Physical and chemical constraints on platinum-group element behavior during crystallization of a basaltic komatiite liquid; example of the Proterozoic Delta Sill, New Quebec, Canada

Abstract

The Delta sill, located in the southern part of the 2.03 to 1.92 Ga Cape Smith fold belt, northern Quebec, is composed of a peridotite, pyroxenite, gabbronorite, and anorthosite suite derived from a basaltic komatiite parental magma (MgO = 13.58-15.66%). Despite its rather limited thickness (215-275 m), it contains three platinum-group elements (PGE) horizons, two slightly enriched horizons, one in the peridotite and one in the lower gabbronorite, and a strongly PGE-enriched horizon (forming a reef) in the upper gabbronorite. The high PGE content in the reef implies a system with an R factor (volume of immiscible sulfide liquid/ volume of silicated magma) of 100 to 1.000. However, the small volume of magma involved suggests that several other processes controlled the PGE behavior. Some experimental works have shown that the CaO and FeO contents in parental magmas influence the sulfide capacity and consequently the sulfur fugacity. The last parameter, as well as the oxygen fugacity, have been previously recognized as having a fundamental influence on the PGE enrichment processes. Because of the relatively high CaO contents (7.21-11.26%) of parental magma and the existence of PGE enrichment, the Delta sill provides an opportunity to test the effects of varying sulfur and oxygen fugacities on the processes of PGE enrichment. Thus, the PGE concentration profiles observed in the two sections transecting the sill are in good agreement with the experimentally determined behavior of these elements in silicate melts as a function of oxygen and sulfur fugacities. First, the relatively high Ca content of the initial magma induced a high sulfide capacity in the liquid. Second, decreasing sulfide capacity (and correlatively, an increase in sulfur fugacity) produced by the precipitation of, calcic phases--notably clinopyroxene and plagioclase--account for the PGE anomalies as well as the late-stage crystallization of sulfides in the reef. In the lowermost PGE anomaly, Ir and Ru enrichment within the peridotitic layer can be clearly related to early ferrochromite crystallization due to increasing oxygen fugacity, whereas subsequent Pt enrichment can be related to the formation of an isoferroplatinum phase (Pt 3 Fe) associated with ferrochromite. In the second PGE anomaly, located in the lower gabbronorite, synchronous Ir and Ru, Rh, Pt, and Pd, and S enrichments are due to the precipitation of small amounts of sulfides under low oxygen fugacity. Both the Ir and Ru and the Rh, Pt, and Pd, left over from the first chromite-forming stage, are complexed by sulfur when the sulfide capacity increases, consequent to CaO removal from the melt by plagioclase fractionation. Finally, the third anomaly in the PGE reef, appears to record the crystallization of a differentiated Fe- and Ti-rich residual liquid containing fluid phases and sulfur. It reflects the migration of an S-bearing liquid following the important lowering of sulfide capacity in the liquid due to the increase of SiO 2 and TiO 2 in the liquid and to the precipitation of Ca-bearing minerals.