Relationships Among Carbonate-Replacement Gold Deposits, Gold Skarns, and Intrusive Rocks, Bau Mining District, Sarawak, Malaysia

MINING GEOLOGY, 40(1), 1•`16, 1990

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 mineralization 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.

of mineralization (HON, 1981) helped establish Introduction a basic understanding of the occurrence of the The Bau mining district is located in gold mineral deposits. Sarawak, Malaysia, on the northwest side of This paper summarizes the key features of Borneo, approximately 24 kilometers south- various styles of mineralization that formed west of the capital city, Kuching (Fig. 1). The the different types of gold deposits and dis- district's recorded production from 1820 to cusses spatial and genetic relationships among 1981 is 37.3 million grams of gold, 79 thou- the carbonate-replacement, vein, and calcic sand tonnes of antimony, and 22,000 flasks skarn gold deposits with Tertiary, calc-alkaline (=748 tonnes) of mercury (HON, 1981). porphyritic intrusions. The Bau gold deposits Gold deposits in the Bau district were first and the Purisima Concepcion deposit in the described by GEIKIE(1906) and general descrip- Yauricocha district of Peru (ALVAREZand tions of the geology and mineral deposits were NOBLE, 1988) are the first documented ex- given by SCRUTTON(1906) and HAMILTON amples suggesting a genetic link between car- (1906). Regional geologic maps (WILFORD, bonate-replacement gold deposits (Carlin-type 1955) and detailed geologic maps (WOLFEN- deposits) and magmatism. DEN,1965; PIMM, 1967) characterized the geologic setting of the district. Mineralogical data District Geology on the arsenic-rich gold ores (LAU, 1970) and The geologic framework of western general information on the physical controls Sarawak includes two subduction melange complexes emplaced upon continental sedi- Received on March 17, 1989, accepted on December mentary and volcanic basement rocks. These 4, 1989 * Nassau Limited are (1) a Lower Jurassic to Lower Cretaceous , Sparks, NV 89431 U.S.A. ** Cougar Metals International complex in extreme western Sarawak, and (2) , Palo Alto, CA 94306, an Eocene complex to the east (HAMILTON, U.S.A. *** U .S. Geological Survey, Menlo Park, CA 94025, 1971). Within the Bau district, the older oce- U.S.A. anic terrane was obducted onto continental Keywords: Gold skarn, Carlin-type deposits, Carbonate- rocks that include shale, sandstone, inter- replacement gold mediate to felsic volcanic rocks, and minor

1 2 T J PERCIVAL, A S. RADTKEand 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- district (Figs. 1 and 2). truded and metamorphosed by pre-late Trias- Structural Geology sic and Miocene inter-mediate to felsic plu- The Bau district lies along the axis of the tonic rocks (WOLFENDEN and HAILE, 1963; east-northeast-trending Bau anticline (Fig. 1). KIRK, 1968). The anticline is symmetrical with a broad axial Stratigraphy crest, steeply dipping limbs, and is flanked by The Bau district is underlain by a sequence synclinal basins to the northwest and south- of Upper Jurassic through Lower Cretaceous east. Several sets of near vertical faults dissect rocks (Fig. 1). The Upper Jurassic Bau the crest of the anticline (Fig. 1). Limestone is conformably overlain by clastic Vertical displacement along several of the sedimentary rocks intercalated with subor- north-and northeast-trending faults created dinate volcanic rocks of the Lower Cretaceous a graben that served as the locus for emplace- Pedawan Formation (Fig. 2). The carbonate ment of several Tertiary intrusions that extend and clastic sedimentary rocks underlie most of southwest into Indonesia. A second fault and the district and are the host rocks for the gold fracture set trends northwest and forms a near- and antimony deposits. ly orthogonal relationship with the northeast- Igneous rocks include the Upper Triassic trending faults (Fig. 1). Serian Volcanics, a bimodal sequence of tuffs, Descriptions of Gold Deposits flows, and breccias that are exposed north and east of the Bau district. Minor amounts of The gold deposits at Bau can be divided into Jurassic and Lower Cretaceous mafic intrusive three types, reflecting different styles of gold and volcanic rocks occur west of the district. mineralization:(1) gold-bearing calcic skarn, Miocene microgranodiorite and dacite por- (2) gold-and base-metal-bearing veins, and (3) phyry occur as stocks, sills, and dikes that in- carbonate-replacement ore bodies. The aerial trude the sedimentary section within the Bau 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 texture to carbonate-replacement gold depos- is a schematic cross sectional view showing the its in the western United States that are com- geologic setting and typical geometric form of monly referred to as Carlin-type deposits . The each type of gold deposit and its spatial Tai Parit carbonate-replacement deposit was association with the porphyritic intrusions. the largest in the district with a recorded pro- The carbonate-replacement gold deposits ac- duction of 15.5 million grams (Hon, 1981). counted for the bulk of the gold production. Other large carbonate-replacement deposits in These deposits are similar in mineralogy and the district are the Tai Ton, Jambusan , and 40(1), 1990 Carbonate-replacement gold deposits in Bau, Sarawak 5

