West Lake map area (105N/9) is underlain by three fault-bounded structural blocks containing tectonically thickened (Late Proterozoic to Mississippian) Selwyn Basin strata. Northeast of the West Lake Fault tightly folded Hyland Group is structurally overlain by Road River Group. Between the West Lake and Wilson Range faults Hyland Group is thrust over Earn Group. In this block the variety of rocks preserved beneath the sub-Earn Group unconformity is greater than surrounding areas. Southwest of the Wilson Range Fault is a tectonically thickened, homoclinal succession of Road River and Earn groups. Silver-bearing quartz veins above and below a thrust fault were mined at the Plata occurrence. Anomalously high Ag, Zn and Pb concentrations in stream sediments result from black shale of the Earn Group exposed in three belts across the map area.