Cathodoluminescence as a Means for Distinguishing Hydrothermal from Pre-hydrothermal Quartz in Sulfide-Bearing Mineral Deposits on the Northern Fringe of the Upper Mississippi Valley Zinc-Lead District, NE Iowa and SW Wisconsin

Sulfide-bearing mineral deposits, located on the northern fringe of the Upper Mississippi Valley Zinc-Lead District, are contained in early Ordovician carbonate rocks that are extensively silicified and dolomitized. Some silica and dolomite appear to be products of the hydrothermal processes that also formed fracture-filling and cavity-lining sulfides and other cogenetic minerals; other silica and dolomite appear to result from low-temperature, pre-hydrothermal regional diagenesis. Distinguishing hydrothermal quartz (jasperoid) from pre-hydrothermal quartz (chert) solely by hand specimen and thin section petrography is difficult because these two types of mineralization are often intimately associated with each other. Polished slabs from several of these deposits were studied using cathodoluminescence (CL). Demonstrable chert exhibits light to dark blue CL, while demonstrable jasperoid exhibits red-brown to tan CL. It appears that intimately-associated jasperoid and chert can be distinguished with CL. A result is that much of what was previously described as jasperoid in the Upper Mississippi Valley Zn-Pb District is very likely non-hydrothermal chert