Item

Abstract

The Hämmerlein Sn-skarn deposit is associated with gneisses and schists which reached peak metamorphic conditions at 340 Ma. The deposit formed around 320-325 Ma, when the Eibenstock granite was emplaced, resulting on the migration of magmatic fluids along the contacts of different units. This induced decarbonation of the marble layers and the development of a series of contrasting skarns. The different units are garnet skarn, amphibole skarn, pyroxene skarn and magnetite skarn, with intercalated layers of schist and gneiss. Cassiterite is the main ore mineral. It occurs as coarse grains (up to 1mm) in the schists, as medium grains (~500 μm) at contacts between amphibole and magnetite skarn and as fine (<100 μm) and disseminated grains in the amphibole skarn. Relatively high content of In has been found in exsolutions of Cu-Fe-rich sphalerite in chalcopyrite patches within the magnetite skarn and in the Fe-rich sphalerite layer beneath or crosscutting the magnetite skarn. Skarns bulk rock compositions are quite similar. Modeling the T -XCO2 conditions of the skarn units from their bulk rock compositions shows that the skarns formed at different XCO2 and temperature. Garnet skarn formed at high temperature and XCO2, whereas amphibole, pyroxene and magnetite skarns formed at lower temperature and lower XCO2. The T-XCO2 conditions have changed with the distance from the source of the fluid and with time. By flowing at the contact between silicate and carbonate rocks, the fluid induced decarbonation and the formation of the different skarn units starting with garnet skarn at high temperature high XCO2. With time the locus of fluid flow at the contact with the carbonate rocks migrated with the newly formed skarn until all the carbonates were consumed, resulting in a sequence of mineralogically contrasting skarns. Fluids responsible of the skarn-forming reactions enriched the rocks in Sn, W, In, Sb, Cd and F. The enhanced contents of Bi, As and U in the skarns are likely to be related to later events such as the important 180 Ma old U mineralization in the region and the multiple later redistribution of U and addition of Bi and As. Major and trace elements as well as the REE seem to have been largely immobile and to be inherited from the protoliths, as indicated by their pattern. All skarns are characterized by similar flat UCC-normalized REE pattern with a positive Eu anomaly, whereas, the gneisses and schists do not have any Eu anomaly. The REE pattern of cassiterite-bearing schists is the same as the one of the unmineralized schists. The εNd values of schists and skarns overlap and correspond to those of unmetamorphosed sedimentary rocks and are clearly lower than those of the granite. The distribution of Sn and In within the different units is heterogeneous, indicating that mineralization is not controlled only by fluid flow alone, but also by precipitation at reaction fronts with selective mineralogically controlled scavenging of ore elements and the reaction history of the fluid, in particular whether the fluid had lost its metal content during earlier fluid-induced reactions.