Abstract

Fe-Zn-Cu sulfide mineralization in the Stone Hill district of the northern Alabama Piedmont is hosted by zones of felsic schist and garnet schist within the Ketchepedrakee Amphibolite. The felsic schist is characterized by quartz-pyrrhotite-biotite-muscovite-plagioclase-rutile-sphalerite + or - chalcopyrite + or - pyrite + or - hornblende + or - epidote assemblages and the garnet schist is characterized by garnet-hornblende-quartz-biotite-pyrrhotite-chalcopyrite + or - sphalerite + or - pyrite + or - chlorite + or - actinolite + or - calcite assemblages. Both schists are well banded and highly deformed. The felsic schist locally contains ellipsoidal, polycrystalline aggregates of quartz-feldspar-biotite that superficially resemble sedimentary clasts but clearly formed tectonically by boudinage of felsic bands. The following petrographic and geochemical data and field relationships suggest that both schists are hydrothermally altered basalt (the precursor of the Ketchepedrakee Amphibolite): (1) the felsic and garnet schist units are very localized lenses (<l-km strike length, <50-m thickness); (2) contacts between the felsic schist and the amphibolite are normally gradational and may be transitional through the garnet schist; (3) both schists contain abundant rutile, distributed in the same way as ilmenite in the schists and the amphibolite; (4) the two schists and the amphibolite are uniformly enriched in compatible transition elements such as Sc, Ti, V, Cr, and Co; (5) ratios of incompatible, high field-strength elements such as Y, Zr, Nb, REE, and Hf are virtually identical in the schists and the amphibolite; and (6) the schists and the amphibolite exhibit similar flat chondrite-normalized rare earth element (REE) abundance patterns (with variable Eu anomalies). Textural data indicate that alteration and mineralization of the basaltic protolith predated metamorphism and deformation. The Fe-Zn-Cu sulfide deposits in the Stone Hill district are interpreted to have formed by synvolcanic hydrothermal alteration in an incipiently rifted basin.

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