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Polyphase enrichment and redistribution processes in silver-rich mineral associations of the hydrothermal fluorite-barite-(Ag-Cu) Clara deposit, SW Germany

Abstract

The silver-copper sulfide mineralization associated with the fluorite-barite vein system at the Clara deposit in SW Germany shows large scale vertical zoning. Low to moderate silver contents prevail in the upper 350 m, whereas high silver contents occur in the subsequent 450 m of the currently known vein system. This change in Ag tenor is related to conspicuous mineralogical changes with depth. A detailed petrographic and fluid inclusion study identifies evidence for five subsequent hydrothermal and one alteration stage—all contributing to mineralogical diversity. The vertical Ag zoning, however, is attributed only to the first of these stages. During this first stage, increasing oxidation of ascending hydrothermal fluids (90–160 °C, 24.2–26.7 wt% NaCl+CaCl2) led to the formation of high-Ag tetrahedrite-tennantite in the lower parts and basically Ag-free enargite in the upper parts of the vein system. The subsequent hydrothermal stage led to significant mineralogical changes, but inherited the pre-existing Ag zonation. In this second hydrothermal stage, which was related to fluids similar in composition to those of the first stage (70–125 °C, 23.1–26.5 wt% NaCl+CaCl2), dissolution of high Ag-tetrahedrite-tennantite resulted in the formation of complex Ag-sulfosalts together with moderately Ag-bearing tetrahedrite-tennantite and chalcopyrite. The first two stages were formed by fluid mixing of a sedimentary and a hot basement fluid. The influx of fluids with high Ag, Bi and Pb activity during stage 3 and 5 resulted in the local replacement of earlier Ag-sulfosalts by galena and Ag-(Bi)-sulfosalts. The fourth stage is marked by partial dissolution of sulfides and sulfosalts by a late, hot, undiluted basement fluid (250 °C, 18.7–20.9 wt% NaCl+CaCl2) precipitating fluorite, barite and quartz. Finally, supergene alteration lead to the dissolution of silver-bearing phases and the precipitation of acanthite and native silver. The study illustrates, how metal tenor and mineralogy are decoupled in vertically extensive, polyphase hydrothermal vein systems. This may be pertinent to similarly zoned polymetallic vein systems.

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Acknowledgments

We are grateful to T. Wenzel and T. Theye for their friendly assistance during EMP analysis, S. Staude, U. Kolitsch, and K. Huck for their helpful input during discussions and interpretation of the results, and S. Schafflick for the professional sample preparation. This work is a contribution of the r4 project “ResErVar—Ressourcenpotential hydrothermaler Lagerstätten der Varisziden” funded by the German Ministry of Education and Research (BMBF; Project reference number 033R129E).

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Correspondence to Maximilian F. Keim.

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Fig. 1

ESM Photographs, showing the exemplary FI petrography including (a) Primary FI in quartz (b) Isolated and primary FI in quartz and (c) Isolated FI in fluorite. (GIF 183 kb)

High resolution image (TIFF 10768 kb)

Fig. 2

ESM Ternary H2O-NaCl-CaCl2 phase diagram including the measured FI as circles. (GIF 40 kb)

High resolution image (TIFF 1403 kb)

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ESM 2

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ESM 3

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ESM 4

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Keim, M.F., Walter, B.F., Neumann, U. et al. Polyphase enrichment and redistribution processes in silver-rich mineral associations of the hydrothermal fluorite-barite-(Ag-Cu) Clara deposit, SW Germany. Miner Deposita 54, 155–174 (2019). https://doi.org/10.1007/s00126-018-0799-z

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