Explained by Leslie J. Cox and Michael F. Diggles
On the choice of deposit models
The rationale for the choice of the porphyry copper-gold model of Cox and Singer (1992) was based on our ability to identify characteristics in the known Arizona deposits that are unique to the porphyry Cu-Au deposit model and on our ability to distinguish a geologic setting for gold-rich porphyries that is different from the general porphyry-copper environment. It is argued that gold-rich porphyries tend to be associated with alkaline igneous rocks (Gilmour, 1982, Lowell, 1989). Wilt (1993), however, contends that gold-rich porphyry deposits occur in calc-alkaline settings as well, but possesses an oxidation state that is determined by the rocks through which the magma rises: the gold-poor porphyries are oxidized, whereas the gold-rich porphyries are weakly oxidized. Our estimates for undiscovered porphyry Cu-Au deposits were guided by Wilt's delineation of oxidized versus weakly oxidized calc-alkaline rocks in Arizona.
On the delineation of permissive tracts
The tract for porphyry copper-gold consists of all six of the general porphyry copper tracts for Arizona. The most favorable rocks consist of porphyritic igneous rocks of Laramide age in the Basin and Range and Transition Zone of Arizona and their interpreted geophysical extensions beneath cover. The team identified and excluded areas that would not be permissive for the occurrence of porphyry copper deposits because of parameters such as geologic setting, rock composition, and depth of cover.
Important examples of this type of deposit
Only a few of the many significant Arizona porphyry copper deposits are gold-rich: Dos Pobres (Langton and Williams, 1982), Ajo-New Cornelia (Hagstrom and others, 1987), and Sanchez. Of the three known deposits, all have tonnages below the median for the porphyry Cu-Au model (Singer and Cox, 1986).
On the numerical estimates made
Estimates for porphyry Cu-Au deposits were weighted toward areas characterized by weakly oxidized calc-alkaline intrusions as defined by ferric:ferrous ratios in unaltered plutons. In southwestern Arizona, the potential for Laramide Cu-Au porphyries in the identified weakly oxidized area is relatively small, because the shallow Laramide intrusions have been largely destroyed by erosion. This is indicated by the fact that volcanic rocks of Oligocene age rest unconformably upon eroded plutons of Laramide age. Only a small part of the tract is favorable for porphyry Cu-Au deposits.
The theory that porphyry Cu-Au deposits are emplaced close to the surface and should be situated within their associated volcanic rocks was considered and Laramide intrusions adjacent to volcanic rocks of the same age were examined. To arrive at estimates of undiscovered porphyry Cu-Au deposits, the team considered first the probability of there being zero deposits, the maximum number likely, and the most likely number. Using this estimation technique, the team reached a consensus of an expected value of about 1 undiscovered porphyry Cu-Au deposit. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 1, 2, 3, and 4 or more deposits consistent with the grade and tonnage model of Singer and Cox (1986).
Cox, D.P., and Singer, D.A., 1992, Distribution of gold in porphyry copper deposits, in DeYoung, J.H., Jr., and Hammarstrom, J.M., eds., Contributions to commodity geology research: U.S. Geological Survey Bulletin 1877, p. C1-C14.
Gilmour, Paul, 1982, Grades and tonnages of porphyry copper deposits in Titley, S.R., ed., Advances in geology of the porphyry copper deposits: Tucson, University of Arizona Press, p. 7-36.
Hagstrum, J.T., Cox, D.P., and Miller, R.J., l987, Structural reinterpretation of the Ajo Mining District, Pima County Arizona based on paleomagnetic and geochronologic studies: Economic Geology, v. 82, no. 4, p. l348-l36l.
Langton, J.M., and Williams, S.A., 1982, Structural petrological, and mineralogical controls for the Dos Pobres ore body, in Titley, S.R., ed., Advances in geology of the porphyry copper deposits, southwestern North America: Tucson, University of Arizona Press, p. 335-352.
Lowell, J.D., 1989, Gold mineralization in porphyry copper deposits discussed: Mining Engineering, v. 41, no. 4, p. 227-231.
Singer, D.A., and Cox, D.P., 1986, Grade and tonnage model of porphyry Cu-Au, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 110-114.
Wilt, J.C., 1993, Geochemical patterns of hydrothermal mineral deposits associated with calc-alkalic and alkali-calcic igneous rocks as evaluated with neural networks: Tucson, University of Arizona, Ph.D. dissertation, 721 p.