Pages: 139 - 148
Abstract: The abandoned Roztoky silver - base metal deposit represents a scarce example of Tertiary sulphide vein-type mineralization spatially and genetically associated with intraplate volcanic rocks of the Cenozoic West and Central European Volcanic Province. The deposit is situated in the Tertiary Roztoky Volcanic Centre (RVC) of the Ceske stredohori Mts., within the Ohre (Eger) Rift. The main ore vein parallels an older bostonite dyke intersecting the monzodiorite body, however, both are intersected by a younger trachyte dike. This suggests that the origin of the mineralization is related to the development of the RVC. The δ13C values of vein carbonates indicate the influence of deep-seated CO2 but the O isotopic composition of hydrothermal fluids (calculated δ18Ofluid values from -3 to -7 ‰ SMOW) shows the dominance of water with relatively shallow circulation. Sphalerite-galena sulphur isotopic geothermometry yielded temperatures between 200 and 250 °C and the presence of cubic hessite indicates temperatures above 155 °C. The calculated δ34Sfluid values vary in the range -1.0 to -2.0 ‰. The presence of banded "colloform" sphalerite and chalcedony suggests lower temperatures. High lead isotope ratios of galena (206Pb/204Pb about 19.03, 207Pb/204Pb about 15.67, 208Pb/204Pb about 39.17) can be a result of mobilization of lead either from Tertiary magmatic rocks, which show similar lead isotope ratios and were probably derived from a sublithospheric HIMU mantle plume, or from upper-crustal rocks, e.g. local Upper Cretaceous sediments. The 87Sr/86Sr isotope ratios of the principal hydrothermal carbonates rhodochrosite (0.70524) and calcite-dolomite (0.70510) are higher than both the local primitive magma derivatives and even the developed rocks of the RVC and indicate an admixture of Sr derived from local crustal rocks.The volcanic rocks of the RVC were the dominant heat source during the formation of the ore deposit. The hydrothermal deposit was formed during relatively shallow hydrothermal circulation of low to medium salinity, low-δ18O fluids supported by CO2 influx of deep-seated origin.
Keywords: Tertiary, Bohemian Massif, epithermal mineralization, stable isotopes, Sr isotopes, alkaline volcanicsDownload PDF document Show Text Version