Platinum mineralisation in the Owendale Uralian-Alaskan-type complex, New South Wales, Australia: The effects of serpentinization on Cu-PGE-Ni sulphides

Abstract

Significant, albeit highly erractic, PGE mineralization is hosted in the ultramafic rocks of the Owendale Complex, a Uralian-Alaskan type intrusion in the Fifield region of central New South Wales, Australia. Three types of PGE mineralization have been identified in drill core samples of these rocks. The first to be recognized was PGE mineralization associated with “P-units” which are narrow (a few millimetre to 2 m wide) lenses of pegmatoidal clinopyroxenites within olivine clinopyroxenites; the best intersection of this type of mineralization averaged 13.19 g/t Pt and 0.90 g/t Pd over 1.57 m, has very low S contents and contains a primary magmatic PGM assemblage. Two types of PGE mineralization occur within dunites and wehrlites, a low-Cu Pt type and a high-Cu Pt type of mineralization. The low-Cu Pt type of mineralization consists mainly of disseminated Cu-Pt-(Fe) alloys that lie along the mesh lines of partially serpentinized olivine; this type of mineralization can have grades of up to 25 g/t Pt but very low amounts of Cu, S, Au and Pd. The high-Cu Pt type of mineralization consists of disseminated, partially oxidised PGE-rich Cu sulphides with Pd-bearing PGM; this type of mineralization has up to 25 g/t Pt and elevated levels of Cu, S, Se, Au and Pd. The Pt:Ir:Rh ratios of the low-Cu Pt and the high-Cu Pt types of mineralization are virtually identical attesting to a common source, but with variable removal of the more mobile Pd, Au, Cu and Se. Some of the high-Cu Pt type sulphides are associated with an unusual Cr-Al-Ti-Mg-rich magnetite that occurs in veins and pods that are up to 15 cm thick. Phlogopite is a common accessory mineral in the dunites and wehrlites, occurring as disseminated grains and pods of mica that are up to 4 cm wide, and as selvedges along the Cr-Al-Ti-Mg-rich magnetite veins. Owendale represents a staging chamber in which olivine cumulates, followed by clinopyroxenite cumulates, were formed and later infiltrated from above by immiscible Fe-rich and Cu-sulphide melts. It is suggested that the phlogopite was produced by K2O-rich fluids released from the monzonites and diorites that intruded the ultramafics and which also drove serpentinisation. Serpentinisation altered the dunite/wehrlites with small amounts of sulphides to remove all the S and most of the Cu, Au, and Pd to leave only Cu-Pt-(Ni-Fe) alloys, forming the low-Cu Pt type of mineralization, whereas the magmatic sulphides in dunites/wehrlites with larger amounts of sulphides were only partially altered, forming the high-CuPt type of mineralization. The Pt alloys produced during serpentinization of the PGE-rich Cu sulphides at Owendale are predominantly Cu-rich, unlike the Pt-Fe alloys found in the placer deposits associated with most Uralian-Alaskan complexes. However, the Owendale Complex was lateritized, as were the Uralian-Alaskan Complexes in the Urals. It is possible that during lateritization, the Cu-Pt-Fe alloys were converted into Pt-Fe alloys similar to those found in the Pt placers.