golden cross (simmons, 2001)

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IntroductionLOW-SULFIDATION epithermal vein deposits are an importantsource of precious metals and are characterized by distinctzones of hydrothermal alteration minerals (e.g., Buchanan,1981; Hayba et al., 1985; Heald et al., 1987; White et al.,1995). Numerous studies of epithermal deposits have focusedon the occurrence, textures, paragenetic sequence, and for-mation of quartz veins and associated ore (e.g., Gemmel etal., 1988; Dong et al., 1995; Shimizu et al., 1998). Relativelyfew studies describe the hydrothermal alteration zones thatsurround epithermal veins (e.g., Conrad et al., 1992; Simeoneand Simmons, 1998), though general summaries exist (Lind-gren, 1933; Buchanan, 1981; White, 1981; Hayba et al., 1985;Heald et al., 1987).

In this study, we describe how the distribution of hydro-thermal alteration minerals relates to mineralized quartzveins at Golden Cross, a volcanic-hosted low-sulfidation ep-ithermal Au-Ag deposit located within the Coromandelpeninsula, New Zealand (Fig. 1).

An earlier investigation by de Ronde and Blattner (1988) de-scribed alteration from surface exposures and diamond drillcore along a single cross section (4850m N), obtained during ex-ploration of the deposit. This study extends their work by docu-menting the occurrence, paragenetic relationships, and spatialdistribution of alteration minerals exposed by open-pit and un-derground mining along three cross sections (5050, 4850 and4650m N), located 200 m apart, which transect the center andperiphery of the Empire vein system. This is complemented bya fluid inclusion study of the major veins and peripheral veinlets.Here, we compare the spatial and temporal distribution of

Hydrothermal Alteration and Hydrologic Evolution of the Golden Cross Epithermal Au-Ag Deposit, New Zealand

MARK P. SIMPSON, JEFFREY L. MAUK,Geology Department, University of Auckland, Private Bag 92019, Auckland, New Zealand

AND STUART F. SIMMONSGeothermal Institute and Geology Department, University of Auckland, Private Bag 92019, Auckland, New Zealand

AbstractGolden Cross, located in the Coromandel peninsula, New Zealand, is a classic example of a volcanic rock-

hosted, low-sulfidation epithermal gold-silver deposit. Gold and silver ore is confined to the Empire vein sys-tem and shallow-level stockwork. The veins are hosted by Miocene to early Pliocene andesites and dacites ofthe Waipupu Formation and the Waiharakeke Dacite that are unconformably overlain by the postoreWhakamoehau Andesite.

Hydrothermal alteration minerals display distinct spatial and temporal zonation around veins, as definedalong three cross sections (1,000 m long 450 m deep), located 200 m apart, which transect the Empire veinsystem. Along these sections replacement quartz, chlorite, and pyrite are ubiquitous with the abundance ofquartz veinlets increasing toward major veins. Replacement adularia envelops the Empire vein system andshallow stockwork in each section. It is coextensive with, and is variably replaced by, illite that progressivelygrades upward and laterally into a zone of interstratified illite-smectite that mantles the deposit. Replacementcalcite and minor siderite formed contemporaneously with, and also overprint, the above minerals, whereaslate barren calcite veins crosscut mineralized quartz veins. Kaolinite pyrite veinlets, together with rare, verylocal alunite, formed during late-stage hydrothermal activity.

Clay mineral zonation is well developed. Illite occurs at depth and close to the veins, grading outward andupward into illite-smectite, with minor smectite occurring ~600 m east of the Empire vein system. This over-all zonation reflects paleothermal gradients of ~150C on the periphery to >220C near the veins, consistentwith the observed Th range of 150 to 240C for fluid inclusions in quartz, platy calcite, and late barren calciteveins. Final ice-melting temperatures for inclusions mostly range from 0.0 to 1.4C, corresponding to ap-parent salinities of less than 2.4 wt percent NaCl equiv. Ice-melting temperatures combined with vapor bub-ble expansion on crushing indicate the presence of dissolved CO2 in some platy calcite and late-stage barrencalcite. The CO2 content is estimated to range from 0.35 to 3.5 wt percent, with the lower limit set by fluid in-clusion vapor expansion during crushing and the upper limit by the absence of any observable clathrates. Depthestimates based on inclusions in platy calcite suggest that the shallow-level stockwork zone formed about 100m below the paleowater table under hydrostatic conditions.

Veins and alteration minerals at Golden Cross formed in the shallow part (

alteration minerals to their distribution in geothermal systemsin order to model the hydrologic evolution of the hydrothermalsystem that formed the Golden Cross deposit.

Regional Geology and Mining History The Golden Cross deposit occurs in the Coromandel

peninsula, the central subaerial sector of a 200-km-long by35-km-wide continental volcanic arc, known as the Coroman-del volcanic zone (Skinner, 1986; Brathwaite and Skinner,1997). The Coromandel volcanic zone rests upon a block-faulted basement of Late Jurassic, low-grade graywackes ofthe Manaia Hill Group that are exposed on the northern andnorthwestern sides of the peninsula. This graywacke base-ment is unconformably overlain by Miocene to early Pliocene(ca. 184 Ma) andesites, minor dacites, and rhyolites (Adamset al., 1994). Volcanism during the Miocene (ca. 189 Ma)was dominated by eruptions of andesites and subordinatedacites of the Coromandel Group. In the late Miocene (ca. 10Ma), volcanism shifted from the west to the east-central sec-tor of the peninsula and changed from andesitic activity to therhyolitic volcanism of the Whitianga Group.

The regional structure of the Coromandel volcanic zone iscontrolled by north-northwest- and northeast- to east-north-easttrending block faults that formed in the Jurassicgraywacke during the Early Cretaceous (Skinner, 1986,1995). The Coromandel volcanic zone is bounded to the west

by the Hauraki rift, a major graben filled by up to 3 km ofPliocene to Pleistocene volcaniclastic sediments (Hochsteinand Ballance, 1993).

The Golden Cross deposit is one of over 50 separate low-sulfidation epithermal gold-silver deposits in the Haurakigoldfield (Christie and Brathwaite, 1986). From 1862 to 1952the Hauraki goldfield produced 44 million ounces (1.4 millionkg) of Au-Ag bullion (Au/Ag ratio of 1/4), with 98 percent ofthis total from just six mines (in order of decreasing produc-tion): Martha Hill, Karangahake, Thames, Golden Cross, Ko-mata, and Kapanga-Hauraki (Figs. 1 and 2). This Au-Ag oreoccurs in steeply dipping quartz veins that fill extensionalfractures hosted mainly within andesites and dacites of theCoromandel Group (Brathwaite and Pirajno, 1993).

Gold and silver in the Golden Cross deposit (Fig. 2) werefirst extracted from quartz veins of the Golden Cross 1 reef(Fig. 3) in 1895 (Bell and Fraser, 1912). Between 1895 and1917, the Golden Cross 1 reef produced 157,184 t of ore at anestimated average grade of 16 g/t Au and 54 g/t Ag (Downey,1935). Exploration drives along strike and to the north inter-sected the stockwork but failed to locate the fault-offset ex-tension of the vein (Keall et al., 1993). However, renewed ex-ploration in 1982 resulted in the discovery and delineation ofthe Empire vein system (Fig. 3) located only ~20 m north ofthe Golden Cross 1 reef. Mining of the underground Empirevein system and associated open-pit stockwork between 1991and 1998 produced 662,000 oz of Au from 5,136,300 t of ore(Au/Ag ratio of 1/3.1). The mine is now closed and the openpit filled and rehabilitated. Combined historic and recentmining produced ~750,000 oz of Au and ~2,325,000 oz of Ag.

Local Geology, Structure, and VeinsThe Golden Cross area (Fig. 2) contains volcanic rocks of

the andesitic Coromandel Group and the younger rhyoliticWhitianga Group. The Coromandel Group consists of threeunits (Fig. 3). From oldest to youngest, these are the WaipupuFormation (ca. 7.9 Ma), the Waiharakeke Dacite (ca. 7.2 Ma),and the Whakamoehau Andesite (ca. 6.76.6 Ma; Brathwaiteand Christie, 1996). The Waipupu Formation consists pre-dominantly of two-pyroxene andesitic lava flows, with local-ized intercalations of volcanic breccias, lithic-crystal tuffs, andminor epiclastic sedimentary rocks. Andesitic lava flows havephenocrysts of plagioclase, hypersthene, and augite, sup-ported in a groundmass of plagioclase laths, Fe-Ti oxides, andinterstitial glass that is either devitrified or hydrothermally al-tered (Brathwaite and Christie, 1996). In the mine, theWaipupu Formation forms the main hanging-wall unit to theEmpire fault and contains four informally named members:Monroe, Empire, Candle, and Golden Cross porphyry(Table 1; Fig. 4; Caddey et al., 1995).

The younger Waiharakeke Dacite comprises dacitic lavaflows and tuff breccia, intercalated with minor lithic-crystaltuffs, ignimbrites, and flow-banded rhyolites (Brathwaite andChristie, 1996). Dacitic lava flows have phenocrysts of plagio-clase, hypersthene, augite, hornblende, and minor quartz, setin a spherulitic glassy groundmass. The Waiharakeke Daciteforms the main footwall unit to the Empire fault and consistsof four informally named members: lower, middle, mid-dle member breccia, and upper (Table 1; Fig. 4; Caddey etal., 1995).

