Isotopic geochemistry of mafic intrusions and related sulfide mineralization: Uitkomst and Kabanga, Africa and the Lady of the Lake intrusion, Montana

Stable isotopic and mineralogic studies of three mafic intrusions located in different tectonic environments have been conducted to evaluate both open and closed system processes that may have influenced the genesis of Cu-Ni sulfide mineralization. The Uitkomst and Kabanga intrusions are both located in southern Africa, whereas the Lady of Lake intrusion is located in the Tobacco Root Mountains of southwestern Montana. Both the Uitkomst and the Kabanga intrusions are thought to have formed in a rift-related environment. In contrast, the Lady of the Lake intrusion developed in a subduction zone. Intrusions at Kabanga and Uitkomst have been previously dated at 2.04Ga and 1.4Ga by U-Pb techniques. U-Pb dating of baddeleyite and zircon from the Lady of the Lake intrusion conducted as a part of this work gives an age of 74.88+/-0.17Ma.;Rock types present in the Uitkomst and Kabanga intrusions include harzburgite, pyroxenite, gabbro, norite, and gabbronorite. Disseminated and massive sulfide ore zones comprised predominantly of pyrrhotite, chalcopyrite, and pentlandite are present at both localities. Sulfur isotopic values of the sulfide minerals differ from the 0+/-2‰ values expected for sulfur of mantle origin (negative in the case of Uitkomst and strongly positive in the case of Kabanga). The sulfur isotopic data confirm the premise that externally derived sulfur is crucial for the generation of magmatic Cu-Ni sulfide ore deposits. Oxygen isotopic values of olivine, pyroxene, and plagioclase from the Uitkomst intrusion are generally in the range of those considered to be normal for uncontaminated mantle-derived melts (∼5-6‰). The decoupling of the O and S isotopic systems indicates that S assimilation by the magmas did not involve extensive partial melting of country rocks, but was more likely related to the generation of H2S during devolatilization of country rocks. A similar process is envisioned at Kabanga, however elevated oxygen isotope values of pyroxenites and gabbronorites suggest that contamination via partial melting of 18O-enriched sedimentary roof rocks occurred after the accumulation of sulfide-bearing harzburgites.;The Lady of the Lake Intrusion (LOL) is a much smaller layered body consisting of melagabbro, gabbro, and diorite that occurs along the contact between the Cretaceous-aged Tobacco Root Batholith (TRB) and Precambrian country rocks. This study was initiated to access the genetic relationship between the mafic rocks of the LOL and the silicic rocks of the TRB, with particular emphasis on the possible importance of mafic rocks in the development of porphyry Cu-style hydrothermal mineralization in the silicic rocks. Both O and S isotopic results indicate that the melagabbro and gabbro are of mantle derivation. Oxygen isotopic data from the TRB is consistent with previously reported radiogenic isotopic data which suggest that the genesis of the TRB involved the mixing of mantle and crustally derived melts. Mineral compositional data and rare earth element (REE) contents suggest that the diorite was produced as a result of fractional crystallization from a magma identical in composition to that which produced the melagabbro and gabbro. Because the diorite is intrusive into the gabbroic rocks, fractionation is thought to have occurred in a separate chamber. The Lady of the Lake Intrusion is genetically unrelated to the Tobacco Root Batholith, but mafic magmas that produced intrusions such as the Lady of the Lake Intrusion supplied the heat which was required to melt crustal rocks and generate the Tobacco Root Batholith.