| 英文摘要: | Nickel (Ni), copper (Cu) and platinum-group elements (PGEs: platinum (Pt), palladium (Pd), rhodium, iridium, ruthenium, and osmium (Os)) are vital to the economic growth of the United States. Ni and PGEs are found almost exclusively in igneous rocks that are rich in iron and magnesium. Copper may occur in a variety of rock types but is also found in association with Ni and PGEs. The ore elements typically occur bonded to sulfur in sulfide minerals, or in some cases PGEs may be found bonded to arsenic (As), tellurium (Te) or antimony (Sb). Although most of these deposits are found within igneous rock bodies, there is a category of mineralization that is composed of nearly pure sulfide minerals that is located outside of the igneous rocks and within enclosing sedimentary rocks. The grade of these occurrences may be high and they represent a poorly understood resource that will be of national importance as our global need for raw materials increases and we strive to maintain a sustainable earth. Because the ore is massive the mineable dimensions are relatively small and hence the environmental footprint is correspondingly small; any necessary reclamation is readily accomplished. Nickel is a strategic ferro-alloy which is essential to the manufacture of stainless steel. Both Pt and Pd are used as active metals within catalytic converters. The United States has only one actively producing Ni mine and only two mines that produce Pt and Pd (both within the same igneous body). The work that will be undertaken as part of this project will increase our understanding of how massive sulfide occurrences form outside of the spatially associated igneous rocks. In order for the project to be successful a collaborative effort between company geologists and scientists at academic institutions is required. A key broader implication of the research is that students from the IU Department of Education frequently undertake research projects in our lab; the experience is beneficial as teachers are prepared to promote student training in science, mathematics and sustainability.
Although massive sulfide mineralization associated with igneous can often be explained via gravitational accumulation of immiscible sulfide liquid in a magmatic system, some occurrences of massive sulfide mineralization associated with Ni-Cu-PGE deposits are more difficult to explain by this mechanism. Mineralization such as that found at Voisey's Bay (Labrador), Eagle (Michigan) and the newly discovered Tamarack deposit (Minnesota) are associated with magma conduits; massive sulfides may have accumulated where magma velocity decreased as the conduit widened, and perhaps flattened. However, discoveries at each of these localities of massive sulfide hosted by metamorphosed sedimentary country rocks are not easily accounted for by such a mechanism. Cu-Ni-(PGE) massive sulfide mineralization hosted by country rocks is also found in association with larger, sheet-like layered intrusions, including intrusions of the Duluth Complex (Minnesota) and the Stillwater Complex (Montana). In both of these localities massive sulfides are found in pelitic rocks of the contact aureoles, with limited or no connectivity with the igneous rocks. Preliminary geochemical data from the massive sulfide mineralization in the Virginia Formation near the contact with the Duluth Complex show strong signatures for partial crustal derivation of S, Os, Cu and lead (Pb). In many localities the country rock-hosted massive sulfides penetrate and crosscut disseminated sulfide-bearing igneous rocks. The origin of the country rock-hosted massive sulfide occurrences is poorly understood; clearly a better understanding is essential for future development of limited natural resources. We plan to undertake a detailed study of the physical and textural form of three such deposits in the Midcontinent region (Eagle, Tamarack, and the Mesaba (Babbitt) deposit of the Duluth Complex, and at the Stillwater Complex), along with a determination of key geochemical characteristics (variations of Ni/Co, S/Se, PGE, Sb and As concentrations and ratios, and Os, Pb, Cu, Ni and S isotopic compositions), that will shed light on the interpretation of whether mantle-derived melts, crustal melts, or hydrothermal fluids have controlled their formation. The well-known Noril'sk Ni-Cu-PGE deposits in Siberia also contain massive sulfide mineralization that occurs in the sedimentary country rocks, separated by several meters from sulfide-bearing igneous rocks. Our Russian colleagues are in the midst of a detailed study of the mineralization at Noril'sk, including the country rock - hosted type which is of particular economic significance. Their study parallels our proposed work and provides an opportunity for comparative evaluation of a poorly understood ore type.
This project is supported in part via the NSF FY15 Sustainable Chemistry, Engineering, and Materials (SusChEM) initiative. |