Etoh - Bladed Calcite at Hishikari (2003)

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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(0–130 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 (0.95–1.78 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 (0.66–1.10 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 (24,000–25,000 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 SEKINE

Mineral 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. 1841–1851

† 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 N50°E 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 µm 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 of–20 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 Ocean

30oN

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 100µm in thickness (Fig. 6). Where the blades have a parallelstructure, the interstices between the blades range in widthfrom 5 to 100 µm (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

Yamada

A1000m0 200 400 600 800

A'

DA-1

HO-2

HonkoHO-2

YamadaHonko-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-7

ZU-1

SH-5

KE-3

HO-1

KI

RY

DA-2

W160B W120B W80B W40B 0B E40B E80B

N40

G0G

S40G

0 400m

9MAHAK-6N

56MAHT-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-5YU-6

YU-7

SE-8

SE-2

SE-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 3–10 <0.1 cal. 9MAHAK-6 35 W220 HLAC-2 Lattice, parallel 3–15 <0.1 cal. Daisen-1 47 E35 SSGQ-1 Lattice 1–18 0.1–2 qtz., adu., cal. 9MAHAK-6 15 W220 HLAQ-2 Parallel 20–60 <0.1–0.2 qtz., adu., sm. Hosen-2 40 W13 SSGQ-3 Lattice 5–35 0.5–4 qtz., adu. Zuisen-1 25 W37 SSGQ-4 Lattice 20–80 0.1–1 qtz., adu. 56MAHT-1 25 E25 SSGQ-5 Parallel 1–3 <0.1–0.3 qtz., adu., sm. Seisen-7 35 W92 HLAQ-6 Lattice 30–100 7–10 qtz., adu., sm. Hosen-2 –5 E2 SSGQ-7 Lattice 5–12 0.5–1.5 qtz., adu. Daisen-1 40 E18 SSG

Abbreviations: adu. = adularia, cal. = calcite, HLA = Hishikari Lower Andesites, qtz. = quartz, sm. = smectite, SSG = Shimanto Supergroup

to 100 mm in diameter and <0.1 to 10 mm in thickness (Fig.7), and consist of anhedral quartz crystals, 10 to 200 µm insize, with rhombic adularia crystals, 5 to 70 µm in size (Fig.8A). Some rhombic adularia crystals of 10 to 50 µm in size

contain minute mineral inclusions (Fig. 8B). Infrared spec-troscopy (JASCO MFT-2000) indicates that the inclusions arecomposed of smectite group minerals. Open spaces, rangingin width from 5 to 2,000 µm, are commonly observed betweenthe individual blades. Fine-grained rhombic adularia occurson the wall of cavity, whereas comb quartz grows away fromthe cavity (Fig. 8C). Although the cavities are often filled withlater quartz, two parallel planes consisting of fine-grainedrhombic adularia outline the walls of the preexisting cavity(Fig. 8D). Figure 9 summarizes these textural relationships.

Fluid InclusionsDoubly polished thin sections, 0.3 to 0.5 mm thick, were

prepared for the study of fluid inclusions in bladed quartz.Whole, bladed calcite crystals with a thickness of less than 0.1mm were also used for the study of fluid inclusions. Primary,pseudosecondary, and secondary fluid inclusions were distin-guished using the criteria of Roedder (1984) and Bodnar et al.(1985), and only primary fluid inclusions were used for this


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Lattice type Parallel type

FIG. 5. Schematic representations of lattice and parallel structural types.

50 µmB

Cal.

5mmA

50 µm

Qtz. & Adu.

D

Cal.

50 µmC

Cal.

FIG. 6. Bladed calcite sample C-1. A. Bladed calcite precipitated in a cavity in andesite lava. B. Lattice-type bladed cal-cite in thin section with crossed polars. Calcite blades of 10- to 50-µm thickness intersect each other. C. Parallel-type bladedcalcite in thin section with crossed polars. Calcite blades of 10- to 50-µm thickness occur in a parallel array at 30- to 100-µmintervals. D. Bladed calcite crystal 20 to 30 µm in thickness. The surface of calcite is coated by an aggregate of adularia andquartz. Sample Q-1. Abbreviations: Adu = adularia, Cal = calcite, Qtz = quartz.


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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 µm

Adu.Qtz.

A

0.5mm

Qtz. & Adu.

Qtz.

C

20 µm

FIG. 8. Bladed quartz in thin sections with crossed polars. A. Blades 10 to 50 µm in thickness in parallel array with cav-ities 10 to 50 µm 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 (0.2–0.6 mm); several slivers of quartz, 10 to 50 µm 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 0.1°C. Tempera-tures of homogenization (Th) and the final melting point ofice (Tm) of liquid-rich fluid inclusions were measured at aheating-cooling rate of 3°C 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 µm. 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 60°C 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 0°C.

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 µm. 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 0°C.

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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Fre

quen

cy

100 15050 200 250 300 350 400

0

2

4

6

8 C-1N=18

0

2

4

6 C-2N=23

10

A

100 15050 200 250 300 350 400

Fre

quen

cy

Q-5N=8

0

2

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

0

2

4

6

8

Q-1N=18

0

2

4

6

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 δ13C and δ18O 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, <200° to 250°C (Izawa et al., 1990), 150° to250°C (Shikazono and Nagayama, 1993), and 175° to 215°C(Etoh et al., 2002). The Tm values of the fluid inclusions wereall 0°C and, therefore, the salinity is very low. Although therange of Th for quartz-hosted fluid inclusions is large, if thefluids responsible for quartz deposition were boiling at a tem-perature of 190°C, the pressure would have been about 13bars or the equivalent pressure of 130 m of overlying water. Ifthe depth of boiling was controlled only by the height of theoverlying water column, then the paleowater table must havebeen at least 130 m above the boiling zone (e.g., Fig. 12).Similar values were obtained from the fluid inclusion study ofcolumnar adularia (Etoh et al., 2002). However, the sug-gested confining pressures are significantly lower than thosereported by Izawa et al. (1990), and may indicate a suddendrop in pressure associated with the formation of bladedquartz, possibly by dilatational fracturing (e.g., Uto et al.,2001). Large-scale dilatational fracturing is also suggested bythe occurrence of large fragments of volcanic rocks in theveins hosted by the basement sedimentary rocks. Nagayama(1993b) also concluded that pressure loss and related boilingphenomena form the Hishikari veins, with adularia/quartzratios decreasing from early to later stage in a single vein.


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20 µm

V.R.

L.R.

FIG. 11. Fluid inclusions in bladed quartz. Predominant, vapor-rich in-clusions and minor, liquid-rich inclusions are seen. Feathery textures are ob-served from the distribution of fluid inclusions (sample Q-6). Abbreviations:LR = liquid-rich inclusions, VR = vapor-rich inclusions.

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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100

150

0

50

200

250

300

Q-6N=33

Q-4N=24

Q-7N=25

Ele

vatio

n ab

ove

sea

leve

l (m

)

Boiling point curve for awater table at 280 mabove sea level

170 m

95 m

50 100 150 200 250 300 350 400

Temperature (°C)

FIG. 12. Hypothetical, boiling point-depth curves for pure water withpossible paleowater tables at elevations of 95, 170, and 280 m above sea level.Curves were adjusted for estimated precipitation temperatures of the bladedquartz Q-6, Q-4, and Q-7, with the boiling zones located at 5, 25, and 42 mabove sea level, respectively.

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