National mineral assessment tract PC40 (Low-sulfide Au-quartz vein)

Tract PC40
Geographic region Pacific Coast
Tract area 103,300sq km
Deposit type Low-sulfide Au-quartz vein
Deposit age Mesozoic

Deposit model

Model code 36a
Model type descriptive
Title Descriptive model of low-sulfide Au-quartz veins
Authors Byron R. Berger
URL https://pubs.usgs.gov/bul/b1693/html/bull8x2r.htm
Source https://pubs.er.usgs.gov/publication/b1693

Estimates

Confidence Number of
deposits
90% 2
50% 6
10% 9
5% 12
1% 15

Estimators: DCox, Albino, Church, Ashley, Diggles, Peterson

Rationale

Explained by William J. Pickthorn and Michael F. Diggles
On the choice of deposit models
Gold-bearing mesothermal quartz veins are localized along major deep-seated through-going structural features in low- to moderate-grade marine metasedimentary and metavolcanic rocks. Many of the type examples used by Berger (1986) in the low-sulfide Au-quartz descriptive model are in the Pacific Coast region.
On the delineation of permissive tracts
The permissive tract was defined principally by the location of low- to moderate-grade regionally metamorphosed marine sedimentary and volcanic rocks of Jurassic and older age, based on the State geologic maps for California and Oregon (Jennings, 1977; Walker and MacLeod, 1991) and the personal knowledge of the assessors. Geophysical evidence was used to extend the tracts into areas of valley fill or thin Quaternary, Tertiary, or Cretaceous cover. In California and Oregon most of the tract contains known gold deposits and includes those deposits in the famed California Mother Lode and the Ashland district in southern Oregon. In southern California, metavolcanic and metasedimentary rocks that enclose, or occur as roof pendants in the major batholiths were also considered permissive. These rocks host small low-sulfide Au-quartz veins in the Julian-Banner district.
Important examples of this type of deposit
The low-sulfide Au-quartz grade and tonnage model is based on deposits containing 100 metric tons or more of gold (Bliss, 1986). Over 50 percent of the deposits in the worldwide model (Bliss and Jones, 1988) are located within this tract and include those of the California Mother Lode and the Grass Valley district in the Sierra Nevada foothills, and deposits in the Klamath, Siskiyou, and Trinity Mountains in northern California and southern Oregon. Several major mines in the tract are currently or have been recently active. Most notable are the Harvard mine near Jamestown and the Royal Mountain King mine near Copperopolis, both in the California Mother Lode. Reserves at the Harvard mine are estimated to be in excess of 100 metric tons of gold (Bliss and Jones, 1988).
On the numerical estimates made
The team concluded that nearly all of the deposits that are exposed at the surface have been discovered. The part of the tract most favorable for undiscovered deposits is that part that is covered by less than 1 km of Cretaceous sedimentary and Tertiary volcanic rocks between the Klamath Mountains and the Sierra Nevada. Estimates for this area were made using deposit density data. The shallow-covered area extending southeast from the Klamath Mountains has an area of 490 km2. The shallow-covered area extending northwest from the Sierra is 1,700 km2. The density of distribution of low-sulfide Au-quartz vein deposits in the Klamath Mountains is 4.0 deposits per 1,000 km2; for the Sierra Nevada, the density is 4.6 according to Bliss and others (1987). Inasmuch as the concentration of deposits tends to drop off with distance from the zones of greatest mineralization, we expect that the density of deposits in the areas beyond the known deposits is half that given by Bliss. These densities (2 and 2.3 per 1,000 km2) multiplied by the areas of covered tract results in an expected value of about one deposit for the Klamath Mountains and about four for the Sierra Nevada.
In the exposed part of the tract, the team estimated only about one undiscovered deposit could exist giving a total expected value of about six deposits. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 2, 6, 9, 12, and 15 or more deposits consistent with the grade and tonnage model of Bliss (1986).
References
Berger, B.R., 1986, Descriptive 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.
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., Menzie, W.D., Orris, G.J., and Page, N.J, 1987, Mineral deposit density—A useful tool for mineral-resource assessment, in Sachs, J.S., ed., U.S. Geological Survey Research on Mineral Resources—1987 program and abstracts: U.S. Geological Survey Circular 995, p. 6.
Bliss, J.D., and Jones, G.M., 1988, Mineralogic and grade-tonnage information on low-sulfide Au-quartz veins: U.S. Geological Survey Open-File Report 88-0229, 99 p.
Clark, W.B., 1976, Gold districts of California: California Division of Mines and Geology Bulletin 193, 186 p.
Jennings, C.W., 1977, Geologic map of California: California Division of Mines and Geology, scale 1:750,000.
Walker, G.W., and MacLeod, N.S., 1991, Geologic map of Oregon: U.S. Geological Survey, 2 sheets, scale 1:500,000.

Geographic coverage

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