UNUSUAL MVT ZN-PB-CU MINERALIZATION IN THE LOWER PORTION OF THE BONNETERRE DOLOMITE, VIBURNUM TREND, SOUTHEAST MISSOURI

The Bonneterre Dolomite (Cambrian) is host to the Mississippi Valley-type (MVT) Pb-Zn-Cu deposits of the Viburnum Trend district of southeast Missouri and is underlain by the comparatively ore-barren Lamotte Sandstone. Sulfide ore mineralization in the district is normally lead-dominant (Pb:Zn > 5) and occurs almost exclusively in the upper portion of the Bonneterre Dolomite, associated typically with the reef-grainstone carbonate facies. Recent drilling in several locations in the Viburnum Trend has encountered economically significant mineralization within the lower Bonneterre Dolomite and in the upper Lamotte Sandstone, more than 30 m below the main ore-bearing horizon of the Viburnum Trend. This mineralization is more zinc- and copper-rich than typical deposits, with some assays ranging from 11 to 29% Zn, < 1 to 8% Cu and 6 to 15% Pb. There is evidence of metal zoning of Cu, Zn and Pb with increasing distance above the Lamotte Sandstone. In one area, mineralization comprises an estimated resource of more than 250,000 tonnes containing greater than 14% combined Zn + Pb. Our current research aims to determine if these stratigraphically lower deposits are part of the known mineral paragenesis for the Viburnum Trend or instead represent a previously unrecognized ore-forming event. Observations of drill cores and mineral textural relationships in polished thin/thick sections indicate that the lower section mineralization contains dominantly early stage Zn- and Cu-bearing sulfides overprinted by main stage Pb-Zn mineralization. The mineralization does not appear to be related spatially to obvious stratigraphic controls, such as pinch-outs of the Lamotte Sandstone against Precambrian knobs. Ongoing studies will address geochemical and structural controls on sulfide precipitation and localization with the hope of developing a predictable exploration model for this unusual mineralization.