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IntroductionTHE GOLDEN Cross Au-Ag deposit is a classic example of avolcanic-hosted, low-sulfidation epithermal vein deposit.From 1989 to 1997, when mining ceased, more than 700,000

oz of gold were produced. Hydrothermal alteration patternsand fluid inclusion data show that the deposit formed in theshallow part of a hydrothermal system at less than 500 mdepth, where boiling-upflow conditions existed (de Ronde andBlattner, 1988; Simpson, C., et al., 1995; Simpson, M., et al.,1995). Late barren calcite veins are a distinctive feature of thedeposit and are most prominent in the Empire zone, whereore was mined by underground methods. The abundance of

Origin of Massive Calcite Veins in the Golden Cross Low-Sulfidation, Epithermal Au-Ag Deposit, New Zealand

STUART F. SIMMONS,

Geothermal Institute and Geology Department, University of Auckland, Private Bag, 92019, Auckland, New Zealand

GREG AREHART,*Institute of Geological and Nuclear Sciences, Wairakei Research Centre, Private Bag 2000, Taupo, New Zealand

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

AND JEFFREY L. MAUKGeology Department, University of Auckland, Private Bag, 92019, Auckland, New Zealand

AbstractAt Golden Cross, andesitic lavas and volcaniclastic rocks host epithermal veins that formed in the shallow

part (

massive calcite veins there diluted the ore. The calcite veinsare also structurally weak, necessitating extra bolting andscreening to secure faces and backs in the stopes. While suchcalcite occurrences are known from other epithermal de-posits (e.g., Kushikino, Japan; Fresnillo, Mexico), their originhas not been investigated. In this paper, we describe the re-sults of our detailed study of the origin of the late, barren cal-cite veins in the Golden Cross deposit using mineral distribu-tion patterns, fluid inclusions, stable isotopes, andmicroprobe analyses. The results strongly suggest that thelate barren calcite was related to the waning stages of hy-drothermal activity and the downward movement of steam-heated, CO2-rich waters.

Geologic SettingThe Golden Cross low-sulfidation epithermal Au-Ag de-

posit is one of 47 known epithermal vein deposits in the Hau-raki goldfield (Fig. 1; Brathwaite et al., 1989). These depositsare hosted by a sequence of andesitic to rhyolitic lavas, tuffs,

and sedimentary derivatives that were emplaced duringMiocene-Pliocene time as part of a volcanic arc, known as theCoromandel volcanic zone, that extended the length of theCoromandel peninsula. Late Jurassic metagraywackes cropout on the western and northern parts of the peninsula andform the basement rocks beneath the volcanic sequence inthe deposit area.

The host rocks (Figs. 2 and 3) belong to the CoromandelGroup and consist of andesitic lavas, breccias, and tuffs, epi-clastic sedimentary rocks, dacitic lavas and breccias, and py-roclastic flow deposits. These rock types are grouped intothree units known as the Whakamoehau andesite, the Wai-harakeke dacite, and the Waipupu Formation (Brathwaiteand Christie, 1996). The igneous mineralogy of these rocksincludes plagioclase, hypersthene, augite, iron-titanium ox-ides, and glass; dacitic rocks additionally contain anhedralquartz phenocrysts with or without hornblende.

Several faults, of which the Empire and Western Boundaryfaults are the most important and continuous (Keall et al.,

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Quaternary alluvial sediments

Jurassic greywacke basement

COROMANDEL

PENINSULA

Waihi

Auckland

Areashown below

Golden Cross

Hauraki Rift

Faults

Epithermal Au-Ag veins

L. Miocene diorite intrusions

L. Pliocene & Mioceneandesitic & dacitic volcanics

Pliocene & U. Miocenerhyolitic & dacitic volcanics(Whitianga Group)

0 200 km

0 25km

FIG. 1. Location of the Golden Cross deposit and the Hauraki gold field, North Island, New Zealand.

1993), transect the deposit (Fig. 2). The Empire fault strikesnortheast, dips steeply to the west, and hosts the mineralizedEmpire vein (Figs. 2 and 3). The Western Boundary faultstrikes north-northeast, dips to the east, and borders the west-ern side of the hanging-wall stockwork veins (Figs. 2 and 3);it is characterized by a 10- to 20-m-wide zone of shearing, al-though the sense of movement and total displacement are un-certain. Several subordinate faults (West Mine, Pillar, andBeefeater) are subparallel to the Western Boundary fault(Fig. 2) and have postmineral displacements of less than 20 m(Keall et al., 1993).

