,FTDATTON TOW .


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EPITHERMAN AND PORPHYRY GOLD GEOLOGICAL MODELS

lithological control. At both these deposits most ore is hosted

within fine variably arsenean yrite depositedas he ore fluid has

rapidly cooled by contact with the wall rocks. Quartz-sulfideAu

:f: Cu depositsare not restricted o magmaticarcs and provide a

link to other intrusion-relatedmineralisation styles, such as the

PaulsensPrecambrianquartz-pyrite odes formed adjacent o a

granitebatholith n the Ashburtondistrict of WesternAustralia.

Varying styles of low sulfidation epithermal Au deposits,

whichcommonly orm in different geological environments, re

distinguished n the basis of vein mineralogy, and display

sufficiently different characteristics o warrant distinction by

explorationists. he group of low sulfidation Au-Ag deposits

with highersulfide contents,although n many nstances nly in

theorderof one o two per cent, display a closer associationwith

intrusivesource ocks. These display transitional relationships

and vary spatially and temporally from early to later in a vein

paragenetic equence, nd generally from deeper o shallower

levels rom: quartz-sulfideAu :t: Cu, to carbonate-base etal Au,

and epithermalquartz Au-Ag deposits. Mechanism of mineral

deposition provides one of the important distinguishing

characteristics, nd is of interest to the explorationist as a

significantnfluenceupon preciousmetal grades.

Exploration implications

Gold is readily liberated from oxidised coarse-grained

quartz-sulfideAu .r.Cu oresand so very low metal gradesmay be

worked as bulk low-grade heap each operations San Cristobal,

Chile). However, explorationists should be aware that

quartz-sulfideAu depositsare notorious for surficial supergene

enrichment,particularly n steeply dipping structuresas sites of

chemicaland mechanical oncentration.Elevatedassay esultsof

surficial samples should be treated with caution. Supergene

settingsare evidenced y boxworks after pyrite, or Au anomalous

jarosite at the surface,baseof oxidation and deep within faults.

Many cases where soil geochemical anomalies cannot be

substantiated uring drill testing, are accounted or by supergene

enriched quartz-sulfide Au .r. Cu mineralisation, even in

associationwith other deposit styles such as adularia-sericite

banded epithermal Au-Ag quartz vein deposits. Fine grained

As-rich ores Lihir and Kerimengen PapuaNew Guinea), ormed

by fluid quenchingdisplay poor metallurgy,and high As contents

in theseoresmay prove o be an environmentaliability. However,

the 'silica gris' may be usedas a vector o buried ores n Andean

settings where extreme topographic variations allow access o

much deeper evel, and possibly more prospective, ein portions

wherehigherpreciousmetalgradepolymetallicoresmight occur.

Higher hypogenegradeores are recognised n settingsof fluid

quenching, either by wall rock reaction, or by mixing with

varying ground waters as evidencedby kaolin (low pH waters),

or more commonly manganese xide (bicarbonatewaters), as a

reflection of the transition to higher crustal evel carbonate-base

metal Au mineralisation.Overprinting epithermal quartz Au-Ag

mineralisationalso provideshigher Au-Ag grades.

Quartz-sulfide Au + Cu deposits

Quartz-sulfide u + Cu depositscharacteristicallycontain iron

sulfideswith a quartz-richgangue,varying to Kfeldspar-rich n

the silica-poor alkaline intrusion-related deposits, and

demonstrateronouncedmineral zonation with varying crustal

levelsof formation.Barite is present n many Andean examples.

At deepest rustal levels these deposits may contain pyrite,

pyrrhotiteand chalcopyrite,with lesser specularhaematiteand

magnetite,n a comb or druzy quartz gangue.Gold is deposited

with sulfides, ypically by fluid cooling, and so slow cooled

coarse-grainedeins often display lower Au grades, but good

metallurgy, speciallywhereoxidised. While most quartz-sulfide

Au deposits ontain pyrite as the main iron sulfide, at elevated

crustal ettingshis may pass o marcasiteand arsenean yrite, in

combinationwith opal to chalcedonyas the silica component.

Many high level deposits are therefore characterisedby an

arsenean yrite bearing Au-As-Ag anomalous 'grey silica'

('silica gris' in Latin America), which commonly displays poor

metallurgyas Au is encapsulated n the sulfide lattice. The

refractory res at the Ladolam deposit Lihir Island, PapuaNew

Guinea, reof this style. Here, Kfeldspar alterationdominates n

thealkalinehost ocks.

As the quartz-sulfideAu .T.Cu deposits orm at deepercrustal

levels hey end o exploit pre-existingstructuresand commonly

displaya close elationship o porphyry Cu-Au intrusions, ocally

representing the porphyry-epithermal transition. Many

quartz-sulfideAu .T. Cu vein systems fit into the D vein

classification f the early porphyry Cu literature (Gustafsonand

Hunt, 1975). n dilational structural settingsoften evidencedby

the presence of sheeted veins, ore fluids may migrate

considerableistancesrom sourceporphyry Cu-Au intrusions o

form wall rock porphyry Au deposits. Consequently,sheeted

veins n someMaricunga Belt (Chile) porphyry Au depositsare

typical of quartz-sulfideAu .T.Cu mineralisation.Similarly, the

giant Cadia Hill wallrock porphyry, Australia, displays

significantly higher Au than Cu contents, than is typical of

porphyryCu-Au deposits,and so represents transition between

quartz-sulfide nd porphyry mineralisation.At La Arena n Peru,

quartz-sulfideAu mineralisation occurs as a fracture coating

pyrite n a quartzite mmediately overlying a porphyry ntrusion.

Sector ollapseat Ladolam initiated the change rom porphyry

Au to epithermalAu mineraldeposition Corbettet ai, 2001).

