Gold grains in laterite weathering profiles of Nilambur, South India and a model for the genesis of supergene gold deposits.

Abstract

Occurrence of dust, coarse grains and even large nuggets of gold in laterite weathering profiles has emerged as a recent challenge to metallogeny. Supergene gold concentrations, as in Nilambur, South India, show morphological and textural characteristics indicative of chemical dissolution, migration and reprecipitation by low temperature solutions in weathering environments. The gold grains from the upper zones have near-spherical shapes with numerous etch pits, while those from the lower zones are of more irregular in shape and are less corroded. Textural features like filamental, dendritic or tuft-like patterns and the petaloids or coatings of “painted gold” over the primary grains indicate that they are the products of incipient growth of secondary gold. Based on their nature of occurrence and grain morphology, we develop a model involving the oxidation of pyrite to form an acidic profile which dissolved gold and transported it as chloride complexes from the primary veins. In the high Eh and neutral to alkaline environment of the oxidized zone, these complexes got dissociated and pure metal was precipitated. Chemical reprecipitation has often led to an increase in the “fineness” (purity) of gold formed in supergene environments.