Fluid inclusions hosted by sphalerite and dolomite cement from the Mississippi Valley-type (MVT) deposits of the Viburnum Trend of the Southeast Missouri district were analyzed by microthermometry and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to determine the composition of mineralizing fluids and to seek insights into ore-forming processes. On the whole, fluid inclusions from throughout the paragenesis have compositions resembling deep sedimentary brines. A population of anomalously Pb-rich fluid inclusions with concentrations reaching 1,500 ppm was identified in some of the paragenetically early and main-stage sphalerite, which is interpreted to represent the influx of a Pb- and possibly overall metal-rich fluid into Southeast Missouri during the time of sulfide mineralization. This influx of Pb-rich fluid appears to have been intermittent, as at least one stage of dolomite cement that hosts Pb-poor fluid inclusions occurs between intervals of sphalerite that host Pb-rich fluid inclusions. If fluid flow into the district was principally from one direction, for example, from the Arkoma basin to the south, then the intermittency in high Pb concentrations could be explained by compositional changes in the source region or along the flow path. Alternatively, if fluid flow into the district was from two or more directions and fluid arriving from each direction had a different composition, then the intermittency in high Pb concentrations could be explained by transient variations in the relative flow rates from each direction.

Lead concentrations on the order of thousands of parts per million make it unlikely that the Southeast Missouri ores were precipitated by any mechanism that involved simultaneous transport of metals and sulfide in the same fluid, as the concentrations of sulfide that could coexist with such high concentrations of Pb would be too low to allow the ores to be deposited within a geologically reasonable period of time, unless the pH was very low or groundwater flow rates were very high. Thus, some type of mixing scenario seems more likely involving either mixing of a metal- with a sulfide-rich fluid or mixing of a metal- and sulfate-rich fluid with a reductant present either in the host rock or in another fluid.

Fluid inclusions hosted by sphalerite have K/Na and Ca/Mg ratios that partly resemble the compositions of both main-stage cuboctahedral galena and less abundant late-stage cubic galena, suggesting that sphalerite was precipitated from a mixture of the fluids that precipitated these two types of galena. However, the fact that aqueous Pb concentration does not correlate with these ratios or other indicators of main-stage mineralization, such as matrix Mn, Fe, Co, Cu, Ag, and Cd content in sphalerite, suggests that more than two fluids were involved in mixing.

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