Manyquartz-sulfideAu .T.Cu depositsoccur as steeplydipping

lodes (Mineral Hill and Adelong, Australia; Bilimoia

[lrumafimpa], Papua New Guinea; Jaing Cha Ling, China;

Rawas, Indonesia) while fracture vein networks (Nolans,

Australia; eeperpartsofPorgera, PapuaNew Guinea) may vary

to more dilational sheeted veins (Kelian, Indonesia). Flat

structuresmay host ore where collapse of volcanic edifices is

interpreted sa trigger for ore formation, suchas he flatmakesat

EmperorGold Mine, Fiji, initiated as a reactivation of bedding

planesduring adjacentcaldera collapse, or the listric faults at

Ladolam ormed by Mt St Helens style sector collapse, and

where ores vary from a deeper structural, to higher level

Carbonate-base metal Au

As the group of sulfide-bearing ow sulfidation intrusion-related

Au depositsdisplay a fluid evolution from quartz-sulfideAu .i:

Cu, to carbonate-basemetal Au, and later epithermal Au-Ag,

many depositsare characterised y all three depositstyles either

grading temporally in overprinting events (Porgera), aterally

(Kelian, Indonesia), or vertically (Kerimenge), while others

display telescoping nto single lodes (Thvatu, Fiji), or fracture

networks Mt Kare, PapuaNew Guinea).Similarly, district scale

vertical zonation may be evident, such as in the Morobe

Goldfield in Papua New Guinea, where the Hamata

quartz-sulfideAu depositoccurs at deepestevel, Hidden Valley,

Kerimengeand Upper Ridges carbonatebasemetal Au deposits

lie at intermediate evels, and the Edie Creek bonanza grade

epithermal Au deposit is mined at an elevatedcrustal setting.

Furthermore,Kerimengedisplaysa vertical zonationover several

hundred metres rom quartz-sulfideAu .i: Cu, to carbonate-base

metal Au and highest and lateral epithermal Au-Ag

mineralisation, all localised at the contact of a fault with the

margin of a diatreme breccia pipe. Although most deposits

display some transitional relationships,a significant portion of

the Au in many southwestPacific rim Au deposits s clearly of

the carbonate-basemetal Au affiliation, as these represent he

most prolific producers n the region. In older more deeply

eroded terranes such as the Ordovician Lachlan Fold Belt of

eastern Australia, quartz-sulfide Au systems attain economic

status due to the overprinting carbonate-base metal Au

mineralisation (Kidston, Lake Cowal and London-Victoria in

Australia). Carbonate-base etal depositsdisplay more efficient

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G CORBETf

surface,Au mineralisationmay occur at breccia pipe marginsat

this level, or within the breccia matrix at depth. The

characteristicMn wad formed by the weatheringof common Mn

carbonatesprovides a ready tool for the recognition of these

deposits. ut may scavenge u.

Epithermal

quartz Au-Ag

Epithermal quartz Au-Ag deposits form at the highest crustal

levels and late stage in the paragenetic sequence of

intrusion-related low sulfidation Au deposits. They consequently

overprint both quartz-sulfide Au (Ladolam, Emperor), and

carbonate-base metal Au deposits (Porgera Zone VII, Mt Kare,

parts of Edie Creek). Some occur marginal to porphyry Cu-Au

deposits (Thames, New Zealand), or at the margins of

carbonate-base metal deposits (Kelian and Kerimenge). Most

display a strong structural control as they form at great distances

from the magma source as fluids deposit to cooler epithermal

crustal settings. Consequently, much of the higher grade ore

commonly occurs in ore shoots formed at preferential sites of

fluid flow or Au deposition (Porgera). One of the most notable

features of these deposits is that the characteristic bonanza Au

grades often overprint existing veins with little new gangue

mineral deposition, and so are difficult to identify. Many deposits

contain anomalous exotic metals such as tellurium (Emperor) or

selenium (Selene, Peru) and minerals such as tellurobismuthinite

are a common associate with Au (Bilimoia). Mineral deposition

is considered to be promoted by rapid fluid cooling, which may

be enhanced by the mixing of ore fluids with groundwaters

entering the deposits at elevated crustal settings.

Recent age data for the Porgera Au deposit (Ronacher et ai,

1999) provides a 5.9 m.y. age for two contrasting mineralisation

events, with a very small age separation (0.26 m.y. within the

error estimation). The initial (Stage I) bulk low-grade

quartz-sulfide grading to a later carbonate-base metal Au event

currently being mined at the Waruwari open pit probably formed

at a deep crustal level as evidenced by the presence of high

temperature pyrrhotite and dark sphalerite. The later (Stage II)

low temperature epithermal quartz Au-Ag mineralisation well

developed in the Zone VII underground is associated with

renewed more felsic magmatism (Corbett et ai, 1995), and

probably formed with at least 600 m less overburden. Recent

exposures from the deepest portion of the mine demonstrate that

the (Stage II) epithermal mineralisation grades rapidly through

quartz-sulfide and low temperature carbonate base metal

alteration, as a renewed mineralisation associated with the later

felsic magmatism. Thrusting exposed within the Porgera open pit

and common throughout the region, is suggested to account for

the rapid unroofing of the Porgera Intrusion Complex and

provision of a trigger for renewed epithermal mineralisation.

mechanismsof Au deposition than the quartz sulfide and so

commonlydisplay higher preciousmetal grades.