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0 20 km

Komata

KarangahakeMartha Hill

Golden Cross

Hauraki R

ift

Thames

37 S0

N

COROMANDELPENINSULA

Tokatea

LEGEND

Major fault

Coromandel Group

Manaia Hill Group

metagreywacke

diorite intrusives(L. Miocene)

(Jurassic)

Alluvial sediments(Quaternary)

Whitianga Group(Pliocene - Pleistocene)

Coromandel Group(L. Miocene - Pliocene)andesites and dacites

rhyolites and ignimbrites

1755E

38 S

176E

Area shownbelow

Auckland

0 200 km

FIG. 1. Geologic map of the Coromandel peninsula, showing the locationof Golden Cross and selected major epithermal gold-silver deposits (afterSkinner, 1986). The insert shows the location of the Coromandel peninsulawithin the North Island of New Zealand.

GOLDEN CROSS EPITHERMAL Au-Ag DEPOSIT, NEW ZEALAND 775

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SCALE0 2 Km

Waihi

Fault

Wai

hi F

ault

Alluvium

LEGEND

Coromandel Group

Quartz vein

Lithologic contact

FaultCoromandel Group

Waiwawa Subgroup

Omahine Subgroup

Minden Rhyolite Subgroup

Coroglen Subgroup

Kaimai Subgroup

Ohinemuri Subgroup

Whitianga Group

Whitianga Group

LITHOLOGY SYMBOLS

Martha Hill

Maratoto

Komata

Huanai

Waitekauri

GOLDENCROSS

Karangahake

WAIHI

Gladstone &Union Hill

MagneticNorth

Map

Grid

No

rth

GraceDarling

17545E 17550E

37 S20

37 S25

FIG. 2. Geologic map of the Waihi district, showing the location of the Golden Cross deposit and other importantepithermal Au-Ag deposits (after Brathwaite and Christie, 1996). The area outlined around Golden Cross is enlarged inFigure 3.

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Open pitoutline

Taranaki

Hippo

vein

(425

mRSL

)

WesternBou

ndaryfault

Fault X

IV4650m N

4800m N

4400m N

4000m N

3900

mE

3500

mE

2700

mE

5200m N

5600m N

Waipupu Formation Drill hole

Waiharakeke Dacite

Whakamoehau Andesite

LEGENDLithology Symbols

Fault

Quartz veinLithologic boundary

Inferred lithologic boundary Cross section

5050m NE510

80

55 58

78

E475

Drainage drive ( 225m RSL)~

4850m N

Magnetic

North

Mine GridNorth

Golde

nCr

oss1reef

(250m

RSL

)

Tramwayve

in(25

0mRSL

)

0 200 m

Cathe

dral

fault

Empire

Southvein

(250m

RSL

)

Empire

vein

(245

mRSL

)

West

Minefault

Slump

3100

mE

62

FIG. 3. Surface geologic map of the Golden Cross mine area, showing the location of the Empire zone, Golden Cross 1reef, and cross sections 5050, 4850, and 4650m N. Drill holes E510, E475, and the drainage drive are also shown; these areprojected onto the 5050, 4850, and 4650m N subsurface cross sections in Figure 4.

The Whakamoehau Andesite unconformably overlies boththe Waipupu Formation and the Waiharakeke Dacite, form-ing an irregular cover that dips gently to the east-northeast(Figs. 3 and 4). The Whakamoehau Andesite consists of an-desitic to dacitic lava flows with phenocrysts of seriate plagio-clase, hypersthene, augite, embayed quartz, with or withouthornblende, supported in a groundmass of plagioclase laths,minor pyroxene, Fe-Ti oxides, and interstitial glass. This unitis typically deeply weathered but unaltered in the mine area.

Prominent faults in the Golden Cross mine area are the Em-pire, Western Boundary, West mine, Pillar-Beefeater, Cathe-dral, and Fault XIV (Fig. 3; Keall et al., 1993). The Empirefault is a north-northeaststriking and steeply west-dippingfault (Fig. 4), with over 300 m of apparent reverse displace-ment (Caddey et al., 1995). The north-striking, east-dippingWestern Boundary fault forms the western limit of open-pitstockwork veins (Fig. 4), but the sense of movement andamount of displacement are uncertain. Striking subparallel tothe Western Boundary fault are the West mine and Pillar-Beefeater faults, which all have normal offsets of less than 20m (Keall et al., 1993).

The four distinct epithermal vein systems in the GoldenCross area include the Empire, Golden Cross 1 reef, Empire

South-Tramway, and Taranaki Hippo (Fig. 3). The GoldenCross 1 reef is the fault-offset southern continuation of theEmpire vein system.

Recent mining, confined to the Empire zone, extracted orefrom both open-pit and underground workings. The under-ground mine extracted ore from both veins and breccias. Al-though at least eight generations of veins occur in the under-ground workings, most of the vein ore was contained incrustiform- and colloform-banded quartz veins (Mauk et al.,1998a). The main vein was the Empire hanging-wall vein,which strikes north-northeast and dips to the west at 65 (Fig.4). Footwall veins lie east of the Empire vein. These dip tothe west at less than 50 above 200 m relative to sea level(r.s.l.), becoming steeper (70-80 west) below that level(David and Barber, 1997). Crosscutting relationships indicatethat the footwall veins formed both synchronously with, aswell as after, the Empire hanging-wall vein.

The open pit, located approximately 100 m west of the Em-pire vein system, exploited a stockwork (Fig. 5) formed be-tween the Western Boundary and Empire faults. In this paper,we use the term stockwork to describe a three-dimensionalnetwork of veinlets that are closely spaced enough so that thewhole mass can be mined, regardless of the orientation of


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TABLE 1. Stratigraphy and Lithologic Units of the Golden Cross Mine Area (Caddey et al, 1995; Brathwaite and Christie, 1996)

Formations and members1 Thickness2 (m) Lithology

Omahine SubgroupWhakamoehau Andesite >150 Two-pyroxene andesitic to dacitic lava flows with minor volcanic breccias and basal

(oa) carbonaceous horizons

Waitekauri beds 15 Lacustrine volcaniclastic sediments at the base of the Whakamoehau Andesite(wb)

Waiwawa SubgroupWaiharakeke Dacite

Upper member >80 Mixed rhyodacitic to rhyolitic pyroclastic air fall and flow deposits; coarse, unsorted(wu) and polymictic lapilli breccia up to 10 m thick predominate and are intercalated with 0.2-

to 1.5-m-thick finely bedded carbonaceous horizons

Middle member breccia 125 Coarse monomictic matrix-supported breccia with clasts of middle member dacite(wmb)

Middle member 290 Massive pyroxene dacitic lava flows with rare distinct subrounded xenoliths(wm)

Lower member >200 Mixed dacitic to rhyolitic pyroclastic air fall and unwelded pyroclastic flow deposits;(wl) poorly sorted, monomictic and polymictic matrix-supported breccia and lapilli breccia

horizons up to 55 m thick predominate and are interbedded with 4- to 12-m-thick horizons of dacitic and/or rhyodacitic tuffs

Waipupu FormationGolden Cross porphyry Feldspar-porphyritic hypabyssal dacitic intrusion characterized by abundant subrounded

(dp) xenoliths

Candle member 85 Andesitic tuff and crystal tuffs with locally developed basal breccia(cc)

Empire member >200 Massive two-pyroxene andesitic lava flows with localized intercalations of volcanic(ce) breccia, lithic tuffs, and volcaniclastic rocks that host discontinuous carbonaceous horizons

Monroe member >200 Feldspar-pyroxene porphyritic andesitic lava flows with volcanic breccia, crystal tuffs, and (cm) local intercalations of volcaniclastic rocks

1Formations and members are listed in order of youngest to oldest2Observed maximum thickness of unit within the mine area

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0 100mSCALE

0 100mSCALE

Cross section 4650m NC

oa

wm

wm

wm

wm

oa

Drainage Drive

wlwl

cm

cedp

dpdp

dp

Pillar Fault

Western Boundary Fault

Em

pire

Fau

lt

Pillar Fault

?

?

?

oaoa

wb

wmb

wl

cc

cc

cm

cm ce

ce

ce

wm

wm

wm

E510

wm

dp

dp

3300

E

2900

E

3100

E

Cross section 5050m N

A

300RSL

200RSL

100RSL

3500

E

3700

E

400RSL

100RSL

3300

E

2900

E

3100

E

300RSL

200RSL

3500

E

3700

E

400RSL

West M

ine Fault


0 100mSCALE

oawb

wmb

wm

wm E475wlwl

wm

wuwlcc

cm

cm

ce

dp

Cross section 4850m NB


Em

pire

Fau

lt

?

100RSL

3300

E

2900

E

3100

E

300RSL

200RSL

3500

E

3700

E

400RSL

Pillar Fault

SYMBOLSLITHOLOGYFault

Drill Hole

Lithologic contactInferred contactQuartz vein

This studyde Ronde and Blattner, 1988

Calcite veinStockwork veins

SAMPLES

Whakamoehau AndesiteWhakamoehau AndesiteoaWaitekauri beds

Waiharakeke Dacite

wb

upper memebermiddle member brecciamiddle memberlower member

wuwmbwmwl

Golden Cross porphyryCandle memberEmpire memberMonroe member

WaipupuFormationdpcccecm

ce

West M

ine Fault

FIG. 4. A, B, and C. West-east cross sections looking north along the 5050, 4850, and 4650m N sections, respectively, show-ing the geology, structure, and veins. Shallow-level stockwork veins are shown in the 4850m N section but are also presentin the 5050 and 4650m N sections. Sample locations along selected drill holes are also shown. Drill holes E510, E475, andthe drainage drive are projected onto the 5050, 4850 and 4650m N sections, respectively.

these veinlets. Open-pit veins strike northeast and dip steeplyto the northwest or southeast. These massive to weaklybanded veins are commonly 10 to 20 cm thick and rarely ex-ceed 0.5 m in thickness. The relative timing of vein formationhere is difficult to determine due to a lack of exposures show-ing crosscutting relationships. Some workers postulate thatthe open-pit veins formed more or less synchronously withthe underground veins (e.g., Begbie, 1997), whereas othersconclude that open-pit veins postdate most undergroundveins (e.g., Keall et al., 1993; Caddey et al., 1995).

