Emplacement of magmatic Cu-Au-Te(-Ni-PGE) sulfide blebs in alkaline mafic rocks of the Mordor Complex, Northern Territory, Australia
journal contributionposted on 16.09.2020 by David Holwell, Daryl Blanks
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Magmatic Ni-Cu-PGE sulfide mineralisation is mostly confined to tholeiiticto komatiiticmafic-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)-rich 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 ultramaficsin the centre of the subcomplexor thicker zones of Cu(-Au-PGE-Te)-rich sulfide hosted by phlogopite-rich shonkinites towards the intrusion margins. This latter stylecomprises blebs of pyrite, chalcopyrite and millerite with 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, are all 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 carbonatein alkaline systems derived from ow 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 settings may both be found in such intrusions.