Evidence from sulfur isotope and trace elements in pyrites for their multiple post-depositional processes in uranium ores at the Stanleigh Mine, Elliot Lake, Ontario, Canada

Abstract

The ca. 2.45 Ga pyritic uraniferous quartz-pebble conglomerate (UQC) of the Matinenda Formation of the Elliot Lake Group, Huronian Supergroup, was used in this study to investigate the origin of pyrite. A laser-microprobe was used for analysis of the sulfur isotopic compositions of individual pyrite grains, and an electron-probe microanalyzer was used for analysis of the trace element compositions of pyrite grains with overgrowth texture. We found a variation in δ34S values among pyrite crystals (73 analyses) of various size and morphologies that occur in a small (∼1 cm3) rock chip: the total range in δ34S is −9.0‰ to +5.5‰ with respect to CDT (Cañon Diablo Troilite) with a mean value of +0.6‰ ±2.1‰ (1σ). The widest range of ∼15‰ is found among euhedral pyrite grains whereas variations of ∼4‰ to ∼6‰ are common in anhedral, subhedral, and rounded grains of pyrite. These values are in marked contrast to the δ34S values of pyrite from the Matinenda Formation that were obtained by previous investigators using bulk-rock sulfur isotope analyses. We found variable concentrations of Co (below detection to 4700 ppm), Ni (to 1900 ppm), and As (to 3400 ppm) among individual pyrite crystals and within single grains with overgrowth textures. These elemental concentrations are markedly different between core and overgrowth parts of pyrite. We demonstrate that the pyrite grains in the Paleoproterozoic UQC have been isotopically, chemically, and morphologically modified by post-depositional processes, suggesting that the pyrite grains have undergone multiple generations. The results of the present study cannot be explained solely by a detrital process. Therefore, one cannot use the preserved morphology and chemistry of pyrite (and possibly uraninite) to represent the original features at the time of deposition to support the hypothesis of an anoxic atmosphere prior to 2.2 Ga.