Geochemistry of intrusive rock suites and related porphyry copper mineralization in the Papua New Guinea - Solomon Islands region

Abstract

In the Papua New Guinea-Solomon Islands region of the southwest Pacific, calc-alkaline intrusive complexes and closely associated porphyry-type copper mineralization of Tertiary and younger age occur in three contrasted tectonic settings: in island arcs, in the Australian continental margin (New Guinea Mobile Belt), and in the Australian continental block. The intrusive complexes of all three settings display a wide range of calc-alkaline mineralogy and chemistry. Geochemical data for the intrusive rocks indicate increasing abundance of 'incompatible' elements from island arc areas, through the continental margin, to the continental block. Regional geochemical variations are also evident in abundance of particular elements in restricted regions, and in overall northwards increase of potassium and related 'incompatible' elements across the Mobile Belt. Genesis of most of the calc-alkaline intrusive complexes involves hybridism of silicic partial melts and refractory material from igneous source rocks in the base of the crust at depths of 20-40 km. Some mafic suites had their source in the upper mantle. The geochemistry of intrusive suites reflects the composition of their source materials. Partial melting is caused by rise of isotherms during prolonged magmatism, and is aided by decrease in load pressure following rapid uplift and erosion. Partial melting beneath the Mobile Belt in Lower to Middle Miocene times was achieved by such means without the participation of a Benioff zone. Mineralized intrusive rock suites were slightly more waterrich than non-mineralized suites. They are characterized by the primary mineral assemblage hornblende + magnetite + sphene, stability of increasingly Mg-rich mafic mineral compositions during crystallization, and lack of mafic inclusions. Later intrusion of mineralized suites in intrusive complexes is a consequence of later partial melting of slightly more water-rich source material at higher levels in the base of the crust. The origin of hydrothermal solutions responsible for porphyry-type mineralization is intimately related to final stages of crystallization of relatively water-rich intrusive suites.