Constraints from Fluid Inclusion Compositions on the Origin of Mississippi Valley-Type Mineralization in the Illinois-Kentucky District

Fluid inclusions hosted by Mississippi Valley-type (MVT) ores in the Illinois-Kentucky district were analyzed using microthermometry, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and Raman spectroscopy to characterize the composition and temperature of mineralizing fluids in order to seek insights into the process of ore formation. Results from the present study support the interpretation of previous microthermometry studies that relate mineralization to mixing of two higher temperature brines and a cooler, dilute meteoric fluid. Fluid inclusions in the Illinois-Kentucky district have overall higher atomic ratios of Ba/Na, Ca/Mg, K/Na, Sr/Na, and lower atomic ratios of Mg/Na compared to inclusions in the Ozark MVT districts and are thus compositionally distinct. These differences may indicate that the Illinois-Kentucky and Ozark MVT ore fluids were derived largely from different source basins or may reflect input from igneous rocks in the Illinois-Kentucky district. Low Ca/Mg ratios of Illinois-Kentucky fluid inclusions indicate that mineralizing fluids were dolomitizing and therefore could not have traveled far through the limestone ore host formations, but instead traveled through deeper nonlimestone units before rising along faults and fractures in the district. Some fluid inclusions in Illinois-Kentucky MVT mineralization yielded strong LA-ICP-MS responses for ore metals. A single sphalerite-hosted fluid inclusion out of 18 analyzed contained detectable Pb, corresponding to a concentration of 420 ppm, whereas numerous fluorite-hosted fluid inclusions contained detectable Pb, Zn, and Cu, corresponding to maximum concentrations on the order of 1,000s of ppm. The high and similar concentrations of Cu, Pb, and Zn in some fluorite-hosted fluid inclusions in the Illinois-Kentucky district are problematic because Cu sulfide mineralization should be much more abundant than Pb and Zn sulfide mineralization owing to the lower solubility of Cu relative to Pb and Zn in the presence of reduced sulfur, which is not observed in the field. This raises the possibility that ore metals may exist as solid phases in suspension or adhering to fluid inclusion walls in some inclusions, rather than as dissolved species in aqueous solution. Raman microprobe analyses of fluid inclusions hosted by minerals occurring throughout the paragenetic sequence detected methane concentrations ranging from about 40 to 1,200 ppm, with most on the order of 100s ppm. Even for the lowest methane concentration determined, redox conditions would have been reducing, at a log of −55.4 at a temperature of 100°C. At such reducing conditions, sulfide would predominate over sulfate; this is supported by high concentrations of Ba in the fluid inclusions. The overall reducing conditions throughout the paragenetic sequence and the possible presence of both metal-poor and -rich fluid inclusions would suggest that mixing between a metal-rich, sulfide-poor fluid and a metal-poor, sulfide-rich fluid were an important mechanism for sulfide mineral precipitation. Minimum depths of mineralization estimated from methane concentrations in fluid inclusions range to about 1,500 m and possibly 2,500 m, which would not have been reached until Late Pennsylvanian to Permian time, i.e., during the Alleghanian orogeny.