The Mt. Mulga barite–magnetite–copper–gold mineralisation, Olary Domain, South Australia

Abstract

The Mt. Mulga barite Cu–Au deposit in the central part of the Olary Domain is hosted by the Palaeoproterozoic Willyama Supergroup of the Curnamona Craton. The lenticular, stratiform ore bodies are hosted by the Ethiudna Subgroup at and near the contact to the underlying Wiperaminga Subgroup. The ore bodies consist of massive, coarse crystalline barite up to 6 m thick interspersed with granular (< 2 cm) magnetite and quartz. In the southern section the massive barite contains Cu–Au mineralisation consisting of chalcopyrite, bornite and native gold. Along strike the mineralised unit grades into a stratiform quartz–magnetite layer which can be traced for several kms. Several smaller (< 1 m thickness) barite lenses occur about 1.5 km north of main barite body in the upper parts of the Ethiudna Subgroup close to the contact to the Saltbush Subgroup. A pronounced positive Eu-anomaly of the mineralisation hosting unit indicates a reducing character of the mineralising fluids and is characteristic for exhalative sediments. 87Sr/ 86Sr-isotope ratios of barite fall into a narrow range of 0.708 to 0.713 suggesting an origin of the mineralising fluids from the underlying Lower Willyama sequence. The δ 34S values of barite from the main ore body range from + 13 and + 16‰, while the barites of the other ore bodies range from + 6 and + 11‰. Chalcopyrite revealed δ 34S values of − 1.3 to + 4.5‰. Barite is suggested to be formed by mixing of a hydrothermal, reduced, hydrothermal Ba-rich fluid with oxidised, sulphate-rich water of a non-marine basin. Although an epigenetic formation of the Cu–Au mineralisation during the Olarian Orogeny is possible, we suggest a cogenetic formation with the barite–iron-oxide mineralisation.