The precious metals occur in quartz-sulfide-bearing veinsassociated with the Empire zone and the hanging-wall stock-work. The ore for both zones is confined to a vertical extentof approximately 300 m (Figs. 2 and 3). The Empire zone isan upward-branching network of crosscutting veins that con-tains ore for approximately 600 m along strike. Poorly miner-alized segments extend at least another several hundred me-ters north. The Empire vein is the steeply dipping structureof coalescing veins that dominates the Empire zone (Fig. 3).A subparallel vein that is part of the same structure (the orig-inal Golden Cross reef mined early in this century; Fig. 2) ex-tends southwest of the Empire zone.

Ore from the Empire zone was mined by undergroundmethods at an average grade of 6 to 7 g Au/tonne. In thehanging wall, closely spaced narrow (

rhodochrosite, and siderite as standards. The concentration ofcarbon dioxide was calculated from stoichiometry.

Occurrences of Hydrothermal CalciteCalcite, along with other hydrothermal phases, formed ei-

ther through replacement of a preexisting phase or throughdirect deposition from an aqueous solution. Hydrothermalmineral occurrences are summarized in Table 1. The degreeof hydrothermal alteration ranges from moderate to intense,with 75 to 100 percent of the rock having been replaced bysecondary minerals. In most samples, original rock texturesare moderately well preserved.

The term replacement is used here to describe calcite oc-currences that are surrounded by other rock-forming miner-als and that appear to have formed through reaction involvinga Ca-bearing precursor phase (commonly plagioclase) andcarbon dioxide. Calcite that formed by direct deposition is in-dicated by its occurrence in a vein or a vug (Fig. 4). Replace-ment and vein calcite are common in samples from sections4650 N and 4850 N, but they decrease in abundance north-ward to section 5050 N (Fig. 5) and disappear north of section5300 N. Calcite was also abundant in the historically minedportions of the Golden Cross 1 reef (Bell and Fraser, 1912;Fig. 1), and small calcite veins have been mapped south of themined area.

Replacement calcite consists of up to 10 percent of the al-tered country rock, although in most cases 5 percent is themaximum. It replaces plagioclase, pyroxene, and amphibolethat occur as phenocrysts and groundmass phases, and it iscommonly accompanied by adularia, illite, and/or chlorite.

Calcite that formed through direct deposition at the time ofore mineralization occurs in trace amounts, most commonlyas tiny grains interspersed in Au-Ag-bearing quartz-sulfideveins (Simpson, C., et al., 1995). However, most calcite occursin late veins, which range from less than 0.02 m to 10 m wide,and which crosscut quartz-sulfide veins (Fig. 4). In these lateveins, calcite consists of more that 99 percent of the massivefilling, forming coarse anhedral crystals that range from 1 cmto greater than 5 cm across (Fig. 4). Smaller crystals formingtrigonal dipyramids line a few open vugs and represent thelast phase of calcite deposition. Platy calcite is found locallyalong the vein contact with host rocks, where minor quartzalso occurs. Other trace phases in the late calcite veins in-clude pyrite and clays. Siderite is found locally on the selvagesof calcite veinlets.

From examination under cathodoluminescence, most cal-cite appears to be compositionally uniform, but zonation ex-ists in a few crystals. The growth zones in replacement calciteindicate that at least some of these grains grew in open spacesfollowing dissolution of a preexisting phenocryst.

Fluid InclusionsFluid inclusions are common in the late massive calcite,

whereas fluid inclusions are sparse in the quartz that is asso-ciated with quartz-sulfide veins. Accordingly, the microther-mometric data set for quartz-hosted fluid inclusions only con-sists of about 150 measurements (de Ronde and Blattner,1988; Simpson, C., 1996). The microthermometric data setfor calcite-hosted fluid inclusions described here exceeds 870measurements.

102 SIMMONS ET AL.

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wf

wf

wf

wd

wd

wa

wd

400 m RSL

200 m RSL

Western Boundary Fault

Em

pire

Fau

lt

West M

ine FaultBeefeater Fault

0 100 m

EW LEGEND

Waipupu FormationWaiharakeke DaciteWhakamoehau Andesite wa

wdwf

LITHOLOGY

SYMBOLS

Fault

Lithologic contact

Quartz vein(Empire Vein)

Calcite vein

Open-pit outline

Stockwork veins

Pillar Fault

FIG. 3. Geologic cross section (4850 N) of the Golden Cross deposit; RSL = relative to sea level.

