Olson, and 3 co-authors, mapped bastnaesite, 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 1 and a half 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 Mesozoic 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-sillimanite 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 preCambrian 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 shonkinite 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 one tenth 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 calcite, dolomite, ankerite, and siderite. The other constituents are barite, bastnaesite, parisite, quartz, and variable small quantities of crocidolite, biotite, phlogopite, chlorite, muscovite, apatite, hematite, goethite, fluorite, monazite, galena, allanite, cerite, sphene, pyrite, chalcopyrite, 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 age 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 preCambrian 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 evaporite, 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 have 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.
INTRODUCTION
In April 1949 a vein containing considerable quantities of bastnaesite, a fluocarbonate of rare-earth metals of the cerium group, was discovered in the vicinity of Mountain Pass in eastern San Bernardino County, Calif, (fig. 1). Small quantities of this rare mineral have been noted at several localities in the
United States, but this deposit was the first at which the mineral was sufficiently abundant to offer promise as a commercial source of rare-earth metals. The district is now known to contain a large supply of the rare-earth elements, and barium, strontium, thorium, and minor quantities of other metals.
The rare-earth metals are used primarily in the form of an alloy called misch metal, a mixture of several rare-earth metals of the cerium group. The principal current uses of the metals depend upon their pyrophoric properties. The rare earths are used in arc lamps, tracer bullets, in the flints of pocket lighters and carbide lamps, in the manufacture of gas mantles, and as deoxidizing agents in metallurgy. They have shown promise in alloys of light metals and in steel.
Chemical compounds of rare earths have a wide variety of uses in industry. Several of the rare earths are excellent absorbers of slow neutrons, and their poisoning effect on the action of nuclear-energy piles has stimulated intensive study of the metals by the Atomic Energy Commission. The rare earths have largely been obtained from monazite placer deposits in various parts of the world, chiefly from beach sands in India and Brazil, but only small quantities of monazite have been produced in this country.
Bastnaesite had been known previously from only about 10 localities in the world. A little bastnaesite was mined in the late nineteenth century at Bastnas, Sweden, and small amounts have been obtained from placer deposits in Belgian Congo and from deposits in central New Mexico. During the period of 1949-51, exploration was intensive but only a few tons of bastnaesite ore were mined at Mountain Pass for metallurgical tests. Many pits and bulldozer excavations, commonly 5 to 10 feet deep, have been made in extensive search for rare-earth minerals. Only two mine workings in the district, the old Sulphide Queen gold mine and the Birthday shaft made in bastnaesite exploration, are as much as 100 feet deep. Present activity (1952) is largely confined to quarrying and drilling at the Sulphide Queen deposit.
LOCATION AND SURFACE FEATURES
The Mountain Pass district is an area about 7 miles long and 3 miles wide centered near Mountain Pass, 60 miles southwest of Las Vegas on U. S. Highway 91, near the northeast corner of San Bernardino County, Calif, (fig. 1). Most of the district is readily accessible from the main highway by dirt roads. The district is 16 miles by paved highway west of Nipton, Calif., a station on the Union Pacific Railroad. Altitudes range from about 4,000 feet at Wheaton Springs to slightly more than 6,000 feet on Kokoweef Peak and the Mescal Eange. Clark Mountain, just northwest of the district, dominates the terrain; its altitude is 7,903 feet. The surface east of Wheaton Springs slopes within a few miles to the floor of Ivanpah Valley, about 2,595 feet in altitude. Mountain Pass, which has an altitude of 4,730 feet, is the highest point on Highway 91 between Las Vegas and Los Angeles.
The district is in part a rolling upland about 5,000 feet in altitude, which is part of a mature surface of moderate relief that has been referred to as part of the Ivanpah upland (D. F. Hewett, 1950, personal communication) . Several peaks and ridges in the district rise about 1,000 feet above this surface. The eastern part of the district is chiefly a rugged, dissected terrain between the Ivanpah upland and the slope into the wide, alluviated Ivanpah Valley. Eock exposures generally are good. The central part of the district, near Mountain Pass, is partly obscured by gravels and alluvium. The gravels are several hundred feet thick in places and have been dissected so that ridges several hundred feet high are composed entirely of gravel deposits. The gravels contain blocks and fragments of the pre-Cambrian gneiss, Paleozoic limestone, dolomite, and quartzitic sandstone, Mesozoic sandstone and volcanic rocks, and igneous rocks, all in various proportions depending upon their distances from the sources of these materials.
The climate is arid. The district is dotted with juniper and Joshua trees, and pinon are found near the north end of the district and on Clark Mountain.
Water is scarce, rainfall is scant, and the principal sources of water are the springs. Waring (1919, p. 76-77) gives data on the water at Mexican Well and at
Mescal, Roseberry, Mineral, and Wheaton Springs in the Mountain Pass district.
FIELD WORK AND ACKNOWLEDGMENTS
The Ivanpah l°-quadrangle, which includes Mountain Pass near its center, was mapped by D. F. Hewett during the period 1924 to 1929. After the discovery in April 1949 of radioactive deposits and bastnaesite, the area of about 900 by 1,500 feet around the discovery (pi. 10), including the Birthday shaft (pi. 11), was mapped at 50 feet to the inch by W. N. Sharp and L. C. Pray between November 15,1949, and January 20,1950 (Sharp and Pray, 1952).
In support of the report, 13 Plates are available: https://pubs.usgs.gov/pp/0261/
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