We have measured 87Sr 86Sr and Rb and Sr concentrations in several minerals, primarily sulfides, spanning the paragenesis of hydrothermal mineralization in the Viburnum Trend in southeast Missouri. Separate measurements were made for fluid inclusions opened by crushing or thermal decrepitation and for the solids. For comparison, measurements were also made on samples of probable local aquifers, the Bonneterre Formation and the Lamotte Sandstone. For some of the samples, concentrations of K, Ca, Cl, Na, and Mg are also reported. In several cases 87Sr 86Sr ratios are different (higher) in the solids than in the fluid inclusions. We have investigated the possibility that either type of sample gives spurious results, e.g., that the fluid inclusions are secondary or contaminated by host dolomite, or that Sr in the solids reflects a detrital rather than an authigenic source. Consideration of mass balance, overall solute chemistry, and examination of non-sulfide dissolution residue, however, suggests that both types of sample reflect primary fluid Sr composition. We thus adopt the working hypothesis that Sr isotopic composition in fluids at the time of hydrothermal mineralization was highly variable. The observed results for the Viburnum Trend do not conform well to expected trends for Mississippi Valley-type (MVT) mineralization based primarily on analyses of gangue carbonates, barite, and fluorite. Fluid inclusion Sr in some of the Viburnum Trend samples is more radiogenic than in the host dolomite but only moderately so; in other samples, notably main-stage octahedral galena, fluid inclusion Sr composition is within the range observed for the host dolomite. In contrast, Sr in some of the sulfides is very radiogenic, much more so than previously reported for MVT minerals, and is very radiogenic early in the paragenesis (pyrite, chalcopyrite, sphalerite), less radiogenic during main-stage ore deposition, and again more radiogenic in later paragenetic stages. ?? 1991.
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Strontium isotopic constraints on the origin of ore-forming fluids of the Viburnum Trend, southeast Missouri