Explained by Sandra H.B. Clark, Joseph A. Briskey, Jr. and Dennis P. Cox
On the choice of deposit models
Large resources of zinc occur in major stratabound Mississippi Valley (MVT) districts that extend from Pennsylvania to Tennessee. Because of the importance and distinctive geological features of the eastern United States deposits, they are the basis for definition of the world-wide Appalachian Zn model (32b of Briskey, 1986). Known Appalachian Zn districts in the east-central United States are stratabound within Lower Cambrian to Lower Ordovician dolostones and limestones that formed as platform carbonate deposits in the Appalachian basin sedimentary sequence. No examples are known in this tract, but carbonate rocks of the Lower and Middle Ordovician Beekmantown and stratigraphically equivalent units are host rocks for deposits in Pennsylvania
On the delineation of permissive tracts
The permissive tract includes Lower and Middle Ordovician carbonate rocks of the Beekmantown Group and stratigraphically equivalent units and their extensions under less than 1 km of cover. No estimate of undiscovered districts was made.
Briskey, J.A., 1986a, Descriptive model of Appalachian Zn, in Cox, D.P., and Singer, D.A., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 222.