In the University of Auckland data set, primary and sec-ondary inclusions were distinguished based on the criteriasummarized by Roedder (1984), and fluid inclusions showingobvious signs of necking were avoided. In the IGNS data set,no distinction between primary and secondary inclusions wasmade, but a large number of measurements were conductedon each sample studied (Table 2); effort was made to restrictmeasurements to groups of inclusions occurring in three-di-mensionally-spaced groups away from healed fractures. Re-gardless, bona fide primary inclusions were found in just afew samples of calcite.

Two types of two-phase (liquid plus vapor) fluid inclusionswere observed at room temperature: liquid-rich inclusionscontaining approximately 80 percent liquid and 20 percentvapor (by volume), and vapor-rich inclusions containing morethan 98 percent vapor. These range in size from less than 5m to about 25 m. In some cases, the coexistence of thesetwo types of inclusions may relate to vapor-saturated condi-tions at the time of fluid inclusion trapping, although we can-not rule out the formation of vapor-rich inclusions by necking,as textural evidence is lacking (Bodnar et al., 1985). Mi-crothermometric measurements were restricted to liquid-rich

inclusions, as reliable measurements of Th on vapor-rich in-clusions were not possible.

Homogenization temperatures range between 125 and227C, with most mean values clustering between 170 and200C (Table 2; Fig. 6). Th data for primary and secondary in-clusions overlap, but the Th data from secondary inclusionstrend toward cooler temperatures. The broad temperaturerange (125 to 230C) can be variously attributed to two-phase fluid trapping in a boiling environment, measurementsof necked inclusions, and to a decrease in temperature overtime.

Ice melting temperatures range from 0.0 to 1.1C, indi-cating that hydrothermal solutions were dilute and contained0 to 2 NaCl wt percent equiv. Crushing experiments were un-dertaken to estimate gas contents, but bubble behavior wasrarely observed. For a few inclusions, vapor bubbles ex-panded slightly, indicating internal pressures of slightly morethan 1 bar and the existence of some noncondensible gas,probably carbon dioxide. Although no gas hydrates were ob-served (Collins, 1979), the ice melting data could be inter-preted solely in terms of dissolved carbon dioxide (Heden-quist and Henley, 1985; Barton and Chou, 1993). The

MASSIVE CALCITE VEINS, GOLDEN CROSS, NEW ZEALAND 103

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FIG. 4. Calcite occurrences at Golden Cross: A. late massive calcite veins crosscutting Au-Ag-bearing quartz-sulfide veins(banded), width of photo represents about 2.5 m; B. late massive calcite-filled veins with offsets at the corners of the vein in-tersection indicating extension in two directions, width of photo represents about 2.8 m; C. massive calcite from late veinshowing rhombohedral cleavage, coin diameter is 2.5 cm; D. platy calcite in late massive calcite veins, coin diameter is 2.5cm.

104 SIMMONS ET AL.

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TAB

LE

1. H

ydro

ther

mal

Min

eral

Occ

urre

nces

at G

olde

n C

ross

Hos

t min

eral

A

bund

ance

M

axim

um g

rain

siz

e Sh

ape

Ass

ocia

ted

min

eral

s

Rep

lace

men

t

Qua

rtz

Plag

iocl

ase

and

pyro

xene

phe

nocr

ysts

, 0

to 6

0%

0.5

mm

A

nhed

ral,

inte

rloc

king

C

hlor

ite-il

men

ite-s

mec

tite-

grou

ndm

ass

adul

aria

-pyr

ite

Illit

e an

d sm

ectit

e

Plag

iocl

ase

phen

ocry

sts,

gro

undm

ass,

adu

lari

a 0

to 1

0%

0.01

mm

?

Qua

rtz-

chlo

rite

-adu

lari

a-ca

lcite

-py

rite

Chl

orite

Pl

agio

clas

e an

d py

roxe

ne p

heno

crys

ts,

2 to

15%

0.

5 m

m

? Q

uart

z-ilm

enite

-adu

lari

a-gr

ound

mas

sca

lcite

-pyr

ite

Kao

linite

Ph

enoc

ryst

s, g

roun

dmas

s ?

? ?

Lat

e ov

erpr

int

Cal

cite

Pl

agio

clas

e, p

yrox

ene,

and

am

phib

ole

0 to

10%

0.

