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Potential role of alkaline aqueous fluids in the formation of supergene and low-temperature hydrothermal mineralizations: experiments and natural analogues

The significance of hot, acidic fluids for the formation of ore deposits, especially in hydrothermal settings, is reasonably well understood. Such fluids are widely recognized as the most important transport media for metals and other solutes in most hydrothermal systems.In contrast, the role of alkaline fluids wth pH >>7, in ore formation is, with few exceptions, only poorly constrained. This is due to the fact that not only are many geochemical parameters and the behavior of critical metals in such fluids not known, but that for many elements even most basic information such as concentration data for natural alkaline waters, are simply not available. This severe knowledge gap prevents any reliable assessment of their potential role during the formation of ore deposits. Alkaline fluids, however, are ubiquitous and occur in many geological settings ranging from endorheic lakes to groundwater to marine hydrothermal fluids. Such fluids could have contributed to the formation of supergene and low-temperature hydrothermal deposits. Here, we intend to constrain the potential contribution of alkaline fluids to the formation of ore deposits by identifying geochemical and isotopic fingerprints of high field strength elements in alkaline solutions and in rocks that have interacted with high-pH solutions. To reach this goal, we will first investigate the distribution of HFSE (e.g., rare earth elements and Y, Sc, Zr, Hf, Nb and Ta) and the isotopic composition of Sr and Nd in alkaline lakes in Europe (e.g., Austria, Hungary), East-Africa, and the US. After having characterized the major, minor and trace solute geochemistry and further important physico-chemical parameters, untreated aliquots of these solutions are used for leaching experiments with a range of different rocks and with potential protores. These experiments will be complemented by experiments using artificial solutions that mimic simplified natural ones. The mobility of HFSE and their isotopes during water-rock interaction in alkaline conditions is not well constrained. This project will, close a significant knowledge gap by providing, for the first time, a comprehensive database of the HFSE and isotope distribution in alkaline waters from several locations. It will provide important information on how such alkaline fluids interact with rocks and how they affect the mobility of critical elements during water-rock interaction. The geochemical signatures in the leachates and the solid residues may be utilized as fingerprints of the involvement of alkaline fluids and will help geochemists and economic geologists alike to assess whether such fluids contributed to the formation of supergene and low-T hydrothermal ore deposits. This study will be complemented by geochemical modeling and finally, a comparison to geochemical data from actual ore deposits, in which the potential contributions of alkaline fluids to the formation of the described deposits will be investigated.