In both underground and open-pit workings, the main Au-and Ag-bearing minerals are electrum and acanthite, withminor pyrargyrite and tetrahedrite. Sulfides consist of pyrite,marcasite, chalcopyrite, sphalerite, arsenopyrite, and galena(Simpson, C., et al., 1995).

Late-stage barren calcite veins crosscut both undergroundand open-pit quartz veins (Simmons et al., 2000). These cal-cite veins are most abundant south of the 4850m N cross sec-tion and are up to 5 m wide.

Intense hydrothermal alteration enveloping the veins,based on geophysical studies, encompasses an area of 3 by 1.5km elongated parallel to the veins (Locke and de Ronde,1987). Alteration and vein mineralization occurred between7.2 Ma, the age of the host Waiharakeke Dacite, and 6.6 Ma,the age of the overlying unaltered Whakamoehau Andesite(Brathwaite and Christie, 1996). These are consistent with40Ar/39Ar ages for replacement and vein adularia of 6.78 0.04 and 6.9 0.1 Ma, respectively (David and Barber, 1997).

Sampling and Analytical TechniquesUnderground workings, the open pit, and over 60,000 m of

drill core provided access to hydrothermally altered volcanicrocks and veins in the Empire zone of the deposit. Sampleswere collected along three cross sections (4650, 4850 and5050m N) perpendicular to the strike of the main veins (Figs. 3

and 4). Additional samples were collected from drill holes E510and E475 and the drainage drive (Fig. 3). Drill-core sampleswere taken every 20 m, on average, and were selected to repre-sent variations in volcanic units, alteration styles, and vein types.

Various petrographic techniques were used to study alter-ation and vein minerals, including hand specimen examina-tion, transmitted and reflected light microscopy of over 150polished thin sections, X-ray diffractometry (XRD), micro-probe analysis, and scanning electron microscopy (SEM).

Over 200 samples were analyzed using XRD. To avoid de-struction of clay crystal-lattice structures, rock samples werecrushed by hand into a fine homogeneous powder, using anagate mortar and pestle. Random mounts of powdered rockwere prepared by back filling an aluminum plate. Clay min-eral separates for 122 samples were prepared by dispersingthe crushed rock in distilled water, with the 15-m fractioncollected by gravitational settling and mounted onto glassslides. A additional 109 clay mineral separates of the 2-m-sized fraction were prepared using a centrifuge and a defloc-culent. A comparison of the 15- and 2-m-size fractions of29 samples showed no significant difference in the clay min-erals identified or their degree of interstratification, althoughthe 2-m fraction produced more intense and better-de-fined X-ray diffraction profiles. Oriented clay mounts were airdried, ethylene glycol solvated (glycolated), and heated at550C for 1 h before analysis. X-ray diffraction patterns werecollected using a Philips PW 1050/25 diffractometer run at 20mA and 40kV, using CuK radiation. Diffractogram data ofReynolds (1980) and Moore and Reynolds (1997) were usedto determine clay mineralogy and degree of interstratification.

SEM studies, combined with energy dispersive X-ray analy-ses (EDS), helped identify minerals, provided qualitativecompositions, and revealed textural relationships.

Fluid inclusion homogenization (Th) and final ice-melting(Tm) temperatures were measured for inclusions in veins andveinlets from underground exposures, the open-pit stock-work, and drill core peripheral to the Empire vein system.Fluid inclusion measurements were made on doubly polishedsections (~100 m thick) using a Fluid Inc.-adapted U.S.G.S.heating and freezing stage. The thermocouple was calibratedat 0.0 and 56.6C, using Syn Flinc fluid inclusion standards,with the data reproducible to 2.0C for homogenizationtemperatures and 0.2C for final ice-melting temperatures.

Hydrothermal AlterationHydrothermal alteration may be characterized by the in-

tensity and pervasiveness of the altered rock. Intensity is thedegree to which susceptible primary minerals are convertedto secondary minerals (Steiner, 1977; Browne, 1978). Perva-siveness pertains to the distribution of alteration as controlledby veinlets at one extreme and the matrix, without regard toveinlets, at the other (Guilbert and Park, 1986). In the Em-pire zone, most volcanic rocks are strongly to intensely al-tered. Intensely altered wall rocks have all of their igneousminerals (>98100%) replaced by hydrothermal minerals, ex-cept for primary quartz, trace zircon, and apatite. Strongly al-tered rocks (7098%) contain relict to unaltered plagioclaseand magnetite. In both intensely and strongly altered rocks,the igneous textures (e.g., porphyritic, spherulitic, flow) aremoderately to well preserved.


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Stockwork veins

Hard bar'

5 Meters

FIG. 5. Photograph of the open-pit stockwork veins. Veins are hosted inintensely altered volcanic rocks of the Waipupu Formation. The dark in-clined band in the center of the open pit represents a hard bar or lens ofmoderately altered rock. Scale: each bench is 5 m high.

The lateral extent of altered rock can be determined formost of the Empire zone except west of the Western Bound-ary fault where there are few drill holes. Wall rock in thehanging wall and footwall of the Empire vein system in the5050, 4850 and 4650m N sections is intensely altered in al-most every sample examined. Along drill hole E475 and thedrainage drive intensely altered rocks grade into strongly al-tered rocks at about 280 and 150 m east of the Empire veinsystem, respectively. In the latter, strongly altered rocks ex-tend for 420 m and grade into moderately altered rocks(4070% altered) at a distance of 540 m from the Empire veinsystem and persist for the remaining length of the drive.Moderately altered rock is also seen in rare isolated lenses ofrock (up to 10 m wide; Fig. 5), termed hard bars (Bell andFrazer, 1912), which occur exclusively in lava flows of theWaipupu Formation and the Waiharakeke Dacite.

Unlike the regular variations of intensity, the pervasivenessof hydrothermal alteration in the Empire zone varies greatly.In some places, such as the areas near the Empire vein sys-tem, rocks are pervasively altered over large distances. Else-where, such as in the drainage drive or the open pit, alterationis selective and clearly controlled by fractures and/or veindensity.

The occurrence and distribution of hydrothermal minerals

A variety of hydrothermal replacement and vein mineralsoccur at Golden Cross (Table 2). In the Empire zone, the mainalteration minerals are quartz, adularia, chlorite, illite, inter-stratified illite-smectite, pyrite, calcite, and kaolinite, togetherwith minor smectite, siderite, marcasite, titanite, leucoxene,and alunite. The occurrence, spatial distribution, and temporal

relationships of individual alteration and vein minerals are de-scribed below.

Quartz

Quartz occurs most conspicuously as multiple generationsof colloform-banded veins in the Empire vein system and ascryptocrystalline veins of the shallow stockwork. Quartz alsooccurs as irregular veinlets that increase in abundance towardmajor veins. However, quartz is most abundant as a replace-ment mineral of the wall rocks, occurring in every samplestudied, and typically comprises 40 to 50 vol percent of therock. Slightly lower quartz contents are present in volcanicbreccias, sporadic hard bars, and moderately altered wall rockfrom the drainage drive. Most quartz replaces the ground-mass and occurs as fine, anhedral interlocking grains inter-grown with chlorite, illite, or illite-smectite, rare adularia, andtrace kaolinite. Quartz and minor pyrite replace pyroxenephenocrysts in wall rocks adjacent to underground and open-pit veins. Cavities and vesicles are commonly rimmed by combquartz and filled by chlorite, rare illite, pyrite, and late calcite.

Adularia

Adularia is the only hydrothermal feldspar at Golden Cross,averaging 2 to 15 vol percent of the wall rocks. It is distrib-uted as a wedge-shaped zone in all three cross sections (Fig.6) that is broadly similar in distribution to illite (describedbelow). The westward limit of its distribution appears to co-incide with the Western Boundary fault, whereas its easternlimit in the 5050 and 4850m N sections is poorly constrained.In the 4850m N section adularia is absent from the middlemember breccia of the Waiharakeke Dacite. It is also absent

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TABLE 2. Alteration and Vein Minerals at the Golden Cross Deposit

Mineral Abundance Origin Occurrence Mineral Abundance Origin Occurrence

Silicates Sulfides and sulfosaltsAdularia A H W and V Acanthite R H VChlorite A H W and V Arsenopyrite R H VCristobalite R H V Bornite R H VCorrensite R H W Chalcopyrite R H V and WHalloysite R S V Electrum R H VIllite A H W and V Galena R H VInterstratified

illite-smectite A H W and V Marcasite M H V and WKaolinite M H V and W Pyragyrite R H VSmectite R H W and V Pyrite A H V and WQuartz A H W and V Pyrolusite R H V

Pyrrhotite R H V and WCarbonates Se acanthite R H VCalcite A H V and W Sphalerite R H VCalcite (Mn) M H V and WSiderite M H V and W Native

Silver R H VSulfates and phosphates Sulfur R H WAlunite R H/S? WApatite R H V OxidesBarite R H V Hematite M H/S W and VChalcanthite R S W Iron oxyhydroxides M S W and VGypsum R S W Leucoxene M H WJarosite R S W Magnetite R H VNatroalunite R H/S? W Rutile M H WTitanite M H W

Symbols: = previously reported and confirmed in present study, = previously reported, unconfirmed in present study, = newly reported from pre-sent study; abundances: A = abundant (>10%), M = minor (110%), R = rare (


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LITHOLOGY

Fault

Lithologic contactInferred lithologic

contactQuartz veinCalcite vein


Waiharakeke Dacite

wb

upper membermiddle member brecciamiddle memberlower member

wuwmbwmwl




?

