Petrogenesis of the Lick Fork Ni-Co prospect, Floyd Co., Virginia

This thesis concerns the petrogenesis of the Ni-Co sulfide mineralization of the Lick Fork body in N.E. Floyd Co., Virginia. The host rock consists of steeply dipping layered ultramafic and mafic units of hornblende-peridotite, gabbro and hornblende-gabbro, in a high-grade metamorphic terrain (granulite facies). Sulfides comprised of pyrrhotite, marcasite, pentlandite, violarite, chalcopyrite, and pyrite form a "net" textured ore, confined to the ultramafic hanging wall unit.

The primary silicates have equilibrated to granulite facies metamorphism as indicated by the average Opx-Cpx
K_{d(Mg-Fe<2+) = 0.57, Mg0 + Fe0 + Fe203 , vs. Al203 in orthopyroxenes; and Ti contents, Al^iv/Al^iv ratios and alkali contents of amphiboles.}

Mg/Mg+Fe variation for pyroxenes, hornblende and biotite is reverse from that expected from typical in situ magmatic differentiation, but systematic variation of Mg/Mg+fe with increasing modal percent mafic minerals suggest an origin by magmatic differentiation rather than metamorphic differentiation.

Prograde reaction textures are rare; however, the comparison of Mg/Fe ratios of coexisting hornblende and pyroxenes, Cpx/Opx ratios and plagioclase compositions are similar to those derived from the theoretical breakdown reactions of hornblende, indicating the anhydrous phases are derived from the prograde dehydration of hornblende. Retrograde reactions of Cpx→Hb, brought about by the volatiles released during dehydration of primary hornblende, occurred at peak metamorphism. The primary silicates were then deformed by tectonic processes which included faulting localized at the hanging wall contact.

The absence of metamorphic sulfide textures, and the textural and chemical disequilibrium of coexisting sulfide-silicate phases argue for a post-metamorphic origin of the sulfides. Movement of hydrothermal fluids along fractures altered the primary silicate phases and was the source of sulfide emplacement as evidenced by replacement textures of silicates by sulfides. Sulfides replaced favorable foliated anhydrous phases resulting in the observed foliated "net" textures. Secondary sulfide mineralization was formed by supergene processes.