Silver and Base Metal Dispersion in Stream Sediments and Waters around an Epithermal Ag-Au-Cu Prospect at Lagalochan, Western Scotlan

Exploration & Mining Geology, Vol. 4, No. 3, 1995

D.A. POLYA, R.A.D. PATTRICK, D.J. MARSH1 and J.-J. BAO2, Geology Department and Geosciences Research Institute, The University of Manchester, Manchester, M13 9PL, United Kingdom.

The utility of high-density stream sediment and stream water sampling to the exploration for silver deposits was investigated by the study of the epithermal Ag-Au base-metal prospect at Lagalochan in western Scotland. The geology of Lagalochan is dominated by a calc-alkaline Caledonian intrusive complex emplaced into Dalradian Supergroup rocks (Harris et al., 1988a). The complex consists of subvolcanic porphyrytic rocks associated with intense hydrothermal alteration and brecciation. Early 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 (op. cit.). Stream sediment sample analysis spatially resolved the Cu-rich and Ag-rich mineralization. Ag and Cu anomalies relative to local background values were found in all size fractions analyzed (-125 µm, +125 µm, +180 µm, +250 µm) but the more pronounced anomalies were found in the -125 µm fraction. The coarser fractions generally revealed 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 when expressed relative to the solute load as Cu/TDS and Ag/TDS. The mineralization at Lagalochan is indicated by Ag, Cu and Pb stream sediment and water concentrations that are anomalous with respect to local backgrounds, but are dif?cult to distinguish from regional backgrounds. This emphasizes the further potential of high-density stream water sampling, in particular, as a viable geochemical exploration tool on a sub-regional scale for epithermal deposits in relatively unpolluted upland terranes.
Keywords: Silver dispersion, Base metal dispersion, Stream sediments, Stream water sampling, Mineralization.