Explained by David A. Brew
On the choice of deposit models
This model (Page, 1986) was selected because it best fit the combination of tectonic environment, deposit mineralogy, and host rocks of this tract and its subtracts. There was some discussion, though, as the fit isn't perfect!
On the delineation of permissive tracts
This tract includes the tracts designated as 09JU, 04MF, 10MF, 01SI, and 08SI by Brew and others (1991, 1996). Those tracts, and therefore this composite tract, were delineated by a combination of criteria: (1)
distribution of known deposits of this type, (2) distribution of geologic map units considered to be permissive for this deposit type, and (3) geophysical signature. Re (1): Each of the subtracts of this composite tract contains deposits or occurrences of the type assigned to the deposit model. Re (2) The boundaries of the permissive map units are the delineating factors. Re (3): Each of these subtracts have known significant
Important examples of this type of deposit
Examples of this deposit type include Bohemia Basin, on Yakobi Island (subtract 01SI), Brady Glacier (subtract 09MF, no undiscovered deposits estimated for this tract, however), and Mertie Lode (subtract 09JU).
On the numerical estimates made
Significant numerical estimates were made only for the areas considered to be most like Brady Glacier (subtract 04MF at Mt. Fairweather) and Bohemia Basin (subtract 01SI). In both of those areas it was estimated that there was a 0.5 probability of one or more deposits and lesser probabilities of more deposits. Other areas were considered to have only lower probabilities. For the entire tract, the mineral resource team estimated the minimum number of undiscovered deposits, consistent with a tonnage and grade model of Singer, and others (1986), to be:
Percentile 90 50 10 5 1
Estimated number of deposits 0 1 3 8 13
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., and Ludington. S.D., 1992, The study of the undiscovered mineral resources of the Tongass National Forest and adjacent lands, southeastern Alaska: Nonrenewable Resources, v. 1, no. 4, p. 303-321.
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.
Brew, D.A., and Drinkwater, J.L., 1991, Tongass Timber Reform Act Wilderness Areas supplement to U.S. Geological Survey Open-File Report 91-10 (Undiscovered locatable mineral resources of the Tongass National Forest and adjacent lands, southeastern Alaska): U.S. Geological Survey Open-File Report 91-343: 56 p.
Brew, D.A, Grybeck, D.J., Taylor, C.D., Jachens, R.C., Cox, D.P., Barnes, D.F., Koch, R.D., Morin, R.L., and Drinkwater, J.L., 1996, Undiscovered mineral resources of southeastern Alaska—Revised mineral-resource-assessment-tract descriptions: U.S. Geological Survey Open-File Report 96-716, 131 p.; one map, scale 1:1,000,000.
Page, N.J., 1986, descriptive model of synorogenic-synvolcanic Ni-Cu: in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 28.
Singer, D.A., Page, N.J., and Menzie, W.D., 1986, Grade and tonnage model of synorogenic-synvolcanic Ni-Cu: in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 28-31.