Discovery of the Mt Bini porphyry copper-gold-molybdenum deposit in the Owen Stanley Ranges, Papua New Guinea - A geochemical case history

The geochemical dispersion train of a porphyry Cu-Au-Mo deposit with overprinted epithermal Au-Ag in a rugged tropical environment 50 km northeast of Port Moresby, Papua New Guinea was successfully located by systematic stream sediment and rock float sampling. A reconnaissance sampling density of 1 site per 10 km(2) was sufficient to locate the deposit. Fanned concentrate (PC) gold and bulk cyanide leach (BCL) gold clearly define significant anomalous drainage basins at the regional scale. A diagnostic Fb signature in material split from the BCL characterised the anomaly, The Au anomaly in the FC's extended farther downstream than that in the BCL. Results of analysis for Au, Cu, and Mo in ridge and spur samples of residual soils, together with rock-chip samples in drainage outcrop, effectively delineated mineralisation. A coincident Au-Cu-Mo anomaly is explained by porphyry-style mineralisation consisting mainly of chalcopyrite (with associated gold) and molybdenite within a potassium-rich quartz diorite. The tenor of the geochemical expression varies as a function of the pyrite content. Quartz veining intensity is seen to be a useful measure of Cu-Au mineralisation. Trenching of the most anomalous soil samples delineated targets which were successfully tested by diamond drilling.