Rare-Earth Mineral Deposits of the Mountain Pass District, San Bernardino County, California
Published:January 01, 1987
J. C. Olson, D. R. Shawe, L. C. Pray, W. N. Sharp, 1987. "Rare-Earth Mineral Deposits of the Mountain Pass District, San Bernardino County, California", Proterozoic Ore Deposits of the Southwestern U.S., Ed DeWitt
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Bastnaeaite, a rare-earth fluocarbonate, was found in the Mountain Pass district in April 1949. Subsequent geologic mapping has shown that rare-earth mineral deposits occur in a belt about 6 miles long and 1½ miles wide. One of the deposits, the Sulphide Queen carbonate body, is the greatest concentration of rare-earth minerals now known in the world.
The Mountain Pass district is in a block of metamorphic rocks of pre-Cambrian age bounded on the east and south by the alluvium of Ivanpah Valley. This block is separated on the west from sedimentary and volcanic rocks of Paleozoic and Mesozoie age by the Clark Mountain normal fault; the northern boundary of the district is a conspicuous transverse fault. The pre-Cambrian metamorphic complex comprises a great variety of lithologic types including garnetiferous mica gneisses and schists; biotite-garnet-sillimanlte gneiss; hornblende gneiss, schist, and amphibolite; biotite gneiss and schist; granitic gneisses and migmatites; granitic pegmatites; and minor amounts of foliated mafic rocks.
The rare-earth-bearing carbonate rocks are spatially and genetically related to potash-rich igneous rocks of probable pre-Cambrian age that cut the metamorphic complex. The larger potash-rich intrusive masses, 300 or more feet wide, comprise 1 granite, 2 syenite, and 4 composite shonkinite-syenite bodies. One of the shonkinite-syenite stocks is 6,300 feet long. Several hundred relatively thin dikes of these potash-rich rocks range in composition from biotite shonklnite through syenite to granite. Although a few thin fine-grained shonkinite dikes cut the granite, the mafic intrusive bodies are generally the oldest, and granitic rocks the youngest. The potash-rich rocks are intruded by east-trending andesitic dikes and displaced by faults.
Veins of carbonate rock are most abundant in and near the southwest side of the largest shonkinite-syenite body. Most veins are less than 6 feet thick. One mass of carbonate rock near the Sulphide Queen mine is 700 feet in maximum width and 2,400 feet long. About 200 veins have been mapped in the district; their aggregate surface area is probably less than onetenth that of the large carbonate mass.
The carbonate minerals, which make up about 60 percent of the veins and the large carbonate body, are chiefly caleite, dolomite, ankerite, and siderite. The other constituents are barite, bastnaesite, parisite, quartz, and variable small quantities of croeidolite, biotite, phlogoplte, chlorite, muscovite, apatite, hematite, goethite, fluorite, monazlte, galena, allanite, cerite, sphene, pyrite, chaleopyrite, tetrahedrite, malachite, azurite, strontianite, cerussite, wulfenite, aragonite, and thorite. The rare-earth oxide content of much of the carbonate rock is 5 to 15 percent; in some local concentrations of bastnaesite the rare-earth oxide content is as high as 40 percent.
The foliation and inclusions in the Sulphide Queen carbonate body, and the discordant contacts between this body and the gneissic wall rocks, show that the carbonate rock was intruded as a mass into its present position. Radioactive age determinations on monazite from the Sulphide Queen carbonate body indicate a tentative age of about 900 to 1,000 million years (pre-Cambrian), and the potash-rich rocks are at least as old and thus are of pre-Cambrian age. Four tentative determinations of 800 to 900 million years for the age of zircon in shonkinite at the Birthday shaft also indicate the pre-Cambrian an of the potash-rich rocks.
The relation of the carbonate rocks to alkalic igneous rocks is Similar to rock associations found in certain other parts of the world. Because of structural reasons, as well as the pre-Cambrian age of the monazite, the rare-earth-bearing carbonate rock could not have originated as sedimentary limestone or dolomite of Paleozoic age or through assimilation of sedimentary rocks of Paleozoic age by the parent magma of the potash-rich rocks. The carbonate rock might have had a sedimentary origin in the pre-Cambrian gneissic complex as limestone or evaporate, subsequently modified and squeezed into discordance with the foliation of the metamorphic rocks. A magmatic origin of the rare-earth-bearing carbonate rock by differentiation of an alkaline magma from shonkinite to syenite to granite, with a carbonate-rich end-product containing the rare elements, is in harmony with the field relations. This late differentiate might rare been introduced either as a relatively concentrated magmatic fluid, highly charged with volatile constituents such as carbon dioxide, sulfur, and fluorine, or as a dilute hydrothermal fluid.
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Proterozoic Ore Deposits of the Southwestern U.S.
This field conference of the Society of Economic Geologists, held in conjunction with the 1987 Annual Meeting of the Geological Society of America in Phoenix, Arizona, investigates various Proterozoic ore deposits in southeastern California and western and central Arizona. The trip starts in Las Vegas, Nevada, and ends in Phoenix, Arizona. Although Proterozoic ore deposits and geology are emphasized, the field trip traverses a variety of tectonic provinces, each containing ore deposits of various ages. Many geologic features and ore deposits that are not Proterozoic in age will be discussed during the course of the trip, but no stops will be made to investigate any of these, features.
The morning part of the trip starts in the Mesozoic foreland fold and thrust belt of Nevada and southeastern California, goes just east of the Mesozoic batholith belt of California, and ends in crystalline basement containing Early and Middle Proterozoic metamorphic and plutonic rocks. Within this basement is exposed the Middle Proterozoic Mountain Pass Carbonatite deposit, the largest source of rare-earth elements in the world. The Mountain Pass Carbonatite will be the focus of our morning stops.
The afternoon part of the trip crosses Early Proterozoic high-grade metamorphic rocks and plutonic rocks east of Mountain Pass. Ore deposits noted along the afternoon portion of the trip include middle Tertiary epithermal, gold-rich veins at Searchlight, Nevada, and disseminated gold deposits in Tertiary volcanic rocks near Hart Peak in the New York Mountains. The trip continues across the Colorado River trough along the border of California and Arizona, where crystalline basement and its cover of Tertiary volcanic rocks and plutons have been greatly extended along listric normal faults and low-angle detachment faults. The afternoon part of the trip ends in Oatman, Arizona, a district famous for its middle Tertiary epithermal, gold-rich deposits. The ore deposits at Oatman will be the subject of a slide show and talk after supper on the first night of the trip.
The first day ends with an evening drive across a middle Tertiary volcanic pile near Oatman. The first night will be spent in Kingman, Arizona.
Assemble at the Las Vegas Convention Center, south of the Hilton Hotel. Take any main street south to Tropicana Avenue. Turn west on Tropicana, past the Las Vegas Strip, and continue to Interstate Highway 15. Turn south on Interstate 15 toward Mountain Pass, California. Mileage starts at 0.0.