Alteration zonesknown andquestioned

FIG. 6. A, B, and C. The distribution of replacement adularia in cross sections 5050, 4850, and 4650m N, respectively.Adularia is notably absent in samples from drill hole E475, which is projected 100 m north onto the 4850m N section.

in samples from drill hole E475 and is seen only in drainagedrive samples close to the Empire vein system.

Adularia is an alteration product of plagioclase phenocrysts.X-ray diffraction analyses further indicate that it is also a sig-nificant replacement mineral in the groundmass. In moder-ately altered rocks adularia appears as rare, minute rhombo-hedral crystals along cracks in weakly altered plagioclasephenocrysts, suggesting that plagioclase has altered directly toadularia. Replacement adularia has undergone variable alter-ation to illite in samples from each of the three cross sections.It is best preserved in the 5050m N section, at depth in thefootwall of the Empire vein system, where it is only slightly al-tered to illite. Elsewhere adularia is extensively replaced, ir-respective of depth, by illite and calcite. In places, this lateralteration may have completely replaced adularia, which mayexplain its absence in drill hole E475.

Adularia commonly fills open spaces as an accessory min-eral within quartz veins and veinlets and occurs as extremelyrare monomineralic veinlets (Simpson, M., et al., 1995). Ittypically forms less than 5 vol percent of the colloform-bandedquartz veins in the Empire vein system, although rare bandscontain up to 60 percent adularia (Simpson, C., et al., 1995).Here, adularia forms disseminated euhedral rhombic crystals,which are typically less than 0.05 mm across. Unlike replace-ment adularia, the adularia in open space has not altered toillite. The composition of replacement and open-space adu-laria, based on electron microprobe analysis, is close to pureKAlSi3O8.

Chlorite

Chlorite is found in almost every sample studied, forming 5to 20 percent of the rock by volume. It is a common alterationproduct of hypersthene, augite, amphibole phenocrysts, andthe groundmass but occurs rarely along cleavage planes inplagioclase. Chlorite is intergrown with fine-grained quartz inthe groundmass. Chlorite fills cavities, vesicles, and veinletsand displays variable crystal morphology, ranging from micro-crystalline to coarsely crystalline radiating masses. In veinletsit is commonly associated with, and is also cut by, later calcite.In thin section, the chlorite that replaces mafic phenocrystsand fills open spaces locally contains murky brown domainsdue to the presence of extremely fine grained illite, as deter-mined from EDS analyses.

X-ray diffraction profiles of chlorite are characterized by re-flections at ~14.2, 7.10, 4.74, and 3.55 . These were not af-fected by glycolation but are affected when heated to 550Cfor 1 h (Bailey, 1991). At Golden Cross, two types of chloriteare recognized based on their stability upon heating (cf. Har-vey and Browne, 1991). After heating, type A chlorite yieldsdiffractogram profiles that lack the 7.10, 4.74, and 3.55 re-flections, while the 14.2 peak shifts and increases slightly inmagnitude (Fig. 7). In contrast, type B chlorite is essentiallyunaffected by heating, although some structural reorganizationis evident from an increase in the magnitude of the 14.2 peak (Fig. 7). Type B chlorite occurs near veins and at depthand is most abundant in the 4850m N section (Fig. 8). Type A

782 SIMPSON ET AL.

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A

B

Air dried

Air dried

Chlorite

Chlorite Chlorite

Chlorite

Chlorite Chlorite

Chlorite

Illite

2 8 14 20

Degrees two theta

26 32

Chlorite

QuartzR.I. = 46

R.I. = 997

R.I. = 133

R.I. = 517

R.I. = 288

R.I. = 44R.I. = 149

R.I. = 133

R.I. = 72

14.2 7.10 4.74 3.55

R.I. = 561

R.I. = 50

R.I. = 342

Chlorite / Kaolinite

Chlorite / Kaolinite

Chlorite & Kaolinite

Chlorite & Kaolinite

Heated550 C,1h

Heated550 C,1h

R.I. = 151Chlorite

FIG. 7. Oriented clay mount (air dried and heated 550C for 1 h) XRD profiles for types A and B chlorite. A. Type A chlo-rite shows complete structural collapse of the 7.1, 4.7, and 3.5 reflections on heating, with some structural reorganizationindicated by a shift and increase in magnitude of the 14.2 reflection. Kaolinite may also be present in this sample. B. Amixture of type B chlorite and kaolinite. The intensity of the 14.2 and 4.7 reflections are essentially unchanged after heat-ing and represent chlorite. Heating has resulted in partial collapse of the 7.1 and 3.5 reflections, indicating the thermal de-composition of kaolinite, with the peaks persisting at 7.17 and 3.58 from chlorite. R.I. = raw intensity.


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Type B ChloriteType A Chlorite


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Fault




Waiharakeke Dacite

wb


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Waipupu Formationdpcccecm

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oundary Fault

Em

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FIG. 8. A, B, and C. The distribution of type A and B chlorites determined by XRD in cross sections 5050, 4850, and4650m N, respectively. Type B chlorite was not detected in samples from drill hole E475, which is projected 100 m northinto the 4850m N section.

chlorite is more widespread, generally occurring at shallowerlevels and distal to the main veins.

Heating chlorite to 550C results in dehydroxylation of thehydroxide sheet with the amount of collapse related to theamount of magnesium present (Bailey, 1991). However, inour samples, XRD profiles of chlorite are characterized byweak odd-order peak reflection (14.2 and 4.74 ) andstronger even-order peaks (7.10 and 3.55 ), suggesting aniron-rich composition with minor magnesium (Moore andReynolds, 1997). The temperature of dehydroxylation is alsoaffected by crystallinity and grain size. We speculate that typeA chlorite may be poorly crystalline, whereas type B chlorite,which is unaffected by heating, may be more highly crystalline.

Pyrite and marcasite

Pyrite is the most abundant sulfide at Golden Cross, occur-ring in veins, breccias, and altered country rocks up to hun-dreds of meters away from the Empire vein system. It typi-cally occurs as disseminated euhedral to anhedral


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WaiharakekeDacite

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oundary Fault

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ault


FIG. 9. A, B, and C. The distribution of replacement illite-smectite (I-S) and illite in cross sections 5050, 4850, and 4650mN. Along drill hole E475, which is projected 100 m north onto the 4850m N section, the distribution of illite and illite-smectite is patchy and, although not shown, a number of samples containing illite also contain illite-smectite. No smectitezone is shown because of the sporadic and rare occurrence of this mineral (cf. de Ronde and Blattner, 1988).

it appears near faults and at depth in drill hole E510. Calcitepartly to fully replaces phenocrysts of plagioclase, pyroxene,and amphibole. It is also disseminated in, and locally floods,the groundmass.

Late-stage calcite forms numerous massive and barren veinsand veinlets in the 4650 and 4850m N sections. These veins,which are up to 5 m wide, cut colloform-banded quartz veinsof the Empire vein system and quartz veins of the shallow

786 SIMPSON ET AL.

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TABLE 3. XRD Results for Interstratified Illite-Smectite and Illite from Golden Cross

PercentageSample no. Section Drill hole Depth (m) Size (m) 2(1) 2(2) Order (R) illite

46534 4650m N DDH21 59.8

stockwork. The calcite is coarsely crystalline with distinctrhombohedral cleavage and contains rare inclusions ofquartz, minor pyrite, and marcasite. In a few places, cavitiesin these veins are coated by a later generation of small (

788 SIMPSON ET AL.

0361-0128/98/000/000-00 $6.00 788

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ReplacementCalcite

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LITHOLOGY

Fault




Waiharakeke Dacite

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Western Boundary Fault Drain

age Drive

FIG. 11. A, B, and C. The distribution of replacement calcite in cross sections 5050, 4850, and 4650m N, respectively. Latebarren calcite veins are present in the 4850 and 4650m N sections and crosscut the underground Empire vein system. Re-placement and vein siderite (not shown) mainly occurs in the 5050m N section and has a distribution similar to calcite.

(4) inclusions homogenized by disappearance of the liquidphase, but neither inclusion type was present in samples thatwe examined.

Fluid inclusions were further classified according to the rel-ative time of their entrapment within the host mineral as pri-mary and secondary, using the criteria of Roedder (1984). Bothprimary and secondary inclusions were recognized, with mostprimary inclusions occurring in well-defined growth zones.

Two-phase liquid-rich fluid inclusions comprise over 99percent of the total inclusion population and range in sizefrom less than 5 to 50 m. Rare isolated vapor-rich inclusionswere only seen in platy calcite, which also contains abundantliquid-rich inclusions. These occurrences along with the platyhabit strongly suggest that at times calcite precipitated underboiling conditions (Browne, 1978; Bodnar et al., 1985; Sim-mons and Christenson, 1994).

