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

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. Introduction The Bau mining district is located in Sarawak, Malaysia, on the northwest side of Borneo, approximately 24 kilometers southwest of the capital city, Kuching (Fig. 1). The district's recorded production from 1820 to 1981 is 37.3 million grams of gold, 79 thousand 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 descriptions of the geology and mineral deposits were given by SCRUTTON (1906) and HAMILTON (1906). Regional geologic maps (WILFORD, 1955) and detailed geologic maps (WOLFENDEN, 1965; PIMM, 1967) characterized the geologic 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 discusses 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 examples suggesting a genetic link between carbonate-replacement gold deposits (Carlin-type deposits) and magmatism. District Geology The geologic framework of western Sarawak includes two subduction melange complexes emplaced upon continental sedimentary 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 oceanic terrane was obducted onto continental rocks that include shale, sandstone, intermediate 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.