S, OS AND CU ISOTOPE VARIATIONS BETWEEN SHEET- AND CONDUIT-STYLE NI-CU-PGE MINERALIZATION IN THE MIDCONTINENT RIFT SYSTEM, USA

The 1.1 Ga Midcontinent Rift System (MRS) hosts multiple types of Ni-Cu-PGE sulfide deposits. The two most common types of mineralization are sheet- and conduit-style. Disseminated sulfides occur within troctolite to gabbro layers in intrusions such as Partridge River and South Kawishiwi in the Duluth Complex of Minnesota and Crystal Lake in Ontario. Disseminated, net-textured, and massive sulfide mineralization occurs in near vertical olivine-rich dike-like bodies, such as the Eagle intrusion in Michigan and the Tamarack Intrusive Complex in Minnesota. The two distinct types of sulfide-rich deposits are characterized by key differences in S, Os and Cu isotope systematics.

Mineralization associated with the sheet-style intrusions exhibit positive δ³⁴S values ranging from 0 to 30 ‰. ¹⁸⁷Os/¹⁸⁸Os ratios reach up to 4.72, and γ_Os values are as high as 1175. δ⁶⁵Cu values of disseminated sulfides in the Duluth Complex range from -0.85 to 0.45 ‰. The S and Os isotope values are similar to those of Proterozoic country rocks and suggest that magma contamination was essential for ore genesis. In contrast, the δ³⁴S values of disseminated, net-textured and massive sulfide mineralization at Eagle and Tamarack fall in the range of 0.3 to 4.6 ‰, and ¹⁸⁷Os/¹⁸⁸Os ratios range from 0.1511 to 1.248 with γ_Os values restricted between 7 and 53. δ⁶⁵Cu values of the mineralization at Eagle and Tamarack are higher than those of sulfides in the sheet-style mineralization and range from 0.69 to 1.84 ‰, outside the range of typical mantle-derived melts.

The conduit-style deposits show S and Os isotope characteristics that may be attributable to isotopic exchange processes in magma conduits, with δ³⁴S and γ_Os values more similar to those of uncontaminated mantle. The strongly anomalous S and Os isotope ratios of sulfide minerals in the sheet-style intrusions are more similar to those found in sulfidic and carbonaceous Proterozoic country rocks, and are indicative of less exchange with uncontaminated mantle-derived magmas. The differences between the Cu isotope compositions of the two types of mineralization are difficult to reconcile by similar mechanisms and must be related either to the presence of distinct Cu reservoirs in the mantle of the MRS, or to Cu isotope fractionation during separation of immiscible sulfide liquids from source magmas.