Cassiterite and Rutile as Indicator Minerals for Exploring the VMS System

Abstract

Cassiterite and rutile strongly resist physical and chemical weathering and can be used as indicator minerals for mineralisation. These minerals are residually concentrated in a near-surface silcrete duricrust capping a deeply weathered profile developed over the Scuddles Cu-Zn-Pb VMS deposit, Golden Grove, Western Australia. These minerals were extracted by panning of rock chips from Reverse Circulation drill holes and studied by optical, scanning electron microscopy and mapped and analysed by electron probe. The formation of silcrete involved destruction of all labile minerals, removal of clays and leaching of alkali, alkaline earth and transition (e.g. Fe, Mn, V, Ni, Co, Cu, Zn) elements. This is followed by residual concentration of high field strength elements in resistate minerals and silica cementation. The main driving force in the formation of silcrete is the oxidation of massive sulphides generating strongly acidic and highly saline fluids. Under these conditions, silica was probably transported as either silicic acid from destabilising aluminosilicates or as an aqueous sol under neutral to low pH conditions. In addition, Au and Ag were remobilised from the massive sulphide mineralisation as a halide complex. Cassiterite, rutile and quartz in silcrete contain inclusions of pyrite, chalcopyrite, sphalerite, galena and argentite identical to those in the underlying VMS. Thus, they are considered as indicator minerals for VMS exploration. Bioturbation and organic acids produced by decay of plants from the upper lateritic profile remobilised clays downward as colloid, recycled Au from Ag halides and attacked resistant heavy minerals. Oxidation of sulphide inclusions in cassiterite, rutile and quartz intensely corroded these minerals and liberated trace elements (i.e., Sn, Ti, Pb, Sb, Bi, Hg, Se, Ge, W, Mo and Te) to form cements of complex chemical compositions. These cements and Au-Ag halides form a multi-element anomaly in silcrete delineating the underlying VMS deposits at Scuddles.