Pb/Pb and U/Pb zircon dating of subvolcanic rhyolite as a time marker for Hercynian granite magmatism and Sn mineralisation in the Eibenstock granite, Erzgebirge, Germany: Considering effects of zircon alteration

摘要

Tin mineralisation is closely related to rhyolite stocks and dykes which occur in the endo- and exocontact of the Eibenstock granite, Erzgebirge, Germany. The same structures which cut the granite control the rhyolite emplacement and the location of ore-bearing greisen bodies. Albitisation and greisenisation related to tin mineralisation as well as sericitisation and argillic alteration may be traced by changes in chemical and mineralogical composition of both rhyolite and granite wall rock. Comprehensive zircon studies by scanning electron microscopy (secondary and backscattered electron as well as cathodoluminescence imaging; EDX measurements) reveal that zircon from rhyolite and from the enclosing granite shows significant changes in chemical composition and crystallinity, including distortion of the U-Pb isotope system when affected by greisenisation. Single evaporation analysis of zircon from rhyolite with little or no greisenisation gave a 207Pb/206Pb age of 290±5 Ma, whereas zircon from little altered granite gave a 207Pb/206Pb age of 320±8 Ma. The single evaporation age of zircon from rhyolite is confirmed by 238U/206Pb SHRIMP ages of 297±8 Ma. The significant time gap of at least 20 Ma between granite intrusion and rhyolite formation suggests that the late magmatic evolution of the Eibenstock granite cannot be regarded as a source for tin-ore forming fluids as previously assumed.