Ore mineralogy and paragenesis of a portion of the Great Gossan Lead district, Virginia

Abstract

The Great Gossan Lead is a Ne-Sw trending belt of sulfide mineralization extending over 28 kilometers through Carroll and Grayson Counties in southwestern Virginia. The zone, which occurs in the metamorphic Late Precambrian Ashe formation, consists of a series of an echelon mineralized pods which pinch and swell irregularly along strike and are essentially concordant with the foliation.

Ore mineralogy is dominated by hexagonal pyrrhotite with minor amounts of sphalerite and chalcopyrite. All other phases--galena, arsenopyrite, pyrite, cubanite, mackinawite, tetrahedrite, stannite, and native bismuth--are disseminated throughout the ores. Pyrite is locally common as corroded primary euhedrae in some are bodies. Sphalerite occurs as disseminated anhedral aggregates commonly containing oriented lamellae and blebs of chalcopyrite and pyrrhotite. Chalcopyrite and galena occur as disseminated grains and as veinlets squeezed into fractured gangue minerals.

A strongly developed metamorphic fabric, the absence of mineral zoning, and the presence of a zone of iron depletion surrounding the ores indicate that the ores and host rocks have been subject to the same metamorphism. At a thermal maximum of 4l5-455°C, the ore assemblage would have developed a sulfur activity of approximately 10-9 to 10-7 atm. and would have been subject to an oxygen activity of about 10 -25 to 10 -27 atm.

The Great Gossan Lead is interpreted to have formed either as synsedimentary deposits or massive pyritic volcanism. Later metamorphism to the epidote-amphibolite facies and subsequent deformation roduced the present mineral assemblage and deformation features.