S-O Stable Isotopes and Geological Considerations of Ba–Sr Deposits from Neuquén Basin, Northwest Patagonia, Argentina

Jurassic–Cretaceous marine–continental carbonate–evaporitic sequences in the Neuquén Basin of Argentina host numerous stratabound Ba–Sr deposits. Mineralization (Sr–barite, Ba–celestine, and minor Pb–Zn–Cu–Fe sulphides) occurs as bedding parallel lenses and crosscutting veins. The stratiform mineralization is formed by replacements of carbonate and gypsum beds and often exhibits typical zebra textures. Dissolution processes associated with Neogene regional uplift produced karstic cavities where a new generation of barite was deposited. Regionally, W to E distribution of carbonates/evaporites and that of Ba–Sr deposits is coincidental. Lower Cretaceous Sr–Ba deposits are spatially related to large N-S reverse faulting, frequently limited to the eastern limb of the folded structures. Average δ18O and δ34S of stratiform and crosscutting vetiform mineralization do not differ significantly, suggesting a common source of sulphate and cations. Deposits spatially linked to areas with magmatic activity and those that are not have similar isotopic values, compatible with bacterial and/or thermochemical reduction of contemporaneous seawater sulphate, although sulphides only occur in deposits with evidence of nearby magmatic activity. Thermal convection of basinal brines leached metals from the Mesozoic sedimentary pile; Ba and Sr were extracted from siliciclastic and carbonate rocks, and sulphur from evaporite layers. Fluids related to Tertiary magmatism helped producing an epithermal mineral association composed of barite, quartz, adularia, and minor sulphides/sulphosalts hosted by veins. Arroyo Nuevo mine (Ba) is different, as it seems to be the product of hydrothermal SedEx deposition onto the anoxic seafloor.