Carbonate-base etal Au depositsare characterised y one to

ten per cent sulfides, commonly as pyrite> sphalerite> galena

with a gangue of carbonate and variable quartz and display

pronounced onation Corbettand Leach, 1998).At deeper evels

the transition o quartz-sulfidestyle may reflect minor pyrrhotite

(Porgera,Kelian). Vertical zonation n sphalerite ype is evident

as a composition-controlled olour change elated o temperature

(depth), varying from black, Fe>Zn, high temperatureat depth,

through brown, red, yellow and locally clear, Zn>Fe, low

temperature phalerite,at highest crustal evels. Carbonates re

zonedas he collapsingweakly acidic bicarbonate luids undergo

a progressive ise in pH with depth by wall rock reaction,and so

vary with increasing depth from carbonatesdominated by Fe

(siderite) at higher crustal levels, to Mn (rhodochrosite),Mg

(ankerite, dolomite) at intermediate evels, and Ca (calcite) at

deepestcrustal levels. Much of the mineral deposition results

from the mixing of rising ore fluids with bicarbonatewaters,

often derived rom high level felsic intrusions.

The form of carbonate-basemetal Au deposits varies

considerably from banded veins (Antamok and Acupan,

Philippines; Cikotok and Pongkor, Indonesia), or composite

lodes (Edie Creek and Woodlark Island, n PapuaNew Guinea;

Tavatu), racture vein networks Porgera,Mt Kare, Lake Cowal),

sheeted ein systems Kelian, Kidston), and matrix to diatreme

breccias Mt Leyshon,Australia; Montana Tunnels, USA, Rosa

Montana, Romania), or brecciated ntrusion margins (Bulawan,

Philippines).This associationwith phreatomagmatic diatreme)

breccias commonly provides a link to magmatic source rocks

(frequently dacite and rhyodacite). At higher crustal levels the

clay altered diatreme brecciasare incompetentand so fracture

mineralisation occurs in the adjacent host rocks (Kelian,

Kerimenge),or at the diatreme-host ock contact (Acupan), and

only at deeper crustal levels does mineralisation reside n the

diatreme matrix (Mt Leyshon, Montana Tunnels), including

cross-cutting racture systems Cripple Creek, USA). Elsewhere,

pre-mineral phreatomagmaticbreccia dykes exploit structures

and provide groundpreparation Woodlark sland).

While in many instancescarbonate-basemetal Au deposits

occur in the same erranesas high sulfidation deposits Rio de

Medio at El Indio in Chile, Victoria at Lepanto in the

Philippines), t is also possible, but only rarely noted, for the

fluids responsible or the formation of high sulfidation Au-Ag

deposits o undergo progressivecooling and neutralisation by

rock reaction to evolve to low sulfidation mineralisation.

Consequently,he Link Zone Prospectprovides a higher grade

and better metallurgy Au mineralisationas quartz-sulfideveins

with a carbonate verprint, ecognised n the margin of the Wafi

high sulfidationdeposit,PapuaNew Guinea.


Carbonate-basemetal Au depositsare important Au producers

but vary considerably,with resultant metallurgical mplications,

especially where overprinting relationshipsare recognisedwith

other styles of low sulfidation Au mineralisation (Kelian,

Porgera).Explorationists should be aware that carbonate-base

metal Au deposits often display very irregular Au distribution

which must be taken into account in ore reserve estimation,

particularly within oxide zones, or where overprinted by

telescoped epithermal quartz Au-Ag mineralisation.

Carbonate-base etal Au depositsdisplay highly variable orms

requiring an estimationof the sub surface hree dimensionsand

ore controls prior to drill testing. It is imperative that drilling

transectsheetedveins or lodes at the best possible angles,and

that kinematic analyses attempt to determine the setting of

structurally controlled ore shoots within throughgoing veins.

While high level diatreme breccia systems may be barren at


The free milling bonanzaAu has ed to the ready dentification

of many epithermal Au-Ag depositsby panning (Porgera,Edie

Creek),but may also promote he presence f artisanminers Mt

Kare, Kelian). The improved metallurgy and higher metal grades

within epithermal quartz Au-Ag mineralisation have enhanced

the economicsof otherwise ower Au grade or metallurgically

difficult quartz-sulfideAu and carbonate-base etal Au deposits

(Porgera,Emperor). Great care must be exercised n the drill

testingof thesedeposits o ensure hat vein oresare ntersected t

the best possible angles, and maintenance of good core

recoveries s essentialwithin mineralised ault zones.Similarly,

in careful drill core sampling, a geologist should mark where

core should be sawn and sampled o ensurebest possibleassay

returns. The bonanza Au grades, commonly with only minor

gangueminerals, are difficult to recogniseand so may provide

challen~es n ore reserve determinations.Explorationists must

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EPITHERMAN AND PORPHYRY GOLD

GEOLOGICAL MODELS

also be aware hat most ore and higher precious metal grades

commonly ccurs n ore shoots ormed as dilatant structuralsites

(flexuresor fault jogs) of enhanced luid flow (PorgeraZone

VII), or enhancedmetal depositionat cross structures Thames),

or hangingwall splits (PorgeraZone VII).

Sediment hosted replacement Au

Sediment hosted replacement Au (Carlin style) deposits are well

documented as major Au producers in the western US (Carlin,

Goldstrike, Cortez), and although less important Au producers in

the southwest Pacific (Mesel, Indonesia; Bau, Malaysia; and

Sepon, Laos), remain as important exploration targets

worldwide, and are well represented in emerging provinces such

as China. They are also recognised in rocks of varying ages

including the Precambrian of the Asburton District of Western

Australia.

