Geochemical dispersion about the Western Tharsis Cu–Au deposit, Mt Lyell, Tasmania

Abstract

The Western Tharsis disseminated Cu–Au orebody, which occurs within the Cambrian Mt Read Volcanics of Western Tasmania, is surrounded by a pyritic halo that extends 100–200 m stratigraphically above and below the ore zone. Although this halo extends laterally along the same stratigraphic position to the south, it probably closes off to the north based on limited surface and drill hole data. The ore zone is characterized by extreme enrichment (the enrichments and depletions referred to herein are relative to background; these have not been established using mass balance techniques) in As, Bi, Ce, Cu, Mo, Ni, S and Se; with the exception of Mo, these elements are also enriched, but at a much lower level, in the pyrite halo. Pronounced depletion in K, Cs and Mg occurs in 20–30 m wide stratiform zones that flank the orebody on both sides within the pyritic halo. These anomalies and depletions in Be, Ga, Rb, Y, MREE and HREE are associated with a pyrophyllite-bearing alteration zone that wraps around the main pyrite–chalcopyrite-bearing ore zone. This zone is also characterized by positive Eu anomalies which persist up to 150 m both into the hanging wall and footwall of the orebody. The depletion of these elements is consistent with the advanced argillic alteration assemblage developed about acid-sulfate Cu–Au deposits. The pyrite halo is surrounded by a peripheral carbonate halo which is highly enriched in C, CaO and MnO, and weakly enriched in Zn and Tl. Zinc and Tl are most enriched in the upper 100–150 m of the stratigraphically lower halo. In the stratigraphically upper halo, Zn and Tl values are anomalously high but erratic. Barium and Sr enrichment, although mainly restricted to the pyrite halo, extends into the stratigraphically lower carbonate halo by up to 100 m. A Na depletion anomaly extends from 150 m below the orebody and to at least the Owen contact (i.e. ≥400 m)in the hanging wall. The dispersion patterns observed at Western Tharsis are quite unlike those of Zn–Pb-rich volcanic-hosted massive sulfide (VHMS) deposits in western Tasmania. Rather, the dispersion patterns observed at Western Tharsis are more akin to those surrounding porphyry Cu deposits and related acid-sulfate Cu–Au deposits.