5 m

m

Anh

edra

l C

hlor

ite-il

men

ite-a

dula

ria-

side

rite

phen

ocry

sts,

gro

undm

ass

Side

rite

Pl

agio

clas

e an

d py

roxe

ne p

heno

crys

ts,

0 to

5%

0.

5 m

m

Anh

edra

l C

alci

tegr

ound

mas

s

K fe

ldsp

ar (

adul

aria

)

Plag

iocl

ase

phen

ocry

sts,

gro

undm

ass

2 to

10%

0.

1 m

m

Subh

edra

l Q

uart

z-ilm

enite

Pyri

te (

mar

casi

te)

M

agne

tite-

ilmen

ite

0 to

5%

0.

4 m

m

Anh

edra

l-euh

edra

l Q

uart

z

Tita

nite

-leuc

oxen

e

Ilm

enite

Tr

ace

0.3

mm

A

nhed

ral

Vein

filli

ng

Abu

ndan

ce

Gra

in s

ize

Shap

e A

ssoc

iate

d m

iner

als

Dir

ect d

epos

ition

Qua

rtz

M

assi

ve, l

amin

ated

, bre

ccia

ted

Up

to 1

00%

maximum possible content of dissolved carbon dioxide is ap-proximately 2.6 wt percent, corresponding to a Tm of 1.1C.

The occurrence of platy calcite in the late massive calcite isa strong indicator of boiling conditions (Browne, 1978; Sim-mons and Christenson, 1994), consistent with the occur-rences of some coexisting liquid-rich and vapor-rich inclu-sions (Bodnar et al., 1985). Therefore, we believe the calciteTh data represent true trapping temperatures, obviating theneed for pressure corrections. Taking the range of 180 to200C and 1.0 wt percent carbon dioxide (Tm = 0.4C) asrepresentative of the calcite-forming solutions, along with ap-propriate Henrys constants (KH ranges from ~6,900 to 6,600)for carbon dioxide (Ellis and Golding, 1963), vapor-saturatedfluid pressures range from 37 to 43 bars. This in turn suggestsa formation depth of about 400 to 500 m below the paleowa-ter table, assuming a hydrostatic pressure gradient as indi-cated by the open, vuggy nature of the veins. The PCO2 for thissolution is about 27 bars (PCO2 = X CO2KH, where X CO2 is molefraction), comprising a significant portion of the total pres-sure, so the depth estimate greatly depends on the estimateof aqueous carbon dioxide.

The calcite Th-Tm data set overlaps the early quartz-sulfidevein Th-Tm data set (Fig. 6) and is broadly consistent with thehost-rock alteration to temperature-sensitive clays (de Rondeand Blattner, 1988; Simpson et al., 1998). These relationshipsindicate that much of the late massive calcite formed at tem-peratures similar to Au-Ag-bearing quartz-sulfide veins.

Based on fluid inclusion studies from active geothermal sys-tems, the Th vs. Tm pattern in Figure 6 can be interpreted asresulting from some combination of gas loss and mixing(Hedenquist and Henley, 1985; Simmons and Christenson,1994). The gas-loss and mixing trends are calculated and po-sitioned to envelop most of the Th-Tm data from calcite; theparent composition is deduced from the intersection of thetwo hydrothermal solution trends (Fig. 6). The compositionof the steam-heated water is determined by analogy to activegeothermal systems, and its formation is discussed below. Fora Tm value of 1.5C, the parent hydrothermal solution couldcontain about 3.7 wt percent carbon dioxide at approximately240C if most of the ice melting depression is due to aqueouscarbon dioxide. The steep Th-Tm data trend for calcite matcha gas-loss trend in which carbon dixoide exsolves from theparent liquid due to phase separation (Fig. 6).

Stable IsotopesThe 18O composition of calcite ranges from 3.8 to 15.4 per

mil (Table 2). Taking the calcite Th data as representative ofthe thermal conditions of isotopic equilibration, the equilib-rium water compositions, calculated on the basis of fractiona-tion factors (ONeil et al., 1969; Friedman and ONeil, 1977),range from 5.1 to 6.2 per mil (Fig. 7A), with most samplesclustering between 2 and 6 per mil. One sample has anequilibrium water composition of 5.1 per mil; the origin ofthis enriched value is unknown and may result from localwater-rock interaction. The 13C composition of calciteranges from 3.1 to 9.0 per mil. The equilibrium 13C com-positions of carbon dioxide for most of these data fall between7 and 9 per mil (Fig. 7B). To test for small-scale isotopicvariations in calcite, microdrilling was used to obtain samplesacross growth zones of several calcite crystals. For calcite

