Explained by William J. Pickthorn and Michael F. Diggles

On the choice of deposit models

Low-sulfide gold-quartz veins (Berger, 1986) are localized in accretionary terranes consisting of belts of low- to moderate-grade regionally metamorphosed marine sedimentary and volcanic rocks, penetratively deformed, and cut by high-angle regional-scale faults.

On the delineation of permissive tracts

This tract was defined principally by the presence of belts of low- to moderate-grade regionally metamorphosed marine sedimentary and volcanic rocks in the Franciscan Complex in the California Coast Ranges. One small district containing low-sulfide Au-quartz vein mines and prospects is reported from the Coast Ranges. Based on the rock type, geologic setting, and the presence of mesothermal gold-bearing vein systems, this tract is considered permissive for undiscovered resources of this deposit type.

Important examples of this type of deposit

Although there are no recognized major low-sulfide Au-quartz vein deposits in the Coast Ranges, small amounts of placer gold are found in almost all streams draining rocks of the Franciscan Complex. In southern Monterey County the Los Burros mining district, which produced approximately 0.16 metric tons of gold from low-sulfide Au-quartz veins, is localized in greenschist facies rocks of the Franciscan Complex that are intruded by serpentine (Hart, 1966). Similar higher-grade metamorphic rocks may be present elsewhere in the Coast Ranges.

On the numerical estimates made

Inasmuch as the largest known deposit is well below the mean size of the model, it seemed appropriate to make a low estimate. For the 90th, 50th, 10th, 5th, and 1st percentiles the team estimated 0, 0, 0, 0, and 1 or more undiscovered 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., 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.

Hart, E.W., 1966, Mines and mineral resources of Monterey County, California: California Division of Mines and Geology County Report 5, 142 p.

Jennings, C.W., 1977, Geologic map of California: California Division of Mines and Geology, scale 750,000.