Structural–geological setting of the Elura-Zn–Pb–Ag massive sulphide deposit, Australia

Abstract

The Elura deposit is located in the Cobar Basin, one of several intracratonic basins developed during Silurian–Devonian time in the central province of the Lachlan Orogen, eastern Australia. The deposit contained a total pre-mining resource of 45 Mt at 8.5% Zn, 5.3% Pb and 69 g/t Ag. The deposit is hosted by an Early Devonian sedimentary sequence, which transits from a shallow-water shelf facies into a deep-water turbidite facies. The host rocks comprise open platform reef limestone, back-reef calcareous mudstone, open platform clastic sediments and distal turbiditic mudstone and siltstone, interbedded with fine-grained sandstone (CSA siltstone). The structural setting of the deposit is at the north–northeastern margin of the half-inverted Cobar Basin, adjacent to the north–northwest trending Elura growth-fault and north–northeast transform–transfer Buckwaroon Fault. These major structures controlled lithofacies distribution during basin formation, flow and focus of metal-bearing fluids during mineralisation, as well as strain distribution during basin inversion. The deposit comprises two mineralised systems: massive sulphide mineralisation hosted in fine-grained turbidites and semi-massive sulphides hosted in limestone. The dominant mineral assemblage in the turbidite-hosted mineralisation is pyrite–sphalerite–pyrrhotite–galena with minor chalcopyrite, whilst mineralisation in the limestone comprises pyrite–sphalerite and minor galena and pyrrhotite. The Elura deposit is interpreted as a syn-diagenetic, replacement massive sulphide deposit formed by mixing of metamorphic fluids derived from basement and basinal brines discharged along the growth-fault. In comparison with other base metal deposits, the Elura deposit is similar to the Irish-type base metals deposits. During basin inversion, the deposit underwent high strain and greenschist metamorphic conditions which caused modification and distortion.