These deposits develop from the interaction of an ore fluid

typical of quartz-sulfide Au style deposits, with reactive host

rocks, typically impure calcareous sediments (marls of the

Popovich Formation, Nevada). Regional scale extensional

structures facilitate the transport of ore fluids from magmatic

source rocks at depth to elevated crustal settings, where mineral

deposition occurs. Deposits of the Carlin trend are localised

along the Post Fault system, and Mesel occurs within a smaller

scale fault jog, where dilatant fractures have focused ore fluid

flow. Sediment hosted replacement Au deposits display

important internal variations from structurally controlled feeder

structures at deeper levels, commonly with higher Au grades, to

lithologically controlled lower grade ores at higher crustal levels.

At Mesel Au contents decline rapidly moving away from dilatant

feeder structures, and in the Carlin trend structurally controlled

higher grade deposits such as Meikle are mined underground,

while the lithologically controlled ores (Carlin, Goldstrike)

represent large open pit mines.

Sediment hosted replacement Au deposits are characterised by

the dolomite-silica-kaolin alteration associated with the

introduction of auriferous arsenean pyrite, with anomalous Hg

and Sb. The early dolomitisation of calcite creates open space

and so provides secondary permeability for ore fluid flow

associated with local silicification. This dissolution is commonly

evidenced as collapse breccias. Silicification is also apparent as

barren jasperoid alteration, common in the upper portions of

these deposits.


At the reconnaissancexploration stage, he resistant asperoid

rocks,which are commonly preserved n the float train, are an

indicationof this style of mineralisationwithin a region. While

barren in outcrop these rocks may provide indicators of

mineralisation t depth (Mesel). Becausesediment hosted Au

deposits recommonly ermedclassic no see em' gold deposits,

goldpanningmay not be reliable, and so advocated eochemical

tools include BLEG stream sediment sampling, followed by

analysesf soil samples or elements uchas As, Sb, W, and Hg.

During evaluation,analysis of structural controls may allow

explorationists o target higher grade ores within feeder

structures t depth, which will compensate or the additional

costsof dealing with thesemetallurgically difficult fine As-rich

pyritic ores. Consequently, xide ores are favoured or mining

operations. he environmental aspectsof the As, Sb and Hg

bearing resshouldbe taken nto account.

Zealand geothermalsystems.While many of these depositsare

well developed n back arc environments Drummond Basin,

Australia; Taupo Volcanic Zone, New Zealand; Argentine

Patagonia; Japan; western US), or some are noted within

intra-arc rifts (Tolukuma, PapuaNew Guinea), other individual

deposits occur within magmatic arcs (El Penon, Chile; Ares,

Peru), or other linear magmatic arcs are dominated by these

deposits (Coromandel Peninsula, New Zealand; Kamchatka

Peninsula,Eastern Russia).This style of mineralisation s also

recognised n the mid oceanic ridge hotspot environment at

Iceland.All theseenvironments isplay characteristic f bimodal

volcanism, commonly as an interpreted associationof Au-Ag

mineralisation with felsic magmatism,commonly hosted with

andesiticor basaltic ock sequences.

The low sulfidation adularia-sericite andedepithermalAu-Ag

quartz veins typically comprise fine interlayers of chalcedony

varying to opal, with lesser adularia, quartz pseudomorphing

platy calcite, and black sulfidic ginguro bands (named by the

nineteenth century Japanese miners). Most genetic

interpretations uggest hat meteoricdominantwaters ise rapidly

up dilatant fracture systems hosted within competent rock

packages and boil to deposit much of the vein mineralogy.

Comparisonswith geothermalsystems ink adularia and quartz

pseudomorphing laty calcite to boiling fluids. However, hese

mineral assemblagesend not to contain Au-Ag mineralisation,

which dominates n the ginguro bands and to a lesser extent

chalcedonyand so, as discussedabove, some workers therefore

invoke rapid cooling, locally aided by mixing of the ore fluid

with varying groundwaters,as a mechanism or deposition of

bonanzaAu-Ag mineralisation.

Adularia-sericite banded epithermal Au-Ag quartz vein

depositsdisplay pronounced ertical zonation.At surficial levels

eruption breccias (Champagne Pool and Puhipuhi in New

Zealand; Toka Tindung, Indonesia; lWin Hills, Australia),

represent sites of venting hydrothermal fluids which form

laminatedsinter deposits.Although anomalousn toxic elements

(Hg, As, Sb, W) many silica sinter deposits are barren with

respect o Au, unless proximal to fluid up flows (Champagne

Pool). Sheetedveins often extend into the deeper portions of

eruption breccias lWin Hills; McLaughlin, USA) and also cap

the upper portions of some issure vein systems Golden Cross

and Karangahake n New Zealand). Most ore systems occur

within deeper fissure veins, commonly localised by dilatant

fractures within competenthost rocks (Pajingo and Vera Nancy,

Australia; Tolukuma; Hishikari and Sado n Japan;Waihi, New

Zealand; Asacha; Eastern Russia). Wall rock clay alteration

varies from illite at deeper evels marginal to fissure veins and

grades vertically and laterally to assemblages ominated by

illite-smectite and thence smectite. Acid sulfate alteration

(alunite, cristobalite, kaolin) forms at near surficial settings by

the reactionwith wall rocks of low pH condensatewaters,which

may also collapse into the ore system to promote mineral

deposition.

