Kopalne jamy krasowe z kruszcami w okolicach Chrzanowa

Abstract

Fossil sinkholes with galena mineralization in the vicinity of Chrzanów (Cracow-Silesia region) The ore-bearing dolomite in the Muschelkalk of the Cracow — Silesian region is the host of sulfide ores. Much of these ores occur as infillings of cavities which, by some authors, are also regarded as produced by hydrothermal solutions („hydrothermal karst” in the meaning of Bogacz et al., 1970). There aire, however, small amount of sulfide ores which seem to be associated with ordinary meteoric karst features developed in the zone of weathering. Such sulfides show no specific spatial relationship to the main body of ores and ore definitely younger than the primary sulfides. The following considerations are devoted to such secondary sulfide ores in sinkholes and are based upon observations from the Matylda mine in the vicinity of Chrzanów, west of Cracow. To begin with, it seems advisable to recall that the ore-bearing dolomite and the sulfide ores resident in it were exposed to intensive subaerial weathering and dissolution during early Tertiary time. Consequently, an extensive karst surface was developed in carbonate rocks throughout much of the Cracow — Silesian region. This surface was veneered, in places, by insoluble residues and subsequently covered by marine clays of Tortonian (Middle Miocene) age (e.g. Radwański 1968, Alexandrowicz 1969). A prominent feature of the above mentioned, pre-Tortonian karst surface is its display of sinkholes (Fig. 1—4). Such sinkholes occur also in the ore-bearing dolomite and are characterized by the presence of sulfides (Fig. 6). The sinkholes under consideration may attain 100 m. in width and 60 m in vertical extent. They are filled with collapse breccias made up of angular dolomite fragments. The lowermost converging walls of sinkholes are sharp solution surfaces. The upper walls are less sharply defined and there may be a gradation between the fractured and crackled dolomite and the clastic fill of sinkholes (Fig. 1). The voids between the rock fragments and blocks in the collapse breccias are filled with disaggregated dolomitic grains, insoluble residuals, calcite and sulfide ores. The insoluble residues, chiefly red or green clays and very fine quarzose sands are obviously derived from the karst surface. These residues were first accumulated on the top of the ore-bearing dolomite and then they were carried downward into the caverns below. The sulfides tend to concentrate in the lower parts of sinkholes (Fig. 6). They are comprised of galena, marcasite, pyrite and brunckite. Sphalerite occurs only in very insignificant amounts. In addition to the above mentioned sulfide minerals the sinkholes contain small amounts of barite. The galena which is the most prominent sulfide mineral in sinkholes is of, two types: 1. detrital and, 2. autigenic. The detrital galena is evidently derived from pre-existing older galena veins and is a part of the clastic fill (compare also Bogacz et al., 1973). The autigenic galena occurs in very subordinate amounts and is associated with coarse crystalline calcite and with accumulations of disaggregated dolomitic grains. Such galena occurs in form of euhedral crystals and resulted from remobilization processes. Two stages in development of sinkholes can be diferrentiated (Fig. 5). The first stage was initiated by formation of „sanded dolomites” through dissolution of crystals edges and appearrance of caverns (compare also Bogacz et al., 1970). A characteristic feature of this stage is that the dissolving solutions were devoid of surfacial contaminations. Therefore, the caverns produced are filled exlusively with disaggregated dolomitic grains derived from the enclosing country rodk. Such internal sediments are characterized by current lamination and have already been described by Bogacz et al., 1973 — as „laminates” . The caves filled with laminates show also galena mineralization (autigenic). Bogacz et al. 1973 leave the nature of solutions responsible for the disaggregation, cavity-making and galena mineralization open to discussion. The present authors assume that all these processes were geneticaly related to the formation of sinkholes and were effected by ordinary karst waters. As solution and brecciation proceeded upwards the cavernization process has reached the karst surface and the insoluble residues accumulated on this surface were carried into the cavities below. By this, the second stage of development was started which finalized in the formation of sinkholes as we see them today (Fig. 1). The process of disaggregation was still progressing and some of the dolomite blocks in collapse breccias became „sanded” or were transformed into a soft quasi-plastic mass of fine particles („shalification” in the meaning of Heyl et al., 1959, compare also Bogacz et al., 1973). Concluding our consideration it may be stated that small amounts of sulfide ores in the ore-bearing dolomite were deposited in karst features produced by the action of meteoric waiter in the zone of weathering. Such sulfide accumulations are partly due to remobilization processes. It should be added that the remobilization processes by ordinary ground waters have been invoked to explain the origin of all the cavity-filling sulfide ores in the Cracow-Silesian region (Smolarska, 1968) and similar ores elsewhere (e.g. Bernard, 1973). This idea is not followed by the present authors. The presence of secondary sulfides in sinkholes proves, however, that remobilization processes are possible and may contribute to the formation of sulfide ores.