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Drill line 4850m N B'B

Replacement calcite

Quartz-sulfide vein

Late massive calcite

3300

E

3200

E

3000

E

2900

E

3100

E

400 RSL

300 RSL

200 RSL

100 RSL

C C'Drill line 4650m N

Replacement calcite

Quartz-sulfide vein

Late massive calcite

2800

E

3200

E

3100

E

2900

E

3000

E

400 RSL

300 RSL

200 RSL

100 RSL

Drill line 5050m NA A'

3300

E

3200

E

3000

E

2900

E

3100

E

Replacement calcite

Quartz-sulfide vein

400 RSL

300 RSL

200 RSL

100 RSL

FIG. 5. Distribution of replacement and vein calcite along sections 4650,4850, and 5050; RSL = relative to sea level.

from massive veins, there is generally less than 1.0 and 0.7 permil variation in 13C and 18O values, respectively. Calcite fromthe veinlets and replacement calcite have more erratic compo-sitional variations; a single veinlet may show up to 1.7 and 5.7per mil variation in 13C and 18O values, respectively, even ifthe veinlet shows no zoning under cathodoluminescence.

The data plot of calcite 13C values vs. calcite 18O values(Fig. 7C) form a positively sloping, linear trend that correspondsto equilibrium between calcite and aqueous H2CO3. The dataalso correspond to equilibrium temperatures of less than200C (Fig. 7C) and provide independent confirmation of cal-cite formation temperatures interpreted from fluid inclusions.

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TABLE 2. Fluid Inclusion and Stable Isotope Data for Late-vein and Replacement Calcite

Location (mine coordinates)2Sample no.1 E N Z ThC range (n)3 ThC mean TmC range (n) 13C () 18O ()

Late massive calciteGC-001 3167 4649 196 150 to 207 (46) 188 0.3 to 0.6 (7) 8.3 5.5GC-002 3070 4620 182 173 to 248 (64) 217 0.4 to 0.6 (10) 8.3 3.8GC-003 3132 4648 177 152 to 217 (36) 192 0.1 to 0.1 (11) 8.9 5.4GC-004A 3122 4650 240 145 to 230 (63) 200 8.2 6.2GC-004B 3122 4650 240 145 to 230 (63) 200 8.5 5.0GC-007 3179 4788 240 125 to 201 (46) 159 7.5 6.5GC-008 3134 4558 27 133 to 169 (54) 144 0.0 to 0.3 (3) 6.0 8.3GC-010 3000 4850 325 151 to 185 (24) 166 0.2 to 0.4 (9) 6.3 8.6GC-011A 3179 4788 240 143 to 230 (35) 166 0.2 to 0.4 (8) 8.1 6.0GC-011B 3179 4788 240 143 to 230 (35) 166 0.2 to 0.4 (8) 8.2 6.2GC-014 3090 4525 240 148 to 152 (7) 150 0.1 to 0.1 (3) 7.1 6.8GC-015 3178 4649 201 141 to 204 (19) 178 0.0 to 0.4 (8) 7.0 5.8GC-102 3103 4756 228 157 to 202 (32) 193 7.5 5.6GC-103 2998 5100 110 141 to 204 (42) 167 8.0 5.9GC-107 2720 3615 269 161 to 237 (48) 205 8.8 5.2GC-108 2680 3622 228 154 to 222 (38) 187 0.0 to 0.2 (7) 5.1 15.4GC-111 2956 3553 81 142 to 206 (30) 162 6.2 8.6GC-112 3431 5344 10 143 to 185 (45) 159 0.0 to 0.4 (9) 7.4 5.8MS-1 open pit 175 to 202 (4) 185 0.3 to 0.4 (3)MS-04 2985 4750 334 180 to 189 (21) 181 0.0 to 0.3 (8) 8.0 6.2

8.1 6.2165 to 177 (5) 171 0.0 (1)

