The Origin of Native Metal-Arsenide Mineralization in the World-Class Schlema-Alberoda Uranium Deposit (Germany): Insights from Arsenide Compositions and Fluid Inclusion Characteristics

The Schlema-Alberoda deposit in the West Erzgebirge region of Germany was one of the largest uranium deposits (extraction of 80 kilotonnes [kt] U) in central and western Europe. It is also a prime example of post-Variscan native metal-arsenide mineralization that is closely associated with uranium mineralization. This study focuses on the nature and composition of native metal-arsenide associations that occur as high-grade ore shoots across the Schlema-Alberoda deposit. Fluid inclusions from gangue minerals genetically related to the native metal-arsenide associations have homogenization temperatures between 126° and 138°C and fluid salinities of ~24.4 to 27.3 wt % (NaCl + CaCl2 equivalent). Fluid inclusion volatiles hosted in gangue minerals indicate that sedimentary and basement fluids mixed during arsenide formation. Fluid mixing occurred in response to the injection of a deep-seated metal-bearing basement fluid into shallower aquifers, triggered by progressive crustal thinning during the Mesozoic. Reduction of these low-temperature and high-salinity basement fluids by carbonaceous rock types is interpreted to have led to the formation of high-grade Co-Ni-Fe-arsenide ore shoots at Schlema-Alberoda. Mineralogical and petrographic observations document a distinct temporal zonation from nickel- and cobalt-rich to cobalt-iron–rich arsenide minerals. There is, however, no evidence of spatial mineralogical zonation on the vein and deposit scale. Nonetheless, skutterudite and nickelskutterudite decrease in S and increase in Fe contents with depth and decreasing distance to the redox barrier. Hence, we propose that the S and Fe concentration of the triarsenides could be a useful vector toward the redox front, which constrains the lower depth limit of mineralization.