EPMA, micro-PIXE, synchrotron microprobe and TEM study of visible and invisible accumulations of Au and PGE in black shale and organic matrix, Kupferschiefer, Poland

Abstract

AbstractThe Kupferschiefcr contains from 0.3 to 30 wt.% of organic matter, with an average value close to 10 wt.%. The concentration of heavy metals increases strongly towards the base of a black shale horizon. Where the black shale rests directly on red sandstone, the bottom few mm of the shale may contain (ppm): Hg 8-1500, mean 174; Ag 310-5780, mean 1222; Au 1-3000, mean 40; Pd 2-1000, mean 37; Pt 2-340, mean 18. The shale, enriched in noble metals, is overlain by thucholite shale. The flotation sulphide concentrate contains 1-3 ppm Au and a detectable Pd and Pt content. Selected gravity concentrates from the black shale contain 4-56 ppm Au, up to 25 ppm Pd, while the Pt content does not exceed 1 ppm. A tightly intermixed clay-organic matrix of the noble metal-bearing shale contains small accumulations of Au, Pt and Pd presumably as organomctallic compounds.Thucholite is present locally. It contains inclusions of uraninite, brannerite and noble metal minerals lcoated at the boundary between optically isotropic and anisotropic components. The optically homogeneous organic matrix of the thucholite contains discrete inclusions of UO2, brannerite, electrum and organic compounds of Au, U, Pt and Ni varying in size from 0.008 to 0.025 μm as shown by TEM. Gold occurs in a continuous Au-Ag series. Refractory Au is present as substitutions in native silver and in the organic matrix of the shale and thucholite. Palladium forms several arsenides and sulphoarsenides. Refractory Pd is present as substitutions in Ni-Co arsenides and in the organic matrix of the shale and thucholite. Platinum is present as substitutions in the organic matrix of the noblemetal- bearing shale and thucholite.The average detection limits of micro-PIXE were 8-10 ppm, and those of the SXRFA from one to a few ppm. A major factor defining detection limits in the studied samples were minor uranium lines overlapping spectral lines of the studied elements.