Mountain Pass - Sulphide Queen

California, USA

Main commodities: REE
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The Mountain Pass barium-rare earth element deposit is located 1.5 km north of the town of Mountain Pass in California, USA, 80 and 20 km SW of Las Vegas and the Nevada border respectively (#Location: 35° 28' 42"N, 115° 31' 56"W).

The deposit was discovered in the early 1950's and has produced rare earth elements since 1954. From the 1960's to the mid-1990's it was the world's largest source of light rare earth elements. After 1998, production from Mountain Pass declined substantially due to environmental constraints and international competition, mostly from China. Mining and rare earth separation ceased by 2002, although extraction of selected REE commodities from stockpiles resumed in 2007 (Castor, 2008). The deposit was owned and operated by Chevron Mining Inc. (originally under the name of Molycorp Inc.).

The Mountain Pass deposit is located within the Palaeoproterozoic Mojave Crustal Province of western Arizona and Nevada and southeastern California. It is one of a number of orogenic provinces that involved newly formed continental crust added to southern Laurentia Craton between ~1.8 and 1.6 Ga. This new episode of crustal accumulation formed ~20% of the North American craton and stretches from southern California to Nova Scotia along a NE trend (Strickland et al., 2013). This crust was added as a series of crustal provinces and terranes that generally young to the south and SE with increasing distance from the Archaean Wyoming and Superior cratons (e.g. Condie, 1982; Bennett and DePaolo, 1987; Karlstrom and Bowring, 1988; Amato et al., 2008). Much of this crust is interpreted as juvenile island arcs and marginal basins formed with limited involvement of pre-existing continental crust (e.g., Condie, 1982; Bennett and DePaolo, 1987; Karlstrom and Bowring, 1988; Wooden and DeWitt, 1991). In contrast, Mojave Province rocks have yielded Nd ages of 2.0 to 2.3 Ga, significantly more radiogenic Pb isotopic compositions, Archaean plutonic and detrital zircon ages and higher Th/U ratios. These data suggest the Mojave Province, unlike neighbouring Palaeoproterozoic crust, such as the adjacent Yavapai Province, likely incorporated a significant amount of older, pre-existing crustal material during its formation from ~1.79 to 1.74 Ga (Bennett and DePaolo, 1987; Wooden and Miller, 1990; Barth et al., 2000; Duebendorfer et al., 2010).

The Mojave Province has been exhumed from mid-crustal depths during Cenozoic tectonic events. It is bounded to the north by the Archaean Wyoming Craton and associated Archaean to Palaeoproterozoic terranes (e.g., the 2.4 to 1.6 Ga Selway Terrane and >2.5 Ga Grouse Creek Block). It is bounded to the SE by the Yavapai Terrane of the Central Plains Orogen, and tapers before terminating to the NE as the northern margin of Yavapai Terrane converges with and abuts the Wyoming Craton.

The oldest exposed unit in the Mojave Province is the 1.84 Ga Elves Chasm tonalite gneiss exposed through a younger window, and interpreted to have intruded possible earlier 1.9 to 2.3 Ga supracrustal sequences. The oldest regionally exposed rocks are metasedimentary gneisses deposited between ∼1.79 and 1.75 Ga (Thomas et al., 1988; Wooden and Miller, 1990). A 1.79 to 1.76 Ga, pre-orogenic magmatic suite which includes gabbro, tonalite, trondhjemite and porphyritic granite is also widespread. These rocks have been subjected to metamorphism of from upper amphibolite to lower granulite facies and migmatites, locally estimated to have reached 675 to ∼740°C and 5 to 6 kb (Thomas et al., 1988; Wooden and Miller, 1990; Barth et al., 2000). A phase of regional metamorphism has been dated at ~1.74 Ga. Magmatism of this age is common in the adjacent Yavapai province, suggesting that the two terranes may have been juxtaposed at or before 1.74 Ga. A further major period of deformation occurred between 1.71 and 1.69 Ga in both the Mojave and adjacent Yavapai provinces (Barth et al., 2000; Duebendorfer et al., 2001; Whitmeyer and Karlstrom, 2007). All of these are cut by postorogenic intrusions at 1.63 to 1.69 Ga (Wooden and Miller 1990). Extensive Mesoproterozoic granite are exposed in SW Nevada, to the east of Mountain Pass (Volborth, 1962; Bingler and Bonham, 1973) and to the SW in California (Kwok, 1987; Anderson and Bender, 1989) with ages of 1.4 to 1.5 Ga (Castor, 2008).

