Explained by Eric R. Force and Leslie J. Cox
On the choice of deposit models
The known Arizona massive sulfide deposits closely resemble the kuroko deposit model of Singer (1986); they are located in metamorphosed rocks of Early Proterozoic age that represent volcanic-arc marine environments. There are at least 41 deposits and occurrences in 13 districts (Donnelly and Conway, 1988) in the Yavapai, Mohave, and Mazatzal terranes in Arizona's Transition Zone.
On the delineation of permissive tracts
Permissive rock units were delineated using the geologic map of Arizona (Reynolds, 1988). Marine felsic to intermediate volcanic rocks of known volcanic-arc association appear to occur only in the Proterozoic rocks of the Yavapai, Mohave, and Mazatzal terranes in Arizona's Transition Zone, and they constitute the permissive tract. Rock assemblages of Jurassic age in southwestern Arizona that would otherwise be permissive are not marine in origin, and, therefore, massive sulfide deposits probably do not occur in them. In addition to areas that outcrop, part of the permissive area was delineated where the geophysical interpretation (M.E. Gettings, written commun., 1991) of gravity and magnetic data (Saltus, 1991) indicated extensions of Early Proterozoic metamorphic bedrock beneath cover materials less than 1 km thick
Important examples of this type of deposit
In the Verde district near Jerome (Anderson and Creasey, 1958), the world-class United Verde deposit produced over 30 million metric tons of ore, averaging 4.8 percent copper (Donnelly and Conway, 1988). The United Verde deposit is approximately five times larger than the next larger deposit in Arizona, the Iron King mine, and 22 times larger than the Old Dick-Bruce mine. Two smaller deposits are the Binghamton and the Antler. Of the 41 deposits and occurrences listed by Donnelly and Conway (1988), more than half had production of less than 5,000 metric tons of ore (E.H. DeWitt, written commun., 1993).
Some deposits in the Bagdad district are difficult to classify. If the granitic sills were to be removed from the section, the Bagdad deposits would be enclosed in pillow basalt lavas on oceanic crust (C.M. Conway, written commun., 1992), a setting more appropriate to Cyprus deposits. Until more work is done to better define this environment, these deposits are considered to be kuroko deposits.
On the numerical estimates made
Our estimate is based on the existence of five deposits closely approximating the tonnage distribution of the kuroko deposit model (Singer and Mosier, 1986). During estimation of undiscovered deposits, much discussion centered around the estimators' beliefs about the possible existence or non-existence of another deposit the size of the United Verde in this tract. Using the grade and tonnage models, one of about 10 estimated undiscovered deposits would have to be the size of the United Verde. Other important considerations included the density of occurrence of known deposits, the belief that many of the Arizona deposits have been tectonically deformed to vertical orientations, beliefs about the intensity of exploration, and the belief that the upper 500 m was fairly well explored by drilling and electromagnetic methods and that the lower 500 m might contain undiscovered deposits. This belief influenced a minority of estimators to postulate that the same number was yet to be found as had already been discovered.
Two separate sets of estimates were made for undiscovered kuroko deposits in Arizona. However, the results of the estimates made using a local Arizona grade and tonnage model provided by Ed DeWitt, of the U.S. Geological Survey, are not included in this report. Estimates based on the worldwide model converged on an expected value of about 2 undiscovered deposits. For the 90th, 50th, 10th,and 5th percentiles, the team estimated 0, 1, 3, and 7 or more deposits consistent with the grade and tonnage model of Singer and Mosier (1986).
Anderson, C.A., and Creasey, S.C., 1958, Geology and ore deposits of the Jerome area, Yavapai County, Arizona: U.S. Geological Survey Professional Paper 308, 185 p.
Donnelly, M.E., and Conway, C.M., 1988, Metallogenic map of volcanogenic massive-sulfide occurrences in Arizona: U.S. Geological Survey Map MF-1853-B, scale 1:1,000,000.
Reynolds, S.J., 1988, Geologic map of Arizona: Arizona Geological Survey Map 26, scale 1:1,000,000.
Saltus, R.W., 1991, Gravity and heat flow constraints on Cenozoic tectonics of the Western United States cordillera: Stanford University, Ph.D. dissertation, 245 p., 66 figs.
Singer, D.A., 1986, Descriptive model of kuroko massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 189-190.
Singer, D.A., and Mosier, D.L., 1986, Grade and tonnage model of kuroko massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 190-197.