Abstract

Fletcher mine is situated on a buried Precambrian ridge that stood as a topographic high during Upper Cambrian Bonneterre deposition. After deposition of the Lamotte Sandstone that surrounds and pinches out against the ridge, algal reefs grew and formed a barrier to open marine shelf deposition. This created offshore, reef, and back reef environments. Clastic carbonates of the Bonneterre Formation were then deposited over the area and became the host for base metal mineralization.Sulfide mineralization, principally galena, sphalerite, and chalcopyrite, follows a pronounced north-south trend within the Fletcher area. The ore zone reaches its widest distribution around the Precambrian ridge that protrudes into the Bonneterre. Ore distribution is facies related, and local emplacement is influenced by porosity, fracturing, brecciation, and the shape of the buried Precambrian high. Most of the mining to date occurs in the middle of the upper clastic grainstone unit. Where the Sullivan Siltstone is brecciated, mineralization extends into the overlying mudstones and grainstones and intothe Davis Formation. Two north-south, narrow, linear breccia trends extend the full length of the mine and contain good sulfide mineralization.The mineralization occurs mainly as open-space and fracture fillings, with minor replacement of dolomite. Dissolution of galena is common and is associated with ore zones that show second-stage enrichment.

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