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IntroductionBladed quartz is common in many epithermal gold
de-
posits (Lindgren, 1933; Urashima, 1956; Simmons andBrowne,
1990), and it often has been observed near high-grade ore (Simeone
and Simmons, 1999; Simon et al., 1999).Bladed calcite, which is
morphologically similar to bladedquartz, also has been observed in
two-phase fluid zones ofsome active epithermal systems (Browne,
1978; Keith andMuffler, 1978; Tulloch, 1982; Simmons and
Christenson,1994). The recognition of bladed calcite in these
systems isimportant because it signifies the initiation of boiling
that hasbeen linked to the mechanism of gold precipitation
(Cun-ningham, 1985; Drummond and Ohmoto, 1985; Henley andBrown,
1985; Brown, 1986; Seward, 1989, 1991). Two majorquestions arise
from observations of bladed quartz in somedeposits. Was bladed
quartz formed from boiling fluids, andwhat is the relationship
between bladed quartz and oregrade? To answer these questions, we
studied the distribu-tion, occurrence, and structure of bladed
quartz, as well asfluid inclusions in bladed quartz, in the
Hishikari epithermalgold deposit, Kyushu, Japan.
Geological BackgroundThe Hishikari low-sulfidation epithermal
vein-type gold
deposit is located in southern Kyushu (Fig. 1), a major
goldmetallogenic province in Japan. A geological map andschematic
longitudinal section of the deposit are shown inFigure 2. Izawa et
al. (1990) and Ibaraki and Suzuki (1993)presented comprehensive
descriptions of the geology, miner-alization, and wall-rock
alteration of the Hishikari deposit,
and our description of the geology is based mainly on
thesestudies.
The Shimanto Supergroup of Cretaceous age comprisesthe basement
rocks beneath Hishikari. These rocks consist ofcarbonaceous shale
and sandstone, which have been sub-jected to low-grade regional
metamorphism. The sedimen-tary rocks contain quartz, albite, Fe
chlorite, and sericite, withminor calcite, pyrite, and carbonaceous
matter. The basementrocks generally are present at elevations of
400 m below sealevel or deeper in the surrounding area, but they
are close tothe surface in the central part of the Hishikari
deposit area(0130 m above sea level).
Quaternary volcanic rocks unconformably overlie the base-ment
rocks. From oldest to youngest, these consist of theHishikari lower
andesites, the Maeda dacites, the Shishimanodacites, the Hannyaji
welded tuff, and the Ito pyroclastic flowdeposits. The Hishikari
lower andesites (0.951.78 Ma; Izawaet al., 1990) consist of
pyroclastic rocks and lava flows of hy-persthene-augite andesites,
and intercalated mudstones, all ofwhich have undergone hydrothermal
alteration. The Maedadacites and the Shishimano dacites (0.661.10
Ma; Izawa etal., 1990) are lava flows of hornblende dacites. The
Hannyajiwelded tuff (0.73 Ma; New Energy and Industrial
TechnologyDevelopment Organization, 1991) is a hornblende dacite
thatwas erupted from the Kakuto caldera (Fig. 1). The lower
por-tion of each of these three units has been hydrothermally
al-tered. The Ito pyroclastic flow deposits (24,00025,000 yr
BP;Machida and Arai, 1992) are hornblende dacite derived fromthe
Aira caldera, and consist mainly of pumice flow deposits.
