Abstract

The mineralogical residence of rare elements is of great academic and practical interest. In situ analytical methods employing largely nondestructive means (electron or proton bombardment) or sputtering by ion beams or laser ablation can be used to measure elemental concentrations directly, without the uncertainties inherent in the preparation of mineral separates. The platinum-group elements (PGE), An, and Ag are strongly fractionated in nature, with concentrations in minerals varying from nearly equal 100 wt percent to <1 ppb (by weight). Using accelerator mass spectrometry (AMS), we can obtain chondrite- normalized PGE patterns from single crystals of common ore minerals which contain precious metals at the ppb level. Preliminary findings for sulfide, oxide, and native element (graphite and native copper) species are illustrated, at levels ranging over some seven orders of magnitude (from nearly equal 0.1%, overlapping the sensitivity range of the electron microprobe, down to nearly equal 0.1 ppb). Complete Au-Ag-PGE analyses are tabulated and plotted for 21 mineral grains of eight species (pyrite, pyrrhotite, pentlandite, chalcopyrite, chromite, magnetite, graphite, and copper). The minerals represent a wide range of lithological and commodity associations, including chromitite, sulfide reef and potholes of mafic-ultramafic layered intrusions, subeconomic magmatic sulfide and pegmatoid facies of smaller mafic bodies, epigenetic sulfide veining, sediment-hosted copper deposits, and skarn-hosted gold mineralization. Quoted values for ore and basalt geostandard rocks, and additional data on historical and modern refined copper are also presented in graphical form.

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