MS-05 2985 4750 334 163 to 190 (6) 179 0.2 to 0.3 (4) 8.1 6.1MS-06 3050 4830 350 191 to 195 (17) 192 0.5 to 0.6 (4) 8.1 6.2MS-07 3050 4830 350 192 to 199 (3) 194 0.1 to 0.2 (2) 8.8 5.9MS-11 2912 4650 335 171 to 185 (17) 182 0.4 to 0.5 (7) 8.0 5.9MS-16 3040 5050 273 163 to 166 (7) 165 0.8 to 1.0 (5) 7.8 6.4MS-17 3060 4850 267 196 to 197 (7) 197 0.4 to 0.6 (5) 8.9 5.3MS-18 3140 4650 244 190 to 198 (10) 192 0.2 to 0.4 (4) 8.7 4.9MS-19 underground 185 to 227 (6) 209 0.5 to 0.6 (3) 8.7 6.8MS-20 2934 4925 370 8.8 5.0MS-36 2925 4650 275 8.9 5.0MS-37 3198 4650 210 190 to 195 (7) 193 0.8 to 1.0 (5) 8.2 5.7

Late calcite veinletsMS-08 3140 4650 244 193 to 203 (11) 200 0.6 to 0.8 (5) 9.0 6.1MS-09 3036 4650 278 193 to 211 (10) 199 0.5 to 1.1 (5) 8.0 5.9MS-12 3070 4850 267 191 to 198 (5) 197 0.3 to 0.4 (2) 8.3 5.8MS-14 3130 5050 250 186 to 198 (13) 195 0.3 to 0.5 (4) 9.0 5.2MS-23 3134 4650 170 7.6 6.5MS-28 3210 4850 96 6.0 7.4

Replacement calciteMS-10 3036 4650 278 7.7 5.5MS-13 3070 4850 267 7.3 6.6MS-15 3130 5050 250 7.1 6.8MS-21 3180 5050 136 Ankerite 3.9 15.4MS-22 3066 5050 213 5.7 8.9MS-24 3128 4650 105 6.6 7.2MS-25 3178 4650 176 7.7 7.0MS-26 3076 5050 353 2.0 13.7MS-29 3270 4850 168 3.1 12.5MS-30 3370 4850 118 7.5 7.1MS-31 3118 4850 206 7.1 5.6MS-32 3140 4650 244 8.0 4.8

1Fluid inclusions in samples denoted GC were measured at IGNS, Wairakei; fluid inclusions in samples denoted MS were measured at University of Auck-land

2E = easting, N = northing, Z = elevation above sea level; distances measured in meters3P = primary, S = secondary

For comparison, the 18O composition of quartz fromquartz-sulfide veins (de Ronde and Blattner, 1988; Simpson,C., 1996) are plotted in Figure 8. The data range from 7.0 to11.7 per mil, but most of these fall between 8 and 11 per mil.Taking the quartz Th data range of 180 to 230C, the equi-librium water compositions, calculated on the basis of frac-tionation factors (Matsuhisa et al., 1979), are bracketed byvalues that range from 4 to 0 per mil (Fig. 8B). These resultsstrongly suggest that the 18O compositions of quartz-equilib-rium waters are slightly greater, in the range of 0 to 2 permil, than the 18O compositions of calcite-equilibrium waters.

Microprobe AnalysesMicroprobe analyses of calcite were made on nine samples

collected from underground workings and drill holes. Fourdifferent types of calcite were analyzed: (1) massive veins (20analyses from one sample); (2) trigonal dipyramid calcite,which formed later than the massive veins (18 analyses fromtwo samples); (3) calcite that replaced phenocrysts in thecountry rock (40 analyses from four samples); and (4) calcite

veinlets, less than 1 cm thick, which occur in country rock dis-tal to the main vein (26 analyses from four samples).

Calcite from Golden Cross is nearly pure, with virtually allanalyses containing greater than 90 mole percent CaCO3.Sodium, Sr, and Ba were at or below the limit of detection innearly all samples, but most samples showed detectable levelsof Mg, Mn, and Fe. Both massive and trigonal dipyramid cal-cites have few impurities and cluster close to the calcite apexof the compositional diagrams in Figure 9, with average CaCO3contents of 99.0 and 97.7 mole percent, respectively. In con-trast, replacement calcite and calcite in veinlets have higherconcentrations of other cations, with average CaCO3 contentsof 97.5 and 94.5 mole percent, respectively (Fig. 9). There-fore, the large calcite veins that occur near the center of thedeposit show less substitution than the replacement calcite orcalcite veinlets located farther from the deposit, although anyof these calcite types may be nearly pure. Siderite is finegrained and intergrown with other minerals, making it diffi-cult to analyze. The available data suggest that siderite con-tains less than 10 mole percent combined Mg, Mn, and Ca.

