Geochemistry and Petrology of Greisenization at the Slavkovsky´ les Mts. (Bohemian Massif)

René Milos˜ Institute of Rock Structure and Mechanics, Academy of Sciences of Czech Republic,

182 02 Prague 8, V Holes˜ovic˜kách 41, Czech Republic

The greisens in the Slavkovsky´ les area are bearers of Sn-W mineralization of Variscan age. They originated through greisenization of predominantly the youngest lithium-topaz, alkaline-feldspar granites of the Krusné hory batholith. The largest greisen accumulations occur in the environs of Horní Slavkov in an area built by the Huber and Schnöd granite stocks. The two stocks belong to the largest granite cupolas in the Saxothuringicum. The greisens form in the upper part of both stocks irregular lenticular horizons with abundant disseminated Sn-W mineralization. The group of the Schöd and Huber stocks is one of the largest granitic structural highs in the Saxothuringian region that has been mined for several centuries for disseminated, greisen-type Sn-W ores.

Occurrence of two phases of alkaline-feldspar granites (commoner medium-grained facies and the less represented, fine-grained porphyritic one) along with the occurrence of fine-grained intrusive breccias argues for a larger number of partial phases within the intrusion lithium-topaz granites that represent the youngest extensional phase of the Variscan granitoid magmatism in the Bohemian Massif.

The occurrence of Sn-W mineralization is controlled by the course and shape of the granite-gneissic country-rock contact. The highest concentration of ore mineralization occur, with some exceptions, mainly in the greisens, less frequently in the highly greisenized and argillitized granites. All the lithium-topaz granite types are affected by more or less intense greisenizatiom and argillitization. The greisenization is reflected by the increase in content of topaz and lithium mica at expense of feldspars. Some of the greisens probably predate the intrusion of the youngest intrusive breccias, in which they occasionally occur as angular xenoliths.

The greisens are mainly represented by topaz-mica to mica-topaz greisens. The topaz-quartz, mica-quartz and mica greisens are less frequent. According to their grain size, they can be subdivided into fine-grained and medium-grained greisens, the latter, nevertheless, predominate. The topaz-mica to mica-topaz greisens are massive, white-grey rocks with macroscopically distinguishable, up to several mm-sized grains and aggregates of quartz, topaz and lithium-mica plates. Genetically, the greisens can be subdivided into greisens I and greisens II. The former form bodies of various size and their grain size depends on that of the parental granites. The greisenization I brought about a gradual increase in the proportion of quartz and topaz and gave rise to large plates and platy aggregates of lithium micas. The greisenization II can be observed only in places where it progressed along fissures and quartz veins, giving rise to greisens surrounding the veins. Microscopically, it can be proved by the arrival of a new generation of topaz, micas and apatite. The origin of the topaz-quartz greisens may be attributed to greisenization II.

An important vertical zoning has been distinguished in the development of the greisenization. Apical part of the Huber stock consists of quartz or quartz-topaz greisens and of intrusive breccia cemented by lithium-topaz, fine-grained granites. The quartz-topaz greisens pass downwards into the topaz-mica ones. Deeper portions of the Huber stock consist of medium-grained alkaline-feldspar granites with minor occurrence of greisenization. The greisenization of lithium-topaz granites brings about a gradual increase in SiO2 and FeO contents and a decrease in Al2O3 content. The granite greisenization is also markedly reflected in the content of normative minerals causing a shift towards normative quartz. The lowest Al2O3 contents have been ascertained in the topaz-mica greisens. The greisens are characterized by an increase in Sn and W contents as well as in Zn, Cu, Sr and Ag contents and a decrease in Ni, Cr, Nb, Y, Zr, Rb, U, Th and Ga contents.

Investigation of cassiterite and quartz homogenization temperatures argues for a greisenization temperature to be between 370 and 380oC. The less frequently determined homogenization temperatures at 500 - 600° rather correspond to the temperature of transport of the ore-bearing

The greisenization was brought about by accumulations of fluorine-rich fluids that were released from the gradually cooling melt of the alkaline-feldspar granites. Thermal energy was released from both the deep-seated magmatic chamber and from the decay of radioactive elements, the highest concentrations of which, particularly those of uranium, occur in the youngest alkaline-feldspar granites.