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

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 (>50000) 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.

Litho- and chemostratigraphy of the Flatreef PGE deposit, northern Bushveld Complex

“…In places, the lower portion of the MZ appears to pinch out, such as on Nonnenwerth and the Aurora project. In the far north, the basement is overlain by rocks with the chemical signature of the central to upper MZ (Manyeruke 2007;McDonald et al 2016). Within the latter, there is a thick, mineralised troctoliticharzburgitic interval that does not have a stratigraphic analogue in the remainder of the Bushveld (van der Merwe 1978).…”

The mineralogy and mineral associations of platinum-group elements and precious metals in the Aurora Cu-Ni-Au-PGE deposit, Northern Limb, Bushveld Complex

“…The Aurora Cu-Ni-PGE-Au deposit is hosted by the Main Zone of the Northern Limb, which it has been suggested represents a type of Main Zone mineralisation which is continued further north in the Waterberg deposit (Kinnaird et al, 2017;McDonald et al, 2017). In this paper we present, for the first time, detailed mineral and trace element characteristics of the base metal sulphide (BMS) and platinum-group mineral (PGM) mineralisation in the Aurora deposit.…”

“…1, Van Der Merwe, 1976). The Northern Limb was originally thought to terminate there: however, the discovery of the Waterberg deposit to the north of the Hout River Shear Zone suggests that either the Northern Limb extends further north (Kinnaird et al, 2017;McDonald et al, 2017;Van Der Merwe, 1976), or that Waterberg represents a distinct magmatic basin not part of the Northern Limb sensu stricto (Kinnaird et al, 2017). The Northern Limb appears to preserve the stratigraphic sequence found in the rest of the RLS, however stratigraphic markers common to the rest of the Bushveld Complex, such as the Pyroxenite Marker (PM), are missing in the Northern Limb.…”

“…Here, we focus on mineralisation hosted in the melanocratic-leucocratic gabbroic cumulates of the Main Zone at Moorddrift, Aurora and Waterberg (Fig. 1, Holwell et al, 2013;McDonald et al, 2017;Kinnaird et al, 2017). The Moorddrift deposit hosts distu ed eef-style mineralisation in gabbronorites, gabbros and anorthosites of the Main Zone which form a mineralised mega-breccia (Holwell et al, 2013;Maier and Barnes, 2010) characterised by extensive hydrothermal alteration and remobilisation (Holwell et al, 2013).…”

“…1) hosted in the higher parts of the stratigraphy that have no known equivalent in the rest of the Bushveld Complex (Holwell et al, 2013;Kinnaird et al, 2017;Maier and Barnes, 2010;McCreesh et al, 2018;McDonald et al, 2017). These are not associated with ultramafic rocks or with high Cr contents like the Platreef and the reef-type deposits of the Eastern and Western Bushveld (McDonald et al, 2017), but instead they are hosted by leucocratic rocks and have been recognised at both the southern and far northern ends of the Northern Limb (Holwell et al, 2013;Kinnaird et al, 2017;Maier and Barnes, 2010;McCreesh et al, 2018;McDonald et al, 2017). This new PGE mineralisation type opens new targets for exploration, but a better understanding of where and how PGE are hosted in this leucocratic unit-associated mineralisation is needed to develop exploration techniques and to aid geometallurgy.…”