Exploration for Platinum-Group Element (PGE) Deposits in Mafic and Ultramafic Rocks Reid R. Keays 1 and Peter C. Lightfoot 2 1 Mineral Exploration Research Center, Laurentian University, Sudbury, Ontario, Canada, P3E 2C6 2 Inco Technical Services Limited, Highway 17 West, Copper Cliff, Ontario, Canada, P0M 1N0 e-mail: rkeays@nickel.laurentian.ca, lightfootpc@inco.com Most economic PGE deposits belong to a continuum of Ni-Cu-PGE sulfide mineralization styles from those that tend to be sulfide-poor and mined mainly for their PGE contents, through to those that are sulfide-rich and are mined mainly for their Ni and Cu contents. These sulfide deposits occur in association with mafic and ultramafic rocks in a variety of settings:  Narrow "reefs" with low sulfide content within layered mafic/ultramafic intrusions (e.g., the Merensky Reef, South Africa; the J-M Reef of the Stillwater Complex, USA; and the Great Dyke, Zimbabwe).  Wider zones of PGE sulfide enrichment within mafic/ultramafic intrusions (e.g., Munni Munni, Australia; River Valley and East Bull Lake Intrusions, Canada).  Stratiform lenses of massive to disseminated Ni-Cu (PGE) sulfides at the base of komatiitic flow units (e.g., Kambalda, Australia; Cape Smith Fold Belt, Canada).  In association with the outer contact phase of the Sudbury Igneous Complex, Canada, and concentrated in early-formed Offset dykes and the proximal country rocks.  Stratabound zones of disseminated Ni- (Cu-PGE) sulfides within komatiitic rocks (e.g., Mount Keith, Australia).  Within feeder zones of magmatic systems (e.g., Voisey’s Bay, Canada).  Within subvolcanic sills which acted as feeders to continental mafic volcanic sequences (e.g., Noril’sk, Russia; and possibly the Thompson Nickel Belt, Canada). Many Ni-Cu sulfide deposits contain appreciable amounts of the PGEs, although the PGE tenor (calculated in 100% sulfide) varies enormously. Most of the mineralization was formed from magmas that were initially S-under- saturated but became S-saturated either by assimilation of S-bearing crustal rocks (e.g., komatiite-associated Ni sulfide deposits) or by magma mixing (e.g., Bushveld). During the fractionation of S-under-saturated mafic magmas, incompatible elements such as Cu, S, Se, Pd, and Pt accumulate in the residual silicate melt, whereas compatible elements such as Ni, Ir, Ru and Os are removed with the early silicate and/or oxide phases (Keays, 1995). Once a magma achieves S-saturation and fractionates even small quantities of immiscible magmatic sulfides, it will become strongly depleted in the PGE because of their very high Nernst partition coefficients (i.e. concentration element in sulfide melt/concentration element in silicate melt). The depletion in the PGE will be much more pronounced than the depletion in the other chalcophile elements such as Cu, S, and Se because the PGE have partition coefficients 1-3 orders of magnitude higher than these siderophile and chalcophile elements. A good example of this is provided by the Ni-Cu-PGE sulfide ores of the Sudbury camp; the noritic rocks of the Main Mass of the Sudbury Igneous Complex overlying the Sudbury ores are depleted in Cu and Ni, and, they are extremely depleted in PGE (Keays and Lightfoot, 1999). In the case of layered intrusions, the ratios of the PGE to elements such as S, Cu and Se are much more useful exploration tools than absolute PGE concentrations. We show that data sets from the Munni Munni Complex, Great Dyke, Stillwater Complex, and the Bushveld Complex exhibit cross-overs in PGE/chalcophile metal ratios proximal to mineralized horizons. This is illustrated for the Merensky Reef of the Bushveld Complex (Fig. 1). The Pd/Cu and Pt/Cu ratios of rocks occurring above the mineralized horizons are all significantly lower than for rocks occurring below the mineralized horizons in these intrusions. The reason for this is that prior to ore formation, the magmas below the mineralized horizons were S-under-saturated whereas those from above it were S-saturated. The advantage of this approach to exploration for PGE deposits is that stratigraphically controlled analyses of ordinary, non-mineralized rocks will provide valuable clues