Abstract

The Great Gossan Lead is a northeast-trending belt of massive to near-massive sulfide pods and lenses extending over 28 km through Carroll and Grayson Counties of southwestern Virginia. The closely spaced en echelon pods and lenses constitute ore zones which are essentially conformable to foliation in the quartz-muscovite and chlorite schist of the late Precambrian Ashe Formation and which range from less than one to more than 50 m in thickness and up to 7.5 km in length.Ore mineralogy is dominated by hexagonal pyrrhotite (>90% by volume) with lesser amounts of sphalerite, chalcopyrite, and galena; other phases--arsenopyrite, cubanite, mackinawite, tetrahedrite, stannite, and native bismuth--are minor but disseminated throughout the ores. Pyrite, except at the northeastern end of the district where it is present as euhedral porphyroblasts, occurs only very locally as highly corroded crystals.A strongly developed metamorphic fabric, the absence of mineral zoning, and the presence of a zone of iron depletion in the silicates surrounding the ores indicate that the ores and host rocks have been subject to the same metamorphism. At a proposed thermal maximum of 415 degrees to 455 degrees C, the ore assemblage would have developed a sulfur activity of approximately 10 (super -9) to 10 (super -7) and would have been subject to an oxygen activity below about 10 (super -25) .Sphalerite occurs as disseminated anhedral aggregates which commonly contain oriented lamellae and blebs of chalcopyrite and pyrrhotite. Sphalerite coexisting with pyrite and pyrrhotite contains as much as 14 mole percent FeS and thus indicates a metamorphic pressure of about 5.5 kb.The Great Gossan Lead is interpreted to have formed as massive, synsedimentary pyritic deposits possibly related to submarine volcanism. Metamorphism to the lower amphibolite facies, intense deformation, and subsequent retrograde effects resulted in the present mineral assemblage and produced the observed tectonic fabric.

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