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FIG. 6. Th vs. Tm data for calcite-hosted fluid inclusions from Golden Cross (Table 2). Points are centered on the meanTh and the median Tm data; horizontal lines represent the range of Th and vertical lines represent the range of Tm. Quartz-hosted fluid inclusion data (de Ronde and Blattner, 1988; C. Simpson, 1996) from quartz-sulfide veins are outlined for com-parison. Calculated mixing and gas-loss curves show how the data can relate to the main processes affecting water composi-tions based on studies of geothermal systems (see Hedenquist and Henley, 1985; Simmons and Christenson, 1994). Thepreboiled parent composition at approximately 240C (Tm ~ 1.5C) represents a composition of about 3.5 wt percent car-bon dioxide. Steam-heated water represents the end-member diluent for the mixing trend; its composition is based on steam-heated groundwater containing about 1 to 1.5 wt percent carbon dioxide, similar to that found in the Broadlands-Ohaaki ge-othermal system (see Hedenquist and Henley, 1985; Simmons and Christenson, 1994). The steep Th-Tm array for calcite databest match a gas-loss trend (not shown, to avoid clutter) starting from water of approximately 200C and containing about2.6 wt percent carbon dioxide (Tm = 1.1). If all the late calcite was derived from a descending CO2-rich steam-heated water,as discussed in the text, then the preboiled solution must have undergone some degree of mixing with the parent solution,causing heating before gas loss was initiated; the dotted arrow traces this reaction path. The inverse solubility of calcite pre-cludes dilution or cooling reaction paths.

Th C

Tm

C

The substitution of Fe, Mg, and Mn into the calcite latticeis a function of several variables, including temperature, rateof precipitation, salinity, Eh, fCO2, pH, and concentrations of

these elements in the parent solution. Most work on elementsubstitution in carbonates has been confined to either high-temperature (>400C) or low-temperature (

Interpretation of the Origin of Massive Calcite Veins

Calcite forms in boiling geothermal systems hosted byvolcanic rocks in three possible ways (Simmons and Christen-son, 1994): (1) through replacement of calcium-bearing alu-minosilicates in the presence of relatively high aqueous car-bon dioxide concentrations at relatively low water-rock ratios;(2) through phase separation of liquid and gas involving exso-lution of carbon dioxide and steam formation (which, for sim-plicity, we call boiling); and (3) through heating of a solutioninitially close to calcite saturation. The data from GoldenCross suggest that all three processes contributed to calciteformation there, reflecting the movement of carbon dioxidein response to boiling, mixing, and condensation. Theprocesses leading to the formation of late massive calcite arediscussed further below using active geothermal environ-ments as an interpretive framework. For background, we de-scribe the occurrence and formation of CO2-bearing waters inthe Broadlands-Ohaaki geothermal system, where the shallowenvironment (

dioxide) to convert calcium-bearing aluminosilicates to calciteplus clay (chlorite, illite, smectite). Unfortunately, we cannotresolve the formation temperatures of replacement calcite todetermine its equilibrium 18O water composition. On thebasis of associated hydrothermal minerals, it is possible thatboth deep chloride and CO2-rich steam-heated waters con-tributed to calcite replacement in the host rocks.

By contrast, we believe that most of the late massive calciteveins formed from marginal and shallow, CO2-rich steam-heated waters that descended into the former upflow zoneduring waning hydrothermal activity (Fig. 10). Two lines ofevidence lead us to this hypothesis. First, deposition of mas-sive calcite in veins along a heating (rather than cooling) flowpath is consistent with the reverse solubility of calcite with re-spect to temperature. For example, the Broadlands-OhaakiCO2-rich, steam-heated waters become saturated with calciteupon heating of only a few degrees centigrade (Simmons andChristenson, 1994). The near total absence of all other min-erals (e.g. quartz, sulfides) requires a selective depositionalenvironment that can easily be achieved by heating. Second,the steam-heated origin is interpreted from the calculatedisotopic compositions of waters. The 18O composition ofwater that deposited the late massive calcite is lower by up to2.5 per mil than the water composition that deposited thequartz in the Au-Ag-bearing quartz-sulfide veins.

