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

The talc deposit at Gopfersgrun (Fichtelgebirge, Germany) was formed by hyd rothermal alteration of the Wunsiedel marble probably during Permian times. The Wunsiedel marble consists of calcite (ccl) and dolomite (dol) layers, Low REE contents, flat shale-normalized REEn patterns with a negative Ce an omaly, delta(13)C(PDB) values of -1.5 to 2 parts per thousand and a low Sr- 87/Sr-86 ratio (0.7086) of the hydrothermally unaltered marble are typical for a sedimentary protolith formed within a marine environment. Hydrotherma l alteration and talc mineralization of the marble occurs along a major fau lt zone and is associated with formation of massive saddle dolomite 2 (do2) replacing eel and dol. Massive dolomitization is followed by vug-filling c arbonates, dolomite 3, calcite 2 and calcite 3. Each carbonate generation h as a distinct chemical composition. Dolomite 2 is enriched in REE, depleted in O-18 (delta(18)O(SMOW) = 10.8 to 11.9 parts per thousand) and shows a h igher Sr-87/Sr-86 value (0.7103) compared to the Wunsiedel marble (delta(18 )O(SMOW) = 14.3 to 18.1 parts per thousand). Late stage vug-filling carbona tes differ from the preceding (replacement) dolomite 2 by higher REE conten ts, a clear roof-shaped REE, pattern, enhanced radiogenic Sr isotope compos ition (Sr-87/Sr-86 = 0.7115 and 0.7117) and lower delta(13)C(PDB) values (- 2.6 to -11.5 parts per thousand). Main talc mineralization result from deca rbonation reactions at low (CO2)-C-X and temperatures between 250 and 400 d egrees C, It is proposed that main stage hydrothermal dolomitization, talc mineralization and late stage carbonate vug-fillings are related to formati on brines or crustal fluids that interact with graphite-bearing metapelites under acid conditions. Changes in chemical composition between (early) rep lacement dolomite and (late) vug-filling carbonates are mainly ascribed to increasing fluid/rock ratios and decreasing temperatures. (C) 1999 Elsevier Science B.V. All rights reserved.