explanation - j-stage

MINING GEOLOGY, 40(1), 1•`16, 1990

Relationships Among Carbonate-Replacement Gold Deposits,

Gold Skarns, and Intrusive Rocks, Bau Mining District,

Sarawak, Malaysia

Timothy J. PERCIVAL*, Arthur S. RADTKE** and William C. BAGBY***

Abstract: Three distinct styles of gold mineralization are spatially associated with Miocene microgranodiorite

porphyry stocks in the Bau mining district, Sarawak, Malaysia. These include: (1) gold-bearing calcic skarns; (2) several varieties of veins near and distal to calcic skarns; and (3) carbonate-replacement ore bodies in sedimentary rocks peripheral to the veins and typically furthest from the stocks. Most of the gold produced to date from the Bau

district originated from the carbonate-replacement deposits. These deposits exhibit strikingly similar mineralogical and geochemical features with Carlin-type deposits that occur in the western United States.

Similarities in key mineralogical and chemicall features of the ores indicate that all three styles of mineraliza-

tion are not only spatially, but genetically, related to the microgranodiorite porphyry stocks. Preliminary fluid inclusion measurements on quartz from the three gold ore types suggest decreasing thermal and salinity gradients with increasing distance from the stocks.

Introduction

The Bau mining district is located in Sarawak, Malaysia, on the northwest side of Borneo, approximately 24 kilometers south-west of the capital city, Kuching (Fig. 1). The district's recorded production from 1820 to 1981 is 37.3 million grams of gold, 79 thou-sand tonnes of antimony, and 22,000 flasks

(=748 tonnes) of mercury (HON, 1981). Gold deposits in the Bau district were first

described by GEIKIE (1906) and general descrip-tions of the geology and mineral deposits were given by SCRUTTON (1906) and HAMILTON (1906). Regional geologic maps (WILFORD, 1955) and detailed geologic maps (WOLFEN-DEN, 1965; PIMM, 1967) characterized the geo-logic setting of the district. Mineralogical data on the arsenic-rich gold ores (LAU, 1970) and general information on the physical controls

of mineralization (HON, 1981) helped establish a basic understanding of the occurrence of the gold mineral deposits.

This paper summarizes the key features of various styles of mineralization that formed the different types of gold deposits and dis-cusses spatial and genetic relationships among the carbonate-replacement, vein, and calcic skarn gold deposits with Tertiary, calc-alkaline

porphyritic intrusions. The Bau gold deposits and the Purisima Concepcion deposit in the Yauricocha district of Peru (ALVAREZ and NOBLE, 1988) are the first documented ex-amples suggesting a genetic link between car-bonate-replacement gold deposits (Carlin-type deposits) and magmatism.

District Geology

The geologic framework of western Sarawak includes two subduction melange complexes emplaced upon continental sedi-mentary and volcanic basement rocks. These are (1) a Lower Jurassic to Lower Cretaceous complex in extreme western Sarawak, and (2) an Eocene complex to the east (HAMILTON, 1971). Within the Bau district, the older oce-anic terrane was obducted onto continental rocks that include shale, sandstone, inter-mediate to felsic volcanic rocks, and minor

Received on March 17, 1989, accepted on December

4, 1989 * Nassau Limited

, Sparks, NV 89431 U.S.A.** Cougar Metals International

, Palo Alto, CA 94306,

U.S.A.*** U

.S. Geological Survey, Menlo Park, CA 94025,

U.S.A.

Keywords: Gold skarn, Carlin-type deposits, Carbonate-

replacement gold

1

2 T J PERCIVAL, A S. RADTKE and W C BAGGY MINING GEOLOGY:

EXPLANATION

Fig. 1 Location and geologic map of the Bau mining district, Sarawak, Malaysia Geology modified from WILFORD

(1955).

