Abstract

The Tamarack Intrusive Complex (1105 ± 1.2 Ma), in northeastern Minnesota, occurs within the Midcontinent rift system and hosts potentially economic Ni-Cu-(PGE) mineralization. The system represents “conduit-style” mineralization and with 1.3 wt % Ni, 0.7 wt % Cu, 0.3 ppm Pt, and 0.25 ppm Pd, is similar in many aspects to the Eagle deposit in Michigan. Sulfur, O, and Os isotopes have been used to evaluate the role of crustal contamination in promoting sulfide liquid saturation. All of the types of mineralization in the Tamarack Intrusive Complex are characterized by δ34S values between –0.2 and 2.8‰, values that are not strongly anomalous relative to uncontaminated mantle values near 0‰. The values are very similar to those from the Eagle deposit, but contrast sharply with values of disseminated sulfides in intrusions of the Duluth Complex and Crystal Lake Gabbro, which may be as elevated as 17‰. Initial 187Os/188Os ratios in the Tamarack Intrusive Complex are between 4 and 44% higher than the same ratio of the undepleted primitive mantle at 1105 Ma and correspond to gamma Os values for all magmatic sulfide types from the Tamarack Intrusive Complex ranging from 10 to 92. These values are consistent with crustal contamination but for S, the isotopic ratios are remarkably lower than those from mineralization in the Duluth Complex, where initial 187Os/188Os ratios are more than 110% higher than that of primitive mantle and γOs values may be in excess of 1,000. Olivine from an unmineralized but sparsely serpentinized portion of the Tamarack Intrusive Complex has O isotope compositions from 5.2 to 5.5‰, indicating a fraction of a percent crustal contamination of the parental magma.

The striking contrast between Os-S isotope systematics in conduit-type deposits associated with the intrusions that formed during the Main stage in the history of the rift is a primary characteristic of these ore systems and as such is important to explain by a geologic model. High Re and Os contents are found in sulfide minerals and organic matter in the sedimentary country rocks. Selective contamination via partial melting and devolatilization involving these phases could have been responsible for S and Re-Os transfer from country rocks to the sulfides without affecting the silicate magma. Therefore, the contamination of the system may have been an order of magnitude greater than that recorded by the silicate portion of the system. Alternatively, the rather low degrees of crustal contamination indicated by the Re-Os and S systems in the Eagle intrusion and the Tamarack Intrusive Complex may also be related to isotopic exchange between contaminated and pristine magmas in the conduit system during the Early stage of the rift development. The high Ni grades of the complex are in part related to the dynamic conduit environment and contamination of picritic magmas promoting sulfide saturation before large quantities of Ni were sequestered by olivine.

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