Epithermal and arc-related layered mafic platinum-group element mineralisation in the mafic–ultramafic rocks of eastern Papua

Abstract

ABSTRACTThis paper describes contrasting platinum-group element (PGE) mineralisation in the mafic–ultramafic rocks of eastern Papua, discovered nearly 100 years after the first record of detrital ‘osmiridium’ in alluvial gold production from the region. The two mineralisation types include: (1) the Doriri Pd–Pt–Ni lode, a new type of epithermal shear zone-hosted PGE mineralisation from the Ada'u Valley; and (2) stratiform Pt–Pd ± Au mineralisation hosted in an arc-related layered mafic intrusion at the Goroa prospect in the Waria Valley. Doriri is the first description of the transport and deposition of PGE in epithermal conditions, confirming the case long made by geochemists for the solubility, transport and deposition of significant concentrations of Pd and Pt in hydrothermal fluids up to 300°C. Doriri's temperature of formation (100–220°C) is based on chlorite thermometry, clearly placing the deposit in the epithermal field. Other epithermal-hydrothermal features of the deposit include formation at a shallow depth, episodic mineral deposition and the development of rhythmically layered chlorite and magnetite, hydrothermal brecciation, cross-cutting vein textures, the presence of low-temperature marcasite in the lode's pyrite–pentlandite–pyrrhotite sulfide mineralisation and a Pd:Pt ratio >7. The stratiform Goroa Pt–Pd ± Au mineralisation contains concentrations up to 311 ppb Pt + Pd over a 3 m interval in a plagioclase-rich gabbro. Grey–black micrograbbo commonly forms alternating thin layers within plagioclase-rich gabbro and many of these mafic layers contain visible sulfides. Sulfide content of the plagioclase-rich gabbro is ≪1 vol%. A Pt:Pd ratio of ∼1 is typical of the cumulate layered mineralisation. The association of Goroa mineralisation with (1) plagioclase-rich gabbroic rocks, rather than ultramafic rocks, (2) trace sulfides, rather than chromite, and (3) the occurrence with Pt–Pd detrital alloys, suggests a genetic relationship with arc-related layered mafic intrusions. The Goroa diorite–gabbro–ultramafic sequence may be genetically linked to late Paleocene–middle Eocene tonalite, quartz diorite and porphyry arc-related stocks present in the lower Waria River district.