MINERALOGY AND RADIOGENIC ISOTOPES OF THE SEVEN MILE BLUFF PROSPECT: AN INVESTIGATION OF A SURFACE MISSISSIPPI VALLEY-TYPE DEPOSIT IN CENTRAL TENNESSEE

Mississippi Valley-type (MVT) deposits are an important source of lead, zinc, fluorite, and barite, as well as gallium and germanium; they are found globally with a range of mineralogic characteristics. These deposits are often interpreted to have formed as a product of foreland basin fluid mobilization during orogenic events, yet there are alternative interpretations and much remains to be understood. Along with being a source for critical metals zinc, gallium, and germanium, Mississippi Valley-type deposits may also be overlooked as a source of critical metals such as indium, rhenium, and platinum-group metals. We anticipate our work here to help investigate the source of these deposits and the larger ore-forming systems. By investigating local occurrences in central Tennessee, such as the Seven Mile Bluff, our goal is to expand on the existing ore-forming history of this region.

The Seven Mile Bluff Prospect is located approximately 16 kilometers (9.7 miles) north of Lebanon, Tennessee along the north bank of the Cumberland River. Historical accounts identify multiple small mineralized veins in the area hosted in Ordovician Carters Limestone. The area is now overgrown with almost no exposures available. We were able to sample loose rocks where a mine site was backfilled. From historical accounts and what we observed, the mineralogy of the veining at this location consists of abundant fluorite, barite, calcite, and galena, and trace amounts of sphalerite, pyrite, and dolomite. In the samples we obtained, the galena is hosted in fractured fluorite veins and those veins are mantled by massive barite.

Lead isotopic analyses of galena have ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios of 19.68, 15.73, and 39.19 respectively. These ratios are very similar to galena from other MVT deposits in central Tennessee. The rare earth element (REE) concentration of a fluorite shows moderate enrichment of the light- to medium-REEs (atomic numbers 57-65), a positive europium anomaly, and slight depletion in the heavy-REEs (68-70). This is consistent with fluorites in central Tennessee. Fluorite Sm-Nd isotope ratios are also consistent with other occurrences in central Tennessee. These geochemical characteristics are distinct from nearby MVT districts suggesting a unique history for our samples.