PETROLOGY AND GEOCHEMISTRY OF ALKALIC INTRUSIVES AT THE PORGERA GOLD DEPOSIT, PAPUA-NEW-GUINEA

The geochemistry of the Middle to Late Miocene mafic intrusive complex at Porgera in the Papua New Guinea Highlands, has been investigated because of its spatial association and possible metallogenic relationship to a major gold deposit. Whole-rock major and trace-element data, combined with electron microprobe analyses of mineral phases, indicate classification of the intrusives as a volatile-rich, alkali basalt→hawaiite →mugearite fractionation suite. Enrichments in incompatible elements including Nb (up to 82 ppm) and the LREE (La/Lu)cn = 13-15) are consistent with an intra-plate, alkaline affinity for the parental magma, and derivation from an enriched garnet lherzolite source in the subcontinental lithosphere. Partial melting probably occurred in response to Middle Miocene uplift of the edge of the Australian craton during collision with an island-arc system, while associated trans-lithospheric faulting (e.g., the Lagaip Fault Zone) may have provided conduits for the ascent of magmas. The numerous small stocks and dykes which make up the intrusive complex were emplaced at high crustal levels, and solidified rapidly with little in situ fractionation. They appear to have been ejected at different times from a large, buried magma chamber, after variable degrees of fractionation of the parental magma. Most of the exposed intrusions are porphyritic: olivine + clinopyroxene ± hornblende phenocrysts occur in alkali basalts/gabbros, whereas hornblende + clinopyroxene + plagioclase occur in more evolved hawaiites and mugearites; apatite and spinel (Cr-rich to Ti-rich magnetite) occur as accessory phenocrysts in most samples. High Fe3+/Fe2+ ratios in whole rocks (average=0.8) and minerals suggest crystallization under high fO conditions. Miarolitic cavities and vesicles are present in most samples, and suggest that a volatile phase was exsolved from the magma during or after high-level emplacement. Compositions of interstitial and miarolitic minerals indicate late-stage enrichments of Fe, Ti and alkalis in residual melts, and evolution of a moderately oxidized, NaCl-bearing aqueous fluid. The mafic, alkaline nature and high fO of the magmas, combined with evidence for extensive fractionation and the evolution of a volatile phase, support theories of a relationship between magmatism and the introduction of gold at Porgera. Parts of the deposit show similarities to other gold occurrences associated with alkaline magmatism (e.g., the Emperor mine, Fiji, and Cripple Creek, Colorado). However, the close association of gold mineralization with structural features rather than specific igneous intrusions still permits the tenure of models involving remobilization of gold by nonmagmatic hydrothermal fluids, or the introduction of gold by deeply circulating fluids ascending along the same faults which controlled emplacement of the alkaline magmas. © 1990.