PLATINUM-GROUP ELEMENTS IN FOX RIVER SILL, MANITOBA, CANADA - IMPLICATIONS WITH RESPECT TO INFLUXES OF FRESH MAGMA AND EXPLORATION FOR PGE DEPOSITS

The Fox River Sill is a Proterozoic (1.88 Ga) intrusion that is locate d along the Churchill-Superior boundary zone in northeastern Manitoba, Canada. It comprises a Marginal Series, a Layered Series and an upper Hybrid Series. The Layered Series is divided into the Lower Central a nd Upper Central Layered Zones (LCLZ and UCLZ). Platinum-group element (PGE)-bearing sulphide mineralization is contained in the lower 600 m of the UCLZ. This interval can be considered to consist of four 'regi mes' with distinctive crystallization characteristics. The PGE content of the bulk rock and in terms of metal in 100% sulphides is low in th e sulphide-rich (0.5-1.5 wt%) cumulates of the LCLZ but increases shar ply in the sulphide-poor cumulates at the base of regime 1. The conten t in 100% sulphides decreases systematically-with three abrupt reversa ls in the trend-over the mineralized zone in regime 1 and falls to a v ery low level above this. Increases occur in regime 2 and, in contrast to regime 1, no clear relationship is apparent between the incoming o f cumulus sulphides and the PGE content in 100% sulphides. PGE content s are low throughout regime 3. Sulphides occur at the base of many of the cyclic units of regime 4 and the PGE content of 100% sulphides, wh ich is high at the base of each cycle, decreases rapidly upwards in ea ch. These observations are interpreted as indicating that sulphide sat uration occurred in the middle of a cyclic unit in regime 1, after whi ch the fractional segregation of sulphide caused a systematic decrease in the PGE content of sulphides. The resulting considerable contrast between the PGE content of sulphides above and below the mineralized z one (< 1000 ppb above compared to 40 000 ppb below) is of practical im portance in exploration for such zones. In regime 2 the magma chamber was open, with frequent influxes of fresh magma, and the PGE content o f the sulphides was not so sensitive to sulphide saturation. In many c yclic units in regime 4 influxes of new magma caused immediate sulphid e saturation and consequent PGE depletion, with the result that, in co ntrast to regime 1, PGE-rich cumulates do not underlie PGE-bearing sul phide zones. Theoretical analysis indicates that regime 1-type behavio ur is likely to characterize intrusions below the level at which plagi oclase has entered as a cumulus phase, whereas regime 4-type behaviour is likely to occur above this.