Porphyry molybdenum mineralization in a continental collision setting at Malala, northwest Sulawesi, Indonesia

Abstract

The Malala deposit in northwest Sulawesi is the only known porphyry molybdenum occurrence in Indonesia. It is typical of the fluorine-poor (quartz monzonite or differentiated monzogranite) class of molybdenum deposits and belongs to the plutonic sub-type. The mineralized system is associated with porphyritic intrusive rocks of predominantly granitic composition (Malala porphyries) which occur as late differentiates in the roof zone of a composite pluton (Dondo batholith). Major oxide, trace element, isotope, and mineralogical data indicate that the various intrusive phases of the Malala-Dondo suite had a common magma source, are magnetite-series, LILE-enriched, (Caledonian) I-type granitoids, and belong to the high-K calc-alkaline series. The intrusive suite forms part of a 600 km long belt of granites and granodiorites, which were emplaced in a continental margin (“Western Sulawesi”) in Late Mio-Pliocene time, during and following the collisions between several microcontinents 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 lithospheric thickening in a continental collision regime. Alteration and mineralization at Malala are erratically, and in most places weakly, developed over an area of 4 km 2, 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 mineralized zone, that has an estimated resource of 100 Mt at 0.14% MoS 2. No well-defined zoning of discrete alteration and sulphide mineral assemblages has been recognized. Petrographic, fluid inclusion and oxygen isotope data suggest that fluids of two origins were involved in the evolution of the mineralized system at Malala. Early hypersaline (40–65 eq. wt.% NaCl), hot (400–700°C) fluids were of magmatic derivation and circulated through fractures while the host intrusion was initially still in a semi-consolidated state, producing potassic alteration, barren quartz veins (stage 1) and quartz-K-feldspar-apatite-molybdenite veins (stage II). This early event was terminated upon introduction of predominantly meteoric, less saline fluids, which flowed through newly formed fractures and older reopened veins at temperatures in the range of 200 to 400°C, resulting in wall-rock alteration to, and vein deposition of, sericite, chlorite, carbonate and base metal sulphides (stage III). Finally, carbonate and kaolinite/dickite were deposited in late fractures (stage IV). Malala shows both significant similarities with and differences from other F-poor porphyry molybdenum deposits. The similarities appear to be in the nature of the cogenetic igneous rocks (granite, quartz monzonite and granodiorite) having relatively high Sr and Ba, and low Rb and Nb contents compared to F-rich Climax-type deposits, the geochemistry of the hydrothermal system (high Cu, low F and Sn), general vein paragenesis and lack of multiple ore shells. Malala differs from most other deposits with respect to its continent-continent collision setting, the late magmatic (“deuteric”) nature of the molybdenum mineralization and the dominance of carbonate alteration.