Explained by Frederic H. Wilson, Stanley E. Church, Richard W. Saltus, Lawrence J. Drew, and W. David Menzie.
On the choice of deposit models
Southeast of the Border Ranges Fault on Kodiak Island, the Chugach terrane, composed of the Kodiak and Shumagin Formations, is permissive for Chugach-type low-sulfide Au-quartz veins (Bliss, 1986, 1992), because such deposits are known in the tract, and because they also occur in the correlative Valdez Group of south-central Alaska.
On the delineation of permissive tracts
This tract consists of Upper Cretaceous graywacke and mudstone of the Kodiak and Shumagin Formations and lower Tertiary plutons on Kodiak, Semidi, Shumagin, and Sanak Islands. The rock units are correlative with the Valdez Group in Prince William Sound, although generally of lower metamorphic grade.
Important examples of this type of deposit
Gold has been reported from several small occurrences, such as at the Amok Gold Mine (Capps, 1937), where a 1-m-thick gold-bearing quartz vein was mined in the early part of the 20th century.
On the numerical estimates made
Appropriate host rocks and a number of known occurrences suggest that additional deposits of this type occur within the tract. The low level of geologic knowledge of the region and the small size of the these deposits, however, contributes to a high level of uncertainty in the estimates.
The minimum number of undiscovered low-sulfide Au-quartz vein deposits, consistent with the tonnage and grade model of Bliss (1992), estimated for this tract is:
Percentile 90 50 10 5 1
Estimated no. of deposits 4 10 30 30 30
Bliss, J.D., 1986, Grade and tonnage model of low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239-243.
Bliss, J.D., 1992, Grade and tonnage model of Chugach-type low-sulfide Au-quartz veins, in, Bliss, J.D., editor, Developments in mineral deposit modeling: U.S. Geological Survey Bulletin 2004, p. 44-46.
Capps, S.R., 1937, Kodiak and adjacent islands: U.S. Geological Survey Bulletin 880‑C, p. 111‑184.