Szakáll Sándor - Fehér Béla: A polgárdi Szár-hegy ásványai (Topographia Mineralogica Hungariae 8. Miskolc, 2003)

Bevezetés (Szakáll Sándor)

Minerals of the Szár Hill, Polgárdi An introduction Sándor SZAKÁLL Department of Mineralogy and Petrology, University of Miskolc, H-3515 Miskolc-Egyetemvaros, Hungary E-mail: askszs@gold.uni-miskolc.hu The flat limestone and phyllite range of the Szár and Somlyó hills extends by 50-60 metres above the flatland of 170-180 m altitude between Polgárdi, Szabadbattyán and Köszárhegy villages. Its highest peak is 228 m a.s.l. The length of the reange is 2.5 km, its width varies between 150 and 500 m (Figs. 1-2). The crystalline limestone of the Szár Hill has been utilized for lime burning for centuries. Large-scale extraction started only in the 1960s when the Dunaújváros Metallurgical Works used this limestone as an additive. The lead ore occurrence, discovered at the northern edge of the range, had been mined since 1938, but it was abandoned in 1954 by the exhaustion of the high-grade ore. It cannot be excluded that the Romans had already utilized both the lead ore and the crystalline limestone. Crystalline limestone The Devonian crystalline limestone (Polgárdi Limestone Formation) is the prevailing rock that occurs on the surface of the Szár Hill. It was completely recrystallized during the Variscan regional metamorphism. Its texture is characterised by a small grain size, anhedral or subhedral crystal shape. Certain beds of the limestone have been dolomitized, as shown by chemical analyses. Another feature of the crystalline limestone is the hydrothermal silicification and the appearance of secondary calcite veins. Calcite crystals, sometimes up to 3-6 cm (Fig. 3) with varied, but predominantly rhombohedral habit occur in these veins. Radially fibrous aggregates of aragonite and tabular crystals of barite are also found rarely in the calcite veins. The fissures of the quartz veins are covered by stubby quartz crystals up to 0.5-2 mm. Globular, reniform aggregates of chalcedony and hyalite can also be found occasionally. The limestone itself sometimes includes pyrite crystals up to 1-2 mm, which has been altered to goethite pseudomorphs. Iron and manganese metasomatism The crystalline limestone has been zonally altered and shows dark brown or dark grey tints. In some places this alteration can be observed in large bodies. An enrichment of iron and manganese can be detected in these zones: manganese content varies between 1-3%, whilst iron content can reach up to 5-10 %. Iron and manganese ions may have been originally incorporated in the calcite. Minerals with ankerite, siderite or rhodochrosite composition has not been found by microprobe so far. During the nearsurface weathering of the limestone the iron and manganese content of the rock migrated