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 skarns at Bau occur as discontinuous Calcic Skarn Deposits pods sporadically distributed along grano- Gold-bearing calcic skarn deposits occur diorite contacts with limestone (Fig. 4). The along microgranodiorite intrusive contacts skarns are composed of wollastonite, andra- with limestone in several areas of the Bau dite-grossular garnet, diopside, epidote, chlo- district (Figs. 1 and 3). Although the calcic- rite, vesuvianite, calcic-plagioclase, sulfides, gold skarns that crop out in the Bau district and late quartz and calcite. The talc-silicate are volumetrically minor compared to the car- minerals form idioblastic crystals and xeno- bonate-replacement deposits, they provide an blastic grains in tight, compact intergrowths. important insight into the genesis of all the Stibnite, pyrite, arsenopyrite, sphalerite, and gold deposits in the district. The calcic skarn pyrrhotite are present and occur as dissemi- deposits in the Gunung A. Bukit and Gunung nated grains and small crystals intricately Bau areas serve as examples for the descrip- intergrown with the talc-silicate minerals. tions presented below. Stibnite also occurs in late calcite as acicular 6 T. J. PERCIVAL, A. S. RADTKEand 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.] 40(1), 1990 Carbonate-replacement gold deposits in Bau, Sarawak 7

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- sive units of the Bau Limestone (Fig. 4). Gold pyrite, sphalerite, and stibnite crosscut mas- is present in all three types and gold-bearing sive skarn. veins are widely distributed throughout the Geochemical analyses of representative Bau district (Fig. 3). Although gold-bearing calcic skarns are shown in Table 1. These veins are abundant, they account for a relative- analyses show elevated concentrations of ly minor portion of past gold production. The gold, arsenic, antimony, and zinc relative to veins are narrow (0.5 to 2.5 meters wide) and unaltered Bau Limestone (Table 2). Some discontinuous along strike. skarns sampled by WOLFENDEN(1965) contain- Type 1 veins are the most widely distributed ed as much as 31 to 125 ppm gold. Other calcic and abundant of the vein types. Veins of Type skarns contained such high concentrations of 1 at the Rumoh, Saburan, and Gunung Krain stibnite that they were mined solely for their deposits were extensively mined for their gold antimony content (WOLFENDEN,1965). and antimony content. Coarsely crystalline Vein Deposits calcite accounts for greater than fifty volume Three types of gold-bearing veins, distin- percent of Type 1 veins, the balance is compos- guished by their mineralogy, occur in the Bau ed of variable amounts of sulfide minerals and district. Type 1 veins are composed primarily quartz. Sulfides include stibnite, arsenopyrite, of either quartz, quartz+calcite, or calcite. sphalerite, and pyrite. Native arsenic and Type 2 veins contain microcyrstalline quartz realgar are also present. The sulfide minerals and calc-silicate minerals. Type 3 veins are are intricately intergrown with quartz and composed predominantly of base-metal sul- calcite. fides. All veins occur in high-angle faults, frac- The gold contents of the veins are variable tures, joints, and bedding planes within mas- and attain high grades (Table 1). Silver con- 8 T. J. PERCIVAL,A. S. RADTKEand W. C. BAGRY MINING GEOLOGY:

