The Porgera gold deposit, Papua New Guinea : geology, geochemistry and geochronology

by Jeremy Peter Richards

Institution: Australian National University
Year: 1990
Keywords: gold ore ; Papua New Guinea ; Porgera area ; Mt Kaijende ; tectonics of Papua New Guinea ; mineralization ; noble metal analyses ; Memorial University of Newfoundland ; University of Saskatchewan in Saskatoon ; X-ray diffraction analyses
Record ID: 1033163
Full text PDF: http://hdl.handle.net/1885/12535


The Porgera gold deposit, located in the highlands of Papua New Guinea (PNG), contains ~410 tonnes Au and ~890 tonnes Ag, distributed between a large lower-grade ore zone (78.6 million tonnes, 3.5 g/tonne Au, 9.9 g/tonne Ag), and a smaller highgrade zone (5.0 million tonnes, 26.5 g/tonne Au, 22.1 g/tonne Ag). Lower-grade mineralization occurs as stockworks and disseminations of auriferous arsenical pyrite associated with strong sericite-carbonate alteration, which cross-cut and overprint a suite of epizonal mafic stocks and dykes of the Porgera Intrusive Complex (PIC) and their sedimentary host rocks (Jurassic-Cretaceous shelf sediments). Minor free gold also occurs in base metal-sulphide veins associated with these alteration zones. Later, high-grade mineralization (locally up to 3000 g/tonne Au) occurs in quartz-oscoelite veins associated with the Roamane Fault (an extensional structure which cross-cuts the intrusive complex and the earlier dissemiriated ore). Abundant visible gold occurs with Au-Ag-Ag-tellurides in these epithermal-style, vuggy, banded veins. Deposition of both types of ore is shown by K-Ar dating of sericite (illite) and roscoelite to have occurred within 1 Ma of the time of emplacement of the PIC at 6.0 ± 0.3 Ma (2a; K-Ar dating of igneous biotite, and 40Ar/39 Ar dating of hornblende). Geochemical, isotopic, and petrographic studies of the PIC indicate that the intrusions represent a comagmatic, volatile-rich alkali basalt/gabbro - hawaiite (trachybasalt) -mugearite (basaltic trachyandesite) fractionation suite, derived from a larger fractionating magma chamber located deeper in the upper crust (aeromagnetic evidence). The intrusions are medium- to coarse-grained, and textures vary from porphyritic to ophitic. Mafic rocks contain olivine (pseudomorphs) and Cr-rich diopside phenocrysts, whereas hornblende (titanian magnesio-hastingsite), Ti-rich salite, and plagioclase phenocrysts occur in hawaiites and mugearites. Fluor-apatite and magnetite (Cr-rich in mafic rocks) occur as phenocrysts throughout the suite. High Fe3+/Fe2+ ratios in whole-rock samples, pyroxenes and amphiboles, and the presence of primary chromite/magnetite microphenocrysts indicate that the magma crystallized under conditions of high fo2. Least-evolved samples are characterized by low Ba/La (8 to 10), La/Nb (0.6 to 0.7) and Sr/Nd (~25) ratios, and LREE-enrichments ([La/Yb]cn = 15 to 19), which are similar to those of intra-plate alkali basalts, and distinguish the PIC from other Late Tertiary K-rich alkaline and calc-alkaline volcanics and intrusions found on the PNG mainland. Isotopic compositions are relatively depleted (ENd= +6, 87Sr/86Sr = 0.7035, 206Pb/204Pb = 18.64, 207Pb/204Pb = 15.55, 208Pb/204Pb = 38.45), and evidence is found for only limited amounts of crustal contamination. These data suggest derivation of the Porgera magmas by partial melting of a garnet-lherzolite source in the upper mantle. The incompatible element-, volatile-rich nature of the magma suggests that the mantle source region had undergone…