Summary:
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 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 defined zone of rare earth mineralization exhibits a strike length of approximately 2,750 ft (850 meters (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. 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 carbonatite and numerous other alkaline intrusives in the vicinity are hosted in Proterozoic gneissic rocks which have been altered through alkali metasomatism (fenitized) by the intrusive carbonatite dikes. Smaller dikes and breccia bodies surround the Sulfide Queen orebody which comprises several different types of carbonatite (sovite, beforsite, dolosolvite, and white sovite) that are interlayered within a relatively large carbonatite package. This deposit is unique in terms of size of the concession, and globally significant in terms of its enrichment in rare-earth minerals.
The southern part of the Sulfide Queen orebody strikes to the south-southeast and dips at 40° to the west-southwest; the northern part of the orebody strikes to the north-northeast and dips at some 40° to the west-northwest. Several post- mineralization faults result in slight offsets to the otherwise simple tabular/lensoid geometry. The total orebody strike length is approximately 2,750 feet (ft) and dip extent is 3,000 ft; true thickness of the more than 2.0% total rare earth oxide (TREO) grade zone ranges between 15 ft and 250 ft.
The main rare-earth-bearing mineral, bastnaesite, is present in all carbonatite subtypes, but in relatively lower concentrations in the breccias and the monazitic carbonatites, which typically occur outside and proximal to the main orebody. Monazite and crocidolite (“blue ore” found on the hanging-wall contact in the northern part of the orebody) are both considered deleterious in the processing plant. In some areas, post-mineral fault zones provide a conduit for water which results in localized hydration and oxidation of the fresh carbonatite. This weathering 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 results in a loosely consolidated high grade bastnaesite sand. The altered ore types are mined, stockpiled separately, and blended to maintain target ore grades in the mill feed blend.