S, O, and Re-Os Isotope Studies of the Tamarack Igneous Complex: Melt-Rock Interaction During the Early Stage of Midcontinent Rift Development

摘要

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.