40(1), 1990 Carbonate-replacement gold deposits in Bau, Sarawak 3

Fig. 2 Stratigraphy of the Bau mining district and adjacent areas. Modified after WOLFENDEN (1965) and PiMM

(1967).

limestone. The continental rocks were in-truded and metamorphosed by pre-late Trias-sic and Miocene inter-mediate to felsic plu-tonic rocks (WOLFENDEN and HAILE, 1963; KIRK, 1968).Stratigraphy

The Bau district is underlain by a sequence of Upper Jurassic through Lower Cretaceous rocks (Fig. 1). The Upper Jurassic Bau Limestone is conformably overlain by clastic sedimentary rocks intercalated with subor-dinate volcanic rocks of the Lower Cretaceous Pedawan Formation (Fig. 2). The carbonate and clastic sedimentary rocks underlie most of the district and are the host rocks for the gold and antimony deposits.

Igneous rocks include the Upper Triassic Serian Volcanics, a bimodal sequence of tuffs, flows, and breccias that are exposed north and east of the Bau district. Minor amounts of Jurassic and Lower Cretaceous mafic intrusive and volcanic rocks occur west of the district. Miocene microgranodiorite and dacite por-

phyry occur as stocks, sills, and dikes that in-trude the sedimentary section within the Bau

district (Figs. 1 and 2).Structural Geology

The Bau district lies along the axis of the east-northeast-trending Bau anticline (Fig. 1). The anticline is symmetrical with a broad axial crest, steeply dipping limbs, and is flanked by synclinal basins to the northwest and south-east. Several sets of near vertical faults dissect the crest of the anticline (Fig. 1).

Vertical displacement along several of the north-and northeast-trending faults created a graben that served as the locus for emplace-ment of several Tertiary intrusions that extend southwest into Indonesia. A second fault and fracture set trends northwest and forms a near-ly orthogonal relationship with the northeast-trending faults (Fig. 1).

Descriptions of Gold Deposits

The gold deposits at Bau can be divided into three types, reflecting different styles of gold mineralization:(1) gold-bearing calcic skarn, (2) gold-and base-metal-bearing veins, and (3) carbonate-replacement ore bodies. The aerial distribution of these distinct types of gold

4 T J. PCRCIVAL, A. S RADTKE and W C BAGBY MINING GEOLOGY

Fig. 3 Geologic map of the Bau district showing the location and aerial distribution of each type of gold deposit discussed in this paper The geologic formations and principal gold deposits (1-10) correspond to those in Figure 1. Additional deposits discussed in the text or listed in the geochemical tables are numbered as follows:(11) Bor-

ing, (12) Gading, (13) Gunung A. Bukit, (14) Gunung Bau and Lucky Hill, (15) Gunung Kolong Bau , (16) Gunung Krian, (17) Kalimantan Lease, (18) Saburan, and (19) Tegora.

mineralization is shown in Figure 3. Figure 4

is a schematic cross sectional view showing the

geologic setting and typical geometric form of each type of gold deposit and its spatial

association with the porphyritic intrusions.

The carbonate-replacement gold deposits ac-

counted for the bulk of the gold production.

These deposits are similar in mineralogy and

texture to carbonate-replacement gold depos-its in the western United States that are com-monly referred to as Carlin-type deposits . The Tai Parit carbonate-replacement deposit was the largest in the district with a recorded pro-duction of 15.5 million grams (Hon, 1981). Other large carbonate-replacement deposits in the district are the Tai Ton, Jambusan , and


Fig. 4 Schematic cross section representation of the various styles of gold mineralization , their typical geometric form and host rocks and their spatial association with the microgranodiorite porphyry intrusions. Abbreviations

are as follows; py: pyrite, qz: quartz, cc: calcite and cs: talc-silicate.

Bidi deposits (Figs. 1 and 3).Calcic Skarn Deposits

Gold-bearing calcic skarn deposits occur along microgranodiorite intrusive contacts with limestone in several areas of the Bau district (Figs. 1 and 3). Although the calcic-

gold skarns that crop out in the Bau district are volumetrically minor compared to the car-bonate-replacement deposits, they provide an important insight into the genesis of all the

gold deposits in the district. The calcic skarn deposits in the Gunung A. Bukit and Gunung Bau areas serve as examples for the descrip-tions presented below.

