A REVIEW OF THE APPLICATION OF MULTIPLE S ISOTOPES TO MAGMATIC Ni-Cu-PGE DEPOSITS AND THE SIGNIFICANCE OF SPATIALLY VARIABLE Δ33S VALUES

Abstract

The determination of Δ 33 S values in conjunction with δ 34 S values provides an effective means of evaluating the importance of S derivation from Archean country rocks in magmatic Ni-Cu-PGE ore genesis, particularly where δ 34 S values of potential S source rocks are near 0‰ and similar to most S of mantle derivation. However, Δ 33 S values of sulfides in some Archean rocks may also be near 0‰; care should be taken in the evaluation of S derivation from Archean rocks based solely on this parameter. Although anomalous Δ 33 S values are found primarily in Archean rocks, rare occurrences of nonzero Δ 33 S values have been identified in sulfides from younger rocks, and hence knowledge of the multiple S isotope composition of potential contaminants is essential for an accurate assessment of S contamination of mantle-derived magmas. Restricted ranges in Δ 33 S values may be characteristic of particular Archean formations, and when used in conjunction with mixing calculations may fingerprint a distinct S source rock. Spatial variability in Δ 33 S values within different types of mineralized systems may be caused by the emplacement of isotopically distinct magma pulses, by locally variable mixing proportions, and the addition of secondary sulfide to the primary assemblage. In the case of conduit-type deposits in the Midcontinent rift system, three-component mixing calculations show that both high and low Δ 33 S values in some units that are similar to end-member values in the Archean country rocks must be a function of assimilation of primarily Archean S even though Proterozoic carbonaceous sedimentary country rocks contain in excess of 1 wt % S and are particularly viable S source rocks.