Microthermometric measurements in this study were madeonly on liquid-rich inclusions that homogenized by disap-pearance of the vapor bubble. No vapor-rich inclusions weremeasured because of the difficulty in observing the homoge-nization of small amounts of liquid (cf. Sterner, 1992). Ho-mogenization temperatures were determined for 371 inclu-sions with the temperature of final ice melting measured for163 of these inclusions (Table 4).

Temperatures of homogenization

The Th of primary and secondary inclusions in quartz, platycalcite, and late barren calcite range from 94 to 260C (Fig.13). Although this range is wide, 80 percent of the fluid in-clusions in individual crystals have a narrow Th range of lessthan 30C, with most inclusion populations displaying near-constant liquid to vapor ratios. Fluid inclusions in latemedium-grained quartz of the Empire vein system homoge-nize between 159 and 240C and average about 190C

(Simpson, C., 1996). Those in surrounding quartz veinletsand in comb quartz coating platy calcite at shallower depthshave a narrow Th range of 171 to 216C and near-identicalaverage of 189C. Although the relative ages of quartz fromthe Empire vein system and peripheral veinlets are unclear,the average Th for inclusions in quartz is relatively constantover the entire ~220-m vertical interval sampled. Fluid inclu-sions in platy calcite have a Th range of 163 to 240C (mostbetween 171 and 192C) and an average of 181C.

Primary and secondary inclusions in late barren calciteshow the widest Th range from 94 to 260C, with major tem-perature peaks at 197 and 212C, and minor peaks at 109and 244C. The 212 and 244C peaks relate to secondary in-clusions in two deep calcite samples (47630 and 47631). The109C peak corresponds to primary inclusions in flat trigonalbipyramids of calcite (47634) that coat late-stage barren cal-cite; these inclusions homogenized between 94 and 122C(Begbie, 1997). In general, the homogenization temperaturesof late barren calcite veins gradually increase by 30C, withincreasing depth over a 160-m vertical interval.

Temperatures of final ice melting

The temperatures of final ice melting (Tm) for all inclusionsrange from 0.0 to 4.7C, with more than 95 percent of themeasurements between 0.0 and 1.4C (Figs. 14 and 15).For most individual fluid inclusion populations (85% of sam-ples), Tm values are within 0.2C of the average. The final ice-melting temperatures for primary and secondary inclusions inquartz of the Empire vein system and surrounding peripheralveinlets range from 0.2 to 1.4C and average about0.8C. An exception is a single quartz veinlet (46659) thathas two inclusions with Tm values of 3.4 and 4.7C. Fluidinclusions in platy calcite have a Tm range of 0.1 to 0.6Cand an average of 0.2C. Final ice-melting temperatures forinclusions in late-stage barren calcite mostly fall between 0.0and 1.4C; one sample (46518) has lower values that rangefrom 2.0 to 2.4C. The average final ice-melting tempera-tures for inclusions in quartz, platy calcite, and late barrencalcite are 0.8, 0.2, and 0.5C, respectively.

Crushing experiments

Crushing studies were undertaken to evaluate the presenceor absence of noncondensable gases (Roedder, 1984; Sasadaet al., 1986). During crushing the vapor bubble in some in-clusions expanded slightly, while others expanded to com-pletely fill the inclusion cavity, indicating the presence of anoncondensable gas, most likely CO2. Inclusions in which thevapor expanded to fill the inclusion contain 0.35 wt percentCO2 (Sasada et al., 1986).

DiscussionThe mineralogic observations and fluid inclusion data can

be used to interpret the chemical and physical conditions pre-vailing during hydrothermal alteration and vein deposition.In the following discussion we draw strongly upon knowledgeof geothermal systems where the relationship between thedistribution of fluid types and alteration patterns is known(e.g., Henley and Ellis, 1983; Hedenquist, 1990; Reyes, 1990;Simmons and Browne, 2000). The data presented here are in-tegrated in the following section to develop a model for the


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Quartz

Pyrite

Adularia

Chlorite

Titanite

Leucoxene

Au-Ag mineralization

Illite

Interstratified I-S

Smectite

Marcasite

Siderite

Calcite

Kaolinite

Alunite

MineralStage Early

Empire vein system Open pit stockwork Overprint

Late

FIG. 12. Paragenetic sequence of alteration and vein minerals in theEmpire zone of the Golden Cross deposit.

hydrothermal system and its evolution. Here, we extend theearlier work of de Ronde and Blattner (1988) by interpretingthe presence and effect of steam-heated CO2-rich waters onthe distribution and timing of hydrothermal mineral deposi-tion at Golden Cross.

Composition of inclusion fluids

The liquid trapped in fluid inclusions provides the only directsamples of waters from which the quartz and calcite veins weredeposited. The composition of the trapped liquid can be esti-mated from the temperature of final ice melting. However,these estimates are complicated by the possibility that the finalice-melting temperature could be affected by the presence ofdissolved gases (dominantly CO2), dissolved salts (expressedas wt % NaCl equiv), or a combination of the two (Hedenquistand Henley, 1985). Assuming that the trapped liquid con-sists only of water and dissolved NaCl, the final ice-melting

temperature range (0.0 to 4.7C) corresponds to salinitiesof up to 7.4 wt percent NaCl equiv (Bodnar, 1993). However,over 95 percent of the inclusions have apparent salinities ofless than 2.4 wt percent NaCl equiv (Fig 15).

Hedenquist and Henley (1985) show that the principalsolute contributing to the freezing point depression of inclu-sion fluids from several New Zealand geothermal systems isaqueous CO2. As gas hydrates were not seen in these samples,the maximum possible concentration of aqueous CO2 is 3.5wt percent, which corresponds to a Tm of 1.5C (Hedenquistand Henley, 1985). Crushing studies of calcite and platy cal-cite indicate the presence of 0.35 wt percent CO2, whichwould depress the Tm by at least 0.15C. The Tm values of0.2 to 1.4C suggest that up to 3.3 wt percent CO2 couldbe present in a very low salinity liquid (Fig 15).

A few of the quartz-hosted fluid inclusions from this study andthat of de Ronde and Blattner (1988) have apparent salinities

790 SIMPSON ET AL.

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TABLE 4. Fluid Inclusion Data for Quartz, Platy Calcite, and Late Massive Calcite from the Empire Zone of the Golden Cross Deposit

Wt % Sample no. Mineral Type Th range (C) n Avg Tm range (C) n Avg NaCl equiv1 Comments

Drill core46655a Calcite S 193211 (10) 199 0.5 to 1.1 (5) 0.9 0.9 to 1.946655a Quartz S 171181 (4) 178 0.6 (2) 0.6 1.146656 Calcite S 171185 (17) 182 0.4 to 0.5 (7) 0.5 0.70.946657 Calcite S 199203 (9) 200 0.6 to 0.8 (5) 0.7 1.11.446658 Calcite S 190195 (7) 193 0.8 to 1.0 (4) 0.8 1.41.746659 Quartz S 178200 (5) 186 0.5 to 4.7 (4) 2.3 0.97.5 Tm = 0.5, 0.5, 3.4, 4.746661 Quartz S 175216 (7) 198 0.7 to 0.8 (4) 0.8 1.21.446662a Calcite S 191198 (5) 197 0.3 to 0.4 (2) 0.3 0.50.7 S inclusions along 246662a Calcite S 155174 (6) 160 0.4 to 0.5 (4) 0.4 0.70.9 different healed fractures46663 Calcite P? 164197 (8) 193 0.4 to 0.6 (5) 0.5 0.7 to 1.146664 Calcite P?S 185198 (13) 195 0.3 to 0.5 (4) 0.9 0.5 to 0.946665 Calcite S 163166 (7) 165 0.8 to 1.0 (5) 0.9 1.4 to 1.6

Open pit46666 Platy calcite P?S 175202 (4) 185 0.3 to 0.4 (3) 0.3 0.50.746667a Platy calcite P 180189 (22) 181 0.0 to 0.3 (12) 0.2 0.00.5 Coexisting L>V and V >L46667a Platy calcite S 165177 (4) 171 ND46667b Quartz S 191204 (11) 193 0.0 to 0.2 (4) 0.1 0.00.7 Inclusions measured for46667b Quartz S 137170 (16) 156 ND 2 different quartz crystals46668 Platy calcite P?S 163190 (6) 179 0.2 to 0.3 (4) 0.2 0.40.5 Coexisting L>V and V>L46669a Calcite P 191195 (17) 192 0.5 to 0.7 (3) 0.6 0.91.1 Inclusions in growth zone46669b Platy calcite P 191192 (3) 192 0.1 to 0.2 (2) 0.1 0.20.446670 Quartz S 171188 (6) 179 0.2 to 0.6 (2) 0.4 0.41.1

Underground464942 Quartz (EVS) S 171188 (6) 180 0.7 to 0.8 (2) 0.7 1.21.4465002 Quartz (EVS) P/S 176200 (20) 190 0.2 to 0.8 (8) 0.5 0.41.4465012 Quartz (EVS) P/S 175230 (13) 200 1.2 to 1.4 (5) 1.3 2.12.4465022 Quartz (EVS) P/S 166204 (18) 183 0.3 to 0.8 (8) 0.6 0.51.4465052 Quartz (EVS) S 159180 (5) 172 1.0 to 1.2 (3) 1.1 1.72.1465152 Quartz (EVS) P 168202 (12) 186 0.7 to 1.0 (9) 0.8 1.21.7465182 Calcite P 191202 (11) 197 2.0 to 2.4 (5) 2.2 3.44.0465212 Quartz (EVS) P 185240 (7) 217 0.9 to 1.0 (2) 0.9 1.61.746671 Calcite S 186198 (10) 192 0.2 to 0.4 (4) 0.3 0.40.746672 Platy calcite P? 185240 (7) 217 0.5 to 0.6 (2) 0.5 0.91.1476303 Calcite S 190260 (30) 230 0.0 to 0.2 (17) 0.1 0.00.4476323 Calcite P 186215 (36) 210 0.2 to 0.6 (10) 0.4 0.41.1476343 Flatten calcite P 94122 (9) 109 0.0 to 0.1 (7) 0.0 0.00.2

Notes: Th = homogenization temperature, Tm = ice-melting temperature, P = primary fluid inclusions, S = secondary fluid inclusions, EVS = Empire veinsystem, L = liquid, V = vapor, L > V = liquid-rich inclusions, V > L = vapor-rich inclusions, ND = not determined

1 Wt percent NaCl equiv calculated from Bodnar (1993)2 Data from Simpson, C. (1996)3 Data from Begbie (1997)

of 5.6 to 14.2 wt percent NaCl equiv. Although these unusu-ally saline inclusion fluids could have derived from a magma(Simmons, 1995), their rare occurrence suggests that theymore likely formed by local open-system boiling to near dry-ness (Simmons and Browne, 1997; Scott and Watanabe, 1998).