Calcic skarns at Bau occur as discontinuous

pods sporadically distributed along grano-diorite contacts with limestone (Fig. 4). The skarns are composed of wollastonite, andra-dite-grossular garnet, diopside, epidote, chlo-rite, vesuvianite, calcic-plagioclase, sulfides, and late quartz and calcite. The talc-silicate minerals form idioblastic crystals and xeno-blastic grains in tight, compact intergrowths. Stibnite, pyrite, arsenopyrite, sphalerite, and

pyrrhotite are present and occur as dissemi-nated grains and small crystals intricately intergrown with the talc-silicate minerals. Stibnite also occurs in late calcite as acicular

6 T. J. PERCIVAL, A. S. RADTKE and W. C. BACBY MINING GEOLOGY:

Table 1 Trace element geochemical analyses of selected samples of various types of vein and calcic skarn from the

Bau Districts Sarawak.

[Analyses by: Hunter Mining Laboratory, Sparks, Nevada. Gold and silver determined by fire assay. All other elements determined by atomic absorption analysis; --, no data. All analyses are reported in parts per million.]


Table 2 Chemical analyses of fresh sedimentary rocks, altered igneous intrusive rocks and various types of gold

ores.

[Analyses by Hunter Mining Laboratory, Sparks, Nevada. Gold and silver determined by Fire Assay methods (1 assay ton). Bracketed values determined by atomic absorption method. All other values determined by emission spectroscopy. <, not detected at detection limit shown; -, no data; gold and silver given in ounces per ton all other values in parts per million]

crystals. Calc-silicate veins containing arseno-pyrite, sphalerite, and stibnite crosscut mas-sive skarn. Geochemical analyses of representative

calcic skarns are shown in Table 1. These analyses show elevated concentrations of

gold, arsenic, antimony, and zinc relative to unaltered Bau Limestone (Table 2). Some skarns sampled by WOLFENDEN (1965) contain-ed as much as 31 to 125 ppm gold. Other calcic skarns contained such high concentrations of stibnite that they were mined solely for their antimony content (WOLFENDEN, 1965).Vein Deposits

Three types of gold-bearing veins, distin-

guished by their mineralogy, occur in the Bau district. Type 1 veins are composed primarily of either quartz, quartz+calcite, or calcite. Type 2 veins contain microcyrstalline quartz and calc-silicate minerals. Type 3 veins are composed predominantly of base-metal sul-fides. All veins occur in high-angle faults, frac-tures, joints, and bedding planes within mas-

sive units of the Bau Limestone (Fig. 4). Gold is present in all three types and gold-bearing veins are widely distributed throughout the Bau district (Fig. 3). Although gold-bearing veins are abundant, they account for a relative-ly minor portion of past gold production. The veins are narrow (0.5 to 2.5 meters wide) and discontinuous along strike.

Type 1 veins are the most widely distributed and abundant of the vein types. Veins of Type 1 at the Rumoh, Saburan, and Gunung Krain deposits were extensively mined for their gold and antimony content. Coarsely crystalline calcite accounts for greater than fifty volume percent of Type 1 veins, the balance is compos-ed of variable amounts of sulfide minerals and

quartz. Sulfides include stibnite, arsenopyrite, sphalerite, and pyrite. Native arsenic and realgar are also present. The sulfide minerals are intricately intergrown with quartz and calcite.

The gold contents of the veins are variable and attain high grades (Table 1). Silver con-

8 T. J. PERCIVAL, A. S. RADTKE and W. C. BAGRY MINING GEOLOGY:

concentrations of the base metals as well as

gold and silver (Table 1). Their genetic and temporal relationships with the spatially associated carbonate-replacement ores remain unclear because of a lack of exposed outcrops and three-dimensional information. However, the mineralogy and chemistry of these veins strongly suggest that they formed from the same hydrothermal system that formed the other gold-bearing deposits in the district.Carbonate-replacement Deposits

Carbonate-replacement deposits occur along the contact of the Bau Limestone with the Pedawan Formation in close proximity to steeply dipping faults (e.g., Tai Parit and Tai Ton deposits, Figs. 1, 3 and 4). These deposits also occur within massive Bau Limestone and shale along brecciated fault zones (e.g., Bidi area, Fig. 1). The sedimentary host rocks in these deposits are brecciated near the faults and the limestone, and to a lesser degree the shale, is silicified. The intensity of silicification is directly proportional to the amount of brec-ciation. Silicification is gradational from minor replacement of matrix calcite to com-

plete replacement (jasperoid*1) of the original rock by microcrystalline quartz. In many areas, pervasively silicified rocks are cut by coarsely crystalline quartz veins and veinlets.

