National mineral assessment tract GP04 (Sediment-hosted Cu, reduced-facies)

Tract GP04
Geographic region Great Plains
Tract area 215,200sq km
Deposit type Sediment-hosted Cu, reduced-facies
Deposit age Permian

Estimates

Confidence Number of
deposits
90% 1
50% 3
10% 10
5% 10
1% 10

Estimators: DCox, Lindsey, Zientek

Rationale

Explained by David A. Lindsey
On the choice of deposit models
Copper deposits in this tract, characterized by the Flowerpot Shale, were assigned to the reduced-facies (shale-hosted or Kupferschiefer-type) model. The Flowerpot copper deposits are hosted by thin (10-50 cm), laterally continuous (as much as 20 km2) reduced shale beds associated with evaporites and red beds (Johnson, 1976). Although not as extensive or as organic-rich (0.09–0.56 percent organic carbon) as the Kupferschiefer, the copper-mineralized beds in the Flowerpot Shale resemble the reduced-facies model more than the red-bed model.
On the delineation of permissive tracts
Deposits occur in shale beds of the Permian (Guadalupian) Flowerpot Shale and stratigraphically equivalent beds of the El Reno Group and San Angelo Formation of Oklahoma and north Texas (Johnson, 1976; Smith, 1976). Favorable shale beds were deposited in the shallow margins of the Midcontinent Permian basin, The tract extends in outcrop from central Oklahoma to central Texas and throughout the subsurface of Panhandle Oklahoma and Texas at a depth of less than 1 km.
Important examples of this type of deposit
The Creta and Mangum, Oklahoma, deposits (Dingess, 1976; Johnson, 1976) have production plus reserves of more than 7 million metric tons each at 1.9 and 1.0 percent Cu, respectively. Two other deposits (Crowell and McClellan mines) in north Texas may contain more than 1.5 million metric tons each of 1 percent Cu (the Crowell deposit is described by Schoenike and Zeballos, 1976).
There is some confusion about the size of the Mangum deposit, MRDS records show a tonnage of 36 million metric tons, which is near the median for world shale-hosted tonnage. However, the Oklahoma Geological Survey (R.O. Fay, written commun., 1993) estimates both Mangum and Creta at slightly more than 7 million metric tons. In conducting estimates of undiscovered copper for the Flowerpot tract, the lower estimate of Mangum tonnage was accepted. Calculations show that the higher estimate can only be reached by assuming a uniform grade of 1 percent copper for the entire thickness and entire known extent of the mineralized bed; this assumption has not been substantiated (Johnson, 1976, p. 13).
On the numerical estimates made
Mineralized localities in outcrop probably number about 10 (Johnson, 1976, lists six; others are described by Stroud and others, 1970). Of these, two 7-million-metric ton deposits have been found (Mangum and Creta). Considering that 10 mineralized localities are represented in outcrop, there may be 30-50 mineralized localities at a depth of less than 1 km in the subsurface of Panhandle Oklahoma and Texas. Of these perhaps two in ten contain Mangum-size deposits, for a total of 6-10 deposits in the subsurface. These hypothetical (undiscovered) deposits will be difficult to find because they are completely concealed; however, our estimate concerns probability of existence, not discovery. There is a good (90th percentile) chance for existence of another Mangum size deposit, in outcrop and probably one or two in the subsurface.
Because the Mangum-size deposit of more than 7 million metric tons is approximately 25 percent as large as the median-size (32 million metric tons) reduced-facies copper deposit, probably (90th percentile) less than one deposit (0.25 times 3 Mangum-size deposits) of the median-size world model could exist in the Flowerpot tract. There may be at least an even chance (50th percentile) of 2 to 3 deposits (0.25 times 0.20 deposits per locality times 40 to 60 localities) and only a 10 percent chance of 10 or more, including deposits that might exist in the subsurface.
For the 90th, 50th, and 10th percentiles, the team estimated, respectively, 0, 3, and 10 or more reduced-facies copper deposits consistent with the grade and tonnage model.
References
Dingess, P.A., 1976, Geology and mining operations at the Creta copper deposit of Eagle-Picher Industries, Inc., in Johnson, K.S., and Croy, R.L., eds., Stratiform copper deposits of the Midcontinent region, a symposium: Oklahoma Geological Survey Circular 77, p. 15-24.
Johnson, K.S., 1776, Permian copper shales of southwestern Oklahoma, in Johnson, K.S., and Croy, R.L., eds., Stratiform copper deposits of the Midcontinent region, a symposium: Oklahoma Geological Survey Circular 77, p. 3-14.
Schoenike, H.G., and Zeballos, R.A., 1967, The geology, exploration, and development of stratiform copper deposit located northwest of Crowell, Texas, in Johnson, K.S., and Croy, R.L., eds., Stratiform copper deposits of the Midcontinent region, a symposium: Oklahoma Geological Survey Circular 77, p. 99.
Smith, G.E., 1976, Sabkha and tidal-flat facies control of stratiform copper deposits in north Texas, in Johnson, K.S., and Croy, R.L., eds., Stratiform copper deposits of the Midcontinent region, a symposium: Oklahoma Geological Survey Circular 77, p. 25-39.
Stroud, R.B., McMahan, A.B., Stroup, R.K., and Hibpsman, M.H., 1976, Production potential of copper deposits associated with Permian red bed formations in Texas, Oklahoma, and Kansas: U.S. Bureau of Mines Report of Investigations 7422, 103 p.

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