Mountain Pass Mine

The Mountain Pass Mine lies at the summit of Mountain Pass near the center of the Ivanpah Mining District. It is operated by the MolyCorp Division of Union Oil Corporation. Access to the mine site is restricted.

The rare earth carbonatite complex at Mountain Pass was discovered almost by accident when two prospectors using a borrowed a Geiger counter staked a series of claims on a radioactive outcrop they thought might have some potential as a uranium resource. Samples were sent to the U.S. Geological Survey for analysis. The “ore” was identified as the rare earth fluoro¬carbonate bastnaesite. Imagine the prospectors disappointment when they realized their ore was virtually worthless. The discovery, however, attracted the attention of the U.S.G.S. They undertook a detailed field survey of the immediate area (Olsen and Pray, 1954). A much larger, non-radioactive deposit of bastnaesite was found on adjoining land. One of the two original prospectors, a metallurgist for MolyCorp, urged the company to claim the land. MolyCorp thus became the owner of a large, essentially worthless, rare earth carbonate complex. MolyCorp spent the next two decades developing a market for rare earth elements.

Some of the elements present and uses for those elements include:

• Cerium used in ultraviolet absorbing glass and lighter flints.
• Lanthanum, samarium and gadolinium used for infrared absorption in glass, improving the refractive index of glass and microwave oven temperature controls.
• Neodymium used to absorb ultraviolet light and with cerium to decolor glass.
• Praseodymium used as a coloring agent in glass when the index of refraction must not change.
• Europium used extensively for the red phosphor in television tubes.

Stratigraphy

Geologic Map of Pine Mountain Area
Geologic Map of Pine Mountain Area

The Mountain Pass Mine lies within a northwest trending block of Precambrian basement bounded on three sides by faults and on the fourth by alluvium of the Ivanpah Valley. The Precambrian host rock consists of a series of gneisses and schists, strongly foliated and sheared. The earliest phases, hornblende schist and biotite gneiss were subsequently intruded by granite (granite gneiss). These rocks were in turn cut by pegmatitic dikes (gneissic pegmatite). The entire sequence was metamorphosed at some time prior to the intrusion of the carbonatite complex about 1.4 BP.

The carbonatite complex consists of a total of eight plugs of alkali igneous rocks ranging in composition from shonkinite to carbonatite. The plugs are all elongated in a northwest direction and dip about 50 to the southwest. They range from 100 to 2000 meters in length and appear to lie unevenly spaced along two nearly parallel northwest trending rows. Associated with the plugs are 200 dikes of carbonatite which also trend northwest.

Phanerozoic rocks to the west of Mountain Pass consist of a nearly complete sequence of marine sedimentary strata from Cambrian through Triassic. Southwest of the district Triassic¬Jurassic sediments are intruded by Jurassic/Cretaceous quartz monzonite (Teutonia batholith) and locally overlain by Delfonte volcanics of uncertain age (Jurassic ?)
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Structure

The Keaney-Mollusk Mine thrust to the west of the mine juxtaposes Cambrian Bonanza King (to the west) and Precambrian basement. The plane of the thrust is clearly visible as the break in slope near the microwave tower at the east end of Mohawk Hill. The thrust strikes north-northwest and dips gently to the west. Burchfiel and Davis (1988) describe the thrust as a decollement type thrust which in the vicinity of Mountain Pass has resulted in the “younger¬over-older” Cambrian Bonanza King above Cambrian Tapeats, Bright Angel and locally, Precambrian basement. South of I¬15, and the South Fault, the more typical thrust sequence of “older¬over-younger” Bonanza King over Mesozoic carbonates, clastics and volcanics can be seen in the Mescal Range.

The South fault to the south¬southwest of Mountain Pass is a northwest striking high angle normal fault of debatable age. Footwall rocks are Precambrian basement while the hanging wall comprises a nearly complete Paleozoic¬Mesozoic sequence. Presumably, it is the Clark Mountain fault of Hewett (1956) and as such can be interpreted as of Cenozoic age. However, Burchfiel and Davis (1988) state that movement along the South fault prior to Mesozoic thrusting was responsible for uplifting the Precambrian block to the northeast . This interpretation places an early to mid Mesozoic age on the fault.

The North fault to the north¬northeast of the mine is poorly defined. Much of this fault lies beneath alluvium and only the geology staff at MolyCorp has accurate maps of the fault. Based on the mapping of Olsen and Pray (1954), the fault has been interpreted as a high angle normal fault. Footwall and hanging wall rocks are presumably Precambrian basement at Mountain Pass, but if the fault continues to the west beneath the Mohawk valley Paleozoic rocks may be involved in the faulting.

