Silver and gold in galena ores from base-metal vein deposits in Bohemian Moravian HighlandsArticle PDF
Silver and gold in galena ores from base-metal vein deposits in Bohemian Moravian Highlands
Galena-rich ores from vein base-metal deposits in the Bohemian-Moravian Highlands were analyzed for Ag and Au. Relatively high quality of ores was confirmed in the Havlíčkův Brod ore district with contents of Ag exceeding 1000 ppm and Au > 1 ppm. Galena ores in the Jihlava ore district are relatively variable in Ag (262–2000 ppm Ag). It is therefore likely that the silver-richest occurrences at Jihlava (Staré Hory dislocation zone) predominantly contained an oxidation zone rich in native silver. The highest silver contents were found in galena impregnations in marble from Čížov, and in galena from quartz from Helenín. Analytical data from the Jihlava area exhibit only low contents of Au; the richest samples have ≤ 0.06 ppm Au. Large variability in Ag contents is evident in a number of small deposits along the southeastern border of the Central Moldanubian Pluton. Dobrá Voda locality is rich in silver and gold; surprisingly high contents of Au (4 ppm) were found in sulfides of the sulphide-poor mineralization at Radlice near Dačice. Deposits in Svratka Dome (Moravicum) have generally high contents of Ag and elevated amounts of Au (>0.5 ppm), markedly in Štěpánov ore district with ore samples containing more than 1000 g Ag/tone; the highest Ag contents were found in barite veins at Olešnička (4406 ppm Ag) and Horní Čepí (5928 ppm Ag). Ores from the historically significant mining area around Rozseč nad Kunštátem also showed high contents of Ag (1572 ppm), whereas elevated Au contents were encountered in the ore from Heroltice near Tišnov. Ore samples from the hydrothermal veins near Měřín provided 1205 ppm Ag and 2.6 ppm Au; high silver contents are present in galena (1474 ppm Ag) and in sfalerite (1843 ppm Ag). In the Třebíč durbachite pluton, the carbonate vein with silver-rich galena (1250 ppm Ag) found at Velké Meziříčí was never encountered by the early prospectors.
The analyzed set of galena-rich ores shows a large variability in Ag and Au contents. With a few exceptions, the richest silver ores (> 1000 ppm Ag) contain small quantities (often < 10 vol. %) of galena in assemblage with sphalerite, chalcopyrite, and pyrite disseminated in quartz and barite veins. The galena is rich in microscopic inclusions of freibergite, tetrahedrite and pyrargyrite. On the contrary, studied high sulfide ores (> 90 vol. % of galena) proved to be rather poor in silver (< 800 ppm Ag, but also < 20 ppm Ag) and were rather mined as a source of lead. It is therefore very likely that the silver ores mined in the Medieval Age at the richest deposits in the studied area were mainly formed by native silver and acanthite.
Vladimír Hrazdil*, Stanislav Houzar, Department of Mineralogy and Petrography, Moravian Museum, Zelný trh 6, 659 37 Brno, Czech Republic. *email@example.com