SILVER AND BASE-METAL DISPERSION IN STREAM SEDIMENTS AND WATERS AROUND AN EPITHERMAL AG-AU-CN PROSPECT AT LAGALOCHAN, WESTERN SCOTLAND

The utility of high-density stream sediment and stream water sampling to the exploration for silver deposits was investigated by the study o f the epithermal Ag-Au base-metal prospect at Lagalochan in western Sc otland. The geology of Lagalochan is dominated by a calc-alkaline Cale donian intrusive complex emplaced into Dalradian Supergroup rocks (Har ris et al., 1988a). The complex consists of subvolcanic porphyrytic ro cks associated with intense hydrothermal alteration and brecciation. E arly Cu-Ag-Mo-rich mineralization is found in the core of the complex with later Pb-Zn-Ag-Au-As-Sb-rich mineralization toward the margins (o p. cit.). Stream sediment sample analysis spatially resolved the Cu-ri ch and Ag-rich mineralization. Ag and Cu anomalies relative to local b ackground values were found in all size fractions analyzed (-125 mu m, +125 mu m, +180 mu m, +250 mu m) but the more pronounced anomalies we re found in the -125 mu m fraction. The coarser fractions generally re vealed more erratic but longer dispersion trails. The separation of Cu -rich and Ag-rich mineralization is also apparent in the distribution of relatively high dissolved Cu and Ag in stream waters, particular wh en expressed relative to the solute load as Cu/TDS and Ag/TDS. The min eralization at Lagalochan is indicated by Ag, Cu and Pb stream sedimen t and water concentrations that are anomalous with respect to local ba ckgrounds, but are difficult to distinguish from regional backgrounds. This emphasizes the further potential of high-density stream water sa mpling, in particular, as a viable geochemical exploration tool on a s ub-regional scale for epithermal deposits in relatively unpolluted upl and terranes.