The Hishikari deposit is composed of the Honko-Sanjinand Yamada
zones. The veins in the Honko-Sanjin zone occurin both the basement
sedimentary rocks and the overlying
BLADED QUARTZ AND ITS RELATIONSHIP TO GOLD MINERALIZATION IN THE
HISHIKARI LOW-SULFIDATION EPITHERMAL GOLD DEPOSIT, JAPAN
JIRO ETOH, EIJI IZAWA, KOICHIRO WATANABE,Department of Earth
Resources Engineering, Graduate School of Engineering, Kyushu
University, Hakozaki, Fukuoka 812-8581, Japan
SACHIHIRO TAGUCHI,Department of Earth System Science, Faculty of
Sciences, Fukuoka University, Nanakuma,Fukuoka 814-80, Japan
AND RYOTA SEKINEMineral Resources Division, Sumitomo Metal
Mining Co., Ltd., Tokyo 105-8716, Japan
AbstractBladed quartz frequently is observed in low-sulfidation,
epithermal gold deposits. Bladed calcite also has
been documented in boiling zones of some active epithermal
systems, and boiling in these systems has beendirectly linked to
gold mineralization. In the Hishikari low-sulfidation epithermal
gold deposit, the distributionand texture of fluid inclusions
within bladed quartz reveal a similar relationship to gold
precipitation.
The formation of bladed quartz at Hishikari involves several
stages: (1) deposition of bladed calcite; (2) pre-cipitation of
fine-grained adularia and quartz on the surface of calcite blades;
(3) dissolution of calcite blades,leaving cavities in the
interstices between aggregates of adularia and quartz; and (4)
infilling of the cavities bylater quartz (i.e., pseudomorphs of the
original bladed calcite). Bladed quartz is present largely in the
deeperpart of the vein system, beneath the high-grade gold ore zone
at Hishikari. This distribution may be explainedby the fact that
the original bladed calcite formed at depth in the system, where
boiling and loss of CO2 ini-tially caused the precipitation of the
calcite, and quartz formed as pseudomorphs of the original calcite
blades.
Economic GeologyVol. 97, 2002, pp. 18411851
Corresponding author, e-mail: [email protected]
-
Hishikari Lower Andesites, whereas the Yamada veins arefound
only in the Hishikari Lower Andesites (Fig. 2). Theseveins have an
estimated reserve of 5.2 Mt of ore at an averagegrade of 45 to 50
g/t Au, including past production. A total of260 t of Au is
contained in the Hishikari deposit, the most pro-ductive gold mine
in Japanese history. Veins in the Hishikarideposit generally strike
N50E and dip 70 north to vertical.The strike length of individual
veins generally ranges from300 to 400 m, and widths are 0.5 to 4 m.
Bonanza zones arelocated at elevations of 50 to 150 m from sea
level (Ibarakiand Suzuki, 1993), and occur in an area of 2.5 by 0.8
km.Chlorite-illite alteration of wall rock is associated with
high-grade gold mineralization at the Honko-Sanjin zone,
whereaschlorite/smectite-illite/smectite mixed-layer clay and
chlorite-illite alteration are associated with the Yamada veins.
40Ar/39Arages of adularia indicate Pleistocene ages of 0.90 to 0.97
Mafor gold mineralization of the Honko-Sanjin zone, and 0.60 to1.15
Ma for that of the Yamada zone (Watanabe et al., 2001).
The veins of the Hishikari deposit consist mainly of
quartz,adularia, and smectite, with minor amounts of kaolinite,
tr-uscottite, and calcite. The principal metallic minerals
areelectrum, naumannite-aguilarite, pyrargyrite,
chalcopyrite,pyrite, and marcasite, with minor amounts of
sphalerite,galena, and stibnite. The veins consist of many bands of
dif-ferent mineralogy and grain size. A typical sequence of
min-eral precipitation, from the wall rock to the center of a
vein,is from adularia, through adularia-quartz, to quartz,
locallyfollowed by smectite (Nagayama, 1993a). This successionoften
is repeated in a single vein.
In the shallow part of the Honko-Sanjin zone, the earliestband
in the veins is commonly represented by a monomineralic
columnar adularia crystals up to 2 cm in length,
elongatedperpendicular to vein wall. The adularia band is followed
byan electrum-rich band, which consists of fine-grained adulariaand
quartz crystals, 5 to 200 m in length, intergrown withelectrum and
sulfide. Bladed quartz associated with electrumalso follows the
adularia band (Imai and Uto, 2002). Themonomineralic adularia band
and electrum-rich adularia-quartz band occur less in the deeper
part of the veins. Theadularia-quartz band is followed by a band of
massive quartzwith a small amount of adularia, and bladed quartz is
very vis-ible in druses of this band, especially in the deeper part
of thevein system. In the veins exposed recently at an elevation
of20 m from sea level, and in a drill core at 50 and 100 m(N.
