Geochemistry of wallrock alteration, Rosebery, Tasmania, Australia

The Rosebery ore deposit consists of two adjacent massive sulphide bodies hosted in an easterly dipping lens of sedimentary rocks within a pile of acid pyroclastic rocks of Cambrian age. Both orebodies are underlain by zones of intense alteration which mark the path of the metal-bearing fluids. The southern vent is exposed on the surface. Its alteration zone extends for up to 2 km in a N-S direction and has a maximum E-W width of ∼ 700 m.

The footwall and, to a minor degree, the hanging wall have been subjected to hydrothermal alteration. In the field, this alteration is marked by plagioclase-free quartz-sericite-(chlorite) schists which exhibit two different facies: siliceous augen schists and thinly cleaved K-feldspar-bearing schists.

Samples can be grouped geochemically into altered and unaltered volcanics despite the large variation in chemical composition within each group. This classification is supported by cluster analysis. In the vicinity of the deposit, concentrations of SiO₂, K₂O and Rb are anomalously high while contents of Al₂O₃, Na₂O, TiO₂, CaO, Sr, Zr, Y and Nb show a significant depletion. At the outer limit of the alteration halo the hydrothermal influence is more easily detected geochemically than geologically.