Source of Pb in orogenic lode-gold mineralisation: Pb isotope constraints from deep crustal rocks from the southwestern Archaean Yilgarn Craton, Australia

Southern Cross was one of the earliest gold mining centres in Western Austr alia. Over 142 tonnes of gold have been produced from the district, and, on a gold per hectare basis, the Southern Cross greenstone belt in the southw estern Yilgarn Craton is the most productive of Western Australia's Archaea n greenstone belts. The SW Yilgarn Craton is characterised by high-grade (a mphibolite- to granulite-facies) metamorphism, extensive granitoid magmatis m and older greenstone volcanism ages, compared to the well-known greenschi st-facies metamorphism and younger (2.7 Ga) eruption ages which dominate in the Eastern Goldfields Province. The Pb-isotope compositions of deep-seate d granitoids in the SW Archaean Yilgarn Craton, which were emplaced coeval with a craton-wide major orogenic lode-gold mineralization event at about 2 .64-2.63 Ga, have been determined for 96 whole-rock and 24 K-feldspar sampl es. The Pb isotope data of the granitoids are consistent with a crustal ori gin for their genesis, probably by reworking (partial melting) of older con tinental crust. The Pb isotope composition of greenstones. which are the ma in host rocks for gold mineralisation, and pyrites from the komatiite-hoste d syngenetic Ni deposits in the amphibolite-facies Forrestania greenstone b elt, have also been determined, with initial Pb-isotope ratios higher than that for the Eastern Goldfields Province. The Pb isotopic character of the orogenic lode-gold deposits in the region is intermediate between coeval gr anitoid and greenstone Pb, indicating that the ore fluids contained metals from both reservoirs. The Pb in the ore fluid of the most deeply formed dep osit, Griffin's Find. overlaps the isotopic composition of co-eval granitoi ds, indicating the deep-seated granitoid magmatism was the primary source f or Pb in the ore fluids.