Explained by Robert J. Kamilli and Leslie J. Cox
On the choice of deposit models
The rationale for the choice of Comstock epithermal veins as analogues for most of the epithermal gold districts in Arizona was based on the reasonable fit between geological and mineralogical features of the known Arizona districts and the description of the model by Mosier and others (1986a). Of 41 districts located worldwide, 3 districts in Arizona are included in the model. Of the 15 districts with pre-production sizes sufficient to be deemed significant, the Kofa, Oatman, Turquoise, Union Pass, and Vulture are above the median of the tonnage distribution. The question of whether the deposits in Arizona are evenly distributed about the means of the tonnage curve remains a matter of debate, although the initial perception that perhaps 60 to 70 districts might lie below the 90th percentile shifted to an acceptance that perhaps only 12 or so of the smaller districts might properly be classified as Comstock type, with the rest better classed as polymetallic or other types.
Gold-silver vein deposits near Prescott are problematic because they occur in Precambrian rocks, yet have many characteristics in common with epithermal vein deposits. These deposits were classified as "Congress-type" in a preassessment of the Prescott quadrangle (Conway and others, 1987).
Arizona epithermal districts generally are all in volcanic rocks of approximately the same age as mineralization. The veins bottom within a few hundred feet and do not connect with larger base-metal deposits at depth. The districts are related to volcanic centers rather than to large intrusions. In Arizona, many of the districts fall in an area south of the Colorado Plateau in a belt that is in line with the Colorado mineral belt.
South of the northeast-trending Holbrook Lineament (Titley and Anthony, 1989, p. 489), gold is generally produced from silver-rich polymetallic deposits whereas between the Holbrook and Bright Angel-Mesa Butte lineament to the north, gold is generally produced from gold-rich quartz veins (Titley, oral commun., 1993).
Examination of the production reported in Keith and others (1983) for approximately 70 districts shown as Type 3 (gold districts with or without copper or lead) shows that for more than half, the ratio of base to precious metal is above 100 (S. Richard, Arizona Geological Survey, written commun., 1993). Some of these were significant gold producers. However, there is no way to discern which of the districts are composed of more than one deposit type’ some of them may contain mostly base metals and may not be of the Comstock type at all.
On the delineation of permissive tracts
The permissive tract was compiled using the 1988 Geologic Map of Arizona (Reynolds, 1988, Map 26). The rocks included Triassic and Jurassic through Quaternary andesites through rhyolites as well as associated subvolcanic intrusions and volcano-sedimentary rocks that are the products of both calc-alkaline and bimodal volcanism. These volcanic fields delineate the permissive tract for epithermal mineral deposits.
The permissive tract was extended under younger units where justified by geologic interpretation of the State map. More speculative extensions were made by assuming a 10° dip and extending the unit either to the point where it would be 1 km deep (a lateral distance of about 6 km) or to a point halfway between the volcanic outcrop and another, nonvolcanic unit. In general, pre-Tertiary units are too discontinuous to extend with confidence under cover. Rocks are not delineated permissive where depth to bedrock is greater than 1 km, based on the Depth-to-bedrock map for Arizona (Oppenheimer and Sumner, 1980). The resulting tract is essentially a map of volcanic rocks, but also includes those parts of epithermal districts that do not plot precisely in the volcanic terranes.
Important examples of this type of deposit
Kofa, Oatman, Turquoise, Union Pass, and Vulture are the most important of the known historic districts in Arizona (Keith and others, 1983). The development in the 1980s of the Frisco deposit raised the Union Pass (McConnico) district above the median of the tonnage curve. The Newsboy deposit in the Vulture district, is a new discovery in the 1980s.
On the numerical estimates made
Information that was available for participants to determine the favorablity of areas within the permissive tract included maps showing the distribution of some of the volcanic centers, maps showing the distribution of As, Sb, and Hg geochemical anomalies, maps showing the distribution of placer gold deposits, and gravity and magnetic anomaly data at sub-regional scales.
The group discussed the perception that exploration activity for gold has been greater in Nevada than in Arizona since the early 1980s. It was estimated that of the 60 or so small districts (both Comstock and polymetallic vein), no more than about half had been looked at three times or more, and thus, some of the districts may be under-explored.
Some estimators thought the potential for undiscovered districts in the upper 1 km of the basins was about the same, area for area, as the number of districts presently exposed.
There are approximately 15,000 km2 of exposed Tertiary volcanic rocks within the permissive tract. The maximum ratio of the area of covered permissive area to exposed area was estimated to be about 0.7.
To promote a detailed analysis, the permissive tract south of the Colorado Plateau was divided into two terranes, southeastern Arizona and northwestern Arizona because of crustal differences in silver-to-gold ratios recognized by Titley (1991). The northwestern area contains ores relatively rich in gold whereas the southeastern area contains ores relatively enriched in silver.
Factors important to the estimators included: (1) the distribution and number of Arizona districts known to be similar in size and grade to those in the Comstock model; (2) amounts of pre- and post-basin volcanic cover; (3) concealment by cover, in that the northwestern area has 3 times as much cover as the southeastern area, and thus, there may be more undiscovered deposits in the northwestern part; (4) the perception that there has been relatively low exploration intensity of exposed volcanic rocks; (5) the fact that unlike Nevada, Arizona is not chiefly known as an important precious metal region; (6) the possibility of the existence of deposits older than Tertiary in age, given that there is apparently a Jurassic epithermal deposit just across the border in Mexico; and (7) the difficulty in finding veins not exposed at the surface.
There remain some differences of opinion among the team members. Some think the possibility of finding deposits having ages older than Tertiary was likely. They believe there is sufficient room in the broadly defined permissive tract to hold the numbers estimated. A more cautious point of view holds that perhaps half of the known districts are mis-classified and that there is not much remaining to be discovered in the rocks now exposed at surface. The final consensus was that for the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 1, 5, 7, 10, and 15 or more deposits consistent with the grade and tonnage model of Mosier and others (1986b).
Conway, C., Hanna, W.F., Hendicks, J.D., Hoover, D.B., Knepper, D.H., Jr., McCarthy, J.H., Jr., Mosier, D., Page, N.J., Pitkin, J.A., and Southworth, C.S., 1987, Planning document for the mineral resource assessment of the Prescott 1° x 2° quadrangle, Arizona: U.S. Geological administrative report, unpublished, 130 msp.
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.
Mosier, D.L., Singer, D.A., and Berger, B.R., 1986, Descriptive model of Comstock epithermal veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 150.
Mosier, D.L., Sato, Takeo, and Singer, D.A., 1986, Grade-tonnage model of Comstock epithermal veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 151-153.
Oppenheimer, J.M., and Sumner, J.S., 1980, Depth-to-bedrock map, Basin and Range province, Arizona: Tucson, Ariz., University of Arizona, Laboratory of Geophysics.
Titley, S.R., 1991, Correspondence of ores of silver and gold with basement terranes in the American southwest: Mineralium Deposita, v. 26, p. 66–71.
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.