Trace element distribution in primary sulfides and Fe–Ti oxides from the sulfide-rich pods of the Lac des Iles Pd deposits, Western Ontario, Canada : Constraints on processes controlling the composition of the ore and the use of pentlandite compositions in exploration

Abstract

There is an on-going debate as to whether the Lac des Iles Pd deposits (Ontario, Canada) are of magmatic or hydrothermal origin. An aspect of the deposits that has not yet been documented is the presence of sulfide-rich pods which occur throughout the host intrusion (the Mine Block Intrusion). The ore mineralogy of the sulfide-rich pods consists of pyrrhotite, pentlandite, chalcopyrite, ± pyrite, magnetite and ilmenite. We present the trace element concentrations of pyrrhotite, pentlandite, chalcopyrite, magnetite, and ilmenite from the pods and compare these results with results from other Ni–Cu–platinum-group element (PGE) deposits. The low concentrations of Si and Ca and high concentrations of V, Ni, and Cr in magnetite are consistent with a magmatic origin of the magnetite. Variations in the V and Cr concentrations indicate that magnetite crystallized from a magmatic sulfide liquid during crystal fractionation of the sulfide liquid. The enrichments in Ni, Co, Os, Ir, Ru, and Rh and depletions in Cu, Ag, Cd, and Zn in pentlandite and pyrrhotite relative to chalcopyrite are also consistent with the formation of the pods by crystallization of a magmatic sulfide liquid. Comparison of pyrrhotite and pentlandite compositions from Lac des Iles with those from other Ni–Cu–PGE deposits shows that pyrrhotite and pentlandite derived from evolved magmas have distinct compositions relative to those derived from more primitive magmas. In addition, this comparison shows that pentlandites from PGE-dominated deposits are richer in Pd and Rh than pentlandites from Ni–Cu sulfide deposits. A plot of Pd vs Rh appears to be effective at distinguishing pentlandites of PGE-dominated deposits from those of Ni–Cu sulfide deposits and could possibly be used to adapt exploration strategies