Near, or adjacent to, areas of silicification, massive, pure carbonate rocks in the Bau Limestone and shales in the Pedawan Forma-tion are argillically altered. The alteration mineralogy of both the shale and limestone in-cludes kaolinite, dickite, illite, minor sericite, calcite, quartz, marcasite, and pyrite. Iron ox-ides are prevalent in weathered altered rocks.

Although limestone was pervasively replac-ed by microcrystalline quartz in the silicified breccias, shale fragments were only partially replaced, resulting in rocks containing quartz, sericite, and fine-grained clay. Quartz veins and veinlets, and zones of intense stockwork

quartz veining cut the silicified breccias, par-ticularly where the breccias occur in shale. The

*1The term "jasperoid" is used here as described by

SPURR (1898) to refer to rocks formed by the epigenetic siliceous replacement of a previously lithified rock.

tents are also variable, with highest concentra- tion associated with abundant manganese ox- ides. Arsenic and antimony may comprise several percent of a given sample. Base metal abundances are generally low, except in isolated samples.

Type 2 veins contain calc-silicate minerals in addition to microcrystalline quartz+calcite and cross cut recrystallized limestone (marble) within the contact metamorphic aureole,

generally outboard from massive calcic skarns (Fig. 1 and 3). These veins are most abundant in the central portion of the district where emplacement of calc-alkaline intrusions was most extensive (e.g., Gunung Bau, Lucky Hill). The veins are narrow and occupy steeply dipping faults, joints, and fractures within marble (Fig. 4). The gold ore zones within the veins are lensoid and pod-like bodies. WOLFENDEN (1965) reported that calc-silicate veins contain up to 125 ppm gold but average 12.5 ppm gold.

The mineralogy of Type 2 veins is similar to that of the calcic skarns with the exception of the higher abundance of microcrystalline

quartz in the veins. Sulfides are finely disseminated within microcrystalline quartz but are also intergrown with wollastonite , garnet, and vesuvianite. In addition to the sulfides, the calc-silicate veins contain native antimony, and aurostibite (AuSb2) and locally abundant sarabauite (CaSb10O10S10; NAKAI et al., 1978). At the Lucky Hill deposit, stibnite and sarabauite account for several percent each of the ore and are intergrown with wollastonite, calcite, and microcrystalline

quartz. Coarse white calcite and drusy and coarsely crystalline quartz line and fill vugs .

Table 1 contains analyses of several Type 2 vein samples. The samples contain anomal-ously high concentrations of gold, antimony , and arsenic. Type 3 base-metal-bearing veins contain sphalerite, chalcopyrite, and galena in association with lesser pyrite, arseriopyrite, and quartz. These veins occur in only a few localities in the district (e.g., Say Seng, Batu Bekajang Lake). The veins cross cut car-bonate-replacement ore bodies and altered microgranodiorite stocks. They contain high

40(l), 1990 Carbonate-replacement gold deposits in Bau, Sarawak 9

Table 3 Trace element geochemical analyses of selected samples of various types of carbonate-replacement ores

from the Bau District, Sarawak.

[Analysis by: Hunter Mining laboratory, Sparks, Nevada. Gold and silver were

determined by fire assay methods. All

other elements determined by atomic

absorption analysis. All analyses are

reported in parts per million.]

veins range from < 1 to > 1 centimeter in thickness. Vugs in the siliceous veins and in the jasperoids are lined with drusy quartz and sulfide minerals.

The abundance of sulfide minerals and native metals in the silicified rocks is directly

proportional to the quantity of introduced silica. Native arsenic, stibnite, arsenopyrite,

pyrite, realgar, sphalerite, and orpiment were introduced both contemporaneous with, and

later than, the microcrystalline quartz. The

abundances of these minerals and the relative

10 T. J. PERCIVAL, A. S. RADTKE and W. C. BAGBY MINING GEOLOGY:

proportions in which they occur are highly variable within and among individual deposits.