tents are also variable, with highest concentra- concentrations of the base metals as well as tion associated with abundant manganese ox- gold and silver (Table 1). Their genetic and ides. Arsenic and antimony may comprise temporal relationships with the spatially several percent of a given sample. Base metal associated carbonate-replacement ores remain abundances are generally low, except in unclear because of a lack of exposed outcrops isolated samples. and three-dimensional information. However, Type 2 veins contain calc-silicate minerals in the mineralogy and chemistry of these veins addition to microcrystalline quartz+calcite strongly suggest that they formed from the and cross cut recrystallized limestone (marble) same hydrothermal system that formed the within the contact metamorphic aureole, other gold-bearing deposits in the district. generally outboard from massive calcic skarns Carbonate-replacement Deposits (Fig. 1 and 3). These veins are most abundant Carbonate-replacement deposits occur in the central portion of the district where along the contact of the Bau Limestone with emplacement of calc-alkaline intrusions was the Pedawan Formation in close proximity to most extensive (e.g., Gunung Bau, Lucky steeply dipping faults (e.g., Tai Parit and Tai Hill). The veins are narrow and occupy steeply Ton deposits, Figs. 1, 3 and 4). These deposits dipping faults, joints, and fractures within also occur within massive Bau Limestone and marble (Fig. 4). The gold ore zones within the shale along brecciated fault zones (e.g., Bidi veins are lensoid and pod-like bodies. area, Fig. 1). The sedimentary host rocks in WOLFENDEN(1965) reported that calc-silicate these deposits are brecciated near the faults veins contain up to 125 ppm gold but average and the limestone, and to a lesser degree the 12.5 ppm gold. shale, is silicified. The intensity of silicification The mineralogy of Type 2 veins is similar to is directly proportional to the amount of brec- that of the calcic skarns with the exception of ciation. Silicification is gradational from the higher abundance of microcrystalline minor replacement of matrix calcite to com- quartz in the veins. Sulfides are finely plete replacement (jasperoid*1) of the original disseminated within microcrystalline quartz rock by microcrystalline quartz. In many but are also intergrown with wollastonite , areas, pervasively silicified rocks are cut by garnet, and vesuvianite. In addition to the coarsely crystalline quartz veins and veinlets. sulfides, the calc-silicate veins contain native Near, or adjacent to, areas of silicification, antimony, and aurostibite (AuSb2) and locally massive, pure carbonate rocks in the Bau abundant sarabauite (CaSb10O10S10;NAKAI et Limestone and shales in the Pedawan Forma- al., 1978). At the Lucky Hill deposit, stibnite tion are argillically altered. The alteration and sarabauite account for several percent mineralogy of both the shale and limestone in- each of the ore and are intergrown with cludes kaolinite, dickite, illite, minor sericite, wollastonite, calcite, and microcrystalline calcite, quartz, marcasite, and pyrite. Iron ox- quartz. Coarse white calcite and drusy and ides are prevalent in weathered altered rocks. coarsely crystalline quartz line and fill vugs. Although limestone was pervasively replac- Table 1 contains analyses of several Type 2 ed by microcrystalline quartz in the silicified vein samples. The samples contain anomal- breccias, shale fragments were only partially ously high concentrations of gold, antimony , replaced, resulting in rocks containing quartz, and arsenic. Type 3 base-metal-bearing veins sericite, and fine-grained clay. Quartz veins contain sphalerite, chalcopyrite, and galena in and veinlets, and zones of intense stockwork association with lesser pyrite, arseriopyrite, quartz veining cut the silicified breccias, par- and quartz. These veins occur in only a few ticularly where the breccias occur in shale. The localities in the district (e.g., Say Seng, Batu Bekajang Lake). The veins cross cut car- *1The term "jasperoid" is used here as described by bonate-replacement ore bodies and altered SPURR(1898) to refer to rocks formed by the epigenetic microgranodiorite stocks. They contain high siliceous replacement of a previously lithified rock. 