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 this tract, Paleozoic carbonate and some clastic rocks are intruded by Cretaceous and Tertiary mesozonal to epizonal subduction- and post-subduction-related calcalkaline granitic plutons and dikes, with local development of extensive thermally altered areas. Zn-Pb skarn deposits (Cox, 1986) are inferred on the basis of their mineralogy, and association with carbonates adjacent to intrusive contacts.
On the delineation of permissive tracts
This tract encompasses 18 separate areas defined by Brew and others (1991) on the basis of the distribution of Paleozoic carbonate and some clastic rocks that are intruded by Cretaceous and Tertiary granitic plutons and dikes, together with the known distribution of Zn-Pb skarns. The detailed information from these individual areas comprise the regional metallogenic factors cited by Brew (1993). Tract boundaries were delineated by the combination of Paleozoic carbonate rocks and Cretaceous and Tertiary intrusive rocks.
Important examples of this type of deposit
Groundhog Basin, east of Wrangell, Alaska is a Zn-Pb skarn deposit (Newberry and Brew, 1989).
On the numerical estimates made
Permissive rock types and associations and appropriate geochemical patterns are the main basis for the estimates. The minimum number of undiscovered Zn-Pb skarn deposits, consistent with the grade and tonnage model of Mosier (1986), is estimated to be:
Percentile 90 50 10 5 1
Estimated number of deposits 0 1 4 5 7
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.
Cox, D.P., 1986, Descriptive model of Zn-Pb skarn deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 90.
Mosier, D.L., 1986, Grade and tonnage model of Zn-Pb skarn deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 90-93.
Newberry, R.J., and Brew, D.A., 1989, Epigenetic hydrothermal origin of the Groundhog Basin-Glacier Basin silver-tin-lead-zinc deposits, southeastern Alaska, in, Galloway, J.P., and Dover, J.H., eds., Geologic studies in Alaska by the U.S. Geological Survey in 1988: U.S. Geological Survey Bulletin 1903, p. 113-121.