Explained by David A. Lindsey
On the choice of deposit models
Structurally-controlled deposits in sedimentary rocks are no different than other red-bed-type copper deposits in that their mechanism of metal transport (oxidizing chloride brines in equilbrium with hematite) and precipitation (reduction by sulfides or organic matter) is the same. However, the path of mineralizing fluids was determined at least in part by structures that cut across stratigraphic horizons, as well as by permeable beds. In addition to mineralized rock in a structure, mineralized beds and lenses (mantos) may extend into favorable formations adjacent to a structure.
Deposits in this tract span the general range of tonnages and grade for red-bed copper deposits. Thus, the world red-bed model is appropriate (Mark3 index 97).
On the delineation of permissive tracts
Structurally controlled red-bed deposits occur where faults and solution-collapse pipes allow basin fluids to ascend through reducing environments. Because existing geologic maps do not show all of the possible structures in which deposits could occur, the tract was delineated by the distribution of the sediment-hosted copper occurrences that appear to have structural control, and thus, may be too restrictive.
Important examples of this type of deposit
The most important examples of fault-controlled red-bed copper deposits are found in Lisbon Valley, Utah (8.6 million metric tons at more than 1 percent Cu) (Morrison and Parry, 1986), and at White Mesa, Arizona (2.2 million metric tons at 1 percent Cu) (Mayo, 1956). Solution-collapse breccia pipes, some of which contain copper with uranium in the Grand Canyon region (e.g., Hack Canyon) (Wenrich, 1985), were excluded from consideration because they differ greatly in form from typical red-bed deposits and because they are primarily uranium deposits. Deposits at four districts in the Grand Canyon region have produced mainly copper and are probably controlled by faults and solution (Van Gosen and Wenrich, in press); these were included within the structurally-controlled tract.
On the numerical estimates made
Estimates are based on entirely on known areas of occurrence. Although the Lisbon Valley, White Mesa, and four Grand Canyon districts have been prospected extensively, the team considered at least one undiscovered copper deposit likely (90th percentile). For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 1, 3, 4, 4, and 5 or more deposits consistent with the grade and tonnage model.
Mayo, E.B., 1956, Copper, in Kiersch, G.A., ed., Metalliferous minerals and mineral fuels—Geology, evaluation, and uses, and a section on the general geology, v. 1 of Mineral resources, Navajo-Hopi Indian Reservation, Arizona-Utah: Tucson, Arizona University Press, p. 19-32.
Morrison, S.J., and Parry, W.T., 1986, Formation of carbonate-sulfate veins associated with copper ore deposits from saline basin brines, Lisbon Valley, Utah: fluid inclusion and isotopic evidence: Economic Geology, v. 81, no. 8, p. 1853-1866.
Van Gosen, B.S., and Wenrich, K.J., in press, Strata-bound copper deposits in the Kaibab Limestone, Coconino County, northern Arizona: U.S. Geological Survey Bulletin.
Wenrich, K.J., 1985, Mineralization of breccia pipes in northern Arizona: Economic Geology, v. 80, no. 6, p. 1722-1735.