Applications of PIXE to mineral characterization

Abstract

This article illustrates the application of the proton-induced X-ray emission (PIXE) technique to detailed documentation of mineral assemblages, with emphasis on base–metal ores. Some of the investigations aided by the PIXE laboratory at Guelph since 1993 include determinations of the distribution of minor and trace elements in magmatic Ni–Cu ores, volcanogenic massive sulphide Cu–Pb–Zn–(Ag–Au) ores and lode Au–(Ag) deposits. Minor elements of importance include possible by-products or co-products of metal refining, as well as deleterious impurities in mill-feed, e.g. Cd, In, Sn, As, Se, Te, Tl and Hg. Weathering products of primary sulphide mineralization, including tropical laterites and other oxidized assemblages, have been analysed successfully and can contain a wide range of minor elements which reflect the bedrock style of mineralization. The iron oxyhydroxide goethite, α-FeO(OH), contains trace levels of many elements, and in some cases 1 wt.% or more of base metals and arsenic, elements which are invisible in reflected-light microscopy. Other metals such as Ag are of sporadic occurrence in oxidized ores: they may be found as discrete mineral species, not incorporated into the dominant oxyhydroxides. A summary of findings from three base–metal deposits in Canada, the Philippines and Portugal serves to illustrate the manner in which PIXE data benefit our knowledge of metal distributions in metallic ores. PIXE can contribute to several facets of mineral-deposit research, such as: (1) the development of ore textures, and specifically the distribution of elements within zoned crystals, or between multiple generations of a particular mineral; (2) the location of precious metals, Ag being in general the simplest case; and (3) pinpointing elements that may have implications for ore genesis, environmental quality or metal refining, such as Cr, As and Se.