Explained by Gilpin R. Robinson, Jr.
On the choice of deposit models
Numerous copper occurrences are known in the early Mesozoic basins of the eastern United States (Robinson and Sears, 1988). Many of these copper deposits and occurrences are sediment-hosted deposit types occurring in Mesozoic red-bed strata (Smoot and Robinson, 1988, table 1A). However, the estimated sizes of most of these occurrences are smaller than the general range of the tonnage curves for sediment-hosted copper, model 30b (Mosier and others, 1986).
The assessment was made using the sediment-hosted copper model 30b of Cox and Singer (1986); this model includes deposits from both red-bed and reduced-facies hosts. The model may not be completely appropriate for assessment of the early Mesozoic basin region for the following reason. A number of deposits in the model have metal grades and tonnages that are enlarged by processes of supergene enrichment, a process not likely to be significant in the eastern region from northern Virginia to Connecticut. Hence the tonnages of copper in the simulation results might be larger than the actual undiscovered tonnage.
On the delineation of permissive tracts
Because of the wide distribution of thick red-bed sequences and numerous small copper occurrences, the Culpeper basin in Virginia, the Gettysburg basin in Pennsylvania, and the Newark basin in New Jersey, and New York are considered to be permissive for small red-bed copper deposits.
In addition, recent work has identified significant occurrences of copper concentrated in reduced lacustrine mudstone units within the sequences of red sandstone and siltstones deposited in the basins (Smoot and Robinson, 1988; Robinson and Smoot, 1989) and the basins are believed to have potential for additional sediment-hosted copper deposits in these reduced-lacustrine facies rocks. Two favorable areas have been postulated for reduced facies sediment-hosted deposits.
In the Culpeper basin, Virginia, two black mudstone intervals, each containing zones of 0.5 to 2 percent copper ranging from 10 cm to more than 1 m in thickness, have been identified (Smoot and Robinson, 1988; Robinson and Smoot, 1989). Indications of mineralization in these beds have been traced along strike for approximately 6 km. Additional favorable host horizons occur in intervals throughout the cyclic stratigraphic section in the tract. This part of the Culpeper basin is underlain by basement rocks containing oxidized continental tholeiitic basalt, a potential source of copper.
Metal-rich mudstones resemble unmineralized mudstones, outcrop exposures in the region are sparse, and the deep humid weathering tends to greatly reduce copper contents in soils and saprolites. These factors suggest that more extensive sediment-hosted copper deposits may have gone unrecognized.
On the numerical estimates made
For the 90th, 50th, and 10th percentiles, the team estimated 1, 2, and 3 or more sediment-hosted copper deposits consistent with the grade and tonnage model of Mosier and others (1986) (Mark3 index 63). The permissive tract is large, poorly exposed, and inadequately explored for reduced facies deposits.
Mosier, D.L., Singer, D.A., and Cox, D.P., 1986, Grade and tonnage model for sediment-hosted copper, in Cox, D.P. and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 205–208.
Cox, D.P., 1986, Descriptive model for sediment-hosted copper, in Cox, D.P. and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 205.
Robinson, G.R., Jr., and Sears, C.M., 1988, Inventory of metal mines and occurrences associated with the early Mesozoic basins of the eastern United States—Summary tables, in Froelich A.J., and Robinson, G.R., Jr., Studies of the early Mesozoic basins of the eastern United States: U.S. Geological Survey Bulletin 1776, p. 265–303.
Robinson, G.R., Jr., and Smoot, J.P., 1989, Stratabound copper and zinc mineralization in Triassic lacustrine beds of the Culpeper basin, Virginia [abs.], in Schindler, K.S., ed., USGS research on mineral resources—1989 program and abstracts, fifth annual V.E. McKelvey Forum on Mineral and Energy Resources: U.S. Geological Survey Circular 1035, p. 57–59.
Smoot, J.P., and Robinson, G.R. Jr., 1988, Sedimentology of stratabound base-metal occurrences in the Newark Supergroup, in Froelich, A.J., and Robinson, G.R., Jr., eds., Studies of the early Mesozoic basins of the eastern United States: U.S. Geological Survey Bulletin 1776, p. 356–376.