-
0361-0128/00/3024/99-14 $6.00 99
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.,
100 SIMMONS 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.
0361-0128/98/000/000-00 $6.00 104
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
MASSIVE CALCITE VEINS, GOLDEN CROSS, NEW ZEALAND 105
0361-0128/98/000/000-00 $6.00 105
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.
106 SIMMONS ET AL.
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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
110 SIMMONS ET AL.
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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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