Jurassic Arc

Region West, Southwest
Mineral systems
Deposit types
Other minerals

Information leading to the delineation of this focus area

Basis for focus area Proposed area includes numerous mining districts in Plumas County, California, and Lyon and Mineral counties, Nevada. No REE resources have been identified. Some mined iron and copper (gold, silver) deposits in Jurassic igneous rocks contain minor amounts of apatite, some of which contains minor amounts of cesium and lanthanum. However, apatite concentrations and compositions have not been published for most deposits, concentrations of other REE have not been published or determined, and other REE minerals have not been recognized. The polygon was drawn to encircle the igneous rocks associated with the Jurassic Arc.
Identified resources Inferred resources and historical production of copper, iron, gold, and silver.
Production California (Lights Creek district, Plumas County): Engels and Superior mines produced 161.5 million lbs Cu, 23,000 oz Au, and 1.9 million oz Ag between 1914 and 1930 (Ghaffari and others, 2018).
Status Past mining. Current (2020) copper exploration by US Copper at Moonlight/Superior/Engels district (Plumas County, California); current mining at the Pumpkin Hollow mine (Lyon County, Nevada).
Estimated resources Pumpkin Hollow deposit (Lyon County, Nevada): Measured and Indicated Fe resource of 209 million tons averaging 37.3% Fe, Inferred resource of 152 million tons averaging 33.83% Fe; Measured and Indicated Cu resource of 30 million tons averaging 1.83% Cu, Inferred 9 million tons of 1.97% Cu; Measured and Indicated Au resource of 0.98 million oz Au and 34 million oz Ag, Inferred 0.47 million oz Au and 21 million oz Ag (Rozelle, 2010). Moonlight Valley deposit (Lights Creek District, Plumas County, California): Indicated resource 252,000,000 short tons, 0.25% Cu, Inferred 109,000,000 short tons, 0.24% Cu (Ghaffari and others, 2018).
Geologic maps Lawrence (1969), scale ~ 1:2,000; Proffett and Dilles (1984), scale 1:24,000; Storey (1978), scale ~1:3000; Moore (1971), scale unknown.
Geophysical data Partly adequate (25%) aeromagnetic coverage. Inadequate aeroradiometric coverage. Small-scale magnetic maps in Lawrence (1969) and Moore (1971). Rank 1 aeromagnetic and aeroradiometric coverage through GeoDAWN in progress.
Favorable rocks and structures The geological environment reflects classic copper skarns in the Basin and Range. Mineralization is associated with I-type, magnetite series, calc-alkaline quartz diorite granodioritic stocks of hypabyssal character. Stock contacts have assemblages of andraditic garnet with diopsidic pyroxene, magnetite with hematite, and moderate to high sulfide contents (pyrite, chalcopyrite, minor tennantite and sphalerite). Large copper skarns are generally associated with altered and mineralized porphyry copper stocks and display extensive retrograde alterations (Rozelle, 2010).
Deposits Nevada: Pumpkin Hollow (MRDS dep_id: 10174173); California: Engels mine (MRDS dep_id: 10034127), Superior mine (MRDS dep_id: 10109418), Moonlight Valley deposit.
Evidence from mineral occurrences MRDS; Rozelle (2010); Ghaffari and others (2018).
Geochemical evidence Smith (1970) reported Zn, As, Sb, Bi, and Sn concentrations in the host plutons of the Plumas copper belt of California. Higher concentrations of Bi and As corresponded to higher Cu concentrations.
Geophysical evidence Unknown.
Evidence from other sources Smith (1970).
Comments California: Dilles and Stephens (2011) suggest Plumas Cu belt deposits display alteration most compatible with IOCG classification but the hydrothermal system may include both magmatic and non-magmatic sources of ore fluids, like Yerington. Porphyry dikes, tourmaline-sulfide veins, and sericitic alteration characterize porphyry Cu associations of a younger and Late Jurassic age. Nevada: Ghaffari and others (2018) state: "Granodiorite to diorite rocks belonging to the Jurassic Yerington batholith intrude the limestones of the Triassic Mason Valley Formation and calcareous argillites and siliceous shales, siltstones and limestones of the Gardnerville Formation. Associated with this intrusive episode is the development of large areas of Iron oxide-copper-gold (IOCG) mineralization dominantly skarn with associated copper and magnetite mineralization with varying levels of gold and silver. The skarn occurs primarily in the middle to lower portion of the Gardnerville Formation and the upper part of the Mason Valley Formation as well as within the intrusive granitoid itself.
Cover thickness and description Variable from place to place.
Authors Peter G. Vikre, Drew L. Siler, Erica Key.
New data needs Geological mapping, geochemical sampling.
Geologic mapping and modeling needs Updated geologic mapping.
Geophysical survey and modeling needs High-resolution, Rank 1 aeromagnetic and aeroradiometric surveys in Nevada and part of California through GeoDAWN project.
Digital elevation data needs Lidar collection in Nevada and part of California through GeoDAWN project (in progress).