The nature and genesis of greisen stocks at Krásno, Slavkovský les area - Western Bohemia, Czech Republic

Abstract

Apical parts of a Li-F granite intrusion, preserved under gneiss cover at the southern margin of the Krudum Massif in the Slavkovský les area, contain greisen stocks locally bearing low-grade Sn-W mineralization. Greisen varieties pass downwards into leucocratic granite and feldspathite zones. The stocks are underlain at depth by the fresh Li-F alkali-feldspar granite, which is known as the Čistá Granite from outcrops at the present surface. Contacts of the rock types in stocks are not sharp and are partly obscured by younger alterations. The upper parts of the stocks are enriched in minor elements including Li, F, Sn, W, As, Cu, and Zn. The rock sequence in the vertical profile is interpreted as a result of a fractionation from a single parental granite magma, which became gradually enriched in volatiles, mainly water and fluorine. Fluid flow, inner differentiation of the magma as well as cooling resulted in crystallization of granite and greisen bodies. Local oversaturation led to fissure filling and, in the same time, alterations of older minerals. Barren leucogranite and feldspathite zones at depth formed apparently from the residual magma, depleted in silica. Such a model does not need to invoke highly unlikely subsolidus (post-magmatic) autometasomatic processes (e.g. albitization and K-feldspathization), which were previously thought to have completely changed the solidified granite.