Ushirone, pers. commun., 2001), bladed quartz is abun-dant, and
electrum-rich bands do not occur. Bladed quartz lo-cally occurs in
the earliest band at such deep levels. A smec-tite-rich band also
commonly occurs in quartz-adularia bandsand, in places, bladed
quartz is present in this band. Brecciasof veins and wall rocks
commonly occur elsewhere in theveins. Fragments of volcanic rocks
up to 1.5 m in diameter arefound locally, several tens of meters
below the contact be-tween the overlying volcanic rocks and the
basement sedi-mentary rocks. In the Yamada zone, vein quartz is
more chal-cedonic and fine grained than that in the Honko-Sanjin
zone,and bladed quartz is present in druses or vugs.
Occurrence of Bladed Quartz and SamplesThe distribution of
bladed quartz and high-grade gold ore
(>50 g/t vein avg) in the Daisen-1 and Hosen-2 veins of
theHonko-Sanjin zone was examined on about 4,500 facesketches
recorded during more than 10 yr of operation at
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Hishikari
Kushikino
Iriki
Ora
Yamagano
Onoyama
Okuchi
Fuke
Aira caldera
KirishimaVolcano
SakurajimaVolcano
0 10 20 km
N
Holocene volcanic rocks
Caldera and basin
Volcanic center
Kyushu
Pacific Ocean30oN
35oN
Graben
Kakuto caldera
Exposure ofthe Shimanto Supergroup
Epithermal gold deposit
130o
E
135o
E
Kagoshimagraben
FIG. 1. Location map of the Hishikari low-sulfidation,
epithermal gold deposit, Kyushu, Japan, after Izawa et al.
(1990).
-
Hishikari (Figs. 3 and 4). The Hosen-2 vein is present only
inbasement sedimentary rocks. Bladed quartz is present in thedeeper
to middle parts of the vein, and high-grade ore is pre-sent mostly
in the upper part of the vein, above the zone ofbladed quartz. The
Daisen-1 vein is hosted by both the base-ment rocks and the
overlying volcanic rocks. High-grade goldore is present in the
shallower to intermediate levels of thevein, and, in the center of
the vein, at deeper levels. Bladedquartz is present in and below
the high-grade gold ore zone.
During this investigation, bladed quartz was collectedfrom 30
locations (Fig. 4). Samples of bladed calcite were col-lected from
three locations. Taking into account the variabil-ity in the size
and texture of blades, seven samples of bladedquartz and two
samples of bladed calcite were selected forfurther study. Table 1
summarizes the location and texturalcharacteristics of these
samples.
Bladed quartz and calcite in the Hishikari deposit can
beclassified into two structural types (Fig. 5), the lattice
type
and the parallel type, according to the criteria of Dong et
al.(1995). The lattice type is characterized by a
house-of-cardsstructure in which individual blades intersect each
other. Theparallel type consists of several sets of parallel
blades, eachhaving a different orientation.
Bladed calcite is locally present in the peripheral parts ofthe
Hishikari vein system, and it occurs there only in druseswithin
veins and cavities in wall rocks. The individual bladesof calcite
are from 3 to 15 mm in diameter and from 10 to 100m in thickness
(Fig. 6). Where the blades have a parallelstructure, the
interstices between the blades range in widthfrom 5 to 100 m (Fig.
6C). Observation under crossed polarsshows that an entire
individual blade becomes extinct at thesame position. Therefore,
each blade consists of a single crys-tal of calcite. In places, the
surface of calcite blade is coveredwith a quartz-adularia aggregate
(Fig. 6D).
