Acta Musei Moraviae

Mineralogy of iron ores of the Lahn-Dill type from the Nová Josefa mine in the Horní Město ore district

Sulphide Pb-Zn-Ag ores and also SEDEX iron ores (traditionally referred to as Lahn-Dill type ores in this area) were mined in the Horní Město ore district. Bodies of both ore types occur in the Devonian volcanosedimentary complex, which is dominated by weakly metamorphosed intermediate and acid volcanics (metatrachytes and metarhyolites) and their tuffs and tuffites. Sulphide ore bodies occur in these intermediate and acid rocks. They are overlain by metamorphosed basic volcanics with iron ore bodies, the mining of which in the Horní Město ore district definitively ended in the second half of the 19^th century. Four mineralogically distinct types of iron ores were distinguished in the material deposited in the heap near the mouth of the Nová Josefa adit in the Horní Město ore district: (i) magnetite ore, (ii) hematite-magnetite ore, (iii) jasper ore, (iv) chlorite-magnetite ore. The first two types of ores are dominant here (about 90% of the ore fragments). They differ basically only in the quantitative ratio of magnetite and hematite. These ores have a banded structure. Bands with a high content of magnetite and hematite alternate with bands in which the content of both Fe-oxides is relatively low. Magnetite and hematite in these ores are accompanied with varying proportions by quartz, carbonates of the dolomite-ankerite series, chlorite of the clinochlo rechamosite series and also muscovite, albite and locally calcite. Common accessory minerals are fluorapatite and pyrite, rarely chalcopyrite, sphalerite, galena, molybdenite, zircon, a TiO2-mineral, tourmaline of the schorl-dravite series, monazite-(Ce) and baryte. Magnetite ore contains about 32 wt. % Fe (median), hematite-magnetite ore about 25 wt. % Fe (median). Compared to similar ore types from iron-ore deposits of the Lahn-Dill type in the Vrbno Group and the Šternberk-Horní Benešov Belt, the low-grade ores occur in the heap near the Nová Josefa adit.