The Mountain Pass deposit is closely associated with the Mountain Pass (or Sulphide Queen) Carbonatite and alkaline intrusions that were emplaced within a suite of Palaeoproterozoic rocks that comprise complexly folded, banded, granulite-facies gneiss and schist with variable quartz, microcline, biotite, garnet, hornblende, orthopyroxene and sillimanite (Castor, 2008). These gneisses include biotite-garnet-sillimanite-hornblende gneisses, biotite granitic gneisses and augen granitic gneisses. Large masses of weakly foliated granitic rocks are also exposed in the Mountain Pass area (DeWitt et al., 1989, Wooden and Miller 1990, Miller and Wooden 1993).

Unlike most other carbonatites, which are usually associated with sodic alkaline rocks, those at Mountain Pass are related to ultrapotassic intrusive rocks that occur in a narrow north-trending belt in southeastern California. These rocks are members of an alkaline suite ranging from mafic (shonkinite) through syenite to granite. In addition, the Mountain Pass carbonatite and associated alkaline rocks do occur as concentric, circular to ovoid masses characteristic of other large alkaline silicate-carbonatite complexes around the world.

The mineralised carbonatites in the Mountain Pass district are associated with ultrapotassic alkaline plutons of similar age, size and orientation, as well as with abundant carbonatite and alkaline dykes, defining a narrow NNW-trending zone of ultrapotassic alkaline igneous rocks at least 130 km long in southeastern California (Castor, 1991; Castor & Nason, 2004 ). However, only ~15 km of this belt of ultrapotassic rocks includes carbonatite intrusions.

Ultrapotassic intrusions in the northern part of this belt, including those of the Mountain Pass district, are unmetamorphosed and have not been modified other than by local hydrothermal alteration. To the south however, the ultrapotassic intrusions and the country rocks are both overprinted by Mesozoic greenschist grade metamorphism. Mafic alkaline rocks in the Mountain Pass area contain >3 wt.% K2O, 3 wt.% MgO, and have K2O:Na2O ratios of >2, satisfying the definition for ultrapotassic rocks of Foley et al. (1987). In the Mountain Pass district, fine- to coarse-grained alkaline rocks occur in plutons as much as 1.8 km long, whilst similar composition dyke rocks are predominantly fine-grained. Shonkinite and melasyenite comprise about 80% of the exposed alkaline rocks in the Mountain Pass district, with only relatively minor syenite, quartz-syenite and granite which all cut the first two dominant intrusive lithologies. However, some late shonkinite and melasyenite dykes do cut cut granite and syenite (Olson et al., 1954), but in places show evidence of co-intrusion with syenite (Haxel 2005).

The shonkinite at Mountain Pass was emplaced at 1410 ±5 Ma, followed by syenite intrusion at 1403 ±5 Ma (U-Th-Pb and
40Ar/39Ar; Dewitt et al., 1987). The granitic rocks have not been dated, but are assumed to be close to 1400 Ma on the basis of mineralogical similarities with the syenite. Carbonatite appears to have been emplacement closely following the alkaline magmatism, but yielded Th-Pb dates of 1375 ±5 Ma on monazite, indicating a significant gap between alkaline and carbonatite intrusion (Dewitt et al., 1987).

The ultrapotassic rocks in the Mountain Pass district show strong LREE enrichment compared to other ultrapotassic suites, reflecting the extreme enrichment of LREE in the Mountain Pass carbonatites. The Mountain Pass ultrapotassic rocks also show extreme Th enrichment to levels regarded as very high for rocks with mafic compositions, and particularly for silica-rich members, some of which contain more than 200 ppm Th (Castor, 2008).

The crudely tabular to lensoid, shallowly west-dipping, NNW trending, ~10 km long intrusive complex that includes the carbonatite suite at Mountain Pass is composed of eight x 100 to 2000 m long plugs of alkaline intrusive rocks (from shonkinites and syenites to carbonatites) and about 200 dykes of carbonatite in NW-trending rows.

The mineralised carbonatite at Mountain Pass includes the large tabular Sulphide Queen carbonatite mass, which is as much as 150 m thick and was about 700 x 150 m in plan at the surface prior to mining (Olson et al., 1954), as well as the carbonatite dykes that range from a few mms to 3 m thick. These dykes are abundant in the vicinity of the Sulphide Queen mass, but are much less common in the related Mineral Hill and Mineral Springs areas several kilometres to the south where mineralised carbonatites are also known.

