Application of the Re‐Os isotopic system to the study of Precambrian magmatic sulfide deposits of Western Australia

Abstract

Mafic and ultramafic rocks that host magmatic Fe‐Ni‐Cu‐platinum‐group element (PGE) sulfide deposits are an important feature of the Archaean Yilgarn and Pilbara Cratons of Western Australia. We have applied the Re‐Os isotopic geochronometer and tracer to four komatiite‐hosted Ni sulfide deposits, one Merensky Reef‐style Cu‐Ni‐PGE disseminated sulfide accumulation in a mafic layered intrusion, and a Cu‐Ni sulfide‐rich siliceous high‐MgO basalt dyke swarm in order to constrain better the genesis of these magmatic ore systems. Significant post‐crystallisation Re and/or Os mobility is observed in the isotopic data for samples from the 3.3 Ga Ruth Well prospect (Pilbara Craton), for cumulates from the 2.9 Ga Munni Munni Complex (Pilbara Craton), for some massive sulfide ore samples from the 2.7 Ga Kambalda deposit (Yilgarn Craton), and for some disseminated ore samples from the 2.7 Ga Perseverance deposit (Yilgarn Craton). We attribute this open‐system behaviour and disturbance of the Re‐Os system to hydrothermal alteration, sulfide recrystallisation during deformation and metamorphism, and/or metasomatism associated with mesothermal lode‐gold mineralisation. However, less tectonised and metamorphosed ores and a host komatiite lava from Kambalda as well as olivine‐sulfide cumulates from Perseverance and Mt Keith yield a magmatic Re‐Os isochron age of 2706 ± 36 Ma, in good agreement with U‐Pb zircon ages of 2709–2702 Ma for metasediments from Kambalda, demonstrating that the Re‐Os isotopic systematics in these samples have remained closed for 2.7 Ga. This isochron can also be used as a tracer of the magmatic processes that brought the host komatiites to sulfide saturation. Assimilation of sulfidic crustal materials via thermal erosion has been proposed as a mechanism by which the Archaean komatiites at Kambalda, Perseverance and Mt Keith achieved sulfide saturation. However, the isochron for the Yilgarn ores yields a precise chondritic initial γ?? value of ‐0.1 ± 0.3 (per cent deviation in 187Os/188Os from chondritic mantle of the same age), whereas a proposed sulfidic sedimentary contaminant from Kambalda yields an extremely radiogenic initial γ?? value of +900. These Os isotopic data in combination with trace element and Nd isotopic data demonstrate that the major ore‐forming komatiites in Western Australia were derived from chemically depleted mantle without significant contamination (< 3%) by crust either prior to eruption or during turbulent flow at the surface. Thus, ground melting and crustal assimilation may not be as important as previously thought in the genesis of these deposits. However, Re‐Os isotopic data for a 2.4 Ga siliceous high‐MgO basalt dyke at Cowarna Rocks (Yilgarn Craton) yield a radiogenic initial Os isotopic composition (γ?? = + 250), suggesting that sulfide saturation may have been linked to crustal assimilation. Thus, the timing and mechanism of sulfide saturation may be subordinate to dynamic magmatic processes in the localisation of economic concentrations of magmatic sulfide liquids. This geodynamic concept has important implications for the discovery of other styles of magmatic ore deposits, such as Voisey's Bay (Labrador, Canada)‐style sulfide mineralisation, in Australia.