Carbonate rocks from W. Bergslagen, Central Sweden: isotopic (C, 0, H) evidence for marine deposition and alteration by hydrothermal processes

Abstract

Carbon and oxygen isotope ratios for calcite and dolomite from stratabound carbonates, carbonate veins and clots in W-(Mo-) and Zn-Pb sulphide skarns, and for a calcite from a Mn-skarn from the Bergslagen region, central Sweden, are presented. Stratabound carbonate carbon isotopes (δ13C - 0‰) imply a marine milieu of deposition. The δ18O values of stratabound carbonates can be divided into two groups: one associated with stratiform iron-oxide deposits (RSC) and the other unrelated (NRSC). Stratabound dolomites of the NRSC are depleted in 18O compared to the 'most Proterozoic dolomite' field. NRSC calcites have a range of δ18O values (+12 to +19‰) similar to the lower half of the 'most Proterozoic calcites' field. Whether the RSC δ18O values (+6 to +11‰) are primary values or values formed by exchange of the RSC with a hydrothermal fluid during the skarn alteration of associated stratiform iron-oxide deposits, is not clear. Calcites and dolomites, from skarn altered iron-oxide deposits, W-(Mo-)skarns, and a sulphide skarn, as late phase veins and clots, have distinguishable δ13C and δ18O values, placing the respective skarns into separate fields. They have, in general, low δ13C (-2 to -8‰) and low δ18O (+6 to +12‰) values, except for the calcite veins in the sulphide skarn. The 18O -depletion of the carbonates most probably occurred during exchange with sea water at temperatures of 300 ± 50°C, or meteoric waters if the temperatures were lower. The types of δ18O values and their range are comparable to well documented sea water hydrothermal systems of younger age from ocean floor ophiolites and many relatively low altitude meteoric-hydrothermal systems. The low carbon and oxygen isotope values of the carbonate veins and clots in W-(Mo-)- and Fe-skarns are typical for such skarns. They indicate the influx of CO2-bearing hydrothermal fluids where the CO2 is either of magmatic origin or derived from sedimentary carbonate formations after major loss of CO2 through decarbonation reactions. Slate hosted calcite from a Ca-Mn-skarn, is isotopically similar to both the skarn associated carbonates and a concretionary carbonate. Diagenetic modifications of organic matter-bearing sediments, exchange of a sedimentary carbonate with hydrothermal fluids, or, more probable, a combination of these two events, were responsible for the isotopic composition of the calcite in this skarn.