Bladed quartz adularia is present in druses as well as infully
cemented parts of the veins. The individual blades are 1
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Alluvial deposits
Ito pyroclastic flow deposits
Hannyaji welded tuff
Shishimano dacites
Maeda dacites
Hishikari lower andesites
Shimanto Supergroup
Quartz veins
SanjinYamada River
N
YamadaA
1000m0 200 400 600 800
A'
DA-1
HO-2
HonkoHO
-2
Yamada Honko-Sanjin zone
1000m0 200 400 600 800
A'SW NE
0
200
-200
(m)400
A
0
200
400(m)
200
FIG. 2. Geological map and schematic longitudinal section of the
Hishikari deposit, after Ibaraki and Suzuki (1993) andNaito et al.
(1993). Abbreviations: DA = Daisen, HO = Hosen, m = meters above
sea level.
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0 100m
0BW10BW20B100
70
40
10
-20
MLNESW
Hosen-2
Bladed quartz
Unconformity between basement and volcanic rocks
High grade gold ore(> 50 g/t)
0 100m
NESW0B E10B E40BE30BE20B
40
MLDaisen-1
100
70
FIG. 3. Distribution of bladed quartz and high-grade gold ore
(>50 ppm vein average) in the Daisen-1 and Hosen-2 veins.Bladed
quartz is present mainly in the deeper to middle parts of both
veins, whereas the high-grade ore occurs largely in theupper part
or above the zone of bladed quartz. The distribution of the
high-grade gold ore zone in Hosen-2 follows Izawa etal. (1990).
Yamada Honko
Sanjin
YU-3
SE-7ZU-1
SH-5
KE-3
HO-1
KI
RY
DA-2
W160B W120B W80B W40B 0B E40B E80B
N40
G0G
S40G
0 400m9MAHAK-6
N56MAHT-1
W200BW240B
C-1Q-1
Q-4
Q-5
C-2Q-7
Q-6Q-2
Q-3
S60G
DA-1
DA-3
HO-2 HO-2
YU-1YU-2
YU-6-2
YU-5 YU-6YU-7
SE-8
SE-2SE-1
FIG. 4. Sampling points for bladed quartz and calcite plotted on
a vein map of the Hishikari gold deposit after Sekine etal. (1998)
and Uto et al. (2001). Circles and triangles represent bladed
quartz and calcite, respectively; filled shapes wereused for this
study. Abbreviations: DA = Daisen, HO = Hosen, KE = Keisen, KI =
Kinsen, RY= Ryosen, SE= Seisen, SH =Shosen, YU = Yusen, ZU =
Zuisen.
TABLE 1. Sample List of Bladed Calcite and Quartz
Length Thickness Constituent Altitude East-west Sample no.
Structural types (mm) (mm) minerals Vein or drill hole (m) grid
Wall rock
C-1 Lattice, parallel 310
- to 100 mm in diameter and
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2cmA 1cmBFIG. 7. A. Lattice-type bladed quartz up to 1 cm in
thickness and 10 cm in length. Surfaces are covered with comb
quartz
(sample Q-6). B. Parallel-type bladed quartz. Several sets of
parallel blades occur in this sample, and each set consists
ofbladed quartz 2 to 6 cm in length (sample Q-2).
Adu.
Qtz.
Sm.
B
0.2mm
Adu.
Qtz.
D
50 m
Adu.Qtz.
A
0.5mm
Qtz. & Adu.Qtz.
C
20 m
FIG. 8. Bladed quartz in thin sections with crossed polars. A.
Blades 10 to 50 m in thickness in parallel array with cav-ities 10
to 50 m in width (sample Q-2). B. Rhombic crystals of adularia and
smectite inclusions are observed (blow up offigure 8D). C. A pair
of blades up to 0.5 mm in thickness observed in a lattice-type
bladed quartz. These blades interleavewith an open cavity created
by the dissolution of bladed calcite (0.20.6 mm); several slivers
of quartz, 10 to 50 m in thick-ness, are present in this cavity,
parallel to the thicker blades. Rhombic adularia occurs near the
wall of the cavity, whereascomb quartz has grown away from the
cavity wall. D. Inside of the blade of sample Q-6. Adularia
crystals arrayed in two par-allel bands (dotted lines) at an
interval of 0.3 mm. Abbreviations: Adu = adularia, Cal = calcite,
Qtz = quartz, Sm = smectite.