Composition of hydrothermal waters from alteration mineral assemblages

The hydrothermal alteration mineral assemblage of quartz,adularia, chlorite, illite, calcite, and pyrite in the Empire zonereflects formation from a near-neutral to weakly alkaline pHchloride water (Table 5). This water is the dominant fluid typein many active hydrothermal systems. It originates fromdeeply circulating meteoric water that is heated and then as-cends from several kilometers depth (Henley and Ellis,1983). The abundance of calcite and the absence of calciumzeolites (e.g., laumontite and wairakite) further indicate thatthese chloride waters contained appreciable concentrationsof dissolved CO2 (Browne and Ellis, 1970), up to 3.5 wt per-cent as evidenced from fluid inclusion studies. Gas loss

(mainly CO2 and H2S) and cooling due to boiling led to thedirect deposition of platy calcite and adularia in veins and theformation of replacement adularia (Browne and Ellis, 1970;Simmons and Browne, 2000). These minerals all indicatehigh permeability (Browne and Ellis, 1970; Browne, 1978;Simmons and Browne, 2000).

The mineral assemblage of replacement calcite, siderite,smectite, illite-smectite, and kaolinite formed from CO2-richsteam-heated water (Table 5) similar to that at the Broad-lands-Ohaaki geothermal system (Hedenquist, 1990; Sim-mons and Browne, 2000; Simmons et al., 2000). At Broad-lands-Ohaaki, peripheral CO2-rich steam-heated wateroriginates through deep boiling of chloride waters from whichCO2 gas partitions into steam. Condensation of this steam,and absorption of the CO2 into cool ground waters at shallowdepths and on the margins of the system, generates a CO2-rich steam-heated water (Hedenquist and Stewart, 1985),with CO2 concentrations up to 2.2 wt percent (Hedenquist,1990). These waters then react with the rocks to generate theassemblage of low-temperature clays and carbonates (Heden-quist, 1990; Simmons and Browne, 2000). Furthermore, ifthe CO2-rich waters are close to calcite saturation, slight heat-ing will result in the precipitation of calcite due to its retro-grade solubility (Simmons and Christenson, 1994).

A third water type, steam-heated acid-sulfate water (Table5), commonly occurs above the boiling upflow zone andforms in the vadose zone by oxidation of H2S gas (Schoen etal., 1974; Rye et al., 1992). This steam-heated acid-sulfatewater reacts with host rocks to produce kaolinite, with cristo-balite, alunite, pyrite, and native sulfur (Schoen et al., 1974).At Golden Cross, abundant kaolinite, together with pyrite,was exposed throughout the open pit. Alunite and native sul-fur, however, were observed at only one location, perhapscaused by a very local and shallow, steam-heated acid-sulfatewater. Most of this shallow alteration style may have beeneroded from the top of the system prior to eruption of theWhakamoehau Andesite.


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10

50 100 150 200 250 300

20

30

40

Temperature of homogenization (C)

Num

ber

of i

nclu

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s

Platy Calcite (n = 46)

Quartz (n = 130)

Calcite (n = 195)

FIG. 13. Fluid inclusion histogram plot of homogenization temperaturevs. number of inclusions for quartz, platy calcite, and late barren calcite fromthe Empire zone.

0.0-0.8-1.6-2.4-3.2-4.0-4.8

14

10

6

2

Temperature of final ice melting (C)

Num

ber

of i

nclu

sion

s


Quartz (n = 53)

Calcite (n = 87)

FIG. 14. Fluid inclusion histogram plot of final ice-melting temperaturevs. number of inclusions for quartz, platy calcite, and late barren calcite fromthe Empire zone.

Tem

per

atur

e of

fina

l ice

mel

ting

(C

)

Wt.

% N

aCl e

qui

vale

nt

Temperature of homogenization (C)

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

50 100 150 200 250 300

1.0

0.0

2.0

3.0

4.0

5.0

6.0

7.0

Calcite (n = 87)


Quartz (n = 53)

FIG. 15. Fluid inclusion plot of Th vs. Tm for individual inclusions inquartz, platy calcite, and late barren calcite from the Empire zone, GoldenCross deposit. The shaded area represents Tm values that can be accountedfor by up to 3.5 wt percent CO2 (the max concentration of CO2 beforeclathrate forms).

Temperatures of vein formation and alteration

Most fluid inclusions in quartz and calcite veins at GoldenCross homogenize between 160 to 240C. The hotter valuesmost likely represent the temperatures during main-stage oredeposition, with the Empire vein system forming at tempera-tures between 190 and 240C (Simpson, C., 1996). Late cal-cite veins formed between 160 and 220C (Simpson, M.,1996; Simmons et al., 2000).

There is a well-developed zonation of clay minerals, with il-lite at depth and close to the veins, that grades outward andupward into illite-smectite, with smectite occurring ~600 meast of the Empire vein system. This scale of zonation is sim-ilar to that in geothermal systems where clay mineralogy cor-relates with temperature; illite forms at temperatures above220C, illite-smectite between 150 and 220C, and smectitebelow 150C (e.g., Steiner, 1968; Reyes, 1990). Thus, theoverall clay zonation at Golden Cross reflects temperaturegradients (~150 to >220C) that extend over ~600 m, fromthe periphery to the center of the Empire vein system.

However, variations in clay mineralogy occur in drill coreand underground workings over the scale of meters to tens ofmeters, with more crystalline clay minerals associated withfractures. Heterogeneities also occur in the hand-sample,thin-section, and even TEM scales, with pore-filling clay min-erals typically more crystalline or in the case of illite-smectite,have greater amounts of illite than the matrix-replacing clayminerals (Tillick et al., 1999, 2001).

To help evaluate the relative importance of temperature incontrolling clay occurrences, we modeled the time requiredfor rocks with an initial temperature of 140C to conductivelyheat up around a fracture filled with 230C fluid (Turcotteand Schubert, 1982). This is a conservative case, as conductiveheating is slower than convective heating. Most geothermal

systems have a time span of 10,000 to 100,000 yr or longer(e.g., Browne, 1978), but this time period would result inheating of the rocks to 220C or more over most of the areaaround the Empire vein system (Fig. 16). This strongly sug-gests that temperature differences alone cannot account forthe variations in clay mineralogy that are observed over thetens of meters to hand-sample to thin-section scales.

Studies elsewhere show that clay mineral transitions clearlyreflect changes in chemistry (e.g., Turner and Fishman, 1991)or intensity of water-rock interactions (e.g., Essene and Pea-cor, 1995; Li et al., 1997) and do not depend on temperature.At Golden Cross, we believe the illite-smectite in the middlemember breccia and in localized hard bars, volcanic breccias,and pyroclastic rocks at depth are examples of such effects. In

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TABLE 5. Comparison of Hydrothermal Water Types and Hydrothermal Alteration

Alkali chloride waters Steam-heated CO2-rich waters Steam-heated acid-sulfate waters

Alteration minerals1,2,3 Adularia, quartz, chlorite, platy calcite, Calcite, replacement calcite, siderite, Kaolinite, alunite, native sulfur, replacement calcite, illite, interstratified smectite, interstratified I-S, kaolinite opal, pyriteI-S, titanite, pyrite, marcasite

Source1,2,3 Deep circulating meteoric ground water Condensation of steam and gas into Oxidation of H2S in the vadose marginal and shallow ground water zone

Chloride (ppm)1 4001,800 low

these instances, the primary control on clay mineral transi-tions is best interpreted in terms of permeability and there-fore clay minerals here may not accurately reflect paleotem-peratures (Simpson et al., 1998; Tillick et al., 1999).

Depth of mineralization and position of the paleowater table

The depth of the deposit has been estimated using alter-ation mineralogy and fluid inclusion data. Low-temperaturealteration minerals at Golden Cross are similar to thoseformed in the shallow parts (

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A

B

Openpitstockwork

Paleowater table~440 m r.s.l.

5050m N

4850m N

4650m N

100m

100m

50m

Erosion surface

Erosion surface

Paleowater table~440 m r.s.l.

