Skarn formation and tin enrichment during regional metamorphism: The Hämmerlein polymetallic skarn deposit

Lefebvre-Desanois, Marie, Romer, Rolf L., Glodny, Johannes and Roscher, Marco (2019) Skarn formation and tin enrichment during regional metamorphism: The Hämmerlein polymetallic skarn deposit. Lithos, 348–349, [105171]. (doi:10.1016/j.lithos.2019.105171).

Abstract

The Hämmerlein Sn-rich polymetallic skarn deposit (Erzgebirge, Germany), which is hosted by Variscan schists and gneisses, includes garnet, amphibole, pyroxene, and magnetite skarns that formed during regional metamorphism at ~340 Ma. The skarn deposit was overprinted during the intrusion of the ca. 320 Ma Eibenstock granite. Tin was introduced into the system during regional metamorphism at 340 Ma (Sn mostly hosted in silicate minerals) and after the intrusion of the Eibenstock granite at c. 320 Ma (Sn hosted in cassiterite). The emplacement of late stage melts of the Eibenstock granite also resulted in the formation of cassiterite-quartz veins in the schists hosting the Hämmerlein skarns and at some distance in the formation of several cassiterite greisen. Major, trace, and rare earth element data of the skarn units and their hosts rocks demonstrate that skarns formed from impure carbonates with variable proportions of silicate minerals. Depending on the original modal compositions of the skarns and their host rocks, the mineralizing fluid selectively added and removed elements, which resulted for the skarns and the schist ore in a general enrichment in Sn, In, Cu, and W and a noticeable depletion in Al and Sr. The Sr, Nd, and Pb isotopic composition of the skarns, schists, and gneisses resembles those of the metamorphic rocks of the Erzgebirge rather than those of the granites, possibly indicating that the geochemical signature of granite-derived fluids had been severely altered during fluid-rock interaction before these fluids reached the skarns, schists, and gneisses. Similarly, the Li and B isotopic composition of the skarn samples reflects the likely range of possible source rocks for metamorphic fluids, rather than the composition of the granite-derived fluid. In contrast, the Li and B signature of greisen samples shows a variable mixture of material derived from the granite and the metasedimentary rocks, respectively. Our data show that a first Sn enrichment took place when the Paleozoic sedimentary rocks were affected by regional metamorphism at ca. 340 Ma. Fluids derived from these siliciclastic sedimentary protoliths mobilized and transported Sn and Fe. These fluids reacted with the impure carbonates, resulting in decarbonation and the formation of calc-silicate minerals and biotite in the various skarn types. The changing fluid composition as well as the incorporation of Fe in skarn silicates and formation of Fe-oxides resulted in the incorporation of Sn into silicate minerals. During or after the emplacement of the Eibenstock granite, the original skarn was to a variable extent retrogressed and Sn was released from Sn-bearing calc-silicates and biotite. During this event additional Sn was added to the skarn and the hosting schists. Released Sn and newly added Sn precipitated as cassiterite.

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Contributors

Author: Marie Lefebvre-Desanois

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