Geochemical exploration using acid insoluble residues of rocks for volcanic-hosted massive sulphide deposits, Rosebery area, western Tasmania

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The Rosebery deposit is one of the major massive sulphide deposits in the Mount Read Volcanic belt of the western Tasmania. The 200×20 km belt of the Mount Read Volcanics, which is one of the world's richest provinces of volcanic-hosted massive sulphide (VHMS) deposits, hosts more than 40 mineral occurrences including major massive sulphide deposits at Mount Lyell, Hercules, Rosebery, Que River and Hellyer. In this study, hot HNO 3 insoluble residues of rocks were used to define the geochemical halos associated with VHMS deposits, and to identify alteration related to VHMS mineralisation. The residues after the acid treatment are composed mainly of quartz, mica and feldspar (plagioclase and K-feldspar) with traces of chlorite and kaolinite in some samples. Quartz, sericite, chlorite, carbonate, etc., are the principal alteration products and relate to both VHMS mineralisation and to metamorphism and deformation in the region. There are no significant differences between the VHMS related and unrelated mineralogical alteration. There are two types of geochemical halos in the Rosebery area. The type-1 halo trends NE–SW, is related to wall-rock alteration, and is defined by enrichment in Cl, and possibly K and Rb, and depletion in Al, Ca, Na, Ti and Sr. It passes discordantly from the west to east across the White Spur Formation, the altered footwall, the host rock of the Rosebery deposit, the hanging wall and the Mount Black Volcanics. The type-2 halo, trending N–S (largely stratabound), is related to massive sulphide mineralisation and is characterised by enrichment in Fe, Mn, Ba, Zn, Pb and possibly K, Rb and F. It outlines the mineralised host rock and footwall alteration zone, and also extends toward the south into the unaltered footwall and hanging wall rocks. Both halos intersect at Rosebery in zones of mineralisation and associated footwall alteration. Recognition of geochemical halos has led to formulation of geochemical indices for identification of alteration related to VHMS mineralisation. Binary relations between (Mn × Ba × F) and (Ca × Na × Sr), and (Mn × Ba) and (Na × Sr), and their ratios (Mn × Ba × F)/(Ca × Na × Sr) and (Mn × Ba)/(Na × Sr) identify the alteration related to massive sulphide mineralisation and distinguish it from alteration unrelated to the mineralisation in the Rosebery area. These geochemical indices are also used for the other deposits which all display similar geochemical alteration signatures. The alteration related to VHMS mineralisation can be distinguished from alteration unrelated to mineralisation by higher values of the geochemical indices of (K × Mn × Ba × Rb), (Mn × Ba × F) and (Mn × Ba).