Age and Genesis of Greisen Mineralization in the Li-Sn(-W) Deposit Zinnwald, Eastern Erzgebirge, Germany

The Zinnwald greisen-type ore deposit forms part of the Altenberg-Teplice caldera in the eastern part of Saxony, Germany. Flat dipping quartz-zinnwaldite-topaz-fluorite-cassiterite greisen ore bodies and veins are hosted in the uppermost part of the “small intrusion” Zinnwald Li-F-granite stock (cf. Seifert & Kempe, 1994). The current investigation was aimed to provide new insight into the metallogenesis of the deposit and its temporal and genetic relation to the late Variscan magmatic evolution of the Altenberg-Teplice Caldera.
Greisen-type mineralization in Zinnwald is the product of high temperature post-magmatic hydrothermal alteration (i.e., greisenization), which affected and overprinted the uppermost part of the narrow Zinnwald granite stock. Pertrographic and mineral paragenetic evidence is used to illustrate that the genesis of the greisen mineralization is related to the interaction of a felsic igneous protolith with aqueous magmatic fluids, highly enriched in alkali metals, incompatible elements and volatiles, such as F. The chemical and mineralogical effects of greisenization are subdivided into three different stages, reflecting in a logical sequence the predicted physicochemical evolution of a magmatic hydrothermal fluid system.
A model is suggested that predicts upward directed fluid flow explaining the characteristic textural and geochemical alteration patterns within the granite stock, as well as high rare metal contents of the Zinnwald granite and provides a possible solution for the problem of fluid access to the solidified granite cap. This holistic metallogenetic model for the origin of the Zinnwald Li-Sn(-W) deposit integrates available field geological, geochronological, petrological and geochemical data and is largely based on the Burnham (1997) model for porphyry deposits, on the Shcherba (1970) model for greisen deposits, but augmented by peculiarities of greisen-type ore deposits of the Erzgebirge.
Zinnwaldite mica separates from the Zinnwald greisen dated in this study yield an average age of 314.1 ± 1.5 Ma. This age is significantly younger than available intrusion ages of similar A-type granitic intrusions into the Teplice Rhyolite (TR) such as 324 ± 2 Ma Re–Os molybdenite age for the Altenberg granite (Romer et al. 2007). The Ar-Ar age is thought to reflect cooling of the greisen below the closure temperature of zinnwaldite, estimated to be about 373 ± 21°C (closure temperature of biotite, Berger & York 1981). In this case, the Ar-Ar age obtained is a minimum age estimate for Li-Sn(-W) mineralization, known to have formed at temperatures above this closure temperature. Ore formation is regarded as an integral part of the development of the Altenberg-Teplice caldera and associated with the intrusion of peraluminous A-type Li-mica granites in an extensional post-orogenic environment during the later stage of the Variscan Orogeny.