Explained by Leslie J. Cox
On the choice of deposit models
The choice of the worldwide porphyry-copper model of Cox (1986) to assess for porphyry copper deposits in Arizona was based on the close fit between geological and mineralogical features of the known Arizona deposits (Titley, 1982) and the description of the deposit model. Of 208 deposits located world-wide, 32 deposits are in Arizona. Most of the porphyry stocks in Arizona that are considered to be the source of large copper ore bodies were probably emplaced in the cores of voluminous fields of andesitic-dacitic to rhyolitic volcanic rocks erupted during Laramide time (Titley and Anthony, 1989).
On the delineation of permissive tracts
The permissive terrane delineated for the Arizona part of the Southern Basin and Range province includes all volcanic and igneous rocks of Jurassic and younger ages as identified on Reynolds' (1988) geologic map of Arizona with the exception of the Early Tertiary to Late Cretaceous muscovite-garnet-bearing peraluminous granites. Igneous rocks of Jurassic age are included because the Bisbee deposit is Jurassic; igneous rocks of the Laramide age are included because almost all of the known porphyry copper deposits in Arizona are generally associated with porphyritic intrusions of the Laramide calc-alkaline series; and igneous rocks of Tertiary age are included because deposits outside of Arizona are associated with Tertiary intrusions.
Areas extending 10 km outward from the Mesozoic and Tertiary plutons, as identified on the geologic maps of Reynolds (1988) and Keith (1984), are also included in the permissive terrane with the following exclusions: areas in which the bedrock is under more than 1 km of cover using a geophysical depth-to-bedrock map for which bedrock was defined as being middle Miocene or older (Oppenheimer and Sumner, 1980). A geophysical depth-to-bedrock map for which bedrock was defined as being Laramide or older (Saltus, 1991) was used to identify covered areas unfavorable for the occurrence of porphyry copper deposits, but these areas were not excluded from the permissive terrane.
The permissive terrane also includes areas where magnetic and gravity expressions indicate shallowly buried permissive bedrock (M.E. Gettings, written commun., 1993), known deposits and districts of this and associated models, and all areas identified in previous U.S. Geological Survey mineral assessments such as for U.S. Bureau of Land Management Wilderness Study Areas and CUSMAP (W.S. Updegrove and M.F. Diggles, written commun., 1993).
Titley and Anthony (1989) defined geologic terranes in Arizona on the basis of underlying basement rocks. This tract is the subset of permissive terrane in Arizona within their subdivision V, the Mesozoic Terrane of the Western Desert.
Important examples of this type of deposit
There are no major deposits in the tract. The lesser extent of Laramide rocks under cover indicated by geophysical evidence suggests that there are only a small number of undiscovered deposits here as well.
On the numerical estimates made
Information supplied to the team included the State geologic map (Reynolds, 1988); mylar overlays emphasizing the distributions of igneous rocks of Jurassic, Laramide, and middle
Tertiary ages (Keith, 1984, and Reynolds, 1988); deep cover as identified by gravity studies for which basement was defined as middle Tertiary (Oppenheimer and Sumner, 1980), and as Laramide (Saltus, 1991); magnetic anomalies (Gettings, written commun., 1991); distribution of anomalies of Sb, Ag, As, Mo, Cu, Pb, and Zn (S.E. Church, written commun., 1993); and the distribution of known deposits (Keith and others, 1983a, and MRDS), all at the scale of 1:1,000,000. In general, consideration was given to the fact that exploration activity in Arizona in the 1960s and 1970s led to the discovery of many new porphyry copper deposits. The team concluded that some areas should be re-examined in light of discoveries about Arizona tectonics in the last 18 years. Some team members felt that some of the districts of middle Tertiary age, particularly in the southeast part of the State, should also be re-examined.
This tract contains only three mineral districts classified 1a (copper and porphyry) by Keith and others (1983a); two have Laramide ages and the other a queried Laramide age. There was much uncertainty about the number, size, and characteristics of deposits in the 1a districts. In addition, there are at least 40 other metallic mineral districts within this tract that are middle Tertiary in age (Keith and others, 1983a). Estimators noted that geophysical extensions of Laramide rock beneath cover and geochemical anomalies are fewer here than in the rest of Arizona.
The estimators thought that nearly all exposed systems have been discovered and that most of the undiscovered deposits are hidden under cover. We attempted to determine the deposit density beneath shallow cover. We assumed that the maximum number of deposits beneath cover would be equal to the number of known porphyry copper deposits in exposed areas times a multiplier: the ratio of the area of shallow cover to the area of exposed rock. In an early stage of the assessment, before we divided southern Arizona into six permissive tracts, we had estimated the ratio of shallow cover to exposed favorable rock in the southern half of the State to be about 2.8, where shallow cover was defined as rocks of middle Tertiary and younger ages 0–1 km thick and the area of favorable exposed rocks was defined as the area 2.5 km beyond and including all igneous rocks of Laramide and Jurassic ages. The definitions of exposed rock and cover were made with the expectation that the undiscovered deposits would have Laramide or older ages. This expectation conflicts somewhat with the parameters used to draw the permissive tract, which allow for the possibility of undiscovered porphyry copper deposits of middle Tertiary age. In retrospect, the construction of separate tracts, one for undiscovered Laramide and one for undiscovered Tertiary deposits, would have facilitated more accurate estimations of deposit densities.
The deposit density multiplier determined for all of southern Arizona was understood to be a very crude guideline, to be adjusted in accord with how similar the rocks beneath cover might be to the rock exposed in each tract. Most estimators reduced the guideline multiplier by about half for this tract where more than half of the exposed rocks are considered favorable and the gravity maps of Saltus (1991) show that more than 10 percent of the shallow cover would be deep cover (>1 km thick) if basement was defined as Laramide rather than as middle Tertiary.
For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 1, 2, and 5 or more deposits consistent with the grade and tonnage model of Singer and others (1986).
Cox, D.P., 1986, Descriptive model of porphyry Cu, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 76.
Keith, S.B., 1984, Map of outcrops of Laramide (Cretaceous-Tertiary) rocks in Arizona and adjacent regions: Arizona Bureau of Geology and Mineral Technology Map 19, 1:1,000,000.
Keith, S.B., Gest, D.E., and DeWitt, Ed, 1983a, Metallic mineral districts of Arizona: Arizona Bureau of Geology and Mineral Technology Map 18, scale 1:1,000,000.
Keith, S.B., Schnabel, Lorraine, DeWitt Ed, Gest, D.E., and Wilt, Jan, 1983b, Map, description and bibliography of the mineralized areas of the Basin and Range province in Arizona: U.S. Geological Survey Open-File Report 84-0086, 129 p.
Oppenheimer, J.M., and Sumner, J.S., 1980, Depth-to-bedrock map, Basin and Range province, Arizona: Tucson, University of Arizona, Laboratory of Geophysics.
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., Mosier, D.L., and Cox, D.P., 1986, Grade and tonnage model of porphyry Cu, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 77-81.
Titley, S.R., 1982, The style and progress of mineralization and alteration and alteration in porphyry copper systems, in Titley, S.R., ed., Advances in geology of the porphyry copper deposits: Tucson, University of Arizona Press, p. 93-116.
Titley, S.R., and Anthony, E.Y., 1989, Laramide mineral deposits in Arizona in Jenney, J.P., and Reynolds, S.J., eds., Geologic evolution of Arizona: Tucson, Arizona Geological Society Digest 17, p. 485-514.
U.S. Bureau of Mines, 1993, The mineral industry of Arizona in 1992: Washington, D.C., U.S. Bureau of Mines, Mineral Industry Surveys, 4 p.