Factors which influence the localisation of higher grade ore

systems nclude, structure,hot rock competency, nd mechanism

of Au-Ag deposition. n many vein systemsmost ore, including

of higher grades,occurs n ore shootsdevelopedas preferential

sites of mineralised fluid flow within flexures in otherwise

poorly mineralised hroughgoingstructures Vera Nancy; Sims,

2000), fault jogs between structures, or splays. Structures

dominatedby strike-slip fault movementhost steeply plunging

ore shoots (Vera Nancy), while those within listric faults will

display flat plunges Sierra Madre, Mexico; Arcata, Peru).Rock

competency nfluences he manner n which host rocks fracture

during vein formation and less competent sequencesmay cap

ore. At Hishikari, the ShimantoGroup shale,which hostsvertical

quartz veins, is overlain by volcanic breccias, and preferred

mineral deposition of bonanza ores has occurred at the

intersectionof theseveins with the lithological contact, esulting

Adularia-sericite banded epithermal Au-Ag quartz

vein deposits

The adularia-sericitebanded epithermal Au-Ag quartz vein

depositsare the most extensively documented ow sulfidation

Au-Ag deposits.particularly using the parallels with the New

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G CORBETT

in the developmentof flat plunging higher grade ore zones.At

Karangahake, he andesite-hostedmineralised fissure veins

become a less well mineralised stockwork in overlying

incompetent rhyolite. While the Chon Aike ignimbrite hosts

veins in the Cerro Vanguardia region, Argentine Patagonia,

elsewhereveins are limited to the competentmargins of felsic

domes at Ares and Asacha (Kamchatka). Enhanced Au-Ag

deposition often occurs at sites of fluid mixing where ground

waters come in contact with ore fluids such as at splay faults

(Tolukuma), hanging wall splits (Asacha), or changes n host

rock competency Hishikari). These may be recognisedby the

presence f kaolin (Ares) or manganese xide (Karangahake)n

the upperand better mineralisedportionsof the vein systems.

to dickite and kaolin clays, and may contain minerals such as

corundum and andalusite, formed in very high temperature

settings. By contrast, at epithermal crustal levels, silica cores

grade to mineral assemblages ominated by alunite and more

marginal dickite and kaolin clays. Erosion of the softer marginal

clays commonly facilitates formation of prominent topographic

highs by preservation f the relict silica coreswhich are resistant

to erosion.

Thesealteration zonesare nterpreted o have developed rom

the reaction with host rocks of magmatic volatiles venting

directly from intrusive source ocks early in their evolution, and

are compared o magmaplumes n geothermal ystems Reyeset

ai, 1993). Alteration zones are recognisedoutcropping on the

margins of porphyry Cu-Au systems where they have been

termed barrenshoulders Corbett and Leach, 1998). mportantly,

the fluids responsible or thesealterationzoneswere rising at the

time of alteration,but have not evolved as the volatile-rich high

sulfidation luids, and so are not mineralised.

Explorationist should note that these alteration zones are

common n the vicinity of porphyry Cu-Au deposits and other

intrusion-related manifestations such as low sulfidation

quartz-sulfide veins. Many exploration case histories include

instances where Au anomalism derived from nearby low

sulfidation epithermal veins has been attributed to these

topographically obvious alteration zones which are themselves

h"rrpn


The early recognitionof thesebarrenalterationzonesmay aid in

prioritisation of exploration programs o focus upon more fertile

alteration systems.Thesebodies of zoned magmaticallyderived

advancedargillic alteration might commonly be recognisedby

the explorationistby the presence f massive ather than vughy

silica, typical of high sulfidation epithermal Au deposits. At

deeper crustal levels high temperature minerals such as

corundumand andalusiteare characteristic, nd minerals suchas

alunite and diaspore may display very coarse-grainedshapes

indicative of slow formation at near porphyry levels. The

distinction of this non-fertile alteration at epithermal crustal

levels is less definitive. Higher temperature dickite clays

dominate over kaolin. They commonly form in settings such as

within eroded volcanic edifices, and contain ledges of

structurally or l ithologically controlled silica in intense

clay-pyrite alteration, which weathers to provide distinctive

liesegang ings and ferricrete deposits. Any Au mineralisation

may be in later cross-cuttingveins or brecciasand importantly

generallyoccursoutside o core of actualalterationzone.


Adularia-sericitebandedepithermal Au-Ag quartz vein deposits

provide attractive exploration targets which commonly contain

bonanza Au-Ag grades, and are amenable o mining in semi

urban areas Hishikari, Waihi) or in difficult terranes Tolukuma

lacks a road ink and s wholly suppliedby helicopter).

Gold panning (Tolukuma; Chatree, Thailand), and BLEG

streamsampling,continue as useful first passprospecting ools,

aided by the recognition of characteristic banded quartz as

stream float downstream (Tolukuma), or at prospect scale

(Chatree, Cerro Vanguadia).Resistive geophysical techniques

(CSAMT) have argetedquartz veins or drill testing (Hishikari),

locally within throughgoingstructures Vera Nancy). PIMA now

replaces lower XRD to readily provide studiesof clay alteration

zonation to target higher temperature llite alteration close to

veins (seeGoldenCross n Corbettand Leach, 1998).

Explorationists should be consciousof the importanceof ore

shootsas sitesof bonanzaAu gradeswithin vein systems.While

many ore shoots display steep plunges (Vera Nancy), flat

plunging ore shoots occur in settings of listric fault control, or

changes n host rock (Hishikari), and the intersectionof hanging

wall splits with main the fault (Asacha). Often unmineralised

eruption brecciasand acid sulfatealteration may cap mineralised

vein systems nd so vector owardsmineralisation.

Careful geological mapping s essential n order to plan drill

testing at the best possible angle, and the angle of veins to the

core axis should be monitored o ensureoptimum drill direction

is maintained.As bonanzaorescomprising ginguro sulfidic vein

portions locally occur with clays, and are commonly

fault-controlled, good drill core recoveries are essential for

accurate ore reserve determinations. Poor ore recoveries

sometimes owngradeAu contents.

BARREN HIGH ADVANCED ARGILLIC

ALTERATION

Many varying styles of alteration result from the interaction of

acid waters with host rocks, and while some alteration systems

clearly vector towardsAu mineralisation, t is imperative or the

explorationist to distinguish prospective from barren

hydrothermal alteration. There is clearly great benefit for

explorationists o adequately nderstand arying styles of barren

advanced rgillic alteration.

