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

The Rosebery deposit is one of the major massive sulphide deposits in the M ount Read Volcanic belt of the western Tasmania. The 200 x 20 km belt of th e Mount Read Volcanics, which is one of the world's richest provinces of vo lcanic-hosted massive sulphide (VHMS) deposits, hosts more than 40 mineral occurrences including major massive sulphide deposits at Mount Lyell, Hercu les, Rosebery, Que River and Hellyer. In this study, hot HNO3 insoluble res idues of rocks were used to define the geochemical halos associated with VH MS 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 prin cipal alteration products and relate to both VHMS mineralisation and to met amorphism and deformation in the region. There are no significant differenc es between the VHMS related and unrelated mineralogical alteration. There a re two types of geochemical halos in the Rosebery area. The type-1 halo tre nds 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 p asses discordantly from the west to east across the White Spur Formation, t he altered footwall, the host rock of the Rosebery deposit, the hanging wal l and the Mount Black Volcanics. The type-2 halo, trending N-S (largely str atabound), is related to massive sulphide mineralisation and is characteris ed by enrichment in Fe, Mn, Ba, Zn, Pb and possibly K, Rb and F. It outline s the mineralised host rock and footwall alteration zone, and also extends toward the south into the unaltered footwall and hanging wall rocks. Both h alos intersect at Rosebery in zones of mineralisation and associated footwa ll alteration. Recognition of geochemical halos has led to formulation of g eochemical indices for identification of alteration related to VHMS mineral isation. Binary relations between (Mn x Ba x F) and (Ca x Na x Sr), and (Mn x Ba) and (Na x Sr), and their ratios (Mn x Ba x F)/(Ca x Na x Sr) and (Mn x Ba)/(Na x Sr) identify the alteration related to massive sulphide minera lisation and distinguish it from alteration unrelated to the mineralisation in the Rosebery area. These geochemical indices are also used for the othe r deposits which all display similar geochemical alteration signatures. The alteration related to VHMS mineralisation can be distinguished from altera tion unrelated to mineralisation by higher values of the geochemical indice s of(K x Mn x Ba x Rb), (Mn x Ba x F) and (Mn x Ba). (C) 1999 Elsevier Scie nce B.V. All rights reserved.