Abstract
Magmatic Ni-Cu-PGE sulfide mineralisation is mostly confined to tholeiitic to komatiitic mafic-ultramafic intrusions, yet there have been an increasing number of occurrences recorded in alkaline-ultramafic, post-collisional magmatic systems, particularly in the lower and middle crust that generally display a characteristic Cu-Au-Te enrichment over more conventional Ni-Cu(-PGE) mineralisation. The Mordor Alkaline Igneous Complex, Australia, is a mid-crustal, zoned alkaline complex comprised of a syenite body with an alkaline mafic-ultramafic subcomplex containing dunites, wehrlites and shonkinites. Sulfide mineralisation is present either in thin, PGE-enriched stratiform ‘reefs’ within layered ultramafics in the centre of the subcomplex, or in thicker zones of Cu(-Au-PGE-Te) sulfide hosted by phlogopite-rich shonkinites towards the intrusion margins. This latter style comprises blebs of pyrite, chalcopyrite and minor millerite and PGE tellurides formed from the cooling of a Cu-dominant sulfide liquid. Primary igneous calcite is present in intimate association with the sulfide. We note that the circular nature of the complex, with a dunite core and shonkinite rim with chalcophile element mineralisation, is comparable to the pipe-like, intracratonic, alkaline-ultramafic Aldan Shield intrusions in Russia. As such, Mordor may have an intracratonic rather than post-collisional affinity. Nevertheless, sulfide mineralisation is typical of other alkaline-hosted occurrences, with a Cu-Au-Te-rich signature, low Ni contents and textural association with calcite, supporting models of chalcophile metal and S fluxing alongside carbonate in alkaline systems derived from low degrees of partial melting of hydrous and carbonated mantle sources. Mordor illustrates that alkaline igneous rocks are prospective for magmatic Cu-Au-PGE-Ni sulfide mineralisation, and the classic ‘marginal base metal– and sulfide-rich’ and ‘stratiform PGE-rich and sulfide-poor’ mineralisation styles may both be found in such intrusions.
Highlights
IntroductionThe most recent drilling by Tanami Gold in 2002 identified ‘reef’ style PGE mineralisation with fine disseminated Cu-Ni sulfides located within the layered ultramafic cumulates in the central part of the MMUI around Mount Doom and Mithril (Fig. 2a); including a 2-m intersection with 1.1 ppm Pt + Pd + Au within an 8-m zone of 0.67 ppm Pt + Pd + Au (Anderson and Smith 2003)
Magmatic Ni-Cu-PGE sulfide deposits are generally located within mafic-ultramafic complexes formed from mantle-Editorial handling: W
We address these features in relation to the wider subject of magmatic sulfides in unconventional settings, alkaline magmatic systems, and how Mordor fits within recent models for this style of mineralisation (Holwell et al 2019; Hou and Wang 2019; Blanks et al 2020); and discuss the origin of the Mordor Alkaline Igneous Complex (MAIC) itself
Summary
The most recent drilling by Tanami Gold in 2002 identified ‘reef’ style PGE mineralisation with fine disseminated Cu-Ni sulfides located within the layered ultramafic cumulates in the central part of the MMUI around Mount Doom and Mithril (Fig. 2a); including a 2-m intersection with 1.1 ppm Pt + Pd + Au within an 8-m zone of 0.67 ppm Pt + Pd + Au (Anderson and Smith 2003). The lowest part of the mineralised zone (which contains the highest PGE/ base metal ratios) is made up of a highly porphyritic rock with large (< 15 mm) K-feldspar crystals surrounded by phlogopite-apatite-sulfide (Fig. 3f).
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