Assessing the Role of Crustal S Contamination in the Generation of PGE Mineralization in the Stillwater Complex, Montana: A multiple S and Re-Os Isotopic Study

Indiana University, Bloomington IN

Investigators

Abstract

The bulk of the world?s supply of platinum is provided by what are known as ?reef-type? deposits. These deposits are thin layers that are found in igneous rocks and are characterized by the presence of 1 to 5 volume percent of disseminated sulfide minerals, principally those rich in iron, copper and nickel. PGE (platinum?group element) sulfide minerals occur in much smaller concentrations but provide the economically important metals such as platinum, and also palladium. Our need for platinum-group elements is projected to increase in coming years, driven particularly by advances in the automotive industry. The largest reserves of platinum are found in South Africa and Zimbabwe, with deposits found in the Bushveld Complex and the Great Dyke. Large reserves are also found in Russia (Siberia) associated with a somewhat different assemblage of igneous rocks that are very rich in sulfide minerals (sometimes nearly 100 % sulfides). The igneous rocks of the Stillwater Complex in Montana host the only two PGE mines in the United States in what is called the J-M Reef. The palladium production from the mines accounts for a few percent of the world total. Because of our utilization of PGEs the United States needs to increase our reserves and reduce our dependence on foreign sources. Research of the type described in this proposal is needed to improve models for both exploration and development of reef-type PGE deposits, similar to those found in the Stillwater Complex. The importance of sulfur derived from sedimentary rocks of the earth?s crust has not been previously emphasized in models dealing with the generation of reef-type deposits, largely because sulfur isotopic values are near those typically assigned to sulfur derived from the earth?s mantle. Because country rocks to the Stillwater Complex are in excess of 2.6 billion years (referred to as Archean age), the utilization of isotopic ratios that involve both 33S and 34S values provides another method to test if country rock derived sulfur has been essential for the production of PGE-enriched magmas. Sulfur from Archean sedimentary rocks may show a distinct departure from a slope of ~ 0.515 in ratio plots involving 33S and 34S; the relation defines what is known as the Terrestrial Fractionation Line (TFL). The origin of the anomalous departure from the TFL is still a matter of debate, but the presence of anomalous sulfur isotopic values in igneous rocks confirms the assimilation, or incorporation, of Archean-aged sulfur of country rock origin. In order to test the hypothesis that crustal sulfur derived from Archean country rocks was involved in the formation of the J-M Reef and other sulfide-rich zones in the Stillwater Complex we propose to collect multiple sulfur isotope data from samples collected in traverses through the Stillwater Complex in two locations that include the current workings at the Stillwater and East Boulder mines. Recent mapping in the Stillwater Mine has clearly defined a number of sulfide-rich lenses that constitute the PGE mineralization in the J-M Reef. Our sampling will include the individual lenses, with detailed petrographic (microscopic) analyses done to interpret interfaces between mineralized and non-mineralized domains. Sulfur concentrations from all samples will also be collected; the combined data set will permit rigorous mass balance calculations that will be used to quantify the importance of crustal sulfur involvement in the Stillwater Complex. The proposed work is required to evaluate if large PGE deposits owe their origins to the interaction of magmas with sedimentary country rocks that supplied sulfur far in excess of that which could have been sequestered from the mantle source of the magmas. Our results will constrain models for further PGE exploration and shed light on the processes of magma-country rock interaction that may be of significance in the generation of several types of metallic ore deposits.