BAU, MALAYSIA: SRHDG DEPOSIT ASSOCIATED WITH MIOCENE MAGMATISM

The Bau district (10x5 km) is on the NW side of the island of Borneo and has produced 37.3 t Au. It is in a Cret. thrust sheet comprised of L. Jur. to M. Cret. limestone and calcareous-clastic rocks and L. Tri. island arc volcanics that is overthrust by L. Jur. to L. Cret. siliciclastic turbidites. Middle Miocene magmatism was due to SE subduction of the Proto-South China Sea under Borneo. Numerous 10-13 Ma, I-type, reduced, intermediate to felsic porphyry stocks intrude a > 30 km long NNE transtensional fault zone. The Au deposits, microdiorite and porphyritic dacite stocks and dikes, and a 5x12 km magnetic anomaly occur at the intersection of the NNE fault system and an ENE-striking anticline. Au production is from proximal calcic skarns, intermediate polymetallic quartz-calcite veins, and distal SRHDG deposits (e.g. Tai Parit 15.5 t Au). Alteration includes decalcification, silicification, argillization, sericitization, and propylitization.  Silicification (±quartz veining) is widespread and spatially associated with Au.  SRHDG deposits have three distinct ore types: siliceous, arsenical, and oxide. They are characterized by varying degrees of silicfication and quartz veining with variable amounts of py, native As, stib, aspy, native Au, real, sph, and their supergene oxidation products.  Ore fluids introduced Si, S, As, Sb, Au, Ag, Mn, Fe, K and minor Zn, Cu, Pb, Mo, Bi, Ba, Hg and removed Ca, CO2 and Mg from carbonate host rocks. Fluid inclusions in Au- and sulfide-bearing quartz from SRHDG deposits have Th from 100-350°C (avg. 225°C) and salinities of 2-4 wt % NaCl. Fluid inclusion water in native As (-60‰) and stib (-81‰) and calculated d¹⁸O of water  (6.2 to 13.6‰) for quartz and calcite from SRHDG deposits are largely magmatic. Sulfides in country rocks have d³⁴S of 5 to 20‰. The calculated d³⁴S of H2S in equilibrium with  (1) py, sph, and gal from porphyry, skarn and polymetallic veins of 0.7 to 5.0‰ is magmatic, (2) disseminated py and aspy (±diagenetic py contamination) from SRHDG deposits of –2.3 to –1.5‰ is largely magmatic, and (3) stib and real from polymetallic vein and SRHDG deposits of 5 to 16‰ is derived in part from country rocks. The evidence indicate that the SRHDG deposits in the Bau district are distal manifestations of a zoned Au-bearing porphyry system with some components derived from the country rocks.