Native arsenic occurs in the quartz matrix as botryoidal crystalline aggregates which exhibit

growth bands and are altered to black, fine-grained arsenolite and other secondary arsenic phases. Arsenopyrite and lesser pyrite occur as euhedral crystals within the quartz matrix with native arsenic, acicular stibnite, and rare realgar. Stibnite is overgrown upon both native arsenic and arsenopyrite.

In many areas, the carbonate-replacement ores are very arsenic-rich and commonly con-tain greater than twenty percent arsenic (Table 3). These ores contain the same minerals as the typical replacement ores with the exception of excessive concentrations of native arsenic.

The carbonate-replacement ores contain very high concentrations of arsenic, an-timony, and gold (Table 3). It is not uncom-mon for these ores to contain many thousands of ppm arsenic and antimony and to contain > 5 ppm gold (Table 3). Mercury, thallium, and zinc are anomalous in these ores and cop-

per and lead occur at very low concentrations. The oxidized equivalents of these ores contain lower concentrations of arsenic and antimony

(Table 3) and were actively exploited for their gold content because of their relative amiabili-ty to gold extraction.Ore Controls

Three physiochemical controls influenced

the locations of the different gold deposit types as well as the intensity of gold mineraliza-tion within the Bau district. These are: region-al and local structures, stratigraphic order and lithology, and chemical reactivity of the rocks.

Faults within a northeast-trending zone defined by an alignment of stocks, hydrother-mally altered rocks, and faults served as the

primary structural control on gold mineraliza-tion processes. This zone, the Bau gold trend, extends to the southwest and includes gold-bearing mercury deposits at Tegora and Gading in Sarawak (Fig. 3) and gold districts in Kalimantan, Indonesia. This regional align-ment of hydrothermal gold deposits is analogous to the Carlin and Cortez trends in

Nevada, U.S.A. (see BAGBY and BERGER, 1985; PERCIVAL et al., 1988). The trends in both countries reflect underlying, deep-seated, linear structural zones within the crust. Several prominent faults are exposed for 3 to 4 kilometers within and parallel to the trend of the Bau gold deposits (e.g. Tai Parit Fault, Fig. 1) and represent exposures of the deep-seated structural zone. Most gold deposits in the Bau district lie within the area where this northeast-trending structural zone intersects the axial zone of the east-to northeast-trending Bau anticline (Fig. 1).

The occurrence of numerous gold deposits along and near the Tai Parit fault suggests that this fault was a major conduit for hydrother-mal fluids (Figs. 1 and 3). At the Tai Parit deposit, the permeable, clastic Krian member of the Bau Limestone was faulted against massive limestone members of the same forma-tion, permitting the development of large, car-bonate-replacment ore bodies. HON (1981) reported that less prominent north- and northwest-trending faults and fractures within the gold trend were intruded by dikes and con-tain narrow veins. These faults and fractures

probably also served as conduits for hydro-thermal fluids.

Stratigraphic sequence, lithology, and che-mical reactivity of the host rocks are all inter-related as controls on formation of the gold deposits. Neither massive, dense limestone members of the Bau Limestone, nor shales of the Pedawan Formation are favorable host rocks for carbonate-replacement ore deposits. However, both units host carbonate-replace-ment ore bodies where the rocks were exten-sively fractured or where faulting has changed the stratigraphic position of the rocks. The relative impermeability and unreactive nature of the shale make it an excellent seal to hydrothermal solutions where the shale is faulted above chemically reactive, brecciated limestone.

Discussion

Intrusions and Contact Metamorphism and

Metasomatism

Miocene calc-alkaline stocks, dikes, and


sills were emplaced into a sequence of Meso-

zoic sedimentary rocks in the Bau district.

Outcrop patterns and areal distribution of

dikes that radiate outward from, yet are con-

nected to the individual stocks, suggest that

these features are the surface expression of

a larger intrusive body at depth. The stocks

appear to have been forcefully emplaced,

resulting in a slight doming of the sedimentary

wall rocks.

