Petrology, geochemistry, and tectonic setting of amphibolites of the Ducktown Mining District, Tennessee

The Ducktown Mining District, located in the Blue Ridge Geologic Province, consists of nine stratabound massive sulfide deposits. These deposits have several characteristics of volcanogenic-exhalative deposits but occur in an overwhelmingly sedimentary environment. Recent isotopic studies have suggested a magmatic component to ore formation. This study is an evaluation of the petrology and geochemistry of late Proterozoic amphibolite units spatially associated with the ore bodies. These amphibolites are the only known manifestation of igneous activity in the Ducktown area and represent one of the few occurrences of igneous rocks in the Ocoee Supergroup.

Microprobe analyses have shown the amphiboles in the amphibolites to vary from magnesio-hornblendes in the garnet zone to tschermakitic-hornblendes in the staurolite zone. Plagioclase compositions vary from calcic oligoclase to sodic andesine. Estimates of temperature and pressure of metamorphism, based on phase assemblages and mineral compositions, are T = ≈ 525°C and P = ≈ 5 Kb.

The amphibolites are interpreted to be igneous in origin based on Niggli trends, relict igneous textures, and bulk chemistry. The protolith was an divine tholeiite, as judged from normative composition, the AFM diagram, and Y/Nb ratios. Discrimination diagrams using immobile trace elements (Ti-Y-Zr, Ti-Zr, Zr/Y-Zr, Ti/V) indicate an affinity with mid-ocean ridge basalts, an interpretation in agreement with that suggested for amphibolites associated with the very similar Gossan Lead deposits (Virginia) in Ocoee-equivalent metasediments (Gair and Slack, 1984), but apparently inconsistent with the ensialic setting of the Ducktown deposits. However, tholeiitic magmas associated with continental breakup may show chemical affinities with MORE, despite their intrusion into continental crust.

The Ducktown amphibolites are interpreted to have been diabasic sills emplaced during deposition of Ocoee sediments and related to intracratonic rifting along the eastern margin of Laurentia during the incipient development of the lapetus ocean. This conclusion is consistent with formation of the Ducktown massive sulfide deposits in a rift environment.