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 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 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,

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Alkalic igneous rocks and related concentrations of thorium, niobium, rare-earth elements, titanium, and other elements have long been known in the Powderhorn mining district and have been explored intermittently for several decades. The deposits formed chiefly about 570 m.y. (million years) ago in latest Precambrian or Early Cambrian time. They were emplaced in lower Proterozoic (Proterozoic X) metasedimentary, metavolcanic, and plutonic rocks. The complex of alkalic rocks of Iron Hill occupies 31 km2 (square kilometers) and is composed of pyroxenite, uncompahgrite, ijolite, nepheline syenite, and carbonatite, in order of generally decreasing age. Fenite occurs in a zone, in places more than 0.6 km (kilometer) wide, around a large part of the margin of the complex and adjacent to alkalic dikes intruding Precambrian host rock. The alkalic rocks have a radioactivity, chiefly due to thorium, greater than that of the surrounding Powderhorn Granite (Proterozoic X) and metamorphic rocks. The pyroxenite, uncompahgrite, ijolite, and nepheline syenite, which form more than 80 percent of the complex, have fair ly uniform radioactivity. Radioactivity in the carbonatite stock, car bonatite dikes, and the carbonatite-pyroxenite mixed rock zone, however, generally exceeds that in the other rocks of the complex. The thorium concentrations in the Powderhorn district occur in six types of deposits: thorite veins, a large massive carbonatite body, carbonatite dikes, trachyte dikes, magnetite-ilmeniteperovskite dikes or segregations, and disseminations in small, anomalously radioactive plutons chiefly of granite or quartz syenite that are older than rocks of the alkalic complex. The highest grade thorium concentrations in the district are in veins that commonly occur in steeply dipping, crosscutting shear or breccia zones in the Precambrian rocks. They range in thickness from a centimeter or less to 5 m (meters) and are as much as 1 km long. The thorite veins are composed chiefly of potassic feldspar, white to smoky quartz, calcite, barite, goethite, and hematite, and also contain thorite, jasper, magnetite, pyrite, galena, chalcopyrite, sphalerite, synchysite, apatite, fluorite, biotite, sodic amphibole, rutile, monazite, bastnaesite, and vanadinite. The ThO2 content of the thorite veins ranges from less than 0.01 percent to as much as 4.9 percent in high-grade samples. The ThO2 content is generally less than 1 percent, however, and is only 0.05 to 0.1 percent in many of the veins examined in the district. Samples of the dolomitic carbonatite of Iron Hill mostly range from 3 to 145 ppm (parts per million) thorium. Thirty samples of the

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