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

Highlights

• The Owendale Complex is a Uralian-Alaskan Complex in New South Wales, Australia.

• The Complex hosts both bedrock and lateritic PGE mineralization.

• The ultimate source of Pt mineralization in the Complex were magmatic sulfides.

• The PGE-rich magmatic sulfides were destroyed during serpentinization.

• The magmatic sulfides were replaced by Cu-Pt-Fe alloys.

• All of the S and most of the Au and Pd in the sulfides were removed by the fluids.

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.

Introduction

Prior to the discovery of the Bushveld Complex, South Africa, the bulk of the world’s Pt production was from secondary placer deposits associated with Uralian-Alaskan-type ultramafic complexes. Very little primary Pt was produced from these complexes (Johan, 2002), although a notable exception is the Nizhny Tagil complex in the Urals (Auge et al., 2005). A limited amount (∼635 kg) of alluvial Pt has historically been recovered from channels that drain the Uralian-Alaskan-type complexes in the southern part of the Fifield Platinum Province, New South Wales, Australia (Hoatson, 2014). One of these complexes, the Owendale Complex, hosts a considerable amount of primary PGE mineralization with grades reaching 24 g/t of Pt (Keays, 2011).

The Owendale Complex shares some common features with other Uralian-Alaskan type Complexes. It is one of a number of zoned igneous bodies comprising both intermediate and ultramafic rocks that occur in long but narrow belts running parallel to subduction zones. In addition, the Owendale Complex, along with many other similar intrusions has been a significant producer of placer Pt that has characteristically high Pt/(ΣPGE) ratios. However, unlike most other Uralian-Alaskan-type Complexes, the Owendale Complex hosts a large primary resource of Pt contained within ultramafic rocks, deep weathering of which has produced a lateritic cap that contains a Pt resource of 12.7Mt averaging 0.28 g/t Pt (Platina Resources, April 2017) and a combined Sc and Pt resource of 35.6.Mt averaging 0.28 g/t Pt, 405 ppm Sc, 0.1% Ni, and 0.06% Co (Platina Resources, August 2018).

In this paper, we investigate the nature of primary Pt mineralisation hosted within ultramafic rocks of the Owendale Complex by presenting detailed petrological, geochemical and mineralogical relationships. In doing so, we discuss the atypical characteristics of Owendale that have been conducive to the formation of a large bulk resource of primary PGE mineralisation in an Alaskan-type intrusion, and post-magmatic processes, in particular serpentinisation, that have modified the style and mineralogy of the mineralisation. We present a genetic model for the emplacement and mineralisation of the Owendale Complex, and explore the implications for Uralian-Alaskan-type intrusions worldwide.