The Th-Tm data (Fig. 6), along with crushing studies, fur-ther support the dominance of aqueous carbon dioxide in

calcite-hosted inclusion fluids. The steep slope on the Th-Tmtrend (Fig. 6) is characteristic of carbon dioxide gas lossthrough boiling and is consistent with local formation of platycalcite early in the filling of the massive calcite veins. How-ever, the composition of the preboiled liquid (~2.6 wt %CO2, equivalent to Tm = 1.1C) falls between the parent andsteam-heated water compositions in Figure 6, suggesting thatmixing preceded phase separation. An analogous situation, inwhich platy calcite precipitated from boiling CO2-rich steam-heated waters, is documented in the Waiotapu geothermalsystem, New Zealand (Hedenquist and Browne, 1989; Sim-mons and Christenson, 1994). The temperature increaserequired to attain boiling conditions in descending waterseasily could have been derived from the residual heatstored in the country rock hosting the veins from the timeof ore mineralization, or through mixing with residual chlo-ride water. Eventually, temperatures precipitating calcitecooled to sub-boiling conditions.

In total, about 2 106 tons of massive calcite were de-posited in the late Empire veins. This requires about 108 to109 tons (0.1 to 1 km3 volume equivalent) of CO2-richsteam-heated water, assuming it contained approximately 1wt percent carbon dixoide at 150C (similar to Broadlands-Ohaaki), and assuming 100 to 10 percent efficiency in cal-cite fixation of the aqueous carbonate. This amount of solu-tion would occupy about 0.5 to 5 km3 of rock, given anaverage porosity of 20 percent, equivalent to a volume of

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FIG. 10. Schematic diagram showing the position and movement of deeply derived chloride waters and shallow CO2-richsteam-heated waters during the formation of (A) precious metal-bearing quartz-sulfide veins (Empire vein and stockworkzones) and (B) late massive calcite veins; RSL = relative to sea level.

Quartz sulfide Au-Ag mineralizationin Empire and stockwork zones

Massive calcite infills late-formed structures

CO2-rich steamheated water

CO2-rich steamheated water

2 (1) 2 (1) 0.5 km. This calculation shows that the de-scending CO2-rich water would have drawn from a relativelylarge area to be focussed through a relatively small volume ofrock represented by the veins today. Based on the decreasingabundance of calcite northward, we believe that the CO2-rich, steam-heated waters were sourced from the southernpart of the deposit.

Implications for Exploration of Low-Sulfidation DepositsEvidence for the existence of CO2-rich steam-heated wa-

ters in a hydrothermal system indicates that boiling condi-tions existed. Thus, one of the features conducive to mineral-ization in epithermal veins, i.e. boiling, can be directlyinferred from the existence of barren calcite veins in low-sul-fidation epithermal prospects.

Barren calcite fills structures on the periphery of the Fres-nillo district (Simmons, 1991) and in the upper parts (0200m below the present surface) of veins that are mineralized atdepth (Gemmel et al., 1988); this pattern is replicated in anumber of other epithermal deposits in the southwesternUnited States and Mexico (Buchanan, 1981). Late barren cal-cite also occurs in the ore-bearing parts of epithermal Au-Agveins at Kushikino, Japan (Matsuhisa et al., 1985). ForGolden Cross, Fresnillo, and Kushikino, the massive calciteveins form within a few hundred meters of epithermal min-eralization. From studies of active systems, we know that thedistribution of CO2-rich steam-heated waters is controlled byshallow hydrology and also by the topographic relief. In low-relief settings, CO2-rich steam-heated water is likely to forma discontinuous carapace over the upflow zone (Hedenquist,1986; 1990), whereas in high-relief settings, this same watermay occupy perched aquifers on the lower flanks of a volcanicedifice (Henley and Ellis, 1983). Thus, the utility of calcitevein occurrences as an exploration guide to low-sulfidationmineralization will be most useful in prospects where a real-istic model of paleohydrology exists. Finally, while barren cal-cite veins may be a positive indicator of conditions conduciveto mineralization, they can be a problem for mining, dilutingthe grade of ore or requiring extra support in undergrounddevelopments.

AcknowledgmentsWe thank Coeur Gold New Zealand Ltd for access to the

mine area, for partial funding, and for permission to publishthis manuscript. Additional funding was provided by theFoundation for Research, Science and Technology (NewZealand) and the University of Auckland. Jan Lindsey ob-tained the fluid inclusion data at IGNS, and Bruce Christen-son made these results available to us. The electron micro-probe analyzer used in this work was acquired under Grant#EAR-82-12764 from the National Science Foundation(United States). We thank Jeff Hedenquist, Patrick Browne,and two Economic Geology reviewers for providing construc-tive comments on an earlier draft of this manuscript. We alsothank Louise Cotterall for drafting the figures.

February 25, September 8, 1999

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