5050m N

4850m N

4650m N

N

I

I

I

II

II

II

Au-

Ag

min

eral

izat

ion

Zon

e of

boi

ling

Steam-heated CO waters2

Direction of fluid flow

Alkali chloride waters

LEGEND

220

220

220

220

Empire vein system

220

FIG. 18. Schematic diagram depicting the evolution of hydrothermal activity at the Golden Cross deposit. A. Mineraliza-tion occurred in two stages during which time the Empire vein system (I) and open stockwork formed (II). Wall-rock alter-ation adjacent to the veins is intense and characterized by the assemblage of quartz, adularia, chlorite, illite, calcite, andpyrite. Contemporaneously, on the margins, steam-heated CO2 rich produced a carapace of illite-smectite. The 220Cisotherm is based on the boundary between illite and illite-smectite. B. Cooling resulted in the collapse of the convecting hy-drothermal system and permitted peripheral steam-heated CO2-rich waters to invade the former zone of fluid upflow. Steam-heated CO2-rich waters that descended fractures heated to produce late barren calcite veins. These waters also reacted withthe wall rock to generate the late-stage clay-carbonate overprint. Local steam-heated acid-sulfate waters (not shown) pro-duced kaolinite, pyrite, and alunite at shallow levels.

in the south and less dominant in the north. Although latekaolinite plus pyrite veinlets may have formed from thesteam-heated CO2-rich waters, it is most likely that theseveinlets formed from descending steam-heated acid-sulfatewaters (de Ronde and Blattner, 1988), as supported by thepresence of local alunite and native sulfur at shallow levels.

Postmineralization emplacement of the Whakamoehau An-desite has effectively prevented erosion, thereby preservingmost of the Golden Cross deposit.

AcknowledgmentsThe authors gratefully acknowledge Coeur Gold New

Zealand Ltd. for providing unlimited access to the GoldenCross mine and permission to publish this manuscript. Wethank Andrew Purvis, Alan McOnie, Peter White, PaulRutherford, Peter Keall, Steve Barbar, Valdimer David, War-ren Cook, and Peter Mitchell for access to unpublished geo-logical cross sections and discussions in the field. Chris Simp-son and Mike Begbie provided some of the fluid inclusiondata. Bob Seal provided sulfur isotope analyses of alunite andpyrite and Manfred Hochstein helped with heat flow calcula-tion. We thank Chris Simpson, Mike Begbie, Paul Hoskin,Wolfgang Irber, Rene Wagner, and David Tillick for helpfuldiscussions.

Financial support was provided by Coeur Gold NewZealand Ltd., the Foundation of Research, Science and Tech-nology (FRST), and the University of Auckland. Finally wethank Patrick Browne, Jeff Hedenquist, Don Hudson, NoelWhite, and an anonymous referee for constructive reviewsthat have improved the paper. August 5, 2000; February 12, 2001

REFERENCESAdams, C.J., Graham, I.J., Seward, D., and Skinner, D.N.B., 1994, Geo-

chronological and geochemical evolution of the late Cenozoic volcanism inthe Coromandel peninsula, New Zealand: New Zealand Journal of Geologyand Geophysics, v. 37, p. 359379.

Bailey, S.W., 1991, Chlorites: Structure and crystal chemistry: Reviews inMineralogy, v. 19, p. 347403.

Begbie, M.J., 1997, Structural aspects of the Golden Cross epithermal veinsystem, Waihi, New Zealand: Unpublished M.Sc. thesis, Auckland, Univer-sity of Auckland, 128 p.

Bell, J.M., and Fraser, C., 1912, The geology of the Waihi-Tairua subdivision,Hauraki, Auckland, New Zealand: New Zealand Geological Survey Bul-letin, v. 15, 193 p.

Bodnar, R.J., 1993, Revised equation and table for determining the freezingpoint depression of H2O-NaCl solutions: Geochimica et CosmochimicaActa, v. 57, p. 683684.

Bodnar, R.J, Reynolds, T.J., and Kuehn, C.A., 1985, Fluid inclusions system-atics in epithermal system: Reviews in Economic Geology, v. 2, p. 7398.

Brathwaite, R.L., and Christie, A.B., 1996, Geology of the Waihi area, scale1:50 000: Institute of Geological and Nuclear Sciences, Geological Map 21.

Brathwaite, R.L., and Pirajno, F., 1993, Metallogenic map of New Zealand:Institute of Geological and Nuclear Sciences Monograph 3, 215 p.

Brathwaite, R.L., and Skinner, D.N.B., 1997, The Coromandel epithermalgold-silver province: A result of collision of the Northland and Colville vol-canic arcs in northern New Zealand: Window on New Zealand Mineralsand Mining Conference, p. 111117.

Browne, P.R.L., 1978, Hydrothermal alteration in active geothermal fields:Annual Reviews in Earth and Planetary Sciences, v. 6, p. 229250.

Browne, P.R.L., and Ellis, A.J., 1970, The Ohaki-Broadlands hydrothermalarea, New Zealand: Mineralogy and related geochemistry: American Jour-nal of Science, v. 269, p. 97215.

Buchanan, L.J., 1981, Precious metal deposits associated with volcanic envi-ronments in the southwest: Arizona Geological Society Digest, v. 14, p.237262.

Caddey, S.W., McOnie, A.W., and Rutherford, P.G., 1995, Volcanic stratigraphy,structure and controls on mineralization, Golden Cross mine, New Zealand:Pacrim Congress 1995, Auckland, New Zealand, November 19-22, 1995,Australasian Institute of Mining and Metallurgy, Proceedings, p. 9398.

Christie, A.B., and Brathwaite, R.L., 1986, Epithermal gold-silver and por-phyry copper deposits of the Hauraki goldfieldsa review: Berlin-Stuttgrat, Gerbruder Borntrager, Monograph Series on Mineral Deposits,v. 26, p. 129145.

Conrad, M.E., Petersen, U., and ONeil, J.R., 1992, Evolution of an Au-Aghydrothermal system: The Tayoltita mine, Durango, Mexico: ECONOMICGEOLOGY, v. 87, p. 14511474.

David, V., and Barber, S., 1997, An integrated underground mining approachto the structural complexity of the Empire vein system, Golden Cross mine,New Zealand: Australasian Institute Mining and Metallurgy Annual Con-ference, Proceedings, p.153163.

de Ronde, C.E.J., and Blattner, P., 1988, Hydrothermal alteration, stable iso-topes, and fluid inclusions of the Golden Cross epithermal gold deposit,Waihi, New Zealand: ECONOMIC GEOLOGY, v. 83, p. 895917.

Dong, G., Morrison, G., and Jaireth, S., 1995, Quartz textures in epithermalveins, Queensland: Classification, origin, and implication: ECONOMIC GE-OLOGY, v. 90, p. 18411856.

Downey, J.F., 1935, Gold mines of the Hauraki district: Wellington, NewZealand, Government Printer, 315 p.

Essene, E.J., and Peacor, D.R., 1995, Clay mineral thermometrya criticalperspective: Clays and Clay Minerals, v. 43, p. 540553.

Gemmel, J.B., Simmons, S.F., and Zantop, H., 1988, The St. Nio silver-lead-zinc vein, Fresnillo district, Zacatecas, Mexico: Pt. I. Structure, vein stratig-raphy, and mineralogy: ECONOMIC GEOLOGY, v. 83, p. 15971618.

Guilbert, and Park, 1986, The geology of ore deposits: New York, Freeman,985 p.

Haas, J.L., 1971, The effects of salinity on the maximum thermal gradient ofa hydrothermal system at hydrostatic pressure: ECONOMIC GEOLOGY, v. 66,p. 940946.

Harvey, C.C., and Browne, P.R.L., 1991, Mixed-layer clay geothermometryin the Wairakei geothermal field, New Zealand: Clays and Clay Minerals, v.6, p. 614621.

Hayba, D.O., Bethke, P.M., and Foley, N.K., 1985, Geologic, mineralogic,and geochemical characteristics of volcanic-hosted epithermal precious-metal deposits: Reviews in Economic Geology, v. 2, p. 129167.

Heald, P., Foley, N.K., and Hayba, D.O., 1987, Comparative anatomy of vol-canic-hosted epithermal deposits: Acid-sulfate and adularia-sericite types:ECONOMIC GEOLOGY, v. 82, p. 126.

Hedenquist, J.W., 1990, The thermal and geochemical structure of theBroadlands-Ohaaki geothermal system, New Zealand: Geothermics, v. 19,p. 151185.

Hedenquist, J.W., and Browne, P.R.L., 1989, The evolution of the Waiotapugeothermal system, New Zealand, based on the chemical and isotopiccomposition of its fluids, minerals and rocks: Geochimica et CosmochimicaActa, v. 53, p. 22352257.

Hedenquist, J.W., and Henley, R.E., 1985, The importance of CO2 on thefreezing point measurements of fluid inclusions: Evidence from active ge-othermal systems and implications of epithermal ore deposition: ECO-NOMIC GEOLOGY, v. 80, p. 13791408.

Hedenquist, J.W., and Stewart, M.K., 1985, Natural CO2-rich steam-heatedwaters at Broadlands, New Zealand: Their chemistry, distribution and cor-rosive nature: Geothermal Resources Council Annual Meeting, Transac-tions, v. 9, p. 245250.

Henley, R.H., and Hedenquist, J.W., 1986, Introduction to the geochemistryof active and fossil geothermal systems: Berlin-Stuttgart, Gerbruder Born-trager, Monograph Series Mineral Deposits, v. 26, p. 122.

Henley, R.W., and Ellis A.J., 1983, Geothermal systems ancient and modern:A geochemical review: Earth Science Reviews, v. 19, p. 150.

Hinkley, D.N., 1963, Variability in crystallinity values among the kaolin de-posits of the coastal plans of Georgia and South Carolina, in Ingerson, E.,ed., Clay and clay minerals: New York, Pergamon Press, p. 229235.

