Gold-Silver Decoupling in Weathering Front: Implications for Gold Exploration in Lateritic Terranes

The differential solubilities of gold and silver in low temperature fluids lead to Au-Ag decoupling in the weathering front. Selective extraction of silver from the primary ores, or differential precipitation of gold at the fluid front can generate supergene gold deposits with enhanced grade. Gold grains formed by each process will be characterized by distinct evolutionary trends in grain morphology and chemistry. In the laterite-hosted gold mineralization of Nilambur, gold fineness (1000 Au/Au+Ag) increases from 921 in the primary veins to an absolute 1000 in laterites, with a concomitant four-fold increase in grain size. The kinetics and growth of gold particles are dictated by the degree of supersaturation, size of the critical nucleus and the free energy of nucleation, together with time-integrated fluid-rock ratio.

Chemical propagation and physical dislocation of Au during weathering place limitations on the application of conventional exploration techniques, and interpretation of anomalies defined from geochemical surveys on bulk samples. Two cases arc identified in this study: "shadow gold" - where overlying prominentanomaly is unrelated to the barren reef bcneath; and "masked gold"- where the potential of the underlying protore is masked by a weak signal in the overlying anomaly. It is suggested that the gradients in Au conlent within individual grains. and the trends in morphological evolution of gold particles are potential pathfinders for gold exploration in lateritic terranes.