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A1
A2
B1
B2
B3
A4
A3
Legend
: Calcite
: Adularia
: Quartz
FIG. 9. Schematic models of the formation process of
lattice-type (A) and parallel-type (B) bladed quartz.
Precipitationof bladed calcite (A1, B1), precipitation of adularia
and quartz (A2, B2), and dissolution of calcite (A3, B3).
Subsequentquartz overgrowth sometimes occurs (A4). A single,
platelike interstice in lattice-type bladed quartz is sometimes
formed bydissolution of several parallel-bladed crystals of
calcite, as shown in Figure 8C.
-
study. Microthermometry was performed using a set ofLinkam
heating-cooling stages (LK-600PM, L-600A, andTH-600RH) at Kyushu
University, in which temperature sta-bility and sensor accuracy
were better than 0.1C. Tempera-tures of homogenization (Th) and the
final melting point ofice (Tm) of liquid-rich fluid inclusions were
measured at aheating-cooling rate of 3C per minute.
Two types of fluid inclusions were observed at room tem-perature
in both bladed calcite and bladed quartz. The firsttype consists of
two-phase, liquid-rich fluid inclusions, con-taining about 70 to 90
vol percent liquid and 10 to 30 vol per-cent vapor, which
homogenize to a liquid phase at an elevatedtemperature. The second
type consists of vapor-rich fluid in-clusions, containing more than
95 vol percent vapor, whichhomogenize to a vapor phase. In contrast
with calcite andquartz, fine-grained rhombic adularia in bladed
quartz con-tained only vapor-rich fluid inclusions.
Fluid inclusions observed in bladed calcite were tetrahe-dral in
shape, sometimes rounded, and the sizes were typi-cally less than
15 m. Liquid-rich fluid inclusions were themost common observed,
and vapor inclusions were less than5 percent of the total
population. The Th of the measured in-clusions varied by as much as
40 to 60C in individual sam-ples (Fig. 10A). The small size of the
fluid inclusions and pooroptical characteristics of the samples
generally prevented themeasurement of Tm. Five liquid-rich fluid
inclusions in onesample, however, indicated a Tm of 0.1 to 0C.
Fluid inclusions observed in bladed quartz were irregularin
shape and elongate, with a feathery structure (Fig. 11). Thesize of
the inclusions was less than 15 m. Vapor-rich fluid in-clusions
accounted for more than 50 percent of all primary in-clusions
observed. The Th of liquid-rich primary inclusionsranged widely
(Fig. 10B), reflecting various liquid-vapor ra-tios. Most fluid
inclusions in bladed quartz were too small forTm measurements. Nine
liquid-rich fluid inclusions in twosamples, however, had a Tm of
0C.
DiscussionBefore examining the conditions of formation of
bladed
quartz, we discuss the origin of bladed calcite, and then
themechanism of formation of bladed quartz, relative to thecalcite.
Finally, we consider how the boiling of ore fluids in-fluenced the
distribution of bladed quartz and high-gradeore.
Browne (1978) described bladed calcite in YellowstoneNational
Park, and discussed how subsurface boiling of fluidscauses changes
in pH through loss of CO2, resulting in pre-cipitation of bladed
calcite. Keith and Muffler (1978) also de-scribed bladed calcite in
well Y-5 of Yellowstone, in which thecalcite existed only within a
few tens of meters of a two-phaseboundary zone, and they also
concluded that the bladed cal-cite was precipitated due to the loss
of CO2. Moreover, Sim-mons and Christenson (1994) used fluid
inclusion and stableisotope data on bladed calcite from
Broadlands-Ohaaki, NewZealand, to show that bladed calcite could
have formed fromboiling of either ascending, chloride-rich water or
descend-ing, steam-heated, CO2-rich water.
