Mobilisierung, Transport und Fixierung von Pb, Zn und Cd in niedrigthermalen Systemen mit Karbonat, Chlorid und Sulfat
The clear-cut geologic and tectonic setting of a Zechstein carbonate hosted Pb-Znoccurrence of the Mississippi-Valley-Type (MVT) allows insights into the genetical processes of the mineralization. The study area is situated at the eastern margin of the Rhenish Schiefergebirge (Germany). Paleogeographically the area formed a restricted marginal basin of the Zechstein sea the basement of which consists of folded Variscan series with intercalations of Carboniferous black shale. The basin was leveled with evaporates and carbonates during the transgression of the Zechstein sea. Based on field evidences, mineralogical studies, and geochemical analyses concepts of thermodynamic models were shaped and the redistribution of Pb, Zn, and Cd in a carbonatic-chloridic-sulfatic environment were calculated. Experimental studies followed the results of the thermodynamic modeling. The studies comprise leaching and precipitation experiments of Pb-, Zn-, and Cd-carbonates under varying ion strengths, temperatures, and CO2 fugacities. A scheme of the mobilization and fixation of Pb, Zn, and Cd in the carbonatic-chloridic-sulfatic environment is presented Positive anomalies of transition elements and ore metals (Pb, Sn, Cd) are restricted to the Zechstein carbonate series above the Carboniferous black shale. They are bound to the uppermost top series of the carbonates (Z 3) whereas the lower parts of the Zechstein carbonates (Z 1, Z 2) which follow directly above the black shale are barren. The correlations of the evaporite elements to the transition elements and ore metals indicate the importance of salt brines for the redistribution of elements. The extent of the mass transport and the proportions of the ore elements are controlled by a) the intensity of the tectonic stress and the accompanying viability for solutions; b) the geometry of the fault systems with maximum adduction of masses in a focusing constellation of the geological frame; c) the geometric constellation of a mineralized carbonate area to depth zones of interrelation between evaporites of the basin series and black shale of the basement. Due to the simple features of the geological frame, the following solutions control predominately the mass redistribution processes: - mobilization and transport by chloride rich connate and/or ascending brines of the Zechstein basin series, saturated with calcite (dolomite); rich in Pb, Zn, and Cd (as chloride complexes); - fixation / precipitation by overlaying meteoric waters, saturated with calcite (dolomite). Up to a certain degree of dissolution of the brines (e.g. three parts meteoric waters, saturated with calcite), the mixture in the contact zone of both solutions yields supersaturation and the precipitation of the carbonates of Pb, Zn, and Cd. The proportions of the precipitated elements vary according to the salinity and degree of dilution, respectively. Because the solubility of the transition metals is controlled also by temperature and pCO2, the parameters will modify the precipitation from the solutions as well. The production of H2S is known to be a common process in sulfate bearing series (evaporites). As demonstrated by the experimental studies the conversion from metal carbonates into sulfides may follow at any time. After the conversion the ores are nearly irreversibly stable in the carbonate environment.
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