Explained by T.L. Klein
On the choice of deposit models
Many of the gold deposits of the southeastern United States are analogous to the low-sulfide Au-quartz vein deposit model (Berger, 1986). There are broad similarities in ore controls (i.e., veins controlled by regional fault systems or folds), in vein and alteration mineralogy, and in their host rocks. Although low-sulfide gold deposits are typically found in low-grade metamorphic terrains, we feel that the metamorphic grade is probably a less important criteria than the character of the protolith and alteration assemblage and the presence of appropriate regional structures. Therefore, the deposits in the Alabama, Dahlonega, and South Mountain districts are classified as low-sulfide Au-quartz veins deposits (model 36a, Berger, 1986), even though they are found in high-grade metamorphic rocks.
On the delineation of permissive tracts
Because gold occurrences are found in nearly every major pre-Mesozoic lithotectonic terrane in the southeastern United States, the entire area of metamophosed igneous and sedimentary rocks in the southern Appalachian Mountains and Piedmont is considered to be permissive for the occurrence of low-sulfide Au-quartz vein deposits. This tract consists of the area remaining after more favorable areas were assigned to tracts SA21–SA27. After the more favorable tracts were removed, the team members believed that the number of undiscovered deposits remaining in this larger permissive tract was small and they were unable to make a numerical estimate.
Berger, B.R., 1986, Descriptive model of low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., eds. Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239.