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The north arm of Sulawesi consists of a Neogene island arc (North Sulawesi Arc) built upon Paleogene volcanic-sedimentary basement and underlain by oceanic crust, which is followed to the west by an arcuate, highly deformed terrane (neck of Sulawesi) characterized by metamorphic rocks and felsic granitoids belonging to the Sundaland continental margin. These two terranes have been contiguous during the Tertiary. The evolution of the North Sulawesi Arc is divided into two stages separated by collision of the north arm with the Sula Platform microcontinent in mid-Miocene time. During the Early Miocene a calc-alkaline andesitic arc developed in relation to west-directed subduction. Arc-continent collision resulted in back-arc thrusting, clockwise rotation of the north arm, and inception of subduction along the North Sulawesi Trench. Post-collisional magmatism in the North Sulawesi Arc produced felsic to mafic volcanic suites that are thought to be related primarily to rifting of the former arc rather than directly to subduction. In the neck of Sulawesi,LILE and LREE-element enriched, potassic granites (Dondo suite) of continental affinity (Sr87/Sr86 0.71) were generated. The north arm of Sulawesi is comparatively well mineralized. Porphyry CuAu and porphyry Mo mineralization is approximately 2–4 Ma old based on preliminary KAr dating and geological relationships. The former developed in an oceanic terrane following collision-related arc reversal, whereas the latter developed in a continental terrane that underwent lower crustal melting during extension following the same collision. Porphyry CuAu mineralization in the North Sulawesi Arc is associatedwith clusters of small hornblende diorite or quartz diorite stocks, but the specific volcanic environment above these intrusions and their magmatic affinity remain unknown. Mineralization related to porphyry CuAu districts includes CuAu skarn, polymetallic vein and high sulphidation epithermal Au (+enargite). Porphyry Mo mineralization in the neck of Sulawesi is related to small stocks or roof apophyses of the Dondo batholith. Cogenetic granites are exposed over 5000 km2 and are intruded in an arcuate belt, more than 400 km long, parallel to the Sula Platform collision zone. Unimodal felsic volcanics of Plio-Pleistocene age, which occur locally along the inner side of the north arm and further to the south, are probably the extrusive equivalents of the Dondo granites. Epithermal Au mineralization (typically quartz-calcite veins ± adularia) is associated mainly with the Early Miocene andesitic arc but there are important exceptions, and in general the age of mineralization is poorly known. A district-wide correlation between epithermal Au mineralization and porphyry CuAu mineralization is not apparent. The most important epithermal Au district (Kotamobagu) is inferred to be associated with a long-lived volcanic center represented by the Moat caldera. Epithermal gold systems in the North Sulawesi Arc are generally eroded deeply, typically with chlorite-epidote wallrock alteration, and little is known about their associated volcanic environment. In addition to high sulphidation Au mineralization related to porphyry systems, other styles and settings of Au mineralization in the north arm are exemplified by the Gunung Pani rhyolitic dome, the Patung diatreme breccia, the Mesel jasperiod, hosted partly by carbonate rocks, and sedimentary replacement Au-base metal mineralization at Doup. Diversity of mineralization in the north arm is further illustrated by VMS mineralization in volcanic basement to the Neogene arc, and by metamorphogenic quartz vein Au mineralization in the neck of Sulawesi. |
