Cu-Ni-PGE mineralisation at the Aurora Project and potential for a new PGE province in the Northern Bushveld Main Zone

Iain Mcdonald,R.E (Jock) Harmer,David A Holwell,Hannah S.R Hughes,Adrian J Boyce

doi:10.1016/j.oregeorev.2016.09.016

Abstract

The Aurora Project is a Cu-Ni-PGE magmatic sulphide deposit in the northern limb of the Bushveld Complex of South Africa. Since 1992 mining in the northern limb has focussed on the Platreef deposit, located along the margin of the complex. Aurora has previously been suggested to represent a far-northern facies of the Platreef located along the basal margin of the complex and this study provides new data with which to test this assertion. In contrast to the Platreef, the base metal sulphide mineralisation at Aurora is both Cu-rich (Ni/Cu <1) and Au-rich. The sulphides are hosted predominantly in leucocratic rocks (gabbronorites and leucogabbronorites) with low Cr/MgO (<30) where pigeonite and orthopyroxene co-exist as low-Ca pyroxenes without cumulus magnetite. This mineral association is found in the Upper Main Zone and the Aurora mineral chemistry is consistent with this stratigraphic interval. Pigeonite gabbronorites above the Aurora mineralisation have high Cu/Pd ratios (>50,000) reflecting the preferential removal of Pd over Cu in the sulphides below. Similarly high Cu/Pd ratios characterise the Upper Main Zone in the northern limb above the pigeonite+orthopyroxene interval and suggest that Aurora-style sulphide mineralisation may be developed here as well. The same mineralogy and geochemical features also appear to be present in the T Zone of the Waterberg PGE deposit, located under younger cover rocks to the north of Aurora. If these links are proved they indicate the potential for a previously unsuspected zone of Cu-Ni-PGE mineralisation extending for over 40km along strike through the Upper Main Zone of the northern Bushveld.

Highlights

The northern limb of the Bushveld Complex of South Africa is one of the most significant mineral provinces for magmatic sulphide deposits containing Ni, Cu and the platinum-group elements (PGE) as it hosts the Platreef, one of the world's largest primary PGE deposits
This study has highlighted several key observations that distinguish the Ni-Cu-PGE mineralisation on the Aurora Project from that found in the Platreef and the stratiform reefs of the Critical Zone
The most PGEenriched zones at Aurora are hosted by rocks that contain both inverted pigeonite and orthopyroxene and these are overlain by pigeonite gabbronorites that are strongly depleted in chalcophile elements

Summary

Introduction

The northern limb of the Bushveld Complex of South Africa is one of the most significant mineral provinces for magmatic sulphide deposits containing Ni, Cu and the platinum-group elements (PGE) as it hosts the Platreef, one of the world's largest primary PGE deposits (see reviews in Kinnaird et al, 2005; McDonald and Holwell, 2011 and Kinnaird and Nex, 2015). Mogalakwena mine complex that extracts and processes Platreef ore is one of the lowest-cost (per PGE ounce) PGE mining operations in the world and its success, coupled with rising costs and limited scope for mechanization associated with deep Merensky Reef and UG2-type deposits, have led to concerted exploration for similar Ni-Cu-PGE orebodies at surface or at reasonably shallow depths elsewhere in the northern limb of the Bushveld Complex This search has included exploration and evaluation of potential NiCu-PGE deposits where the associations and genetic links with stratiform reef-type or Platreef-style mineralisation are not immediately clear, and in parts of the magmatic stratigraphy where the potential for Ni-Cu-PGE has traditionally been thought to be low (see recent reviews by Maier et al, 2013; and Kinnaird and Nex, 2015). McDonald et al / Ore Geology Reviews 80 (2017) 1135–1159

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Discussion

Conclusion