Rare earth element and isotope (C, O, Sr) characteristics of hydrothermal carbonates: genetic implications for dolomite-hosted talc mineralization at Gِpfersgrün (Fichtelgebirge, Germany)

The talc deposit at Göpfersgrün (Fichtelgebirge, Germany) was formed by hydrothermal alteration of the Wunsiedel marble probably during Permian times. The Wunsiedel marble consists of calcite (cc1) and dolomite (do1) layers. Low REE contents, flat shale-normalized REEn patterns with a negative Ce anomaly, δ13CPDB values of −1.5 to 2‰ and a low 87Sr/86Sr ratio (0.7086) of the hydrothermally unaltered marble are typical for a sedimentary protolith formed within a marine environment. Hydrothermal alteration and talc mineralization of the marble occurs along a major fault zone and is associated with formation of massive saddle dolomite 2 (do2) replacing cc1 and do1. Massive dolomitization is followed by vug-filling carbonates, dolomite 3, calcite 2 and calcite 3. Each carbonate generation has a distinct chemical composition. Dolomite 2 is enriched in REE, depleted in 18O (δ18OSMOW=10.8 to 11.9‰) and shows a higher 87Sr/86Sr value (0.7103) compared to the Wunsiedel marble (δ18OSMOW=14.3 to 18.1‰). Late stage vug-filling carbonates differ from the preceding (replacement) dolomite 2 by higher REE contents, a clear roof-shaped REEn pattern, enhanced radiogenic Sr isotope composition (87Sr/86Sr=0.7115 and 0.7117) and lower δ13CPDB values (−2.6 to −11.5‰). Main talc mineralization result from decarbonation reactions at low XCO2 and temperatures between 250 and 400°C. It is proposed that main stage hydrothermal dolomitization, talc mineralization and late stage carbonate vug-fillings are related to formation brines or crustal fluids that interact with graphite-bearing metapelites under acid conditions. Changes in chemical composition between (early) replacement dolomite and (late) vug-filling carbonates are mainly ascribed to increasing fluid/rock ratios and decreasing temperatures.