Hochstein, M.P., and Ballance, P.F., 1993, Hauraki rift: A young active, intra-continental rift in back-arc setting, in Ballance, P.F., ed., South Pacific sed-imentary basins. Sedimentary basins of the world 2: Elsevier Science Pub-lishers, p. 295305.

Hoskin, P.W.O., Seal, R.R., and Mauk, J.L., 1994, Sulphur isotope geochem-istry of FeS2 at the Golden Cross mine, Coromandel peninsula, NewZealand: Geological Society of New Zealand Miscellaneous Publication80A, 93 p.


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Inoue, A., and Utada, M., 1983, Further investigations of a conversion seriesof dioctahedral mica/smectites in the Shinzan hydrothermal alteration area,northeast Japan: Clays and Clay Minerals, v. 31, p. 401412.

Keall, P.C., Cook, W.C., Mathews, S.J., and Purvis, A.H., 1993, The geologyof the Golden Cross orebody: Complex veining and evolving mining re-sponses: New Zealand Branch of the Australian Institute of Mining andMetallurgy Annual Conference, 27th, Wellington, New Zealand, August2427, 1993, Proceedings, p. 143160.

Li, G., Peacor, D.R., and Coombs, D.S., 1997, Transformation of smectite toillite in bentonite and associated sediments from Kaka point, New Zealand:Contrast in rate and mechanism: Clay and Clay Minerals, v. 45, p. 5467.

Lindgren, W., 1933, Mineral deposits, 4th ed.: New York, McGraw Hill, 930p.

Locke, C., and de Ronde, C.E.J., 1987, Delineation of gold-bearing hy-drothermally altered rocks using gravity dataa New Zealand example:Geoexploration, v. 24, p. 471481.

Mauk, J.L., Begbie, M.J., and Sprli, K.B., 1998a, Structural setting of theEmpire vein system of the Golden Cross deposit, New Zealand: Aus-tralasian Institute of Mining and Metallurgy Annual Conference, 31st, Pro-ceedings, p. 2434.

Mauk, J.L., Hoskin, P.W.O., and Seal, R.R., II, 1998b, Morphology of pyriteand marcasite at the Golden Cross mine, New Zealand, in Arehart, G, B.,and Hulston, J.R., eds., Water-rock interaction 9: Balkema, p. 557560.

Moore, D.M., and Reynolds, R.C., 1997, XRD and the identification andanalysis of clay minerals, 2nd ed.: Oxford University Press, 378 p.

Reyes, A.G., 1990, Petrology of Philippine geothermal systems and the ap-plication of alteration mineralogy to their assessment: Journal of Volcanol-ogy and Geothermal Research, v. 43, p. 279309.

Reynolds, R.C., 1980, Interstratified clay minerals, in Brindley, G, W., andBrown, G., eds., Crystal structures of clay minerals and their X-ray identi-fication: Mineralogical Society of London, p. 249303.

Roedder, E., 1984, Fluid inclusions: Reviews in Mineralogy, v. 12, 644 p.Rye, R.O., Bethke, P.M., and Wasserman, M.D., 1992, The stable isotope

geochemistry of acid sulfate alteration: ECONOMIC GEOLOGY, v. 87, p.225262.

Sasada, M., Roedder, E., and Belkin, H.E., 1986, Fluid inclusions from drillhole DW-5, Hohi geothermal area Japan: Evidence of boiling and proce-dure for estimating CO2 content: Journal of Volcanology and GeothermalResearch, v. 30, p. 231251.

Schoen, R., White, D.E., and Hemley, J.J., 1973, Argillization by descendingacid at the Steamboat Springs, Nevada: Clay and Clay Minerals, v. 22, p.122.

Scott, A.M., and Watanabe, Y., 1998, Extreme boiling model for variablesalinity of the Hokko low-sulfidation epithermal Au prospect, southwesternHokkaido, Japan: Mineralium Deposita, v. 33, p. 568578.

Shimizu, T., Matsueda, H., Ishiyama, D., and Matsubaya, O., 1998, Genesisof epithermal Au-Ag mineralization of the Koryu mine, Hokkaido, Japan:ECONOMIC GEOLOGY, v. 93, p. 303325.

Simeone, R., and Simmons, S.F., 1998, Mineralogical and fluid inclusionstudies of low-sulfidation epithermal veins at Osilo (Sardinia), Italy: Miner-alium Deposita, v. 34, p. 705717.

Simmons, S.F., 1995, Magmatic contributions to low-sulfidation epithermaldeposits: Magmas, fluids and ore deposits: Mineralogical Society of CanadaShort Course Notes, v. 23, p. 455477.

Simmons, S.F., and Browne, P.R.L., 1997, Saline fluid inclusions in sphaleritefrom the Broadlands-Ohaaki geothermal system: A coincidental trapping offluids being boiled toward dryness: ECONOMIC GEOLOGY, v. 92, p. 485489.

2000, Hydrothermal minerals and precious metals in the Broadlands-Ohaaki geothermal system: Implications for understanding low-sulfidationepithermal environments: ECONOMIC GEOLOGY, v. 95, p. 9711000.

Simmons, S.F., and Christenson, B.W., 1994, Origins of calcite in a boilinggeothermal system: American Journal of Science, v. 294, p. 361400.

Simmons, S.F., Arehart, G., Simpson, M.P., and Mauk, J, L., 2000, Origin ofmassive calcite veins in the Golden Cross, low-sulfidation epithermal Au-Ag deposit, New Zealand: ECONOMIC GEOLOGY, v. 95, p. 99112.

Simpson, C.R.J., 1996, The formation of banded epithermal quartz veins atthe Golden Cross mine, Waihi, New Zealand: Unpublished M.Sc.thesis,Auckland, University of Auckland, 120 p.

Simpson, C.R.J., Mauk, J.L., and Arhart, G., 1995, The formation of bandedepithermal quartz veins at the Golden Cross mine, Waihi, New Zealand:Pacrim Congress 1995, Auckland, New Zealand, November 1922, Aus-tralasian Institute of Mining and Metallurgy, Proceedings, p. 545550.

Simpson, M.P., 1996, Hydrothermal alteration in the Empire zone of theGolden Cross epithermal Au-Ag deposit, Waihi, New Zealand: Unpub-lished M.Sc. thesis, Auckland, University of Auckland, 133 p.

Simpson, M.P., Simmons, S.F., Mauk, J.L., and McOnie, A., 1995, The dis-tribution of hydrothermal alteration minerals at the Golden Cross epither-mal Au-Ag deposit, Waihi, New Zealand: Pacrim Congress 1995, Auckland,New Zealand, November 19-22, Australasian Institute of Mining and Met-allurgy, Proceedings, p. 551556.

Simpson, M.P., Simmons, S.F., and Mauk, J.L., 1998, The occurrence, distri-bution and XRD properties of hydrothermal clays at the Gold Cross ep-ithermal Au-Ag deposit, New Zealand: New Zealand Geothermal Work-shop, 20th, Auckland, New Zealand, Proceedings, p. 163168.

Skinner, D.N.B., 1986, Neogene volcanism of the Hauraki volcanic region:Royal Society of New Zealand Bulletin, v. 23, p. 2047.

1995, Geology of the Mercury Bay area: Institute of Geological and Nu-clear Sciences Geological Map 17, scale 1:50,000.

Steiner, A., 1968, Clay minerals in hydrothermally altered rocks at Wairakei,New Zealand: Clays and Clay Minerals, v. 16, p. 193213.

1977, The Wairakei geothermal area, North Island, New Zealand: NewZealand Geological Survey Bulletin 90, 136 p.

Sterner, S.M., 1992, Homogenization of fluid inclusions to the vapor phase:The apparent homogenization phenomenon: ECONOMIC GEOLOGY, v. 87, p.16161623.

Tillick, D.A., Mauk, J.L., and Peacor, D.R., 1999, SEM and TEM investiga-tion of a dioctahedral clay mineral series in the Golden Cross epithermaldeposit, New Zealand: Preliminary results: Australasian Institute of Miningand Metallurgy Annual Conference, 32nd, Proceedings, p. 131140.

Tillick, D.A., Peacor, D.R., and Mauk, J.L., 2001, Genesis of dioctahedralphyllosilicates during hydrothermal alteration of volcanic rocks: I. TheGolden Cross epithermal ore deposit, New Zealand: Clay and Clay Miner-als, v. 49, p. 126140.

Turcotte, D.L., and Schubert, G., 1982, Geodynamics: Applications of con-tinuum physics to geological problems: New York, Wiley, 450 p.

Turner, C.E., and Fishman, N.S., 1991, Jurassic Lake Toodichi: A large al-kaline, saline lake, Morrison Formation, eastern Colorado Plateau: Geo-logical Society America Bulletin, v. 103, p. 538558.

Watanabe, T., 1981, Identification of illite/montmorillonite interstratifica-tions by X-ray diffraction: Journal of the Mineralogical Society Japan, Spe-cial Issue, v. 15, p. 3241 (in Japanese).

White, D.E., 1981, Active geothermal systems and hydrothermal ore de-posits: ECONOMIC GEOLOGY 75TH ANNIVERSARY VOLUME, p. 392423.

White, N.C., Leake, M.J., McCaughey, S.N., and Parris, B.W., 1995, Ep-ithermal gold deposits of the southwest Pacific: Journal of Geochemical Ex-ploration, v. 54, p. 87136.

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golden cross (simmons, 2001)

Documents

cross sections

major veins

described alteration

university of auckland

new zealandmark

single cross section

mation of quartz veins

empire vein system