National mineral assessment tract PC08 (Porphyry Cu)

Tract PC08
Geographic region Pacific Coast
Tract area 143,100sq km
Deposit type Porphyry Cu
Deposit age Mesozoic

Deposit model

Model code 17
Model type descriptive
Title Descriptive model of porphyry Cu
Authors Dennis P. Cox


Confidence Number of
90% 0
50% 0
10% 0
5% 1
1% 1

Estimators: DCox, Ludington


Explained by Dennis P. Cox and Roger P. Ashley
On the choice of deposit models
The Lights Creek porphyry copper deposit (Storey, 1978) is located in the northernmost part of the Sierra Nevada, in the Plumas County copper belt of Knopf (1935). Lights Creek consists of two mineralized zones 3 km apart. It differs from typical porphyry copper systems in that magnetite is more abundant than pyrite and chlorite is the main alteration mineral. Zoned potassic and phyllic alteration typical of porphyry copper deposits is indistinct. The copper belt includes the Engels and Superior copper vein deposits near Lights Creek (Anderson, 1931) and the Walker vein deposit 20 km to the southeast. The vein deposits are large compared to polymetallic vein median tonnage, and contain magnetite, tourmaline and actinolite in addition to chalcopyrite, bornite and other sulfides. Deposits in the Plumas County copper belt are all related to stocks of gabbroic to granodioritic composition that are older than the major batholiths in the northern Sierra Nevada.
The abundance of magnetite in these deposits suggests an affinity with porphyry copper-gold systems, but no gold grades for the deposits are available. The general porphyry copper grade and tonnage model (Singer and others, 1986) was used in the assessment.
On the delineation of permissive tracts
The permissive tract includes all the major plutons of the Sierra Nevada, Salinian block, and Klamath Mountains. Plutons permissive for porphyry copper deposits are believed to be emplaced at shallow levels in the crust, but because we have no way to distinguish this environment on a regional scale, the entire tract is considered permissive. Despite the apparent scarcity of pluton-related deposits in the Klamath Mountains, this part of the tract is considered permissive for several types of pluton-related deposits. Of these, polymetallic veins might be indicators of concealed porphyry copper systems. The only vein deposits that clearly fit the polymetallic vein model are quartz veins rich in silver and base metal sulfides in the South Fork district, located in the Shasta Bally pluton at the southern edge of the Klamath Mountains (Silberman and Danielson, 1993). Some occurrences in the Gold Hill, Ashland, and Applegate districts of Oregon may also be polymetallic veins.
Important examples of this type of deposit
The Lights Creek bodies contain 315 million metric tons of mineralized rock that average 0.34 percent copper. The Engels and Superior veins together produced about four million metric tons of ore averaging 1.79 percent copper (Storey, 1978). In the Klamath Mountains, the most important polymetallic vein deposit is the Silver Falls-Chicago Consolidated mine in the South Fork district, which produced $1,000,000 worth of metal, mainly silver (Hotz, 1971).
On the numerical estimates made
Because of the scarcity and restricted extent of porphyry copper environments in the Sierra Nevada and Klamath Mountains, a low estimate of undiscovered deposits was made. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 1, and 1 or more porphyry copper deposits consistent with the grade and tonnage model of Singer and others (1986) (Mark3 index 4). Most of the undiscovered resource is believed to be in the northern Sierra Nevada.
Anderson, C.A., 1931, Geology of the Engels and Superior mines, Plumas County, California (with a note on the ore deposits of the Superior mine): California University Geological Sciences Bulletin, v. 20, no. 8, p. 293-330.
Hotz, P.E., 1971, Geology of lode gold districts in the Klamath Mountains, California and Oregon: U.S. Geological Survey Bulletin 1290, 91 p.
Knopf, Adolph, 1935, The Plumas County copper belt, California, in Copper resources of the world: 16th International Geologic Congress, Washington D.C., v. 1, p. 241-245.
Silberman, M.L., and Danielson, Joanne, 1993, Gold-bearing quartz veins in the Klamath Mountains in the Redding 1o x 2o degree quadrangle, northern California: California Geology, v. 46, p. 35-44.
Singer, D.A., Mosier, D.L., and Cox, D.P., 1986, Grade-tonnage model of porphyry copper, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 77-81.
Storey, L.O., 1978, Geology and mineralization of the Lights Creek stock, Plumas County, California in Proceedings of the porphyry copper symposium: Arizona Geological Society Digest, v. XI, p. 49-58.

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