Explained by David A. Brew
On the choice of deposit models
All of these tracts consist of ultramafic and mafic rocks that are the "type" Alaskan-type mafic-ultramafic complexes. As such, the deposits and occurrences in these tracts define the model.
On the delineation of permissive tracts
This tract includes the tracts designated as by Brew and others (1991, 1996) as 08CR, 11CR, 09KC, 15KC, 10PE, 17PE, 03PR, 13SK, 15SK, 02SD, and 07SD. 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): Known deposits with calculated resources that occur in these rocks include Union Bay, Kasaan-Salt Chuck, Yellow Hill, Blashke Islands, Duke Island, Snettisham, Klukwan, and Haines. Re (2): As described above, all of these deposits are in Alaskan-type mafic-ultramafic complexes, whose boundaries define the tracts. Re (3): All of these host mafic-ultramafic bodies have prominent steep-sided aeromagnetic and gravity anomalies associated with them.
Important examples of this type of deposit
Union Bay, Kasaan-Salt Chuck, Duke Island, Snettisham, and Klukwan are all examples of Alaskan PGE deposits.
On the numerical estimates made
Consistent numerical estimates of 0.05 probability of one or more deposits were made for the Union Bay (08CR), Snettisham (02SD), and Turn Mountain (07SD) tracts. These estimates were largely based on the extent of exploration and focused on the Fe and Ti content of the deposits.
Percentile 90 50 10 5 1
Estimated number of deposits 0 0 0 1 2
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.