BIOGEOCHEMICAL PROSPECTING FOR PLATINUM GROUP METALS IN EASTERN SIBERIA

Abstract

Analysis of more than 25 000 plant samples, mostly rotted stumps and cones of Scots pine (Pinus sylvestris), has resulted in the discovery of several new types of platinum group metal (PGM) mineralization in weathered bedrock. Mineralization occurs in intrusions dominated by hybrid syenites of Palaeozoic age within the deeply faulted Gil'bera area of eastern Siberia. Plant ash contains up to 5000 ppb (ng g -1) Pt, in contrast to background levels of <1 ppb Pt. An initial survey revealed 12 biogeochemical anomalies, 1-10 m wide, with concentrations of 50-500 ppb Pt in plant ash. Subsequent trenching disclosed seven types of PGM mineralization: (1) stockworks up to several hundred metres in width; (2) smaller-scale PGM-rich zones (30-80 m wide) within these stockworks; (3) complex mineralized zones, 2-30 m wide, containing Au, Ag and from 2 to 6 of the PGM; (4) steeply-dipping zones, 0.2-3 m wide, within (3) above, containing only one PGM in one or two trees; (5) PGM in xenoliths of hydrothermally altered and skarnitized rocks; (6) zones of secondary alteration 1-5 m thick that blanket unweathered rock; and (7) lenses of secondary alteration. Of these seven categories of rocks hosting PGM, the most interesting occurrences fall within the weathered bedrock of group 6 from which PGM were mined c. 100 years ago.