Magmatic-derived advanced argillic alteration

Magmatic arcs contain common bodies of barren advanced

argillic alteration which typically outcrop as resistive ledges

developed by alteration within structures or permeable

lithologies, and extending from porphyry to epithermal crustal

levels. While these alteration systems display characteristic

zonation outwards from silica cores, mineral assemblages vary

with depth such that at deeper levels mineral assemblages are

dominated by alunite, pyrophyllite and diaspore grading laterally

HIGH SULFIDATION Au - Ag - Cu DEPOSITS

High sulfidation epithermal Au deposits result from the

interaction with host rocks of magmatically-derived re fluids,

which have evolved to attain a characteristic very acidic

character. n simple terms, a volatile-rich fluid (dominantly SOb

but also containing COb H2S,HCI) leaves he magmatic source

and becomes depressurisedduring the rapid migration to

epithermal crustal levels, causing the volatiles to come out of

solution and oxidise (as O2 and H2O also evolve from the same

depressurisingluid) to form a hot acidic fluid. The fluid has not

interactedwith host rocks or groundwaters uring rapid upward

migration. Consequently he fluid has evolved during rapid

ascent, rom a near neutral luid in the porphyry environment, o

strongly acidic at epithermal levels, where host rock reaction

results n the developmentof the characteristichigh sulfidation

alterationzonationand mineralisation.

High sulfidation deposits commonly develop without the

repeated activation of dilational structures, which in low

sulfidation systems drive hydrothermal cells of

?11 Arlol,,;rlo ~A 10

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EPITHERMAN AND PORPHYRY GOLD - GEOLOGICAL MODELS

in remote sensing imagery as colour anomalies, and more

recently using multispectural lay analyses.

The enargite ores often display difficult metallurgy and so

many deposits are preferentially worked in the oxide zone

(Sipan, El Guanaco,La Coipa) and the bulk low-grade ores are

conducive to extraction as heap leach operations (Sipan,

Yanancocha).Acid mine waters producedby weatheringof the

pyritic clay alteration, and the high As content of sulfide ores,

and local Hg by products, may all provide environmental

concerns.

One of the most mportant explorationproceduress the useof

careful PIMA studies to supplement ield identification of the

alteration zonation n order to vector to the central vughy silica

which hosts most mineralisation. t is possible or silica-alunite

to form resistant barren caps and so mask mineralised vughy

silica. In some cases he barren alteration has previously been

mined for industrial minerals (pyrophyllite at Peak Hill,

Australia). The Nansatsuhigh sulfidation deposits n Japanhave

beenmined or silica flux and he Gidginbungsilica was nitially

used or road construction.

PORPHYRY COPPER-GOLD

Porphyry Cu-Au depositsdevelop as a result of focusing of the

mineralising luids at depthsof I - 2 km in the cooler apophyses

to magmatic sources at greater depths, and so extend from

intrusion host rocks into the wall rocks. Some of the better ore

systems Grasberg, ndonesia;Oyu Tolgoi, Mongolia; Ridgeway

and Goonumbla n Australia) are characterised y multi phase

intrusion emplacement into spine-like vertically attenuated

intrusion complexes. Overprinting intrusions provide multiple

events of mineralisation and locally recycle ore minerals into

settings with higher metal grades, but may also overprint and

obliterate mineralisation related to earlier porphyry Cu-Au

intrusions, hereforedowngrading he total ore system Figure 3).

meteoric-dominated aters to facilitate banded quartz vein

formation.Rather, he developmentof high sulfidation deposits

might be promoted as single magmatically dominated

hydrothermal ventsduring transient elaxation n compressional

magmatic rcs.The kinematics of individual deposits herefore

commonly ontrastwith observed egional ectonics.

The volatile portion of the high sulfidation fluid travels more

rapidly han he fluid-rich portion, and reactswith the host rocks

to produce he characteristicalteration zonation. At the core of

thealteration one he host rocks undergo ntense eachingby the

extremelyacidic fluid to produce a rock composed almost

entirelyof silica, termed residual or vughy silica, indicative of

the characteristic pen space exture. As the hot acidic fluid is

cooled and neutralisedby rock reaction the zoned alteration

grades utwardshrough alterationassemblagesharacterised y

alunite, pyrophyllite, diaspore, and dickite/kaolin, to neutral

clays such as illite/smectite, and eventual marginal porphyritic

alteration chlorite-carbonate).Alteration zonation also varies

according o, proximity to the fluid up flow, host rock

permeability, nd crustal evel (eg dickite at deeper1evels asses

to kaolin n higher evel cooler settings).

High sulfidation fluid flow is influenced by permeability

controlsclassedas structural, lithological and breccia. While

many deposits are localised either on major throughgoing

structuresGidginbung,Australia, on the Gilmore Suture; Waft,

PapuaNew Guineaon a transfer structure;Mt Kasi, Fiji), or on

dilational ractures between throughgoing faults (Lepanto and

Nenaeach ie on splays; EI Indio occurs within a sigmoidal

loop), he contactsof thesestructureswith permeable ithologies

(Sipan,Peru; EI Guanaco, Chile; Nena and Gidginbung), or

diatreme breccias (Lepanto), provide suitable settings for

development f alteration zones. Other deposits are wholly

controlledwithin permeable ithologies in volcanic sequences

(Pierina,Peru;La Coipa Chile). Many high sulfidation deposits

are associatedwith phreatomagmaticbreccias (Wafi; Pascua,

Chile; Veladero,Argentina; Miwah, Indonesia),which no doubt

facilitate he rapid rise of ore fluids from porphyry to epithermal

levels,and so some high sulfidation deposits also occur within

felsicdomes Mt Kasi), or dome/breccia omplexes Yanacocha,

Peru).Felsic domes are an important link to magmatic source

rocks at depth, and therefore commonly associatedwith high

sulfidationAu deposits.Somedepositsoccur as veins (EI Indio)

while otherdeeper evel systems ollapse upon porphyry Cu-Au

depositsMonywa,Myanmar;Tampakan,Philippines).

