Mineralogy of the U - Se-polymetallic deposit Niederschlema-Alberoda, Erzgebirge, Germany. IV. The continuous clausthalite-galena solid-solution series

Abstract

In a sample from the Niederschlema-Alberoda U - Se-polymetallic deposit, western Erzgebirge, Germany, the entire PbSe - PbS solid-solution series was observed associated with uraninite, coffinite, hematite, acanthite, sphalerite, chalcopyrite, pyrite, and lollingite. Early deposited, Se-rich members of the Pb(Se, S) series occur as fracture fillings inside spherical uraninite or on its surface or form anhedral to subhedral grains precipitated in the immediate neighbourhood of the U minerals. Later crys- tallized, S-rich members of the series are affiliated with the sulfide minerals. The solid-solution series covers the range PbS1.00 - Pb(S0.04Se0.96)Σ1.00 virtually free of gaps, consistent with a temperature of formation of ≥ 100 uC. The PbSe - PbS solid solutions were likely deposited from hydrothermal fluids that became successively depleted in Se and enriched in S. The fugacities of sele- nium and sulfur covered the range -17 < logƒSe2 < - 26 and -17 < logƒS2 < - 22, respectively, implying ƒSe2/ƒS2 ≤ 1. The spher- ical texture of the uraninite, as well as its U-Th - total Pb age (192 ± 21 Ma), imply deposition of the Pb(Se, S) series during the Jurassic, contemporaneous with the formation of the bulk of the other selenium minerals. The electron-microprobe data from this study confirm earlier inferences on complete miscibility between clausthalite and ga- lena deduced from X-ray patterns of PbSe - PbS solid solutions from different uranium-vanadium deposits of the Colorado Pla- teau (Coleman 1959). In Niederschlema-Alberoda, the entire clausthalite - galena series occurs in a single section.