Low-temperature alteration of the Davis Formation of the Elvins Group and its relationship to the regional Mississippi Valley-type ore genesis in the southeast Missouri Lead District

Low-temperature hydrothermal alteration of carbonates is the subject of intense research due to a myriad of evidence for a relationship between the alteration of carbonates and Mississippi Valley-Type (MVT) ore deposit formation. Correlation between altered carbonate units, particularly those that have been dolomitized, and MVT deposits has been firmly established, although the exact nature of the relationship remains unclear. The Viburnum Trend and the Old Lead Belt of the southeast Missouri lead districts follow this trend and are hosted by gangue dolomite in which Fe-enrichment has been observed, indicating overall metal-rich fluid alteration. The overlying unit, the Davis Formation, is a sandy dolostone and shale unit with minimal, though present, occurrences of sulfides, and it exhibits evidence of repeated episodes of hydrothermal fluid alteration. However, much of the previous research focuses on the shales of the Davis Fm. rather than on the carbonates and fails to provide any explanation as to why the Davis Fm. was only sparsely mineralized in comparison with the Bonneterre. The relationship between the low-temperature alteration minerals such as K-feldspar, dolomite, ferroan dolomite, and sulfide minerals is also unclear. Powder X-ray diffractometry, scanning electron microscopy, and X-ray fluorescence analyses were performed on the carbonate rocks of the Davis Fm. in order to determine mineral compositions, textural relationships between minerals, and whole-rock geochemistry, respectively, to elucidate how the Davis Fm. was affected by hydrothermal fluids flowing upwards from the underlying Bonneterre Fm. X-ray diffraction confirmed the presence of two distinct potassium feldspars, sanidine and microcline. In backscattered-electron imaging (BSE), the feldspar grains exhibited core-and-rim textures. Energy-dispersive spectroscopy (EDS) indicated that the cores were Ba-rich and the rims, conversely, were Ba-poor. The cores also emitted blue fluorescence in cathodoluminescence (CL), further confirming the presence of Ba. These textural and chemical data are interpreted as being the result of two distinct periods of crystallization. Multiple generations of carbonate precipitation were also observed in BSE imaging. A late-stage Fe-enriched ankeritic phase as well as an arrested dedolomitization reaction were identified via BSE in some samples and confirmed using X-ray diffraction. Rietveld refinements indicated a variable degree of Fe-enrichment up to ~27% Fe occupancy of the M1 site of Fe-enriched samples; in dedolomitized samples, calcite comprises between 18 and 70 wt. % of the sample and dolomite comprises between 2 and 55 wt. %. Samples from a given outcrop exhibit the same pattern of Fe-enrichment, indicating that metal-rich fluid(s) only infiltrated the Davis Fm. locally. Dedolomitization reactions were only observed in a single location, further indicating that certain fluid alteration events only locally affected the Davis Fm. Although no direct dolomitic and/or Pb-Zn mineralization of fractures and joints was observed, outcrops along Highway 8 were sometimes found to have thin crusts of calcite crystals on joint surfaces. Panno et al. (1988) asserted that fractures/joints surrounding the main ore body in the Viburnum Trend provided pathways for fluid migration up into the Davis Fm. and were responsible for leaching of metals from the Davis shales. Fractures and jointing were observed in the Davis Fm. at the outcrops used in this study and could have been the pathways by which fluids entered and locally altered the Davis Fm. Therefore the source of these fluids is interpreted as solutions upwelling through cracks and joints through the Bonneterre Fm. into the Davis Fm.