Uwagi o śladach mineralizacji w utworach dewonu w Przegini na SE od Olkusza

Abstract

REMARKS ON MINERALIZATION TRACES IN DEVONIAN FORMATIONS AT PRZEGINIA, SE OF OLKUSZSummaryThe article presents new data on the Devonian formations pierced at Przeginia, near Olkusz (Fig. 1). The Devonian is here part of the Variscian anticline of Cracow. Under the Jurassic deposits, dark-grey crystalline dolomites have been encountered at a depth of 118.80 m. The dolomites, which contain limestone and conglomerate-like dolomite intercalations, reveal fauna of amphipores and corals. Siltstones, as well as dolomitic and clayey marls are less abundant. The thickness of these formations is unknown, since all the drillings were completed in them at a depth of 300 metres only. The lithological development of these rocks points to a considerable analogy to the rocks of Middle Devonian age, found in the vicinity of Siewierz (S. Śliwiński, 1956). The great change in lithological development of the deposits at Przeginia proves the differentiated sedimentary conditions. The Devonian formations at Przeginia are strongly cracked, brecciated and reveal karst phenomena. Tectonical fractures, probably of several generations, are accompanied by pyrite, haematite, dolomite, calcite and chalcedony mineralization. The mineralization appears in the form of veins and of filled-in caverns. Here, two stages of mineralization have been distinguished. Most probably, the older of them is related to hydrothermal processes. These are veinlets of grey dolomites with chalcedony, as well as veins and geodes of white or pink coloured dolomites, in which increased Sn and Cu contents have been ascertained. The younger stage is represented by thin veins of white calcite, at places with haematite, and by veinlets and druses of yellow coloured calcite that, most probably, is related to the lateral circulation of waters in the cracked and karst-like massif. The karst phenomena appear in the Jurassic and Devonian formations mainly in dolomites. They are greatly responsible for the concealing of the existing mineralization. The results of spectral analyses of both sedimentary and vein rocks merit here a special attention, mainly because of the presence of the increased Sn, Cu, Zn, Pb and Ba contents, anomalous in relation to the clark of carbonate rocks. Tin occurs here in an amount from 0,011 to 0.015%, accompanied frequently by copper, from 0.01 to 0.1%. It should be stressed here that so far the presence of this chemical element has not been ascertained to appear in the known mineralization occurrences within the area between Cracow and Olkusz. This trace mineralization of tin in the Devonian sedimentary and vein rocks at Przeginia is interesting mainly due to an additional fact that in the heavy mineral assemblage of the Jurassic deposits, at Rzeszotary, cassiterite has been encountered. The area of the cassiterite occurence is situated in the southern continuation of the Variscan anticline of Cracow (M. Krysowska, 1962).