Morphology of Gold Grains in Laterite Profiles of Nilambur, Kerala: Implications for the Genesis of Supergene Gold Deposits

Abstract

Laterite weathering profiles in the Nilambur gold prospect of northern Kerala contain dust, grains and nuggets of gold in the various zones. A close examination of the morphological parameters of gold particles separated from various horizons reveal that those occurring towards the upper part of the weathering profiles have nearspherical or ovoidal shapes with regular grain contours and dull lustre. A porous network of numerous etched pits characterise the grain surface. Towards the lower horizons, the degree of etching becomes less and the grains are xenomorphic with plane crystal faces and irregular to jagged contours. Grain-size in the laterite gold is almost four times larger than that of primary gold in vein quartz, while in individual weathering profiles grain-size shows a relative iDcrease from top to bottom. suggesting dissolution. Various textural patterns including arborescent dendrites, tufts. filaments and petalites of secondary gold have been identified. Spectacular incipient growth of painted gold with increased fineness and bright lustre characterise many of the grains, typifying low temperature chemical reprecipitation. An evaluation of the mechanisms of supergene gold formation suggests that ferrolysis reaction involving oxidation of pyrite was fundamental in mobilizing gold in the Nilambur weathering profiles, testified to by the intimate association between iron oxides and supergene gold grains. Migration was effected by capillarity and fluctuations in water table level. Precipitation was triggered by the reduction of anionic complexes in acidic profiles. The nature of occurrence, morphology and texture parameters of gold grains in the Nilambur laterites indicate that supergene gold genesis is dictated by distinct chemical reactions related to natural weathering processes.