LIMITS ON THE METAL CONTENT OF FLUID INCLUSIONS IN GANGUE MINERALS FROM THE VIBURNUM TREND, SOUTHEAST MISSOURI DETERMINED BY LASER ABLATION ICP-MS

The hydrothermal fluids that deposited the ores of the Southeast Missouri district have long been recognized to be sedimentary basinal brines. The major element composition of these brines has been well characterized, however their metal content had never been measured and is only loosely bounded by theoretical constraints. Reported upon here is an effort to assess the metal content of these mineralizing brines by analyzing fluid inclusions from gangue minerals in the Viburnum Trend subdistrict of Southeast Missouri using laser ablation ICP-MS and microthermometry.

Fluid inclusions in dolomite, quartz, and calcite from throughout the Viburnum Trend and from across the paragenesis were analyzed. Detection limits varied from inclusion to inclusion and were controlled by the mass of analyte within the brine volume. None of the inclusions was found to contain measurable quantities of Pb, Zn, or Cu and only a few contained measurable quantities of Ba, which ranged from about 100-500 ppm. These results showed that fluid depositing dolomite, quartz, and calcite gangue in the Viburnum Trend is unlikely to have had Pb and Zn concentrations higher than on the order of 10² ppm and Cu concentrations higher than on the order of 10¹ ppm, with Pb and Zn concentrations as high as on the order of 10³ ppm and Cu concentrations as high as on the order of 10² ppm allowed during deposition of stage 3 dolomite. These concentration maxima, with the principal exception of those in stage 3 dolomite fluid inclusions, are of similar magnitude as the concentration maxima for Pb, Zn, Cu, and Ba found in typical sedimentary brines. Thus, the brines responsible for depositing gangue minerals in the Viburnum Trend are unlikely to have had anomalously high ore metal concentrations compared to typical basinal brines. The lack of temporal overlap between gangue and metal sulfide deposition, and differences in major element composition between fluid inclusions in the gangue and metal sulfides leaves open the possibility that the metal content of fluid that deposited sulfides was different from that responsible for depositing gangue. At a minimum, the data show that fluid entering the Southeast Missouri district was not pervasively metal-rich over time, and that any more metal-enriched fluid that may have entered the district was limited to discrete intervals.