Vertical Zoning in Hydrothermal U-Ag-Bi-Co-Ni-As Systems – A Case Study from the Annaberg-Buchholz District, Erzgebirge (Germany)

The Annaberg-Buchholz district is a classic occurrence of hydrothermal five-element (U-Ag-Bi-Co-Ni-As) veins in the Erzgebirge (Germany) with a historical production of ~8700 t of Co ore, 496 t of U ore and 26.9 t of Ag. Multiple mineralization stages are recognized in polyphase veins hosted by Proterozoic paragneiss. Fluorite-barite-Pb-Zn mineralization occurs across the entire vertical profile of the district, whereas U and five-element stages are restricted to the upper 400 m below surface, coinciding with a graphite-rich gneiss lithology.
Here, we present field and petrographic observations, electron probe microanalysis and fluid inclusion data, as well as thermodynamic calculations to characterize fiveelement and fluorite-barite-Pb-Zn associations, and to constrain the origin of the vertical zoning in the Annaberg-Buchholz district. Microthermometric analyses of fluid inclusions related to the fluorite-barite-Pb-Zn stage yield homogenization temperatures between 78 and 140°C and salinities between 21.9 and 27.7 % eq. w(NaCl-CaCl 2 ).
A correlation of fluid inclusion Na/(Na+Ca) ratios with salinity suggests fluid mixing as a likely precipitation mechanism and relates ore-formation tentatively to regional tectonics of the Mesozoic opening of the Atlantic.Thermodynamic calculations indicate that U is more sensitive to reduction than As,
predicting that arsenide minerals are precipitated more distally relative to uraninite upon reduction along the fluid flow path. This implies that the observed vertical zoning is not a primary feature but is the result of hydrothermal remobilization. The observations made in the Annaberg-Buchholz district have general importance to the understanding of U-rich five-element mineralization and may be relevant for exploration targeting in unconformity-related U deposits.