Abstract

Strata-bound barite-hyalophane sulfidic orebodies recently discovered at Rozna (in the Moldanubian zone of the Bohemian Massif, Czech Republic) are hosted within the high-grade metamorphosed complex composed of migmatitic biotite or sillimanite-biotite gneiss and amphibolite, with rare intercalations of marble and calcsilicate gneiss. The orebodies and the host rocks have a complex metamorphic history. The last Variscan amphibolite facies metamorphic event is recorded in the quartz-biotite oxygen isotope fractionation in the host gneiss, which indicates temperatures from 600 degrees to 700 degrees C. Metamorphic textures in the orebodies include granoblastic annealing-recrystallization of barite and sulfides, pressure solution seams, and local development of shear zones near the margins of the orebodies. Despite intense metamorphism, the ore is generally strata bound, with sharp host-rock contacts. At a few places, tectonically modified offsets (usually enriched in sulfides) extend up to 2 m across the foliation of the surrounding rocks.Based on ore mineralogy and texture, the orebodies can be divided into three types: (1) massive baritehyalophane sulfidic rock, (2) calc-silicate gneiss with minor barite and sulfides, and (3) disseminated sulfides in marble. Massive barite ore, which is the only type of mineralization of economic interest (with estimated reserves of over 2 million tons [Mt]), contains an average of 62.2 percent barite, 3.2 percent Zn, and 1.8 percent Pb. The relatively simple mineral assemblage of massive ore includes barite, hyalophane, calcite, quartz, pyrite, pyrrhotite, sphalerite, and galena, as well as minor amounts of pyrargyrite, Ag tetrahedrite, fluorite, pyrophanite, and ilmenite. Calc-silicate gneiss with sulfides and barite usually forms the outer parts of the barite orebodies hosted in gneiss. It consists mainly of pyroxene, hyalophane, and quartz, with minor garnet, calcite, forsterite, minerals of the zoisite group, and sulfides. Disseminated Pb-Zn mineralization in marbles occurs only near the contact of marbles with massive barite orebodies.The delta 34 S values of sulfide minerals are heavy in all mineralization types (7.2-14.8ppm), probably indicating their formation in an anoxic, restricted marine environment, where the isotopic composition of sulfate and sulfide gradually shifted to more positive values as a result of the fractionation of S isotopes during bacterial reduction of sulfate to sulfide and the removal of isotopically light S by sulfide sedimentation. The restricted character of the sedimentary environment during barite formation is supported by the elevated content of graphite in the host rocks (0.01-2.0%) and by very heavy delta 34 S values of pyrite and anhydrite in a small (up to 1 m thick) anhydrite lens that occurs in a host-rock complex. The 87 Sr/ 86 Sr ratios in barite ore (0.7077-0.7079) are close to those for anhydrite (0.7080). Because the anhydrite probably represents a meta-evaporite, the isotopic composition of both barite and anhydrite strontium is close to that of contemporaneous seawater. The isotopic composition of lead in galenas from massive barite and calc-silicate ore is very homogeneous ( 206 Pb/ 204 Pb from 17.65 to 17.66, 207 Pb/ 204 Pb from 5.47 to 15.51, and 206 Pb/ 204 Pb from 37.36 to 37.45), and lead isotope ratios prove the pre-Variscan age of the orebodies.It is proposed that the Ron strata-bound mineralization formed by mixing of the hydrothermal solutions carrying Ba, Zn, and Pb with seawater in a restricted oceanic or intra-arc trough filled with clastic sediments derived mostly from contemporaneous volcanic rocks. The restricted nature of the trough resulted in low oxygen contents in the bottom waters and low recycling rates for the organic matter. During the subsequent thermal events, the mineralization and its host rocks were deformed and metamorphosed to the upper amphibolite grade and the carbonate-enriched orebodies were transformed to calc-silicate ore. Following deformation and retrograde metamorphic events at rather superficial tectonic levels caused retrograde shearing at the margins of the orebodies. At a waning stage of the metamorphism, the ore and host rocks were metamorphosed under greenschist facies conditions.

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