The Magellan Pb deposit, Western Australia; a new category within the class of supergene non-sulphide mineral systems

The Magellan Pb deposit is located in an outlier of the Palaeoproterozoic Earaheedy Group overlying the southeast Yerrida Basin. A feasibility study of the Magellan prospect was completed in 2001 with, at that time, proven and probable ore reserves of 8.5 Mt grading 7.12% Pb. The deposit is currently exploited from two open pits; Magellan and Cano. The lead orebodies are hosted in a silicified dolomitic collapse breccia, and underlying altered and weathered sandstone and siltstone. The host succession comprises, from top to bottom: a lateritic unit overlying a zone of silicified (silcretized) quartz–clay breccia, overlying saprolitic clay breccia and siltstone (Palaeoproterozoic Earaheedy Group and Basin). The footwall consists of black shales of the Maraloou Formation (Palaeoproterozoic Yerrida Basin). Magellan is a non-sulphide mineral system with cerrusite and anglesite as the major ore minerals and plattnerite, coronadite, pyromorphite, and plumbogummite as minor to negligible ore components. The lack of sulphides and the sole presence of oxide minerals suggest that the ore was formed under physico-chemical conditions conducive to the precipitation of Pb carbonate and Pb sulphate. It is proposed that the Magellan lead deposit represents the supergene modified residue from Mississippi Valley-type (MVT) mineralisation hosted in the Sweetwaters Member of the Yelma Formation of the Earaheedy Group that underwent prolonged weathering, which led to dissolution, volume reduction, and oxidation of the galena and sphalerite, as well as leaching out of highly mobile Zn, leaving behind the Pb carbonate and sulphate mineralisation. The Magellan mineral system likely represents a new category within the non-sulphide class of ore deposits.