The origin of a mississippi valley-type deposit in the Viburnum Trend, Southeast Missouri

Abstract

Petrologic and isotopic studies of the base metal sulfide mineralization in the Buick mine have established the following paragenesis: an early stage of fine-grained pyrite and marcasite replaced the dolostone host rock; the iron disulfides and dolostone were replaced by sphalerite during the main stage of sphalerite precipitation (delta 34 S sp 16.5 to 19.0ppm). Following the sphalerite, two main stages of galena were precipitated--a cuboctahedral stage (delta 34 S gn 21.3 to 10.0ppm), during which galena replaced dolostone and sphalerite and grew in vugs in the dolostone and in the sphalerite, and a cubic stage (delta 34 S gn 10.5 to 0.9ppm). The two stages of galena precipitation were separated by a period during which the main stage of sphalerite and the cuboctahedral galena were partially dissolved and bladed marcasite (delta 34 S mc 2.3 to 4.1ppm) was precipitated, and followed by a period of mild etching of the cubic galena. Minor amounts of pale sphalerite (delta 34 S sp 14.0 to 16.5ppm) were then precipitated. White and colorless rhombs of dolomite were precipitated at the same time as some of the main stage of sphalerite, the cubic galena, and the pale sphalerite. After the pale sphalerite was precipitated, chalcopyrite, quartz, and siegenite were precipitated, followed by calcite and finally kaolinite. The paragenetic and textural data for both galena and sphalerite in the Buick mine strongly suggest that these minerals were precipitated and dissolved repetitively during ore formation. Similar textural evidence for galena reported by other workers for different mines in the Viburnum Trend implies that repetitive precipitation and dissolution of galena was probably an important process during the formation of the entire Viburnum Trend.Fluid inclusions in late quartz crystals coprecipitated with chalcopyrite have filling and freezing temperatures within the ranges reported for sphalerite from other parts of the Viburnum Trend, demonstrating that the ore-forming solutions in the Viburnum Trend were probably at temperatures in the range 105 degrees to 141 degrees C (assuming a maximum pressure correction of about 11 degrees C) and were extremely saline, containing about 4.0 to 5.0 molal chloride. Consideration of typical chemical analyses of fluid inclusions from other Mississippi Valley-type deposits suggests that the maximum likely concentration of dissolved CO 2 in the ore-forming solutions was about 0.05 molal. At these concentrations, the ore-forming solutions that travelled to the site of deposition could have had pH9s about two units less than neutral and still have been in equilibrium with dolomite or calcite.The ranges of sulfur and lead isotopic compositions of galena from the Buick mine are very similar to those established by earlier studies of the Old Lead Belt and adjacent subdistricts. However, the correlation of isotopically lighter sulfur with more radiogenic lead is much stronger for the Viburnum Trend, suggesting that galena was precipitated from many solutions, each of which carried lead and sulfur together and had different lead and overall sulfur isotopic compositions.The textural, paragenetic, and isotopic data for the Buick mine cannot be easily explained either by a sulfate reduction model in which sulfate was reduced by organic matter or methane, or mixing models in which base metals and reduced sulfur were transported separately to the site of deposition. Instead, the data strongly support a reduced sulfur model for ore formation according to which the base metals were transported together with reduced sulfur (as H 2 S), in the same solutions, to the sites of deposition. The reduced sulfur model is also supported by a theoretical calculation of the solubilities of galena and sphalerite, which demonstrate that low, but economically significant, concentrations of lead or zinc could have been transported as chloride complexes, together with equivalent concentrations of reduced sulfur, at temperatures of 100 degrees to 150 degrees C and pH9s of one to two units less than neutral, and could have been precipitated by pH increases, cooling, or dilution at the sites of deposition.