An integrated lithogeochemical approach to detecting and interpreting cryptic alteration around the Elura Zn-Pb-Ag deposit, New South Wales, Australia

Abstract

ABSTRACT Hydrothermal alteration around the sediment-hosted Elura Zn-Pb-Ag sulphide deposit has produced detectable and systematic chemical changes that are also reflected in subtle mineralogical features (cryptic alteration). Iron carbonate development accompanied by potassic alteration, the destruction of albite and the absence of chlorite are the dominant mineral alteration effects in the surrounding turbidites. Key elements enriched in the primary dispersion zone are Zn, Pb, Ag, As, Rb, Tl, and particularly Sb. Sodium is strongly depleted in the alteration system. Cryptic alteration and primary dispersion can be detected up to 350 m from the orebody below the weathering front. Pearce Element Ratio (PER) and General Element Ratio (GER) techniques more clearly identify and quantify this alteration and could assist in vectoring towards high intensity alteration adjacent to ore during exploration drilling. These data assessment techniques avoid the problems of closure when comparing element contents for altered and unaltered samples and allow modelling of the mineralogical controls on chemical variation. There are important differences in the alteration effects observed in the fine and coarse fractions of the enclosing turbidite units. For vectoring purposes, the best lithotype to sample is the shale component.