AN ATYPICAL OREBODY IN THE BRUSHY CREEK MINE, VIBURNUM TREND, MVT DISTRICT, MISSOURI: EARLY Cu-(Ni-Co)-Zn-RICH ORES AT THE LAMOTTE SANDSTONE/BONNETERRE DOLOMITE CONTACT

Abstract

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 of the Brushy Creek mine, more than 30 m below the main ore-bearing horizon of the district. These ores are more zinc and copper rich than typical deposits (Zn/Pb commonly >2 and Cu up to 8 wt %), with notable enrichments in Ni, Co, and Ag. There is pronounced zoning of Ni-Co, Cu, Zn, and Pb with increasing distance above the Lamotte Sandstone. Observations of drill cores and samples from active mine faces, and mineral textural relationships in thin section indicate that the lower ore mineralization contains dominantly early Zn- and Cu-bearing sulfides, which are overprinted by main-stage Pb-Zn mineralization. The lower ores contain multiple generations of chalcopyrite and sphalerite that display frequent breccia textures at the meso- and microscale. This style of breccia is uncommon for the Viburnum Trend and is composed dominantly of sulfides supported by clay (insoluble residue), which are cemented by subsequent generations of sulfides and carbonates, resulting in massive, high-grade ore. Cathodoluminescence microscopy reveals that ore is associated with two generations of dolomite cement that appear to predate the regional dolomite cement associated with main-stage Pb-Zn mineralization in the Viburnum Trend. The stratigraphic position, massive character, metal contents, mineral zoning and textures, and gangue dolomite cements of the orebody are unusual for the Viburnum Trend. Mineralogically and stratigraphically, these ores are more similar to the historically mined ores of the Indian Creek and Fredericktown-Mine La Motte districts to the north and east of the Viburnum Trend. In the Brushy Creek mine, the lower ores are not related to obvious stratigraphic controls, such as pinch-outs of the Lamotte Sandstone against Precambrian knobs, but appear to be localized along early zones of fracture-enhanced porosity and permeability within the Lamotte Sandstone, which promoted extreme dissolution of host rocks at the base of the Bonneterre Dolomite and development of high-grade Cu-(Ni-Co)-Zn-rich ore. The presence of distinct Ni- and Ni-Co-rich areas and the spatial variability of sphalerite types across the orebody indicate that there were likely multiple sites of ore fluid introduction whose influence varied with time during development of the ores.