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 proportional to the quantity of introduced thickness. Vugs in the siliceous veins and in silica. Native arsenic, stibnite, arsenopyrite, the jasperoids are lined with drusy quartz and pyrite, realgar, sphalerite, and orpiment were sulfide minerals. introduced both contemporaneous with, and The abundance of sulfide minerals and later than, the microcrystalline quartz. The native metals in the silicified rocks is directly abundances of these minerals and the relative 10 T. J. PERCIVAL, A. S. RADTKEand W. C. BAGBY MINING GEOLOGY: proportions in which they occur are highly Nevada, U.S.A. (see BAGBYand BERGER,1985; variable within and among individual PERCIVALet al., 1988). The trends in both deposits. countries reflect underlying, deep-seated, Native arsenic occurs in the quartz matrix as linear structural zones within the crust. botryoidal crystalline aggregates which exhibit Several prominent faults are exposed for 3 to 4 growth bands and are altered to black, fine- kilometers within and parallel to the trend of grained arsenolite and other secondary arsenic the Bau gold deposits (e.g. Tai Parit Fault, phases. Arsenopyrite and lesser pyrite occur as Fig. 1) and represent exposures of the deep- euhedral crystals within the quartz matrix with seated structural zone. Most gold deposits in native arsenic, acicular stibnite, and rare the Bau district lie within the area where this realgar. Stibnite is overgrown upon both northeast-trending structural zone intersects native arsenic and arsenopyrite. the axial zone of the east-to northeast- In many areas, the carbonate-replacement trending Bau anticline (Fig. 1). ores are very arsenic-rich and commonly con- The occurrence of numerous gold deposits tain greater than twenty percent arsenic (Table along and near the Tai Parit fault suggests that 3). These ores contain the same minerals as the this fault was a major conduit for hydrother- typical replacement ores with the exception of mal fluids (Figs. 1 and 3). At the Tai Parit excessive concentrations of native arsenic. deposit, the permeable, clastic Krian member The carbonate-replacement ores contain of the Bau Limestone was faulted against very high concentrations of arsenic, an- massive limestone members of the same forma- timony, and gold (Table 3). It is not uncom- tion, permitting the development of large, car- mon for these ores to contain many thousands bonate-replacment ore bodies. HON (1981) of ppm arsenic and antimony and to contain reported that less prominent north- and > 5 ppm gold (Table 3). Mercury, thallium, northwest-trending faults and fractures within and zinc are anomalous in these ores and cop- the gold trend were intruded by dikes and con- per and lead occur at very low concentrations. tain narrow veins. These faults and fractures The oxidized equivalents of these ores contain probably also served as conduits for hydro- lower concentrations of arsenic and antimony thermal fluids. (Table 3) and were actively exploited for their Stratigraphic sequence, lithology, and che- gold content because of their relative amiabili- mical reactivity of the host rocks are all inter- ty to gold extraction. related as controls on formation of the gold Ore Controls deposits. Neither massive, dense limestone Three physiochemical controls influenced members of the Bau Limestone, nor shales of the locations of the different gold deposit the Pedawan Formation are favorable host types as well as the intensity of gold mineraliza- rocks for carbonate-replacement ore deposits. tion within the Bau district. These are: region- However, both units host carbonate-replace- al and local structures, stratigraphic order and ment ore bodies where the rocks were exten- lithology, and chemical reactivity of the rocks. sively fractured or where faulting has changed Faults within a northeast-trending zone the stratigraphic position of the rocks. The defined by an alignment of stocks, hydrother- relative impermeability and unreactive nature mally altered rocks, and faults served as the of the shale make it an excellent seal to primary structural control on gold mineraliza- hydrothermal solutions where the shale is tion processes. This zone, the Bau gold trend, faulted above chemically reactive, brecciated extends to the southwest and includes gold- limestone. bearing mercury deposits at Tegora and Gading in Sarawak (Fig. 3) and gold districts Discussion in Kalimantan, Indonesia. This regional align- Intrusions and Contact Metamorphism and ment of hydrothermal gold deposits is Metasomatism analogous to the Carlin and Cortez trends in Miocene calc-alkaline stocks, dikes, and 40(1), 1990 Carbonate-replacement gold deposits in Bau, Sarawak 11 sills were emplaced into a sequence of Meso- skarn deposits leads us to believe that they zoic sedimentary rocks in the Bau district. formed from evolved, modified metasomatic Outcrop patterns and areal distribution of fluids. By establishing the conditions of calc- dikes that radiate outward from, yet are con- silicate formation, we can begin to understand nected to the individual stocks, suggest that the genetic link between the carbonate-replace- these features are the surface expression of ment, skarn, and vein gold deposits at Bau. a larger intrusive body at depth. The stocks Environment of Formation of Calcic Skarn appear to have been forcefully emplaced, The mineralogic and petrologic features of resulting in a slight doming of the sedimentary calcic skarn and Type 2 vein deposits are wall rocks. strikingly similar and we believe that their

