The world-class Schlema-Alberoda U-(Ag-Co-Ni) deposit (Germany): mineralogy and fluid characteristics

The Schlema-Alberoda deposit represents one of the largest uranium deposits in Europe with uranium ores closely associated with native metal-arsenide mineralization. All mineralization styles occur in veins and stockwork zones that crosscut carbon-rich Devonian to Silurian metasediments and metabasites. This study investigates the native metal-arsenide veins in the district and provides an update on mineral paragenesis and novel fluid inclusion data. Native metal-arsenide mineralization occurs as vein-hosted ore shoots with native metals (Ag, Bi, As) and Co-Ni-Fe arsenides. Within the native metal-arsenide stage, five mineral associations are identified: (i) bismuth-skutterudite-safflorite, (ii) silver-rammelsbergite-skutterudite, (iii) arsenic-silver-loellingite, (iv) loellingite and (v) arsenic-sulfosalt-sulphide. Fluid inclusions measured in dolomite-ankerite that occur as gangue minerals have homogenization temperatures of ~115-150°C with fluid salinities of ~24.4-27.3 wt% (NaCl+CaCl2) eq. The spatial relationship between native metal-arsenide across the deposit and carbon-rich lithologies suggest reduction of the ore fluid as a decisive precipitation process. Microthermometric data indicate mixing of a sedimentary and a basement brine, which is also documented for other occurrences of native metal-arsenide mineralization across Europe that are all related to Mesozoic continental rifting.