PORPHYRY MOLYBDENUM MINERALIZATION IN A CONTINENTAL COLLISION SETTINGAT MALALA, NORTHWEST SULAWESI, INDONESIA

The Malala deposit in northwest Sulawesi is the only known porphyry mo lybdenum occurrence in Indonesia. It is typical of the fluorine-poor ( quartz monzonite or differentiated monzogranite) class of molybdenum d eposits and belongs to the plutonic sub-type. The mineralized system i s associated with porphyritic intrusive rocks of predominantly graniti c composition (Malala porphyries) which occur as late differentiates i n the roof zone of a composite pluton (Dondo batholith). Major oxide, trace element, isotope, and mineralogical data indicate that the vario us intrusive phases of the Malala-Dondo suite had a common magma sourc e, are magnetite-series, LILE-enriched, (Caledonian) I-type granitoids , and belong to the high-K calc-alkaline series. The intrusive suite f orms part of a 600 km long belt of granites and granodiorites, which w ere emplaced in a continental margin (''Western Sulawesi'') in Late Mi o-Pliocene time, during and following the collisions between several m icrocontinents and the Mesozoic-Tertiary western magmatic arc/eastern subduction complex which forms the island of Sulawesi. The granitoids have initial Sr isotope ratios of 0.71-0.72, and are interpreted to be the results of partial melting of lower crust (possibly underthrusted continental crust of Precambrian to Paleozoic age) due to lithospheri c thickening in a continental collision regime. Alteration and mineral ization at Malala are erratically, and in most places weakly, develope d over an area of 4 km2, predominantly as a ''shell'' up to 50 m thick at the intrusive contact. Highest grades are found in the East Zone, an elongate, steeply dipping, NW trending, fault controlled mineralize d zone, that has an estimated resource of 100 Mt at 0.14% MoS2. No wel l-defined zoning of discrete alteration and sulphide mineral assemblag es has been recognized. Petrographic, fluid inclusion and oxygen isoto pe data suggest that fluids of two origins were involved in the evolut ion of the mineralized system at Malala. Early hypersaline (40-65 eq. wt.% NaCl), hot (400-700-degrees-C) fluids were of magmatic derivation and circulated through fractures while the host intrusion was initial ly still in a semi-consolidated state, producing potassic alteration, barren quartz veins (stage I) and quartz-K-feldspar-apatite-molybdenit e veins (stage II). This early event was terminated upon introduction of predominantly meteoric, less saline fluids, which flowed through ne wly formed fractures and older reopened veins at temperatures in the r ange of 200 to 400-degrees-C, resulting in wall-rock alteration to, an d vein deposition of, sericite, chlorite, carbonate and base metal sul phides (stage III). Finally, carbonate and kaolinite/dickite were depo sited in late fractures (stage IV). Malala shows both significant simi larities with and differences from other F-poor porphyry molybdenum de posits. The similarities appear to be in the nature of the cogenetic i gneous rocks (granite, quartz monzonite and granodiorite) having relat ively high Sr and Ba, and low Rb and Nb contents compared to F-rich Cl imax-type deposits, the geochemistry of the hydrothermal system (high Cu, low F and Sn), general vein paragenesis and lack of multiple ore s hells. Malala differs from most other deposits with respect to its con tinent-continent collision setting, the late magmatic (''deuteric'') n ature of the molybdenum mineralization and the dominance of carbonate alteration.