IMPLICATIONS OF PB-RICH FLUID INCLUSIONS FROM MISSISSIPPI VALLEY-TYPE ORE DEPOSITS OF THE VIBURNUM TREND, SOUTHEAST MISSOURI

Fluid inclusions in sphalerite and dolomite cement from Mississippi Valley-type (MVT) deposits in the Viburnum Trend of the Southeast Missouri district were analyzed using microthermometry and laser ablation inductively coupled plasma mass spectrometry. Results of the analyses confirmed that the fluids involved in MVT mineralization closely resemble typical deep sedimentary basinal brines. Total dissolved solids contents in fluid inclusions from the present study averaged about 205,000 parts per million (ppm) and elemental concentrations averaged as follows: Cl = 126,000, Na = 61,000, Ca = 13,000, K = 2600, Mg = 1900, Sr = 640, and Ba = 17 ppm. Ore metals in all dolomite-hosted fluid inclusions analyzed existed below the detection limits of 10’s to 100’s of ppm for Zn and Cu and 1’s to 10’s of ppm for Pb. Concentrations of Zn and Cu could not be quantified in sphalerite-hosted fluid inclusions because of interferences caused by high concentrations of these elements in the mineral matrix. However, Pb is present in much lower concentrations in the sphalerite matrix and could be quantified in many sphalerite-hosted fluid inclusions, revealing a population of anomalously Pb-rich fluid inclusions with concentrations of 10’s to 100’s of ppm, with a maximum of 340 ppm. These results point to the influx of a Pb-rich fluid into the Southeast Missouri district during times of sphalerite (and probably galena) precipitation but interrupted during times of dolomite cement precipitation. The Pb-rich fluid may have been present in the district in even greater proportion during precipitation of cuboctahedral galena, the predominant sulfide ore mineral, as sphalerite-hosted fluid inclusions have K/Na and Ca/Mg ratios intermediate to those hosted by cuboctahedral galena and a later and less abundant cubic galena, also suggesting that sphalerite precipitated from a mixture of the fluids that precipitated these two types of galena. Considering the low sulfide concentrations that could coexist with Pb in concentrations of 100’s of ppm, these data suggest ore formation via some type of mixing mechanism involving either a metal-rich fluid and a sulfide-rich fluid, or a metal- and sulfate-rich fluid and a reductant-rich fluid, rather than via a mechanism that required simultaneous transport of metals and sulfide in the same fluid.