Mine Geology

Within the mine area three major “older” metamorphic rock types have been recognized; biotite granite gneiss, granitic augen gneiss and sillimanite-biotite-garnet gneiss. These represent only the dominant lithologies in a highly variable sequence of gneisses, schists and minor marble. Olsen and Pray (1954) suggest that the volcanic and sedimentary protoliths of the sillimanite-biotite-garnet gneiss were intruded by bioite granite producing a migmatite, later deformed to biotite granite gneiss. Subsequently, the complex was intruded by synkinematic leucogranite and mafic dikes. Regional metamorphism culminated at about 1.7 BP.

The carbonatite complex consists of eight alkali plugs ranging in composition from shonkinite to carbonatite (Woyski,1980). The plugs transect the metamorphic foliation, but are themselves undeformed. Shonkinite was the first alkali phase to be intruded, followed by syenite, granite, fine grained shonkinite dikes and finally carbonatite. Shonkinite is defined as a mafic, alkali rock composed of greater than 50% mafic minerals. At Mountain Pass shonkinite characteristically contains 25¬40% biotite, 5¬40% sodic pyroxene and 40¬50% microcline (Woyski, 1980). Apatite is a common and often abundant accessory. The early shonkinite phase is typically medium to coarse grained, while later dikes are fine grained. Granite and syenite have essentially similar mineralogies to shonkinite, lacking however, the high percentage of mafic phases. Granite and syenite are differentiated on the basis of quartz content, the latter with less than 5% quartz. Typically, both are coarse grained and occasionally porphyritic, the phenocrysts aegerine, biotite or phlogopite.
South of the mine a nearly complete sequence of Paleozoic¬Lower Mesozoic sedimentary rocks lies within the footwall block of the South fault. Striking, cross¬bedded Aztec Sandstone caps the sedimentary section. Delfonte volcanics of probable Jurassic age cut the sedimentary units.

Ore Mineralization and Alteration

Carbonatite is an intrusive igneous rock composed predominantly of calcite and/or dolomite. At Mountain Pass the main ore body occurs as a large plug and smaller peripheral dikes within the sheared gneissic host. Warhol (1980) states that the ore body has a strike length of 750 meters, thickness of 75 meters and dips 40 to the west. The ore consists of 60% carbonate (calcite, dolomite, siderite and ankerite), 20% sulfate (barite and celestite), 10% bastnaesite (rare earth fluoro¬carbonate) and 10% silicate phases (chiefly quartz).

Bastnaesite is the principle ore mineral, but minor monazite and perhaps apatite are also recovered in the milling process. The following (from Warhol, 1980) is the best approximation of the rare earth elemental breakdown in the ore:

• Cerium 50.0%
• Lanthanum 34.0%
• Neodymium 11.0%
• Praseodymium 4.0%
• Samarium 0.5%
• Gadolinium 0.2%
• Europium 0.1%
• Others 0.2%

Alteration consists of fenitization and minor supergene hemitization. Fenitization is an alkali, metasomatic alteration characterized by secondary K¬spar as well as sodic amphiboles and pyroxenes. The net result is to alter the host to a rock resembling a syenite. Thus, it often becomes difficult to distinguish the alkali intrusives from metasomatically altered gneiss. Hematite is a common mineral on the dumps where it occupies fractures and coats rocks. Its relationship to the other minerals suggests it is of supergene origin. Large botryoidal masses of calcite filling open spaces in the carbonatite complex are also of secondary origin.

REFERENCES CITED

Burchfiel, B.C., and Davis, G.A., 1988, Mesozoic thrust faults and Cenozoic low-angle normal faults, eastern Spring Mountains Nevada, and Clark Mountains thrust complex, California in This Extended Land: Geological journeys in the southern Basin and Range, Field Trip Guidebook, Geological Society of America, Western Cordilleran Section, Las Vegas, Nevada, p. 87-10.

Hewett, D.F., 1956, Geology and mineral resources of the Ivanpah quadrangle, California and Nevada: U.S. Geol. Survey Professional Paper 275, 172 p.

Olsen, J.C., and Pray, L.C., 1954, The Mountain Pass rare earth deposits, in The Geology of Southern California, Calif. Bureau of Mines and Geology Bull. 170, Chap VIII, p. 23-29.

Warhol, Warren N., 1980, Molycorp’s Mountain Pass Operations, in Geology and mineral wealth of the California Desert, Fife, D.L. and Brown A.R., Eds., South Coast Geological Society, p. 359-366.

Woyski, Margaret S., 1980, Petrology of the Mountain Pass carbonatite complex – A review, in Geology and mineral wealth of the California Desert, Fife, D.L. and Brown A.R., Eds., South Coast Geological Society, p. 367-378.