APPLICATION OF THE RE-OS ISOTOPIC SYSTEM TO THE STUDY OF PRECAMBRIAN MAGMATIC SULFIDE DEPOSITS OF WESTERN-AUSTRALIA

Mafic and ultramafic rocks that host magmatic Fe-Ni-Cu-platinum-group element (PGE) sulfide deposits are an important feature of the Archaea n Yilgarn and Pilbara Cratons of Western Australia. We have applied th e Re-Os isotopic geochronometer and tracer to four komatite-hosted Ni sulfide deposits, one Merensky Reef-style Cu-Ni-PGE disseminated sulfi de 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-crystallisati on 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 fro m the 2.9 Ga Munni Munni Complex (Pilbara Craton), for some massive su lfide ore samples from the 2.7 Ga Kambalda deposit (Yilgarn Craton), a nd for some disseminated ore samples from the 2.7 Go Perseverance depo sit (Yilgarn Craton). We attribute this open-system behaviour and dist urbance of the Re-Os system to hydrothermal alteration, sulfide recrys tallisation during deformation and metamorphism, and/or metasomatism a ssociated with mesothermal lode-gold mineralisation. However, less tec tonised and metamorphosed ores and a host komatiite lava from Kambalda as well as olivine-sulfide cumulates from Perseverance and Mt Keith y ield a magmatic Re-Os isochron age of 2706 +/- 36 Ma, in good agreemen t with U-Pb zircon ages of 2709-2702 Ma for metasediments from Kambald a, 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 t racer of the magmatic processes that brought the host komatiites to su lfide saturation. Assimilation of sulfidic crustal materials via therm al erosion has been proposed as a mechanism by which the Archaean koma tiites at Kambalda, Perseverance and Mt Keith achieved sulfide saturat ion. However, the isochron for the Yilgarn ores yields a precise chond ritic initial gamma Os value of -0.1 +/- 0.3 (per cent deviation in Os -187/Os-188 from chondritic mantle of the same age), whereas a propose d sulfidic sedimentary contaminant from Kambalda yields an extremely r adiogenic initial gamma Os value of +900. These Os isotopic data in co mbination with trace element and Nd isotopic data demonstrate that the major ore-forming komatiites in Western Australia were derived from c hemically 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 importan t 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 Cowarn a Rocks (Yilgarn Craton) yield a radiogenic initial Os isotopic compos ition (gamma Os = + 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 i n the localisation of economic concentrations of magmatic sulfide liqu ids. This geodynamic concept has important implications for the discov ery of other styles of magmatic ore deposits, such as Voisey's Bay (La brador, Canada)-style sulfide mineralisation, in Australia.