Unravelling the paragenesis of the Archean Gilbeys Gold Deposit, Murchison Domain, Yilgarn Craton

Through core logging, polished thin section petrography, lithogeochemical analysis, HyLogging and scanning electron microscopy, the controls on mineralisation at Gilbeys gold deposit have been established. The Archean Gilbeys deposit located in the poorly-studied Dalgaranga greenstone belt, Murchison Domain, has features consistent with orogenic-style mineralisation. The deposit contains an average grade of 1.3g/t, with over 1.1Moz Au resources remaining after 229koz Au extraction in the late 1990’s. This research identifies that at least two stages of gold mineralisation are present at Gilbeys. The bulk of gold mineralisation formed during Stage-I, concomitant with protracted hydrothermal alteration, and is characterised by disseminated pyrites and inclusion-free pyrite rims. Gold mineralisation (electrum) associated with this event is lithologically controlled, as (1) rheological contrasts facilitated intense shearing of the dacite porphyry-black shale package which enabled hydrothermal fluid flow, and (2) gold precipitation resulted from destabilisation of Au(HS)2- complexes during wallrock sulphidation in the relatively Fe-rich black shale. Stage-II mineralisation involved the remobilisation, and subsequent reprecipitation of electrum within pyrite cracks of late sulphide veins in the black shale. The host rocks of the deposit have been metamorphosed from upper greenschist to lower amphibolite facies. The textural relationships between peak metamorphic minerals, alteration minerals, and electrum, the undepleted Ag content of wallrock electrum, and the crosscutting nature of the electrum-bearing late sulphide veins are lines of evidence consistent with a post-peak metamorphic age of mineralisation at Gilbeys. Rare alkali alteration indices ([Rb+Cs]/Th)N, elevated K abundance attributed to proximal biotite ± sericite alteration, an increase in Mg/Fe ratios of alteration minerals, and an increase in concentration of Mo, Bi, and Au itself, are potential vectors to gold mineralisation. The strong structural-lithological control on mineralisation exemplified in the geochemically heterogeneous, sheared dacite porphyry-black shale package, indicates areas that contain units with high rheological contrasts and chemical variations (i.e. Fe content) should be targeted during future gold exploration in the Dalgaranga greenstone belt.