A liquid-dominated fluid component enters the zoned

alteration ia the sameplumbing systemas supplied he volatile

portion of the ore fluid, and so deposits sulfides comprising

pyrite, enargite (including the low temperature polymorph

luzonite)and additional alunite, along with barite and ate stage

sulfur,mainly within the vughy silica, locally extending nto the

adjacentsilica-alunite portion of the zoned alteration. High

sulfidationdepositsdisplay a zonation from Cu-rich at deeper

levels,grading o Au-rich at higher crustal evels. Ores occur as

veins, breccia matrix, or filling vughy silica. While most

southwest acific depositscontain very little Ag, many Andean

deposits re Ag-rich and higher level deposits may contain Hg

andTe.


The characteristic iliceous float provides a ready prospecting

tool, which may be recognisedn the float train well downstream

and traced to source (Miwah). Covered silicification is

discernibleon resistivity geophysical studies such as CSAMT

(Mt Kasi; Maragorik, Papua New Guinea), most apparent n

higher evel systemswhere there is a strong contrast with the

adjacent onductive clay alteration. The high sulfidation clay

alterationmay be identified on geophysicalstudiesas regionsof

magnetite estructionand coincident electrical conductivity, or

PA~RIM ?nn4 4rloloirlo ~4 10 - J'J ~ontomho, 'JnnA


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G CORBETT


Explorationsitsneed o gain an understanding f the subsurface

anatomy of the overprinting intrusion-hydrothermalsystem to

target ore, generally best developed n stockwork and sheeted

quartz veins about the intrusion carapaceand wall rocks. The

magneticand zonedprogradealterationprovidesvectors owards

the intrusion carapace. However, magnetite destruction and

chargeability anomalies recognised in association with the

strongly pyritic retrogradephyllic-argillic and advancedargillic

alteration styles need not display a direct relationship with

mineralisation,and require an understanding f the overprinting

events.Regionalstructuralanalysismay be projected nto the ore

environmentcombine with careful geological mapping n order

to plan the best direction for drill testing of preferentially

orientedsheeted ein systems,which both host and ransportore

as tension veins. While multiple intrusion events may upgrade

somesystems, ost-mineral ntrusionscan also stopeout ore and

so explorationistsshould carefully consider he implications of

barrendrill holes within mineralised ones.

Explorationistsshould not neglect he importanceof skarns n

porphyry exploration.Many important porphyry systems Frieda

River and Ok Tedi in PapuaNew Guinea) were identified by

follow up of skarn loat well downstream,while the presence f

skarns helped maintain interest in the Cadia alteration system

during exploration, and eventual dentification of the Ridgeway

deposit,and the Grasbergdiscovery n West Papuaaroseduring

the mining of the Ertsbergskarn.

CONCLUSION

Geological models assist he in the prioritisation of exploration

projects and play an important role in the estimation of the

subsurface anatomy of hydrothermal systems in order plan

effectivedrill testing.

Porphyry Cu-Au mineralisation s best developedat intrusion

apophyses nd high sulfidation Au mineralisation s focused n

the central portions of zonedalteration.Once categorised, arren

high zones of advancedargillic alteration can be avoided. An

understanding f the different styles of porphyry and epithermal

Au mineralisation allows the exploration implications of each

style of mineralisation o be applied o individual projects,and so

provide a framework for mineral exploration in magmatic arc

settings.For instance,quartz-sulfideAu depositsoften undergo

surficial supergene nrichmentand so surfaceassaydata mustbe

treated with caution. The alteration zonation n high sulfidation

Au depositsvariesaccording o crustal evel, relationship o fluid

up flow and style of host rocks. Similarly, the application of the

understanding of the processes of ore formation assist in

accessing whether an alteration system warrants continued

evaluation, and locally aid in provision of vectors to guide

further exploration. In low sulfidation gold deposits shallow

circulating meteoric waters may deposit banded chalcedony

veinswhich areessentiallyunmineralised.

While many exploration successesormerly relied upon the

recognition of mineralised float during first pass prospecting

(Tolukuma, Chatree, Miwah), where ore systems have been

exposed by erosion, it is often now necessary o focus upon

techniques which aid in the identification of buried

mineralisation,either not exposedby erosion (Nena,Ridgeway),

or obscuredby post-mineral cover (Raffertys Porphyry, Wafi;

Vera Nancy). It is hoped a better understanding f the nature of

porphyry and epithermal Au mineralisation will assist in the

evaluationof subsurface eologyduring exploration.

In generally compressional subduction-related magmatic arcs,

changes in the nature of convergence may act triggers to

facilitate the emplacement of vertically attenuated intrusions

from magmatic source rocks at depth, into higher crustal level

dilational structural settings (Corbett and Leach, 1998; Corbett,

2002b). Ore fluids then continue to evolve from the magmatic

source into higher level ore settings utilising dilational fracture

systems such as sheeted quartz veins. Many quality porphyry

Cu-Au deposits are not linked to associated extrusive volcanic

rocks (Grasberg; Oyu Tolgoi; Bingham Canyon, USA)

suggesting volatiles and mineralisation may have been retained

within the cupola rather than vented.

