Weathering-induced Sb isotope fractionation during leaching of stibnite and formation of secondary Sb minerals

In this work, we investigated the extent of antimony (Sb) isotopic fractionation during weathering of stibnite at supergene conditions. Antimony isotope data have been obtained from secondary Sb minerals collected from Pezinok, Dobšiná (both Slovakia) and Allchar (North Macedonia) deposits and mine tailings. The Sb isotope compositions of sulfides and secondary Sb minerals formed on the primary stibnite Sb2S3 or in mine tailings grains were compared with each other. Furthermore, we experimentally investigated Sb isotope fractionation during stibnite leaching with different acids. Our study reveals a large isotopic range for δ123Sb (from −0.50 to +0.69 ‰) for secondary Sb minerals. They are either isotopically indistinguishable or isotopically lighter than the primary stibnite. Isotopically indistinguishable weathering products likely formed by quantitative Sb transfer from stibnite to the secondary minerals, such as brandholzite Mg(H2O)6[Sb(OH)62] from Pezinok. Isotopic fractionation towards lighter δ123Sb was observed for adsorption of Sb onto iron oxides. Distinctly isotopically lighter δ123Sb was observed in secondary Sb minerals tripuhyite FeSbO4, chapmanite Fe3+2Sb3+(Si2O5)O3(OH), hydroxyferroromeite Fe2Sb2O6(OH), and stibiconite Sb3O6OH that either replace stibnite or formed in mine tailings from the pore solutions. These secondary minerals were likely generated by partial precipitation of Sb from aqueous solutions produced by dissolution of stibnite. In the leaching experiments with HCl and oxalic acid, Sb was leached without significant isotope effects during the first 2–3 days, followed by a drop of the dissolved Sb concentration associated with Sb isotope fractionation towards high δ123Sb in the leachate (by up to 0.5 ‰) after 4–7 days. We interpret these observations to be related to the precipitation of secondary Sb oxides with low δ123Sb, resulting in an isotopically heavy dissolved Sb pool. These findings are in agreement with previous results of isotopically heavy groundwater and mine drainage water with δ123Sb > +0.36 ‰ that may suggest that the 'truly' dissolved (operationally defined as <0.45 μm) Sb fraction in general may be isotopically heavy.