The intrusions all exhibit porphyritic tex- genesis is the same. The intergrowth of tures suggesting emplacement at relatively wollastonite, garnet, and idocrase with stib- shallow (epizonal) crustal levels. BURNHAM nite, native antimony, arsenopyrite, and gold

(1979) estimates that porphyritic textures are a strongly supports the interpretation that product of physiochemical processes that oc- skarn-forming fluids also transported and cur at magma temperatures of 750•‹to 850•Ž deposited the ore minerals. within the range of 1 to 2 kilobars lithostatic The porphyritic textures of the micrograno- pressure, corresponding to depths of between diorite stocks indicate emplacement and cry- about 4 and 8 kilometers. stallization of magmas at depths less than eight The contact metamorphic aureoles sur- kilometers. The upper stability limit of wolla- rounding the stocks at Bau are of limited size; stonite in this low pressure environment is be- they are usually less than 35 meters wide but tween about 500•‹to 550•Ž (WINKLER, 1976; locally extend up to 350 meters from the intru- EINAUDI et al., 1981). These temperatures as- sive contacts. Recrystallization of limestone is sume a mole fraction of CO2 of approximately the dominant product of contact metamor- 0.10, considered by EINAUDI et al. (1981) to be phism. However, calcic skarn occurs adjacent a reasonable estimate for the epizonal contact to stocks and grades outward to a zone of metamorphic environment. The 550•Ž is a wollastonite-, garnet-, idocrase-, and clinopy- maximum temperature for calc-silicate miner- roxene-bearing rocks and veins (Type 2) that al formation associated with epizonal stocks. occur primarily along joints, fractures, and Temperature stability limits for calc-silicate faults in limestone. The mineralogy, spatial, mineral assemblages from epizonal skarns pub- and textural relationships of the calcic skarn lished by REVERDATTO (1973) and EINAUDI et and Type 2 veins to the intrusions support a al. (1981) suggest that the temperature range metasomatic origin for the calc-silicate miner- of 400•‹ to 500•Ž is more likely for the Bau als. The presence of gold in the calcic skarns skarns. and Type 2 veins indicate that the emplace- The upper stability limits for aurostibite ment of silicic magmas was an integral part of and sarabauite also fall within the same the ore-forming hydrothermal system. Other temperature range. Aurostibite melts at 460•Ž examples of gold-bearing skarns related to calc- (HANSEN and ANDERKO, 1958) and sarabauite alkaline intrusions include McCoy, Nevada undergoes phase transformations above

