Platinum-group element and Au geochemistry of Late Archean to Proterozoic calc-alkaline and alkaline magmas in the Yilgarn Craton, Western Australia

Abstract

The Yilgarn Craton of Western Australia is one of the largest Archean cratons in the world and is well-known for its metal endowment. In order to provide new insights into its metallogenic fertility and the nature of the upper mantle that lies underneath the craton, this study investigates the poorly constrained platinum-group element (PGE) and gold signatures of a selected suite of calc-alkaline lamprophyres, ultramafic lamprophyres, carbonatites and orangeites, ranging in age from the Proterozoic to the Late Archean. Proterozoic ultramafic lamprophyres and carbonatites within the Eastern Goldfields Superterrane (EGS) of the Yilgarn Craton have anomalously low PGE contents (Ir = 0.1–1.3 ppb; Ru = <0.08 to 3.6 ppb; Rh = <0.04 to 0.4 ppb; Pt = <0.17 to 3.3 ppb; Pd = <0.12 to 4.4 ppb) and variable Au contents (<0.4 to 9.8 ppb). Based on their low PGE contents and unfractionated PGE patterns with (Pd/Ir)N ratios up to ~6, it is suggested that these magmas may derive from extremely low-degree partial melting of the convective mantle. Conversely, the Late Archean calc-alkaline lamprophyres in the EGS exhibit comparatively fractionated PGE patterns with (Pd/Ir)N ratios up to ~27 and variable Au concentrations, including localised anomalous enrichments (up to 13.8 ppb). The fractionated PGE patterns are explained by incongruent melting of mantle sulfides with potential contribution from PPGE- and Au-bearing alloys, which would preferentially contribute Pt, Pd and Au over Ir, Ru and Rh. Combination of elevated Au and volatile contents in these rocks is consistent with derivation from a mantle source that was metasomatised by slab-derived fluids along a laterally extensive Late Archean subduction setting. This process may have provided a first-order control on the exceptional gold endowment of the Eastern Goldfields Superterrane.