The coexistence of liquid-rich and vapor-rich fluid inclu-sions
in bladed calcite from the Hishikari deposit suggests cal-cite
precipitation from a two-phase fluid, though the vapor-rich
fluid inclusions are less abundant. This may indicate eitherthat
vapor-rich inclusions could not be easily trapped in cal-cite
growing from the boiling fluids (Loucks, 2000), or thatthe bladed
calcite only precipitated near the depth at which
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Freq
ue
ncy
100 15050 200 250 300 350 400
024
68 C-1
N=18
0246 C-2
N=23
10
A
100 15050 200 250 300 350 400
Freq
uen
cy
Q-5N=8
02
Q-4N=24
0
2
4
6
8
Q-3N=13
0
2
Q-2N=14
0
2
4
Q-6N=33
0
2
4
6
Q-7N=25
02
4
6
8
Q-1N=18
02
46
B
Homogenization temperature (C)
Homogenization temperature (C)
FIG. 10. Temperature of homogenization (Th) of fluid inclusions
inbladed calcite (A) and bladed quartz (B). The Th is from primary,
liquid-richfluid inclusions that homogenized to a liquid-phase.
Dotted and gray areasshow the vein-formation temperature ranges
from Shikazono and Nagayama(1993) and in Etoh et al. (2002),
respectively.
-
fluids initiated boiling. The low temperatures (Fig. 10A) andlow
salinity obtained by microthermometry of fluid inclusionsfrom
calcite in areas peripheral to the Hishikari vein systemsuggest
that this calcite precipitated from descending fluidsupon loss of
CO2 due to boiling. Elsewhere in the system, pre-cipitation of
quartz-adularia on the bladed calcite suggests thatboth may have
been precipitated from SiO2-rich, ascendinghydrothermal fluids. In
addition, Imai and Uto (2002) exam-ined 13C and 18O of calcite from
Hishikari, including bladedcalcite, and concluded that the
hydrothermal fluids responsi-ble for gold mineralization
equilibrated isotopically with sedi-mentary basement rocks. Their
results also suggest the pre-cipitation of calcite from ascending
fluids in Hishikari.
Both lattice-type and parallel-type bladed quartz havebeen
observed in hand specimens from the Hishikari deposit,and cavities
reminiscent of platelike calcite are common in in-terstices between
bladed quartz. The shape of cavities impliesthe dissolution of
preexisting bladed calcite. Other bladedminerals occur in
epithermal environments, such as anhydriteand barite, but these
have not been reported in the Hishikarideposit or any other
low-sulfidation, epithermal gold depositsin southern Kyushu.
Truscottite shows radiating orientationsof blades, and this
structure is different from that of bladedquartz in the Hishikari
deposit. Therefore, we conclude thatcavities in the interstices
within bladed quartz in the Hishikarideposit were formed by
dissolution of bladed calcite.
Another prominent feature of bladed quartz is the pres-ence of
adularia crystals along the walls of the cavities (Fig.8C), which
indicates that quartz and adularia coated the for-mer calcite
blades. The presence of adularia is considered di-agnostic of
boiling (Browne and Ellis, 1970; Browne, 1978),and Dong and
Morrison (1995) concluded that rhombic adu-laria might form under
particularly vigorous boiling condi-tions. Smectite inclusions in
rhombic adularia (Fig. 8B) mayindicate that the codeposition of the
two minerals is a resultof rapid precipitation, and the coexistence
of smectite and
adularia may reflect the increase in pH due to continued
boil-ing and loss of CO2. The vapor-rich primary fluid inclusions
inthe adularia and the coexistence of liquid-rich and
vapor-richfluid inclusions in the surrounding quartz also suggest
thatadularia and quartz were precipitated on the calcite bladesfrom
boiling fluids.
