Mountain Pass

Location: California, United States (35.477, -115.531 approx.)

Commodities: rare earth elements; gold

Deposit model: carbonatite · carbonatite deposits

Tracked facility at this deposit: MP Materials Mountain Pass

Geology

“Sovitic carbonation forms the basal part of the orebody, and most of the ore where the carbonatite narrows in the north part of the pit. In the thick, southern part of the orebody, it makes up less than half of the ore thickness. The sovitic ore contains relatively coarse, early-formed bastnasite, along with recrystallized barite phenocrysts, in a matrix of fine-grained calcite and barite. Where unaltered, it is pink to mottled white and reddish brown rock that typically contains about 65% calcite, 25% strontian barite, and 10% bastnasite. However, relative amounts of these three phases vary considerably (table 6), and alteration following magmatic crystallization produced more complex mineral assemblages.” (Castor and Nason (2004)) “Dolomitic soviet (dolosovite) ore occurs in a 30- to 60-m-wide zone between the sovitic and beforsitic ore types. It contains both dolomite and calcite in variable amounts, and the carbonate minerals show evidence of secondary redistribution such as calcite veining and dolomitization. Dolosovite is generally limonitic, commonly as dark brown pseudomorphs after dolomite rhombs. Barite phenocrysts are typically white to pink and recrystallized. Some dolosovite contains coarse bastnasite as in the sovite, and some has fine-grained, late beforsite-style bastnasite.” (Castor and Nason (2004)) “Bastnasite in sovitic ore is typically coarse grained (fig. 10); on the basis of data from 126 samples collected during pit mapping, average bastnasite crystal diameter is about 300 microns. Bastnasite in the sovite generally forms hexagonal prisms strongly elongated along the c crystallographic axis. Parisite, which occurs locally in the sovite, commonly forms fan-like intergrowths with bastnasite (fig. 11). For the most part, monazite occurs sparingly in the sovite, generally as small primary euhedral and patches of radial secondary needles.” (Castor and Nason (2004)) “Much of the sovitic ore has been altered. Silicification is common; fine-grained, anhedral quartz comprises as much as 60% of the rock locally (table 6), generally as pervasive flooding or stockwork veining. The silicification was mainly at the expense of calcite, although partial replacement of barite and bastnasite also took place. Sovite with weak to severe talc replacement (table 6) occurs in the north part of the pit, mostly as schistose gray-green rock with brick-red barite augens and deformed bastnasite prims. Allanite occurs locally in talc-altered sovite. Other alteration minerals are chlorite, phlogopite, and magnesioriebeckite, which generally occur in xenoliths, but locally in the carbonatite. Strontianite replacement of calcite and strontian barite is common, and celestine occurs as bladed replacements and overgrowths of strontian barite and as late veins. Iron hydroxide is locally abundant, particularly in silicified ore.” (Castor and Nason (2004)) “Mineralization occurs entirely within the Sulfide Queen carbonatite within the Project area. This has been defined through drilling and mapping. Grade distribution internal to this mineralized zone is variable. Higher grade zones (>10% TREO) tend to occur in lenses parallel to the hanging wall/foot wall contacts, both down dip and along strike. Continuity of mineralization internal to the carbonatite zone is well defined both along strike and down dip. The currently defined zone of rare earth mineralization exhibits a strike length of approximately 2,750 ft in a north-northwest direction, and extends for approximately 7,000 ft down dip from surface. The true thickness of the >3.0% TREO zone ranges between 15 ft to 250 ft. The principal economic mineral at the Project is bastnasite, a rare earth fluorocarbonate with the chemical formula LNCO3F where LN is a variable representing yttrium or any rare earth ratio but usually lanthanum or cerium. This naming convention is applied throughout this resource report. The bastnasite composition at the Project is dominated by cerium, lanthanum, and neodymium, with smaller concentrations of praseodymium, europium, samarium, gadolinium, dysprosium, terbium and the heavier rare-earth elements. Bastnasite mineralization at the Project is entirely restricted to carbonatite rocks which were subdivided by Castor (1988, 2008) as described below. Non-mineralized rock types within the open pit area are also described.” (Bair and others (2012)) “Beforsitic ore was only found in the southwestern corner of the pit in the early 1980s, but on lower levels it was found to stretch further to the north along the hanging wall of the orebody. This ore type typically overlies sovitic ore and is separated from it by dolosovite. The beforsitic ore contains ferroan dolomite as the major carbonate phase. The average mode is about 55% dolomite, 25% barite, 15% bastnasite, and 5 % calcite (table 6). It is light-gray to pale-brown or pale-pinkish-brown rock that contains abundant gray, white, or pale-red to pink barite phenocrysts, commonly as single crystals rather than recrystallized aggregates. Barite-rich zones noted in drill core and during pit mapping may represent cumulates along internal intrusive contacts in the beforsite. The dolomite, which occurs as pale-yellowish-brown to brown rhombs, crystallized after formation of the barite phenocrysts. The dolomite rhombs are set in fine-grained, pale-yellow, interstitial material that consists of bastnasite, calcite, and barite. Along the south wall of the pit, the beforsite contains crude, nearly vertical banding that consists of braided discontinuous veins of late bastnasite+calcite. The texture may have formed by upward streaming of residual REE- and Ca-rich fluids.” (Castor and Nason (2004)) “Rare earth element mineralization at Mountain Pass occurs in the Sulphide Queen body, which was about 700 meters (m) by 150 m prior to mining (Olson and others, 1954). Detailed mapping and petrography of the Sulphide Queen carbonatite showed that it is composed of a group of carbonatites having distinct mineralogical or textural characteristics (Castor, 2008b).” (Verplanck and others (2014))

