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

ABSTRACT

This 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.

KEYWORDS:

• arc-related layered mafic PGE complexes
• epithermal PGE
• hydrothermal PGE
• Papuan Orogen
• Papuan Ultramafic Belt
• platinum-group elements
• shear zone-hosted PGE

Acknowledgements

The author wishes to acknowledge the contribution of John Kirakar and Richard Moore in the 1:1000 scale geological mapping and sampling of the Goroa gabbro sequence. Richard kindly supplied the Goroa photographs. John Kirakar and Richard Moore assisted me during the detailed 1:250 scale geological mapping and sampling at Doriri. Thomas Sorulen was with me to inspect and log the intriguing drill core from the first drill holes at Doriri. Karua Imale of Amau village, Cloudy Bay district, land leader for the Aula'abula clan (Ba'u tribe) of the Doriri area, and Elvin Nobake, Ba'u tribe land mediator, are thanked for their support and interest in our exploration work. Peter Goldner is thanked for his review of an early draft of the manuscript. Several anonymous reviewers provided numerous suggestions resulting in improvements to this paper. The patience of Associate Editor Tony Webster is also very much appreciated.

Disclosure statement

No potential conflict of interest was reported by the author.