Figure 9 shows the process of formation of bladed quartzin the
Hishikari deposit, inferred from the observationsabove. First,
bladed calcite was precipitated from boiling flu-ids to make a
framework of either lattice (A1) or parallel (B1)type. Fine-grained
adularia and quartz precipitated on thesurface of calcite blades
during boiling (A2, B2). Then, thebladed calcite dissolved, leaving
cavities in the interstices be-tween the adularia-quartz aggregates
(A3, B3). Although thesurface of the calcite blades was coated and
armored by adu-laria and quartz, the edges of the blades probably
were incontact with the fluids. Cooling due to continued boiling
mayhave caused the dissolution of calcite. As described by Sim-mons
and Christenson (1994), once most dissolved CO2 waslost, calcite
undersaturation would result from continuedcooling. Finally, quartz
was deposited on the adularia-quartzaggregation and filled the
platelike interstices (A4).
The coexistence of vapor-rich and liquid-rich inclusions
inbladed quartz in the Hishikari deposit is consistent with
pre-cipitation from a boiling fluid. Although necking down iscommon
in these irregularly shaped, elongate inclusions, thepresence of
abundant, vapor-rich inclusions cannot be ex-plained solely by the
process of necking down. Izawa et al.(1990), Nagayama (1993a), and
Etoh et al. (2002) also re-ported trapping of boiling fluids in
minerals in Hishikari.Samples of bladed quartz were collected from
various bandsin different veins, which probably formed from fluids
withvarying temperature. Quartz-hosted fluid inclusions of
bladedquartz show wide ranges of homogenization temperatures(Fig.
9B), and the formation temperature of quartz cannot beestimated
accurately. However, the mode of the Th histogramis within ranges
of previously reported temperatures of veinformation, that is,
-
The distribution of bladed quartz and high-grade gold oreshows
that bladed quartz is present largely in the deeper partof the vein
system beneath the high-grade ore zone (Fig. 3).Assuming that the
bladed quartz is indicative of boiling (i.e.,as pseudomorphs of
bladed calcite), this relationship is bestexplained by the
precipitation of gold above a deep boilingzone. Henley (1984)
pointed out that CO2 partitions rapidlyinto the vapor phase during
boiling, whereas H2S may remainin solution longer. At the depth
where boiling is initiated,rapid loss of CO2 and the resulting
increase in pH will causethe solubility of gold as a bisulfide
complex to increase (e.g.,Seward, 1989), and causes precipitation
of bladed calcite.Cooling and gradual loss of H2S to the vapor
eventually re-
duce the solubility of gold and cause gold deposition abovethe
boiling zone.
ConclusionsWe have described the relationship between bladed
calcite
and bladed quartz in the Hishikari gold deposit, and
haveconcluded that both formed from boiling fluids, with
bladedquartz forming from boiling of an ascending, deeply
derivedfluid.
The formation of bladed quartz at Hishikari involves sev-eral
stages: (1) deposition of bladed calcite; (2) precipitationof
fine-grained adularia and quartz on the surface of calciteblades;
(3) dissolution of calcite blades, leaving cavities in
theinterstices between aggregates of adularia and quartz; and
(4)infilling of the cavities by later quartz.
Bladed quartz is present largely in the deeper part of thevein
system, beneath the high-grade gold ore zone. The dis-tribution may
be explained by progressive loss of gasses fromascending, two-phase
boiling fluids.
AcknowledgmentsWe are grateful to the Sumitomo Metal Mining Co.,
Ltd.,
for permission to publish this paper. This paper was improvedby
Drs. F.G. Sajona and L.P. James. We wish to thank S. Ka-jino for
helping to study extensive company files of under-ground notes.
Constructive comments by Economic Geologyreviewers are highly
appreciated. We also would like to ac-knowledge colleagues at
Kyushu University for their helpfulcomments.
December 27, 1999; July 22, 2002
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Q-6N=33
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Elev
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sea
leve
l (m
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Boiling point curve for awater table at 280 mabove sea level
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