The intrusions all exhibit porphyritic tex-

tures suggesting emplacement at relatively

shallow (epizonal) crustal levels. BURNHAM

(1979) estimates that porphyritic textures are a

product of physiochemical processes that oc-

cur at magma temperatures of 750•‹to 850•Ž

within the range of 1 to 2 kilobars lithostatic

pressure, corresponding to depths of between

about 4 and 8 kilometers.

The contact metamorphic aureoles sur-

rounding the stocks at Bau are of limited size;

they are usually less than 35 meters wide but

locally extend up to 350 meters from the intru-

sive contacts. Recrystallization of limestone is

the dominant product of contact metamor-

phism. However, calcic skarn occurs adjacent

to stocks and grades outward to a zone of

wollastonite-, garnet-, idocrase-, and clinopy-

roxene-bearing rocks and veins (Type 2) that

occur primarily along joints, fractures, and

faults in limestone. The mineralogy, spatial,

and textural relationships of the calcic skarn

and Type 2 veins to the intrusions support a

metasomatic origin for the calc-silicate miner-

als. The presence of gold in the calcic skarns

and Type 2 veins indicate that the emplace-

ment of silicic magmas was an integral part of

the ore-forming hydrothermal system. Other

examples of gold-bearing skarns related to calc-

alkaline intrusions include McCoy, Nevada

(KUYPER, 1988) and Muara Sipongi, West

Sumatra(BEDDOE-STEPHENS et al., 1987). ORRIS

et al. (1987) have tabulated about thirty occur-

rences worldwide of gold-bearing skarns, in-

cluding those in the Bau district.

The carbonate-replacement deposits con-

tain the same ore minerals as the skarns. The

spatial relationships of the carbonate-replace-

ment deposits to the intrusions and to the

skarn deposits leads us to believe that they formed from evolved, modified metasomatic fluids. By establishing the conditions of calc-

silicate formation, we can begin to understand the genetic link between the carbonate-replace-ment, skarn, and vein gold deposits at Bau.Environment of Formation of Calcic Skarn

The mineralogic and petrologic features of

calcic skarn and Type 2 vein deposits are

strikingly similar and we believe that their

genesis is the same. The intergrowth of

wollastonite, garnet, and idocrase with stib-

nite, native antimony, arsenopyrite, and gold

strongly supports the interpretation that

skarn-forming fluids also transported and

deposited the ore minerals.

The porphyritic textures of the micrograno-

diorite stocks indicate emplacement and cry-

stallization of magmas at depths less than eight

kilometers. The upper stability limit of wolla-

stonite in this low pressure environment is be-

tween about 500•‹to 550•Ž (WINKLER, 1976;

EINAUDI et al., 1981). These temperatures as-

sume a mole fraction of CO2 of approximately

0.10, considered by EINAUDI et al. (1981) to be

a reasonable estimate for the epizonal contact

metamorphic environment. The 550•Ž is a

maximum temperature for calc-silicate miner-

al formation associated with epizonal stocks.

Temperature stability limits for calc-silicate

mineral assemblages from epizonal skarns pub-

lished by REVERDATTO (1973) and EINAUDI et

al. (1981) suggest that the temperature range

of 400•‹ to 500•Ž is more likely for the Bau

skarns.

The upper stability limits for aurostibite

and sarabauite also fall within the same

temperature range. Aurostibite melts at 460•Ž

(HANSEN and ANDERKO, 1958) and sarabauite

undergoes phase transformations above

420•Ž (NAKAI et al., 1978). The assemblage

aurostibite+gold melts at 360•Ž, which sug-

gests that ore mineral deposition continued to

lower temperatures if these two minerals were

deposited in equilibrium. Other evidence for

continued ore mineral deposition at lower

temperatures is given by cross cutting micro-

crystalline and coarsely crystalline quartz+

calcite veins with identical ore mineralogy as


Fig. 5 Northwest-southeast cross section through the Bau district showing geologic and spatial relationships

among deposit types and igneous intrusions, changes in bulk mineralogy, geochemistry, and fluid inclusion

characteristics with respect to geology. Abbreviations: Fl=fluid inclusions, dm=daughter minerals; minerals,

wo=wollastonite, gr=grossular, an=andradite, vs=vesuvianite, ep=epidote, and pl=plagioclase; ore minerals,

as=native arsenic, st=stibnite, asp=arsenopyrite, py=pyrite, rl=realgar, orp=orpiment, aur=aurostibite,

sar=sarabauite, sp=sphalerite, and Au=native gold.

the skarns.

