Rare-Earth Mineral Deposits of the Mountain Pass District, San Bernardino County, California.

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 iy2 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 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 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 296635—54———2 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