Crocoite and associated minerals from Tasmania

The world's best and most plentiful crocoite specimens come from Tasmania, particularly the Adelaide and Red Lead silver-lead mines in the South Dundas mineral field, Dundas. The Dundas Extended, West Comet, Kosminsky and Platts Prospect, al lfrom this same mineral field, have also produced striking finds, as has the Kapi mine from the North Dundasfield. A third area between Savage River and Waratah, about 50 km north-north-east of the Dundas district, contains the Heazlewood (where crocoite was first found in Tasmania), Whyte River, Magnet and Washington Hay mines, all of which have produced some crocoite. There are also minor occurrences at Zeehan. These fields are associated with Cambrian serpentinites and/or amphibolites and serpentinite-derived greywackes; the silver-lead lodes lie mostly along the brecciated contacts of these bodies. Chromite, stichtite and chromian muscovite in the serpentinites and serpentine-derived sediments are the source of chromian in the crocoite. The mines originally worked Ag- and Pb-rich sulphide and carbonate-rich primary ores. Near the surface the lodes are mostly represented by deeply weathered, goethite rich vuggy gossans. Other secondary minerals include cerussite, anglesite, dundasite, petterdite, philipsbornite, coronadite, mimetite, pyromorphite, chlorargyrite and chalcophanite. The main factors in the genesis of the supergene minerals of the Dundas district are the super position of Cr-rich rocks with galena-rich veins, and deep weathering. Galena readily oxidises to form a number of supergene minerals, depending on pH, and the local availability of other anions in solution. Anglesite is usually first to form and is then overgrown or replaced by cerussite, crocoite or other lead minerals. Mimetite, pyromorphite, coronadite and other Pb minerals result from various local chemical processes in the gossans. Cerussite is less common in the Dundas district perhaps because of prevailing low pH groundwaters. These conditions led to an abundance of crocoite as it is more stable than most lead minerals at low pH.