A chemical study of the silicate minerals of the Great Gossen Lead and surrounding rocks in southwestern Virginia
Samples of garnet-grade Ashe formation schists, gneisses, and amphibolites from two drill holes through the Great Gossan Lead in southwestern Virginia have been studied petrographically and chemically. Metamorphic temperatures determined from the muscovite-paragonite solvus, the muscovite-calcite-quartz system, and Fe⇄Mg distribution in biotite and garnet are approximately 400-460°C. Using the sphalerite geobarometer, metamorphic pressures were found to have been equal to or greater than 4 kbar.
Fine- to medium-grained garnet-chlorite-biotite-quartz-muscovite schists and gneisses predominate. Other lithologies include thin layers of hornblende amphibolite, sulfide ore, quartz veins, thin layers of hornblendic gneiss, marble lenses and minor solution cavities. The coarser-grained ore zone lithologies are dominated by chlorite, hornblende and actinolite-tremolite with cummingtonite rims, calcite, and quartz. Garnet-chlorite and garnet-biotite selvages are also occasionally found in the ore zone.
The ferromagnesian minerals show marked iron depletion within 2-5 feet of the sulfide ore. Ore zone chlorite and biotite show a significant increase in magnesium; garnets show a similar increase in manganese.
The cores of garnets from the ore zone show the influence of the ore even at the earliest stages of garnet growth, indicating that the ore was present prior to the peak of metamorphism. The distribution coefficients for Fe - Mg exchange reactions for garnet-biotite and garnet-chlorite also indicate that the ore and the surrounding rocks were metamorphosed together. Therefore, hypotheses for a synsedimentary or early hydrothermal origin for the ore are favored over those suggesting a post-metamorphic hydrothermal origin.