Petrogenesis and Ni-Cu-(PGE) prospectivity of the Mount Ayliff Complex in the Karoo Igneous Province: new insights from the Ingeli and Horseshoe lobes

Abstract

The Mount Ayliff Complex comprises five cognate mafic-ultramafic bodies emplaced along the southern margin of the Kaapvaal Craton. This study examines the Ingeli and Horseshoe lobes and assesses their magmatic sulphide prospectivity with respect to the Insizwa lobe, which hosts massive sulphides at its Waterfall Gorge occurrence. Ingeli consists of 465 m of olivine-chromite cumulates overlain by 340 m of (olivine-)gabbronorite, while Horseshoe consists of 45 m of olivine gabbronorite overlain by 5 m of gabbro. Ingeli possesses sparsely disseminated sulphides at its mafic-ultramafic transition, whereas disseminated sulphides are present throughout Horseshoe. A petrogenetic model is proposed, whereby magma accumulated and fractionated nickeliferous olivine and chromite in an upper-crustal staging chamber hosted by Proterozoic basement rocks. Magma then ascended and deposited olivine-chromite cumulates at the level of the complex. Prolonged magma flux in the staging chamber instigated the assimilation of basement rocks, triggering sulphide saturation and the crystallisation of Ni-poor olivine. Contaminated magma then ascended and intruded pre-existing cumulates, depositing sulphide melt that may have backflowed as magmatic activity waned. Basaltic magma then flowed over the ultramafic cumulates, depositing disseminated sulphides whilst undergoing closed-system fractionation. Basal depressions and underlying feeder structures are the most prospective locations for magmatic sulphide mineralisation.