Tectonic setting and origin of intrusive rocks and related porphyry copper deposits in the Western Highlands of Papua New Guinea

Abstract

In the Western Highlands of Papua New Guinea, Tertiary and younger calc-alkaline intrusive complexes and related porphyry copper mineralization occur in two contrasting tectonic settings: the New Guinea Mobile Belt to the north and the Australian Continental Block to the south. Geologic evidence supports the theory that the Mobile Belt developed adjacent to the northern Australian continental margin. Ages of intrusive complexes and mineralization are dominantly Middle Miocene (15-10 m.y.) in the Mobile Belt, and Late Miocene to Pleistocene (7-1 m.y.) in the Continental Block. Transcurrent- and block-faulting appear to have directly controlled the location and emplacement of the intrusive systems. Simplified cooling theory applied to rapidly uplifted and eroded horst blocks reveals that temperatures in the central parts of such blocks remain of the order of 100°C higher than surrounding areas for periods of several million years. Such locally elevated isotherms, together with the decrease in load pressure following rapid erosion, are considered to have caused partial melting events in the basal parts of the crust which eventually yielded the intrusive complexes now observed. The tectonic and geochronologic data are synthesized in a model featuring sequential uplift and erosion of, first, the Mobile Belt, and later the contiguous parts of the Continental Block, in response to interaction of the northern Australian continental margin with island arc structures further to the north.