Intrusive rocks and porphyry copper occurrences of the Papua New Guinea-Solomon Islands region; a reconnaissance study

Abstract

A collection of 141 Tertiary and younger intrusive igneous rocks from the Papua New Guinea-Solomon Islands region, representing barren and porphyry copper-mineralized intrusions from island-arc, continental margin, and continental settings, have been studied petrographically and analyzed for a wide range of major and minor elements. Samples from individual complexes have been grouped into one or more suites, each of which is mineralogically and geochemically distinct. Field and geochronological data, where available, support these distinctions.Geochemical variation (principally potassium) permits recognition of low K, normal K, and high K calc-alkaline suites having 2.5 percent K 2 O at 62 percent SiO 2 , respectively. High K and normal K suites occur in the Australian Continental Block, while the whole range of suite chemistries occurs in the New Guinea Mobile Belt (continental margin) and Outer Melanesian island-arc settings. The highest K suite occurs in the continental setting (Ok Tedi Complex) and the lowest K suite occurs in an island-arc setting (Koloula Igneous Complex).Porphyry copper mineralization is intimately associated with all types of calc-alkaline suites. However, where different suites occur in a single complex, mineralization is associated with a later, lower K suite. Amphiboles from mineralized suites display Mg enrichment toward rims with an intermediate compositional gap, while amphiboles from barren suites usually show steady Fe enrichment toward rims. Higher oxygen fugacities are inferred for mineralized suites.Statistical analysis of the intrusive rocks shows an increase of K, Rb, La, Ce, Y, Th, U, and Pb and a decrease of Ni, Co, Cu, and Zn from island-arc settings, through the continental margin, to the continental setting. These geochemical variations characterize continental margins in other areas of the world.Geochemical and textural features of these rock suites are consistent with their derivation by segregation of mafic crystalline material (relic crustal source rock) from silicic partial melt. The degree of segregation determines the extent of chemical variation within the suite and determines in part its ability to evolve a hydrothermal (porphyry copper) system. The genesis of porphyry copper systems is thus interpreted to be a natural extension of magmatic phenomena in certain intrusive suites.In many instances, calc-alkaline magmas were intruded soon after extensive andesitic volcanism and rapid regional uplift. It is suggested that volcanism and its probable effects at the base of the crust, rapid uplift and erosion, and subsequent generation of calc-alkaline magmas from crustal sources are all intimately related geologic phenomena.