Although the Sulphide Queen carbonatite exhibits considerable mineralogical and chemical variability, it is predominantly composed of bastnäsite-barite sövite (calcite carbonatite) and bastnäsite-barite-dolomite carbonatite (beforsite), or of rock that is intermediate between these two types (bastnäsite-barite dolomitic sövite). These rock types generally comprise ore (5% or more REO) and locally contain as much as 25% REO over 2 m drill-core intervals. The dolomitic carbonatites (beforsite and dolomitic sövite) are more abundant than sövite (Castor, 2008). The bastnäsite at Mountain Pass is predominantly bastnäsite-Ce, i.e., (Ce,La)CO

Bastnäsite-barite sövite forms the basal part of the Sulphide Queen carbonatite, and most of the carbonatite in the northern part of the pit. It also commonly occurs as a thin zone along the hanging wall of the carbonatite. In the thick, southern section of the orebody, sövite comprises less than half of the ore thickness. The sövitic ore contains early-formed bastnäsite, along with generally recrystallised, but locally single-crystal phenocrysts of barite, in a groundmass of fine- to medium-grained calcite and barite. When unaltered, the sövite is a pink to mottled white and reddish brown rock that typically contains ~65% calcite, 25% strontian barite, and 10% bastnäsite, although the relative amounts of these three phases vary considerably. Much of the sövitic ore has been altered, with fine grained, anhedral quartz locally comprising as much as 60% of the rock, generally as pervasive flooding or stockwork veining. Silicification was mainly at the expense of calcite, although partial replacement of barite and bastnäsite also took place. Other alteration minerals include talc and allanite with chlorite, phlogopite and magnesioriebeckite, generally occurring in xenoliths, but are locally also in the carbonatite itself (Castor, 2008).

At the surface, beforsite (dolomitic carbonatite) was only found in the southwestern corner of the Sulphide Queen orebody (Olson et al., 1954) and was not significant during early mining, but in the 1980's, it became an important type of ore. On the lower levels of the mine, it was found to extend to the north along the hanging wall of the orebody. It typically overlies sövitic ore, and is separated from it by dolomitic sövite. Ferroan dolomite is the major carbonate phase of the beforsite. The average mode is ~55% dolomite, 25% barite, 15% bastnäsite and 5% calcite. It occurs as a light grey to pale brown or pale pinkish brown rock that contains abundant grey, white, or pale red to pink phenocrysts of barite, commonly occurring as single crystals rather than recrystallised aggregates. The dolomite, which predominantly occurs as brownish grey to pale yellowish brown rhombs, is locally oxidised and dark brown. It crystallised after the formation of the barite phenocrysts. Dolomite rhombs are set in a fine-grained, pale yellow to light pink or nearly white interstitial material composed of of bastnäsite, calcite and barite. Bastnäsite crystals in the beforsite ore is relatively fine grained, averaging ~87µm; in diameter. Bastnäsite apparently crystallised late from residual fluid in the beforsite, after barite and dolomite crystallisation, as opposed to in the sövite, in which it was comparatively early. The monazite content of beforsite ore is variable, locally as much as 5%, occurring mostly as irregular veinlets of 'bone' monazite with a microcrystalline granular to radiating acicular texture. The beforsite generally contains a little quartz as late interstitial grains that, with calcite, postdate the formation of bastnäite.

In addition to the ore types, several other carbonatite varieties are found in, and adjacent to, the orebody, including parisite-barite sövite and monazite-bearing sövite, dolomitic sövite and beforsite. The orebody is further complicated by the presence, particularly in the hanging wall, of breccia containing variable amounts of carbonatite matrix and altered country-rock clasts at its northern and southern ends (Castor, 2008).

The Mountain Pass REE deposit, within the Sulphide Queen carbonatite, had reserves in 2008 of >20 Mt @ 8.9% REO (Castor and Nason 2004). The ore typically contains 10 to 15% bastnäsite-(Ce), and is mostly composed of calcite, dolomite and barite.
In 1987, Mountain Pass reserves were calculated at 29 Mt @ 8.9% REO at a 5% REO cut-off grade (Castor 2007).

Mountain Pass bastnäsite assays: 30.37% La; 45.50% Ce; 4.65% Pr; 15.82 Nd; 1.83% Sm; 0.35% Eu; 0.74% Gd; 0.05% Tb, 0.16% Dy; 0.02% Ho; 0.03% Er; <0.01% Tm; 0.49% Yb; <0.01% Lu; and <0.01% Y (Castor, 2008).

The most recent source geological information used to prepare this summary was dated: 2008.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Castor, S.B.,  2008 - Rare Earth Deposits of North America: in    Resource Geology   v.58, pp. 337-347.
Castor, S.B.,  2008 - The Mountain Pass rare-earth carbonatite and associated ultrapotassic rocks, California: in    The Canadian Mineralogist   v.46, pp. 779-806.
Pirajno, F.,  2014 - Intracontinental anorogenic alkaline magmatism and carbonatites, associated mineral systems and the mantle plume connection: in    Gondwana Research   v.27, pp. 1181-1216.
Smith, M.P., Moore, K., Kavecsanszki, D., Finch, A.A., Kynicky, J. and Wall, F.,  2016 - From mantle to critical zone: A review of large and giant sized deposits of the rare earth elements: in    Geoscience Frontiers   v.7, pp. 315-334.

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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