Vein and Carbonate-replacement Deposits

Figure 5 shows the relationship between

mineral-deposit type and distance from the

porphyritic stocks. Vein deposits occur out-side of the calcic skarn zone but, irl general,

are closer to the intrusions than the carbonate-

replacement deposits. The lack of permeable

host rocks near the stocks explains the restric-

tion of veins to fault zones and their close

spatial association with dikes that also occupy

faults. The sulfide mineralogy of Type 1 veins


is virtually identical to that of calcic skarn and

Type 2 veins. Most of the carbonate-replace-

ment deposits occur along the Tai Parit fault

(Fig. 1) where it intersects limestone in contact

with shale. These deposits are the most distal

gold deposits to the intrusive rocks and exhibit

lower temperature silicate mineral assem-

blages and textures than those of the vein and

skarn deposits.

Several mineralogical and geochemical

changes occur in carbonate-replacement

deposits with increasing distance from the

stocks. Native arsenic, arsenopyrite, realgar,

and stibnite are more abundant, As:Sb in-

creases, and there is a marked increase in the

abundance of microcrystalline quartz as

distance from a stock increases. The presence

of realgar in association with native arsenic

places an upper limit of 281•Ž on the

temperature of deposition of these minerals in

the carbonate-replacement deposits (HALL and

YUND, 1964).

Fluid Inclusion Measurements

A preliminary examination of fluid inclu-

sions in quartz in the calcic skarn and Type 2

vein material indicates that many of the

primary inclusions are liquid+vapor type con-

taining cubic and acicular daughter minerals.

The cubic daughter mineral is halite. Prelim-

inary measurements indicate the average ho-

mogenization temperatures range from Thv=

280-315•Ž and Ths=185-190•Ž for the halite-

bearing inclusions (PERCIVAL et al., 1989).

These high salinity, moderately high tempera-

ture inclusions suggest, based upon their close

spatial relationships with the porphyry stocks,

that the ore fluid originated in part from a

magmatic-hydrothermal source. However, no

KCl or K-Fe-Cl daughter minerals have been

identified, therefore a large magmatic fluid in-

put has not been proven. In contrast, later

formed microcrystalline quartz which cross

cuts the calcic skarn and Type 2 veins con-

tains very small (<I micron) liquid+vapor

fluid inclusions that are typical of low tempera-

ture quartz (e.g., see BODNAR et al., 1985).

A reconnaissance study of the fluid inclu-

sions in the carbonate-replacement ores in-

dicates that microcrystalline quartz typically

contains very small (approximately 1 micron

or less) primary liquid + vapor inclusions

(PERCIVAL et al., 1989). The morphology and

petrologic relations of the inclusions and the

mosaic texture of the microcrystalline quartz

are identical to those studied by BODNAR et al.

(1985) from many Carlin-type deposits in the

western United States. Most inclusions of this

type are too small for microthermometry and

those which have been measured by BODNAR et

al. (1985) typically homogenize at 200•Ž or

less. Preliminary studies of measureable inclu-

sions from the carbonate-replacement ores at

Bau yield homogenization temperature ranges

of Thv=210-240•Ž and Tmice= -1.0 to

-2.6•Ž (PERCIVAL et al., 1989). The inclu-

sions are of low salinity and similar tempera-

ture range as those for Carlin-type deposits in

Nevada, U.S.A. that have been studied in de-

tail. For example, geological and geochemical

studies at Carlin by RADTKE et al. (1980) and

RADTKE (1985) indicates gold deposition occur-

red between 175-200•Ž from fluids with a salin-

ity of approximately 3 equivalent weight per-

cent NaCl. Similar studies at Cortez (RYTUBA,

1977), Mercur, Utah (JEWELL, 1984) and recon-

naissance of other Nevada deposits by NASH

(1972) are in close agreement. More recent

studies of the Jerritt Canyon deposit by Ho-

FSTRA et al. (1987) indicates that fluid inclu-

sions in quartz from gold ore stage minerali-

zation homogenize at temperatures ranging be-

tween 200-300•Ž and contain between 3-10

equivalent weight percent NaCI. Therefore,

we conclude the carbonate-replacement

deposits at Bau, with similar mineralogy and

geochemistry to deposits in the western U.S.

