Scavenging of uranium in experimental and natural samples

Abstract

The hypothesis that interface coupled dissolution-reprecipitation reactions (ICDR) can play a key role in scavenging minor elements has been investigated via exploring the fate of U during the experimental sulfidation of hematite to chalcopyrite and the exsolution of chalcopyrite from bornite digenite solid solution (bdss) under hydrothermal conditions. The results of experiments with two kinds of Uranium (U) sources; either as solid UO₂₊ₓ(s) or as a soluble uranyl complex, differed from the U-free experiments. In the reactions from hematite to chalcopyrite under 220-300°C hydrothermal conditions, pyrite precipitated initially, before the onset of chalcopyrite precipitation. In addition, when UO₂₊ₓ(s) was included in the experiments, enhanced hematite dissolution led to increased porosity and precipitation of pyrite+magnetite within the hematite core. However, in uranyl nitrate bearing experiments, abundant pyrite formed initially, before being replaced by chalcopyrite. Uranium scavenging was mainly associated with the pyrite precipitation, as a result that a thin U-rich layer along the original hematite grain surface precipitated out. In the reactions of chalcopyrite exsolution from bdss during annealing under hydrothermal conditions in a solutions nominally containing Cu(I) and hydrosulfide in a pH₂₅°C [°C subscript] ~6 acetate buffer, a similar U-rich rim was observed along the original grain when uranyl nitrate as U-source was included in the reactions. The precipitation of uranium was related to the presences of HS- in buffer. Chemical mapping and X-ray absorption near edge structure (XANES) spectroscopy showed the UO₂₊ₓ(s) was the mainly restricted to the U-rich layer. The two sets of experiments demonstrate that the presence of minor components can affect the pathway of ICDR reactions. Reactions between U- and Cu-bearing fluids and hematite or chalcopyrite can explain the Cu-U association prominent in some iron oxide-copper-gold (IOCG) deposits. In this study, synchrotron-based X-ray fluorescence (SXRF) mapping was used to trace the distribution of uranium in natural samples from different geological contexts (sandstone-hosted U-deposit; IOCG) for investigating the deportment of uranium and its paragenesis in the context of thin-section scale textural complexity. It has been confirmed that the enrichment of U occurs via late dissolution-reprecipitation reactions in the bornite ores of the Moonta and Wallaroo IOCG deposits (South Australia), and that the U distribution in the ores of sandstone-hosted U-deposit is complex. Image analysis also revealed a number of new results for other minor elements, e.g. (i) the distribution of μm-sized Pt-rich grains and evidence for Ti-mobility during the formation of schistosity at the Fifield Pt prospect (New South Wales, Australia); (ii) the presence of Ge contained in organic matter and of Hg minerals associated within quartzite clasts in the Lake Frome U ores (South Australia); and (iii) confirmation of the two-stage Ge-enrichment in the Barrigão deposit, with demonstration of the presence of Ge in solid solution in the early chalcopyrite (Portuguese Iberian Pyrite Belt).