Initial intrusion emplacement is characterised by zoned

prograde alteration grading outwards as potassic (magnetite,

secondary biotite and Kfeldspar), to inner propylitic (actinolite,

epidote), and outer propylitic (chlorite, carbonate) alteration

(Figure 3). In sodic rocks albite alteration may dominate over

potassic-propylitic alteration assemblages. The early disjointed,

high temperature ptygmatic A-style quartz veins (in the

terminology of Gustafson and Hunt, 1975) develop within the

cooling intrusion, while quartz-magnetite (M-style) quartz veins

dominate within the prograde magnetite-bearing alteration, and

locally contain chalcopyrite-bornite-pyrite mineralisation.

As also recognised in active geothermal systems (Reyes et at,

1993), volatiles venting from intrusions early in the cooling

history react with host rocks to produce barren advanced argillic

alteration, often migrating laterally as altered ledges, described

above.

Hydrothermal fluids accumulate at the apophyses of cooling

intrusions which become overpressured and eventually fracture.

The resulting pronounced pressure drop promotes the

development of B-style quartz veins (in the terminology of

Gustafson and Hunt, 1975). Whereas traditional geological

models utilise an excess of fluid pressure over rock tensile

strength to promote fracture development.. structural systems

which localise intrusions may also crack an overpressured

carapace. Consequently, sheeted (parallel) quartz veins display

dilational tensile fracture/vein relationships, and so may

transport ore fluids from magmatic sources at depth, to the cooler

carapace, and extending into the wall rocks where mineral

deposition occurs. At Cadia Hill, Australia, wall rock hosted

sheeted quartz veins vary little through 700 m vertical extent.

Low pH condensate waters develop in the upper portion of the

hydrothermal system and react with the host rocks to promote

retrograde alteration of the prograde assemblages, including

demagnetisation, typically as phyllic (sericite-silica-pyrite)

grading to argillic (chlorite-kaolin-carbonate) alteration, as the

collapsing fluids are progressively cooled and neutralised by

rock reaction (Figure 3). Strongly acid fluids will also promote

the development of advanced argillic alteration, which contains

additional alunite-pyrophyllite in addition to the phyllic

alteration assemblage. This, and early venting magmatic

volatiles, contribute towards the development of lithocaps

recognised in the upper levels of porphyry systems. Cooling of

the intrusion apophysis promotes collapse of the hydrothermal

system and re-entry of ground waters into the porphyry

environment and development of extensive pyrite-bearing acid

alteration.

While sulfides may be exsolved from the major intrusion source

at depth throughout the life of the cooling porphyry, mineral

deposition is promoted as the apophysis cools in the at later stages

of the porphyry evolution, especially if ore fluids come in contact

with low pH condensate waters collapsing into the hydrothermal

system. Consequently, sulfides (chalcopyrite-bornite-pyrite)

commonly cross-cut or occur in the central portions of quartz

veins, and are focused within breccias. Dilational structural

settings promote the evolution of ore fluids from the intrusion

source at depth to form intrusion-related epithermal deposits

(including D veins) at higher crustal levels.

ACKNOWLEDGEMENTS

Too manycolleaguesnd clients o mentionover 25 yearsof

Pacific im travel are thanked or their participationsn the

22

Adelaide, SA, 19 - 22 September 2004

PACRIM 2004


9/9

EPITHERMAN AND PORPHYRY GOLD

-

GEOLOGICAL MODELS

of the conceptspresentedherein. Mike Smith and

.-- . commentedon the manuscriptand Denese

figures. I thank Dale Sims of The AuslMM for

."~~-'" ~ paper, and the Australian Institute of

for permission o provide it as an update of the

;,2002a).

Leach, T M and Corbett, G J, 1993. Porphyry-related arbonatebase

metal gold systems: The transition between the epithermal and

porphyry environments, n Second National Meeting, Specialist

Group in Economic Geology, Geological Society of Australia

Ab.~tracts, Vol 34, pp 39-40.

Leach, T M and Corbett, G J, 1994. Porphyry-related carbonate-base

metal gold systems in the southwest Pacific: characteristics, in

Proceedings PNG Geology. Exploration and Mining Conference,

(Ed: R Rogerson)

p 84-91

(The Australasian nstitute of Mining and

Metallurgy: Melbourne).

Leach, T M and Corbett, G J, 1995. Characteristics of low sulfidation

gold-copper systems in the southwest Pacific, in Proceeding.~

PACRIM 95, pp 327-332 (The Australasian Institute of Mining and


Reyes, A G, Giggenbach, W F, Saleros, J D M, Salonga, N D and

Vergara, M C, 1993. Petrology and geochemistry of Alto Peak, a

vapour-cored hydrothermal system, Lyete Province, Philippines, in

Geothermal Systems of the Philippines: Geothermics, (Eds:

D Sussman, J R Ruaya, A G Reyes and J W Hedenquist) 22:479-519.

Ronacher, E, Richards, J P, Villeneuve, M E and Johnston, M D, 2002.

Short life span of the ore forming system at the Porgera Gold Deposit

PNG: Laser 4() r/39Ar dated from roscoelite, biotite and hornblende,

Mineralium Deposita, 37:75-86.

Sillitoe, R H and Hedenquist, J W, 2003. Linkages between

volcanotectonic settings, ore-fluid compositions, and epithermal

precious metal deposits in volcanic, geothermal, and ore-forming

fluids: rulers and witnesses of processes within the earth, Special

Publication No 10. Society of EconlJmic Geol(lgist.~, pp 315-345.

Sims, D, 2000. Controls on high-grade gold distribution at Vera Nancy

Mine: Northern Queensland Exploration and Mining 2000,

AI/.~tralian In.~titute (ifGe().~cienti.~t.~ Bulletin 3/, pp 51-60.

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