Mountain Pass is a carbonatite hosted rare earth deposit (USGS Deposit Model 10; Singer, 1986). The mineralization is hosted principally in carbonatite igneous rock. Mountain Pass is the only known example of a rare earth deposit in which bastnaesite is mined as the primary magmatic economic mineral in the world (Haxel, 2004).

Mineralization occurs entirely within the carbonatitic portion of the currently drilled geologic sections, although grade distribution internal to this mineralized zone is variable. Higher grade zones (>10% TREO) tend to occur in lenses parallel to the hangingwall/footwall contacts, both downdip and along strike. Continuity of mineralization internal to the carbonatite zone is well defined both along strike and downdip. The currently defined zone of rare earth mineralization exhibits a strike length of approximately 2,750 ft (850 m) in a north-northwest direction and extends for approximately 3,000 ft (930 m) downdip from surface. The true thickness of the >2.0% TREO zone ranges between 15 to 250 ft (5 to 75 m).

Globally, carbonatites are subdivided into two main groups: apatite-magnetite bearing, mined for iron and/or phosphorus ± various by-products, and rare-earth bearing carbonatites. Many other commodities may be present in economically significant concentrations, such as uranium, thorium, titanium, copper, vermiculite, zirconium, niobium, and phosphorus (Singer, 1986). The majority of carbonatite complexes display a series of variable carbonatitic magma compositions, the majority of which are not significantly enriched in rare earths. Mountain Pass is unique in that the carbonatite does not exhibit such variation and has significant intervals of elevated rare earths throughout its entirety.

The main rare-earth-bearing mineral, bastnaesite, is present in all carbonatite subtypes, but in relatively lower proportions in the breccias and the monazitic carbonatites which typically occur mainly outside of and close to the main orebody. Monazite and crocidolite (“blue ore” found on the hangingwall contact in the northern part of the orebody) are both undesirable in the processing plant. In some areas, post mineral fault zones provide a conduit for water which results in localized alteration of the fresh carbonatite. Alteration dissolves the calcite and dolomite gangue minerals, leaving behind elevated concentrations of bastnaesite with limonite resulting in what is referred to as brown and black ore types, the most altered of which become a loosely consolidated very high grade bastnaesite sand. The altered ore types are mined, stockpiled separately and blended at a minorproportion to maintain target ore grades in the mill feed blend.

The Mountain Pass deposit is mined by open pit mining methods. Surface mining operations include:
• Drilling and blasting to remove overburden material;
• Loading and haulage;
• General maintenance and services.

The open pit that forms the basis of the mineral reserves and the LoM production schedule is approximately 3,100 ft from east to west and 3,800 ft from north to south with a maximum depth of 1,400 ft. Total mining is estimated at 216 million st comprised of 30.4 million st of ore and 186 million st of waste, resulting in a strip ratio of 6.1 (waste to ore). Ore grade averages 6.35% TREO yielding over 2.05 million dry st of recoverable 60%TREO concentrate. SRK designed four pit pushbacks that adhere to proper minimum mining widths. Bench sinking rates are approximated to no more than six benches per year per pushback.

Slope design constraints assume a 15 ft model block height. Mining production will be conducted primarily on 30 ft bench heights. Most areas of the mine are in competent rock mass, and it is envisioned that in these areas the mining in the final wall will be finished to a 30 ft face or a 60 ft face height. Using a multiple-bench final wall configuration permits a steeper IRA in competent ground. The maximum inter-ramp slope height (bench stackheight) is 500 ft. A geotechnical berm, or haul ramp, with a minimum width of 65 ft is required between bench stacks.

The minimum catch bench width is developed using the modified Ritchie Criteria (Ryan and Pryor, 2000). The minimum catch bench width for a 60 ft-high bench face is 24 ft using the Ritchie Criteria.

For a 30 ft-high bench, the minimum width is 15 ft. Bench face angles vary by sector and are based on average obtained values by mapping. The measured bench face angle using highwall controlled blasting procedures results in average bench face angles ranging from66° to 68°. For the given slope design parameters and limited subsurface data, dual ramp access is required to ensure access to ore material for each mining phase. With the ramps and the recommended IRAs, the final wall overall slope angle maximum is 45°. Stability of the pit slope, including hydrogeological inputs, is documented in the CNI, 2022 report. SRK has reviewed the results, and stability of the pit slope using these design parameters meets a slope acceptance criterion with a minimum FoS of greater than 2.0. These FoS results are within the guidelines of the current reclamation plan, and also meet the criteria outlined in Guidelines for OpenPit Slope Desing (Read & Stacey, 2009).

MP Materials has contracted for drilling and blasting services. The contractor will provide all equipment, supplies, and labor to complete the services. It is MP Materials’ intention to continue with contractor drilling and blasting services for the foreseeable future. Accordingly, SRK has included a provision in the mining cost estimate for drilling and blasting services for the LoM timeframe.

Drilling is based on a 15 ft blasthole spacing and a 15 ft burden. The designed hole depth is 30 ft with a 4 ft subdrill. Dry blastholes will be loaded with ammonium nitrate fuel oil (ANFO). It is assumed that there will be 20% additional holes for pre-splitting, and 10% of blastholes will be loaded with emulsion (wet conditions).

The blasting contractor transports blasting accessories to site and stores these separately in a suitable explosives magazine. The blasting contractor has an explosives truck (ANFO/emulsion), which delivers bulk explosives to the open pit blast sites during daylight hours. Stemming material is 3/4 inch rock. The blasting contractor manages and conducts the blasting operations.

The loading, hauling, and support equipment operations are performed with a fleet that is owned and operated by MP Materials. The primary loading equipment is front-end loaders (17 yd3), which were selected for operational flexibility. Rigid frame haul trucks (102 wet st) were selected to match with the loading units.

The mine operations schedule includes one 12-hour day shift, seven days per week for 365 days per year.

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Existing Crushing and Concentrating Operations
MP Materials operates a 2,000 t/d flotation concentrator that produces concentrates that are currently sold to customers who further process the concentrates to produce separated rare earth oxides. The concentratorflowsheet includes crushing, grinding, rougher/scavenger flotation, cleaner flotation, concentrate thickening and filtration and tailings thickening and filtration followed by dry stack tailings disposal. Significant improvementsin concentrator performance have occurred since inception of operations, which are attributed primarily to new reagent and ore blending schemes as well as the introduction of steam boiler to support process kinetics.During 2020 TREO recovery averaged 66.8% into concentrates containing an average of 60.6% TREO. During 2021 (January – September) TREO recovery has averaged 69.8% into concentrates averaging 61.2% TREO,reflecting ongoing operational improvements in the concentrator.

Flotation