(Table 4), formed from fluids of similar

physiochemical properties.

Summary

The different types of gold deposits in the Bau district are genetically related to middle

Miocene calc-alkaline magmatic activity. The emplacement of intermediate to silicic mag-mas formed a contact metamorphic aureole which was overprinted by a hydrothermal sys-

tem that formed calcic skarns. Gold was de-

posited with calc-silicate minerals at high tem-

14 T. J. PERCIVAL, A. S. RADTKE and W. C. BAGEY MINING GEOLOGY:

Table 4 Comparison of important geological and chemical feature of carbonate-replacement ores at Bau with

similar sedimentary rock-hosted gold deposits in the western United States.

peratures in the epizonal environment. As the hydrothermal system cooled and evolved, veins and carbonate-replacement gold deposits were formed. This conceptual model is supported by preliminary fluid inclusion measurements, textural and mineralogical characteristics of the deposits, and the spatial associations of the deposits to the porphyritic stocks. Additional fluid inclusion studies and stable isotope re-search are now in progress to test the model.

Textural, mineralogical, and geochemical features of the carbonate-replacement depos-its at Bau are similar to gold deposits at Carlin (RADTKE, 1985) and Jerritt Canyon (BIRAK and HAWKINS, 1985; HOFSTRA and ROWE, 1987) and numerous other sedimentary rock-hosted pre-cious-metal deposits in Nevada, U.S.A.

(BAGBY and BERGER, 1985). Table 4 is a com-parison the Bau deposits with several very similar deposits in Nevada. The conceptual

model that we suggest genetically links the

skarn, vein, and carbonate-replacement gold

deposits in the Bau district may also be valid

for the carbonate-replacement deposits in Ne-

vada. The discovery of deep gold ore associ-

ated with skarn in the Carlin district of Ne-

vada provides evidence that the conceptual

genetic model for the Bau gold deposits may indeed have broad application.

Acknowledgements: We thank C. G. CUNN-

INGHAM, Jim BLISS, and Greg MCKELVEY of the

U. S. Geological Survey, and F. W. DICKSON

of the University of Nevada, Reno for critical

and helpful reviews of earlier versions of the

manuscript. We also thank Jeffery HEDEN-

QUIST and a second reviewer for Mining Geology for their comments. Permission to

publish company data was given by Blakeney STAFFORD, Managing Partner, Nassau Ltd.

palo Alto, California.


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マレーシア・サラワク州バウ鉱山地帯の炭酸塩岩交代鉱床 ,

金を伴うスカルン鉱床と貫入岩体との関係

要旨:マレーシア・サラワク州バウ鉱山地帯にある第三

紀の細粒花崗閃緑斑岩の貫入岩体は周囲に三種の異なっ

た金鉱化作用を伴っている.それらは,(1)岩体周辺の金

を伴うスカルソ鉱床,(2)このスカルソの近傍ないしそれ

より離れた地域に見られる鉱脈鉱床,(3)鉱脈をとりまい

て貫入岩体から離れた地域の堆積層中に見られる,炭酸

塩岩を交代した交代鉱床,の三種である.これらの鉱床

は米国西部に見られるカーリン型金鉱床と鉱物学的,地

球化学的に非常に類似した特徴を示す.これらの類似は

鉱床と貫入岩体との関係が単に位置的なものではなく,

成因的なものであることを示している.バウ地帯の鉱床

中の石英の流体包有体に対する予察的測定データによる

と,包有物の均質化温度,塩濃度は貫入岩体から遠ざか

るにつれて低下することが判明した.

explanation - j-stage

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