Abstract

The margins of layered igneous intrusions can host ore deposits of nickel, copper, and platinum group elements (PGEs). These marginal deposits are characterized by a complex geologic history, which obscures direct evidence for the mineralization process. Mass-independent fractionation of sulfur isotopes is a distinguishing feature of Archean sedimentary rocks, and reflects processes operating solely in the Archean surface environment. As a result, mass-independent fractionation of sulfur isotopes is a chemically conservative tracer that indicates the involvement of crustal sulfur in marginal Ni-Cu-(PGE) deposits. In this study we use mass-independent fractionation of sulfur isotopes to evaluate the sulfur budget of the Platreef—the marginal PGE-bearing horizon of the northern limb of the Bushveld Complex, South Africa. Our multiple sulfur isotope measurements demonstrate that crustal sulfur in the Platreef originated from a restricted stratigraphic horizon (the Duitschland Formation) in the Platreef footwall. As this signature is spread out over the ≈100-km strike length of the Platreef, it does not reflect sulfur inputs from the local footwall. Nonlocal crustal sulfur isotope anomalies are unexpected given the high metal tenors in the Platreef. These features can be reconciled if a preformed Ni-Cu-(PGE)–rich sulfide melt assimilated sulfur in a staging chamber bounded completely by the Duitschland Formation, and then was emplaced into the Platreef. Such a model is supported by long-standing observations of lithologic, geochemical, and stratigraphic similarities between the Merensky Reef and the Platreef, as well as by recent suggestions that the northern limb represents an “escape structure” for the rest of the Bushveld Complex. Crustal sulfur assimilation is often thought to initiate the ore-forming process by producing an immiscible sulfide melt that can collect base metals and PGEs. In the Platreef, however, crustal sulfur assimilation apparently diluted a preexisting PGE-rich sulfide melt instead.

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