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
Small placer Au-PGE deposits have been exploited throughout southeastern Alaska, particularly in association with low-sulfide- gold-quartz vein districts, but the Porcupine Creek area is the only one that continues to be of interest.
On the delineation of permissive tracts
This tract includes the Porcupine Creek drainage basin and is defined by the distribution of deposits and occurrences, and the distribution of extensive recent stream-gravel deposits (Brew and others, 1991). Because of both steep topography and the scouring effects of the Pleistocene ice sheets and glaciers, other parts of southeastern Alaska generally do not have large accumulations of gravels in proximity to lode gold deposits and past placer operations have been generally small and short-lived.
Important examples of this type of deposit
Porcupine Creek and McKinley Creek are examples of placer gold deposits in this tract.
On the numerical estimates made
Permissive sediment types and the occurrence of known placer deposits were the main basis for the estimate. The estimated minimum number of placer gold deposits, consistent with the grade and tonnage model of Orris and Bliss (1986), expected to occur in this tract is:
Percentile 90 50 10 5 1
Estimated number of deposits 0 0 0 0 1
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.
Orris, G.J., and Bliss, J.D., 1986, Grade and tonnage model of placer Au-PGE, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 261-264.