(KUYPER, 1988) and Muara Sipongi, West 420•Ž (NAKAI et al., 1978). The assemblage Sumatra(BEDDOE-STEPHENS et al., 1987). ORRIS aurostibite+gold melts at 360•Ž, which sug- et al. (1987) have tabulated about thirty occur- gests that ore mineral deposition continued to rences worldwide of gold-bearing skarns, in- lower temperatures if these two minerals were cluding those in the Bau district. deposited in equilibrium. Other evidence for The carbonate-replacement deposits con- continued ore mineral deposition at lower tain the same ore minerals as the skarns. The temperatures is given by cross cutting micro- spatial relationships of the carbonate-replace- crystalline and coarsely crystalline quartz+ ment deposits to the intrusions and to the calcite veins with identical ore mineralogy as MINING GEOLOGY: 12 T. J. PERCIVAL, A. S. RADTKEand W. C. BAGBY

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. are closer to the intrusions than the carbonate- Vein and Carbonate-replacement Deposits replacement deposits. The lack of permeable Figure 5 shows the relationship between host rocks near the stocks explains the restric- mineral-deposit type and distance from the tion of veins to fault zones and their close porphyritic stocks. Vein deposits occur out- spatial association with dikes that also occupy side of the calcic skarn zone but, irl general, faults. The sulfide mineralogy of Type 1 veins 40(1), 1990 Carbonate-replacement gold deposits in Bau, Sarawak 13 is virtually identical to that of calcic skarn and contains very small (approximately 1 micron Type 2 veins. Most of the carbonate-replace- or less) primary liquid + vapor inclusions ment deposits occur along the Tai Parit fault (PERCIVAL et al., 1989). The morphology and (Fig. 1) where it intersects limestone in contact petrologic relations of the inclusions and the with shale. These deposits are the most distal mosaic texture of the microcrystalline quartz gold deposits to the intrusive rocks and exhibit are identical to those studied by BODNAR et al. lower temperature silicate mineral assem- (1985) from many Carlin-type deposits in the blages and textures than those of the vein and western United States. Most inclusions of this skarn deposits. type are too small for microthermometry and

Several mineralogical and geochemical those which have been measured by BODNAR et changes occur in carbonate-replacement al. (1985) typically homogenize at 200•Ž or deposits with increasing distance from the less. Preliminary studies of measureable inclu- stocks. Native arsenic, arsenopyrite, realgar, sions from the carbonate-replacement ores at and stibnite are more abundant, As:Sb in- Bau yield homogenization temperature ranges creases, and there is a marked increase in the of Thv=210-240•Ž and Tmice= -1.0 to

abundance of microcrystalline quartz as -2.6•Ž (PERCIVAL et al., 1989). The inclu- distance from a stock increases. The presence sions are of low salinity and similar tempera-

of realgar in association with native arsenic ture range as those for Carlin-type deposits in

places an upper limit of 281•Ž on the Nevada, U.S.A. that have been studied in de- temperature of deposition of these minerals in tail. For example, geological and geochemical the carbonate-replacement deposits (HALL and studies at Carlin by RADTKE et al. (1980) and YUND, 1964). RADTKE (1985) indicates gold deposition occur- Fluid Inclusion Measurements red between 175-200•Ž from fluids with a salin- A preliminary examination of fluid inclu- ity of approximately 3 equivalent weight per-

sions in quartz in the calcic skarn and Type 2 cent NaCl. Similar studies at Cortez (RYTUBA, vein material indicates that many of the 1977), Mercur, Utah (JEWELL, 1984) and recon-

primary inclusions are liquid+vapor type con- naissance of other Nevada deposits by NASH taining cubic and acicular daughter minerals. (1972) are in close agreement. More recent The cubic daughter mineral is halite. Prelim- studies of the Jerritt Canyon deposit by Ho- inary measurements indicate the average ho- FSTRA et al. (1987) indicates that fluid inclu- mogenization temperatures range from Thv= sions in quartz from gold ore stage minerali- 280-315•Ž and Ths=185-190•Ž for the halite- zation homogenize at temperatures ranging be- bearing inclusions (PERCIVAL et al., 1989). tween 200-300•Ž and contain between 3-10

