MINERALOGY OF THE NIEDERSCHLEMA ALBERODA U Se POLYMETALLIC DEPOSIT, ERZGEBIRGE, GERMANY. I. JOLLIFFEITE, NiAsSe, THE RARE Se-DOMINANT ANALOGUE OF GERSDORFFITE

Abstract

The Niederschlema–Alberoda uranium deposit, in the Erzgebirge region of Germany, contains an uncommon assemblage of metallic minerals, in particular selenides, sulfides, arsenides, tellurides, and native elements, in addition to uraninite and coffinite. The complex mineralogy resulted from the superposition of several mineralizing events over the time interval from the Permian to the Cretaceous; these events introduced and redeposited a great variety of metallic elements within the hydrothermal uranium deposit (Pb, Ag, Cu, Hg, Tl, Bi, Co, Ni, As, Sb, Se, S, Te). One of the exotic minerals is jolliffeite, an arsenoselenide with endmember composition NiAsSe, so far only known from Lake Athabasca, Saskatchewan, Canada. A single, small, anhedral grain of jolliffeite from Niederschlema–Alberoda is included and partly replaced by sulfurian eskebornite. Associated minerals comprise hematite, Ni–Co–Se-bearing lollingite, clausthalite, tiemannite, mercurian hakite–giraudite solid solutions, sulfurian berzelianite, sulfurian umangite, hessite, Ni–Co–As-bearing pyrite, and Se-rich chalcopyrite. The sulfurian jolliffeite has the empirical formula (Ni0.85Cu0.09Co0.05Fe0.02Ag0.01)� 1.02As0.98(Se0.77S0.23)� 1.00 and differs from type jolliffeite mainly by substantial substitution of Cu (2.6–3.3 wt.%) for Ni and S (3.2–4.1 wt.%) for Se. Substantial S-for-Se substitution in jolliffeite implies extensive and probably complete miscibility between NiAsSe and its S-dominant analogue, gersdorffite-Pa3 (NiAsS). We suggest that a localized accumulation of Ni and As in the Se–(S)-bearing hydrothermal fluid gave rise to the crystallization of jolliffeite at some rare locations at a late stage of formation of the Jurassic selenide assemblage.