FUNDAMENTAL GEOCHEMICAL CHARACTERISTICS OF THE MOUNTAIN PASS CARBONATITE, SOUTHERN CALIFORNIA

The carbonatite at Mountain Pass (MP) is famous for its extraordinary enrichment in light rare earth elements (6 mass %, 10⁵ times chondritic), and its essential role in the early development of modern REE technology. However, the remarkable abundance of LREE at MP has overshadowed several other characteristics of the carbonatite that are as or more unusual, and arguably of equal or greater petrogenetic significance.

The 1.4-Ga Mountain Pass carbonatite is a carbonate-sulfate igneous rock, with abundant strontian barite, as phenocrysts and in the groundmass. Modal content of sulfate minerals is typically 10–30 percent. Average molar (SO₄)^2- is nearly one-quarter that of (CO₃)^2-. Thus, representative samples of minimally altered sövite and beforsite have respective formulas (CaCO₃)₇₆(BaSO₄)₁₄(LnCO₃F)₉(SiO₂)₁ and (CaMg[CO₃]₂)₃₂(CaCO₃)₁₀([Ba,Sr]SO₄)₃₈(LnCO₃F)₁₅(SiO₂)₅ {where LnCO₃F = bastnäsite, the ore mineral}. Despite the renowned concentration of LREE at MP, enrichment in S and Ba is actually several times greater, relative to both depleted mantle and ordinary Ca-Mg carbonatites.

Two other notable geochemical attributes evidently were critical in governing LREE mineralization at MP: low phosphorus and high fluorine. Unlike most carbonatites, the MP carbonatite is depleted not enriched in the HFSE Ti, Nb, and P. Average F/P is ~ 20 times that of ordinary carbonatites. Accordingly, low P limited early crystallization of monazite and fluorapatite, leaving F and LREE available to form later magmatic and hydrothermal bastnäsite.

The fundamental task in elucidating the origin of the MP carbonatite is envisioning or reconstructing a mantle source region exceptionally rich in S, Ba, Sr, F, and LREE. In this source, Ba and F presumably resided in phlogopite, and LREE and Sr hypothetically in titanate minerals of the crichtonite group [compositionally complex; e.g., (Sr,Ca,Ln,U)(Ti,Fe,Cr)_20–21O₃₈]. Sulfur must also have come from the mantle, as Mesoproterozoic or older S-bearing crustal rocks are absent from the MP region. The mantle source of so much oxidized sulfur remains a mystery. Are the extraordinary large concentrations of S, Ba, and LREE at Mountain Pass unrelated, or are they linked in some manner not yet fully appreciated?