These high salinity, moderately high tempera- equivalent weight percent NaCI. Therefore, ture inclusions suggest, based upon their close we conclude the carbonate-replacement

spatial relationships with the porphyry stocks, deposits at Bau, with similar mineralogy and

that the ore fluid originated in part from a geochemistry to deposits in the western U.S. magmatic-hydrothermal source. However, no (Table 4), formed from fluids of similar KCl or K-Fe-Cl daughter minerals have been physiochemical properties. identified, therefore a large magmatic fluid in- Summary put has not been proven. In contrast, later formed microcrystalline quartz which cross The different types of gold deposits in the cuts the calcic skarn and Type 2 veins con- Bau district are genetically related to middle tains very small (

sions in the carbonate-replacement ores in- tem that formed calcic skarns. Gold was de- dicates that microcrystalline quartz typically posited with calc-silicate minerals at high tem- 14 T. J. PERCIVAL, A. S. RADTKEand 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 model that we suggest genetically links the hydrothermal system cooled and evolved, veins skarn, vein, and carbonate-replacement gold and carbonate-replacement gold deposits were deposits in the Bau district may also be valid formed. This conceptual model is supported for the carbonate-replacement deposits in Ne- by preliminary fluid inclusion measurements, vada. The discovery of deep gold ore associ- textural and mineralogical characteristics of ated with skarn in the Carlin district of Ne- the deposits, and the spatial associations of the vada provides evidence that the conceptual deposits to the porphyritic stocks. Additional genetic model for the Bau gold deposits may fluid inclusion studies and stable isotope re- indeed have broad application. search are now in progress to test the model. Acknowledgements: We thank C. G. CUNN- Textural, mineralogical, and geochemical INGHAM, Jim BLISS, and Greg MCKELVEY of the features of the carbonate-replacement depos- U. S. Geological Survey, and F. W. DICKSON its at Bau are similar to gold deposits at Carlin of the University of Nevada, Reno for critical (RADTKE,1985) and Jerritt Canyon (BIRAKand and helpful reviews of earlier versions of the HAWKINS,1985; HOFSTRAand ROWE,1987) and manuscript. We also thank Jeffery HEDEN- numerous other sedimentary rock-hosted pre- QUIST and a second reviewer for Mining cious-metal deposits in Nevada, U.S.A. Geology for their comments. Permission to (BAGBYand BERGER,1985). Table 4 is a com- publish company data was given by Blakeney parison the Bau deposits with several very STAFFORD, Managing Partner, Nassau Ltd. similar deposits in Nevada. The conceptual palo Alto, California. 40(1), 1990 Carbonate-replacement gold deposits in Bau, Sarawak 15

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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は米国西部に見られるカーリン型金鉱床と鉱物学的,地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球化学的に非常に類似した特徴を示す.これらの類似はた金鉱化作用を伴っている.それらは,(1)岩体周辺の金鉱床と貫入岩体との関係が単に位置的なものではなく, を伴うスカルソ鉱床,(2)このスカルソの近傍ないしそれ成因的なものであることを示している.バウ地帯の鉱床より離れた地域に見られる鉱脈鉱床,(3)鉱脈をとりまい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て貫入岩体から離れた地域の堆積層中に見られる,炭酸と,包有物の均質化温度,塩濃度は貫入岩体から遠ざか塩岩を交代した交代鉱床,の三種である.これらの鉱床るにつれて低下することが判明した.