Explained by David A. Brew, Cliff D. Taylor, Robert C. Jachens, Richard D. Koch, Donald Grybeck, James L. Drinkwater, David F. Barnes, and Larry J. Drew
On the choice of deposit models
In southeastern Alaska low-sulfide Au-quartz vein deposits occur in Mesozoic and Paleozoic low- to medium-grade metamorphosed clastic, volcanic, and some carbonate rocks along or near large through-going high-angle faults that are close to Cretaceous and Tertiary mesozonal to epizonal subduction- and post-subduction-related calcalkaline granitic plutons and dikes. Additional low-sulfide gold-quartz vein deposits are expected.
On the delineation of permissive tracts
This tract includes 20 separate tracts that were delineated, evaluated, and estimated individually by Brew and others (1991) and summarized by Brew (1993). The tract boundaries were delineated mainly by combining the distribution of deposits and occurrences, the distribution of the plutons, metaclastic units and large faults, and the distribution of favorable geochemical patterns.
Important examples of this type of deposit
The Alaska-Juneau, Treadwell, Chichagof, Hirst-Chichagof, and Helm Bay mines are examples of low-sulfide Au-quartz vein deposits.
On the numerical estimates made
Permissive rock types and associations, geochemical patterns, and the numerous low-sulfide Au-quartz vein occurrences were the main basis for the estimates. The expected minimum number of undiscovered low-sulfide Au-quartz vein deposits consistent with the grade and tonnage model of Bliss (1986) is:
Percentile 90 50 10 5 1
Estimated number of deposits 1 5 13 14 17
Bliss, J.D., 1986, Grade and tonnage model of low-sulfide gold-quartz veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239-243.
Brew, D.A., 1993, Regional geologic setting of mineral resources in southeastern Alaska, in Godwin, L.H., and Smith, B. D., eds., Economic mineral resources of the Annette Islands Reserve, Alaska: U.S. Dept. of the Interior, Bureau of Indian Affairs, Division of Energy and Mineral Resources Publication, p.13-20.
Brew, D.A., Drew, L.J., Schmidt, L.M., Root, D.H., and Huber, D.F, 1991, Undiscovered locatable mineral resources of the Tongass National Forest and adjacent areas, southeastern Alaska: U.S. Geological Survey Open-File Report 91-10, 370 p., 15 maps at 1:250,000, 1 map at 1:500,000, 11 figs.