Low-temperature transport of platinum-metals and selenium

Increasing demand for platinum-group elements (PGE) used mainly as auto catalysts, in jewellery and electronics, and a high price (> $ 1.000/oz) phase for palladium have stimulated research on PGE deposits and on PGE mineralogy and geochemistry in recent years. Much of this has gone into the development of petrologic concepts, assisted by experimental mineralogy, to explain the genesis of major magmatic (high-temperature) deposits such as the Bushveld, South Africa, Stillwater, USA, the Great Dyke, Zimbabwe, and Norilsk, Siberia, all of which are associated with mafic-ultramafic igneous rocks. Limited work only, however, has been done on the low-temperature solubility, transport and deposition of PGE, although experimental and field evidence now points to PGE mobility at temperatures well below 100 C. Possible reasons for this deficiency include resistance of the "orthomagmatic" school to the acceptance of fluids as possible transport agents, and the fact that most of the world`s platinum is recovered from deposits linked to mafic/ultramafic rocks. The low-T environment is important not only for understanding the late stages of development of "magmatic" PGE deposits, but also for concentrations of PGE in hydrothermal ("deposited from warm water") occurrences, in weathering zones, laterites and placers. To test these novel concepts, we intend to investigate one remarkable PGE-Au deposit which is unrelated to mafic- ultramafic rocks (the "standard hosts" of PGE) and has very likely formed from meteoric waters and at low temperatures. This is Coronation Hill in the Northern Territory of Australia, where PGE-Au mineralisation occurs in Proterozoic metasedimentary and igneous rocks in the vicinity of a major fault system. Selenides play a significant role in the mineral association, and the derivation of not only Se, but also of Pt, Pd, Au and small amounts of base metals has, so far, not been explained. In addition to established mineralogical and geochemical methods we therefore intend to use the novel approach of selenium isotope analysis in an attempt to determine the source of Se. Only during the past five years has it been recognized that biological processes lead to fractionation of Se isotopes. This may assist in determining the role of black shales in the study area in concentrating and releasing Se. The respective work will be undertaken in cooperation with the Woods Hole Oceanographic Institute, USA, one of only two institutions world-wide where Se-isotope determinations can presently be undertaken. The occurrence of parts of the mineralisation in carbonaceous shales necessitates investigation of maturity of the organic matter present. Coronation Hill is one of very few low-temperature PGE deposits world-wide; others are Waterberg, South Africa, and Serra Pelada, Brazil. Extensive drill core material from the exploration stages of the deposit is now housed at the Northern Territory Geological Survey (NTGS) core library in Darwin, and NTGS has offered us collaboration and significant geoscientific (geophysical data, 3-D model, etc.) and logistical (maps, archives, field vehicles) support. It is a unique opportunity to advance our understanding of the behaviour of PGE and Se at low temperatures employing novel techniques such as selenium isotopes, with the Coronation Hill deposit as a test site. NTGS will provide access to drill cores and to the field area and also offers logistical support which significantly reduces the costs of the project. We also intend to maintain and expand, by projects such as this, European know-how in mineral deposits geology. This has been several endangered and curtailed by the decline of mining and exploration activities in Europe. Association with GEODE successor projects of the European Science Foundation (ESF) and IGCP (UNESCO) is planned.