Published resource estimates

As compiled by USGS USMIN from the cited studies. Resource classes follow the source; "approximate" flags values USMIN reports as approximations. These are not current company reserve statements.

Recorded production

• Mountain Pass, Mountain Pass Mine: produced 0.0 metric tons of gold (Au), containing about 0.0 metric tons Au, 1939–1942 (Bair and others (2012), Pages 17-18)
• Mountain Pass, Mountain Pass Mine: produced 614,000 metric tons of rare-earth oxide (REO), containing about 614,000 metric tons REO, 1952–2014 (Bair and others (2012); Braun and others (2010); Woodmansee (1973); Woodmansee (1974); Woodmansee (1976); Kerns (1977); Smith (1977); Moore (1979); Moore (1980))

Exploration and site history

• Active — Production; Mining resumes; Molycorp prepares to resume milling; Production from stockpiled feedstock, 2010 (Gambogi and Cordier (2012))
• Inactive — Molycorp shuts down Mountain Pass; Mountain Pass transitions back to care and maintenance (1), 2015–2018 (Jamasmie (2015))
• Active — Care and maintenance - The existing mill entered care and maintenance in 2002; Production - Processed and sold previously mined bastnasite-bearing ore; Production estimated to be 5,000 metric tons of REOs of bastnasite (1), 2002–2003 (Bair and others (2012))
• Active — Intermittent Production - Production of rare earth oxides at the Project began in 1952, using the old gold plant, a new ball mill, and flotation cells from MCA’s Urad, Colorado molybdenum property; Molycorp ceased mining in the 4th quarter of 2015 (1), 1952–2015 (Bair and others (2012))
• Active — Care-and-maintenance; Production - Sold from stockpiles; Molycorp was not actively mining, but sold from its stockpile of bastnasite concentrates, intermediate concentrates, and separated compounds previously processed at its open pit (1), 2003–2009 (Hedrick (2004))
• Discovery — Discovery of REE mineralization at Mountain Pass was made in April of 1949 by prospectors searching for U, 1949 (Bair and others (2012))

Record imported from USGS USMIN ver. 4.0 (June 2019). Developments after that date appear on facility records, not here; see the methodology.