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THE GENESIS OF SULFIDE ASSEMBLAGES IN THE FORMER WILHELMINE MINE, SPESSART, BAVARIA, GERMANY

Martin Okrusch^1,, Joachim A. Lorenz² and Stefan Weyer³

¹ Mineralogisches Institut, Universität Würzburg, Am Hubland, D–97074 Würzburg, Germany
² Graslitzer Str. 5, D–63791 Karlstein am Main, Germany
³ Institut für Petrologie, Geochemie und Lagerstättenkunde, Universität Frankfurt am Main, Senckenberganlage 28, D–60325 Frankfurt am Main, Germany

E-mail address: okrusch{at}mail.uni-wuerzburg.de

The vein-type copper mineralization in the abandoned Wilhelmine copper mine, Sommerkahl, Spessart, northwestern Bavaria, Germany, is hosted by metamorphic rocks of the Spessart Crystalline Complex. These are overlain by Permo-Triassic sedimentary rocks, including the stratabound base-metal mineralization of the Kupferschiefer. Ore textures in the sulfide veins demonstrate three stages of mineralization: (i) An early stage (I) is characterized by colloform textures, documented by spherical, cockade-like or garland-shaped, monomineralic or polymineralic aggregates of tennantite I, enargite I, pyrite I, chalcopyrite I, bornite I and digenite I. (ii) During a subsequent stage of recrystallization (II), the colloform textures were overgrown by, or enclosed in, anhedral grains of tennantite II, enargite II, bornite II, digenite II, pyrite II and chalcopyrite II. Minimum temperatures of about 175°C for this stage can be estimated from the bulk composition of fine-grained to submicroscopic digenite–bornite intergrowths, exsolved from an initial 1a solid solution with up to 55 mole % of bornite. Conversely, the assemblage pyrite + chalcopyrite + tennantite provides an uppermost temperature limit of ~440°C [at an a(S₂) = 10^–5]. (iii) A late stage of alteration (III) led to the replacement of the primary sulfides by yarrowite, spionkopite and rare covellite, together with goethite, under decreasing temperatures and rising activity of sulfur. The close spatial association of the Sommerkahl vein-type Cu deposit with the overlying Kupferschiefer suggests that this metal-rich bituminous shale played an important role in the formation of the ore veins. Sulfur isotope analyses yielded negative ³⁴S of –12.8 to –23.9, which indicate that the sulfur was derived from the overlying Kupferschiefer, presumably by hydrothermal leaching. Such a low-temperature mobilization could also be discussed for all or part of the metals forming the Wilhelmine ore, which later on recrystallized in response to hydrothermal activity (modified hydrothermal model). However, we prefer a strictly hydrothermal model, according to which all or most of the metals were derived from deep-seated sources, transported upward by hydrothermal fluids, and precipitated by thermochemical reduction of sulfate due to interaction with the sulfur-bearing organic matter and the pyrite of the Kupferschiefer. Formation of the sulfide ore veins in the former Wilhelmine mine is related to post-Variscan hydrothermal activity that affected the Spessart area in Middle Jurassic to late Early Cretaceous time.

Keywords: vein-type Cu mineralization, Kupferschiefer, tennantite, bornite, digenite, anilite, yarrowite, spionkopite, sulfur isotopes, hydrothermal activity, thermochemical sulfate reduction, Spessart, Germany.