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MINERALOGY OF THE NIEDERSCHLEMA–ALBERODA U – Se – POLYMETALLIC DEPOSIT, ERZGEBIRGE, GERMANY. V. WATKINSONITE, NEVSKITE, BOHDANOWICZITE AND OTHER BISMUTH MINERALS

Hans-Jürgen Förster^1,, Gerhard Tischendorf² and Dieter Rhede³

¹ Institute of Earth Sciences, University of Potsdam, D–14415 Potsdam, Germany
² Bautzner Strasse 16, D–02763 Zittau, Germany
³ GeoForschungsZentrum Potsdam, Telegrafenberg, D–14473 Potsdam, Germany

E-mail address: forhj{at}gfz-potsdam.de

The uranium deposit at Niederschlema–Alberoda, Germany, contains a rich variety of Bi minerals deposited between the Permian and the Cretaceous; these have been studied for paragenetic relations, composition, and conditions of formation. Particular attention is given to the rare Bi selenides watkinsonite, nevskite, and cuproan bohdanowiczite. Whereas watkinsonite and nevskite only occur intergrown with clausthalite, bohdanowiczite is more widespread and also is associated with Cu selenides. Watkinsonite from this second confirmed locality worldwide has an average composition (Cu_1.47Ag_0.49)_1.96(Pb_1.01Hg_0.01 Fe_0.01)_1.03Bi_3.98(Se_7.98S_0.05)_8.03, ideally (Cu,Ag)₂PbBi₄Se₈. These findings suggest that the empirical formula of watkinsonite originally proposed for the type specimen from the Otish Mountains uranium deposit in Quebec [Cu_2+xPb_1+xBi_4–x(Se,S,Te)₈, x 0.3] requires revision. The composition of nevskite is (Pb_0.06Bi_0.95)_1.01Se_0.99, on average. Bohdanowiczite from the Cu-selenide assemblage shows extensive substitution of Cu⁺ for Ag⁺, expressed by the crystallochemical formula (Ag_1.80–0.94Cu_0.16–1.05Pb_0.00–0.05)_1.98–2.07Bi_1.97–2.03Se_3.96–4.04. This observation seems to argue for the natural existence of Cu₂Bi₂Se₄, the Se-dominant analogue of emplectite. The Bi selenides were deposited at temperatures of about 100°C, in the Jurassic. The lack of thermodynamic data for all the Bi selenides limits reliable inferences on the fugacities of selenium and sulfur that prevailed during their formation. Other Bi minerals from this locality comprise members of the bismuthinite–aikinite solid-solution series of Permian age and, more importantly, native Bi and Bi sulfides (matildite, bismuthinite, wittichinite), deposited in the Cretaceous.

Keywords: selenium minerals, watkinsonite, nevskite, bohdanowiczite, matildite, wittichenite, bismuthinite, aikinite, solid solution, uranium deposit, Niederschlema–Alberoda, Erzgebirge, Germany.