Atypical MVT, Zn-Cu-rich mineralization in the lower portion of the Bonneterre Dolomite, Viburnum Trend, southeast Missouri, U.S.A.
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Mississippi Valley-type (MVT) deposits of the Viburnum Trend are typically lead-dominant (Pb:Zn > 5) and occur mainly in the reef-grainstone facies of the upper Bonneterre Dolomite (Cambrian). Recent drilling has encountered economic mineralization within the lower Bonneterre Dolomite and underlying Lamotte Sandstone, more than 30 m below the main ore-bearing horizon of the district. In one area of the Brushy Creek mine, a currently mined orebody of this Zn-Pb-Cu-(Co-Ni)-rich mineralization comprises a resource of more than 250,000 tonnes containing > 14% Zn + Pb. The lower ores in the Brushy Creek mine are not related to obvious stratigraphic controls, such as pinch-outs of the Lamotte Sandstone against Precambrian knobs, and do not fit into the traditional exploration models for the Viburnum Trend. This study investigates the relationship of this unusual lower orebody at Brushy Creek mine to the typical, overlying MVT deposits of the Viburnum Trend through a combination of petrographic, cathodoluminescence, fluid inclusion, and stable isotope studies. The lower ore mineralization contains discernible zoning with increasing distance above the Lamotte Sandstone of multiple, dominantly early generations of Ni-Co-, Cuand Zn-bearing sulfides that were frequently brecciated and successively overprinted by later mineralization, including main stage Pb-Zn mineralization. The breccia is composed dominantly of sulfides supported by clay (insoluble residue) resulting in massive, high-grade ore with only rare gangue minerals. Cathodoluminescence microscopy reveals that the ore is associated with two generations of dolomite cement, early LOZ Bright and later LOZ Moderate that appear to pre-date the regional dolomite cement associated with main stage Pb-Zn mineralization in the Viburnum Trend. Microthermometry in the lower ore body yields a wide range of temperatures and salinities compared to the data from more typical deposits of the Viburnum Trend. Each stage of the sphalerite paragenesis in the lower orebody displays specific salinities and Th values across the paragenesis. Dolomite cements indicate higher Th values and largely lower salinities than the data from the regional 4-zone dolomite cements. The range of salinities and the higher temperatures of the lower ore zone cements and some of the sphalerites, especially schalenblende, emphasize that there are multiple distinct fluids, including high-temperature fluids, involved in the deposition of the lower orebody. Fluid inclusion LA-ICP-MS analysis of sphalerite-hosted fluid inclusions indicates that the lower orebody records greater atomic ratios of K/Na and Mg/Na than those published for the Viburnum Trend, and elevated ranges of Sr/Na and Ba/Na. Thus, the fluids are overall geochemically distinct from those that formed the upper orebodies. Paragenetic trends distinguish multiple fluids with distinct compositions and reflect possibly different migration pathways that varied with time. Carbon and oxygen isotope studies of the lower ore zone dolomite cements identified by cathodoluminescent studies found that the earlier of the two, LOZ Bright, has an isotopic signature unique from the later dolomite cement in the lower ore zone— LOZ Moderate—and from those of the individual zones of the regional, 4-zone cement, likely reflecting a different fluid source. Additionally, the lower ore zone cements and their calculated δ18Owater values show that the paragenetic trend of decreasing δ18O values from LOZ Bright to LOZ Moderate is not due to temperature effects, but instead indicates that the dolomite cement-depositing fluids are distinct from each other and from the fluids responsible for the regional 4-zone cements of the Viburnum Trend formed. The lower ore zone shows variations of δ34S values that indicate the presence of multiple sulfur sources whose influence varied with depth and time. This further illustrates that the lower orebody at Brushy Creek mine did not form from a single evolving fluid or a constant fluid mixture, but is instead the product of multiple fluids with distinct and evolving sources, which mixed at the site of deposition. This study indicates that fault activity enhanced porosity and permeability, via brecciation, thus promoting voluminous dissolution of carbonate rock. Faults localized multiple, metal-bearing fluids and sulfur sources in the same place, and the localization permitted mixing of distinct fluids that resulted in the accumulation of unique, high-grade ores at the Bonneterre Dolomite-Lamotte Sandstone contact in the Brushy Creek mine. These faults tapped into local fluid reservoirs that interacted with basement rocks prior to the onset of regional flow in southeast Missouri. This structural control reflects the importance of fault and fracture networks to ore formation in the Viburnum Trend and southeast Missouri.