SULFUR ISOTOPE INVESTIGATION OF SULFATE AND SULFIDE MINERALS IN THE MOUNTAIN PASS CARBONATITE INTRUSION, CALIFORNIA

The Mountain Pass carbonatite is very unusual in that it is an economic REE orebody and a carbonate-sulfate igneous rock. The origin of the sulfur required to form this assemblage has been uncertain. Moreover, little work has been conducted to constrain temperature and oxygen fugacity (fO₂) of the carbonatite melt. Sulfate occurs primarily as barite (BaSO₄), in solid solution with celestine (SrSO₄). Barite phenocrysts commonly show increasing Sr toward the rims and are occasionally crosscut by Sr-rich (celestine-dominant) veins. Sulfates constitute on average 20–30 volume % of the carbonatite, and locally up to 65%. Many carbonatite samples contain sulfides in accessory (pyrite, galena) to trace (chalcopyrite) abundances in textural association with sulfates. The carbonatite is associated with intrusive alkaline (ultrapotassic) silicate stocks and dikes, some of which also contain trace barite and pyrite. This study presents new in-situ geochemical and S isotopic analysis of the sulfate and sulfide minerals of the Mountain Pass Intrusive Suite.

Owing to fractionation of S isotopes between SO₄^2- (sulfate) and HS^- (sulfide), neither sulfate nor sulfide minerals represent the bulk δ³⁴S of the source of the carbonatite magma. Sulfur isotope ratios suggest sulfides and sulfates were typically in chemical equilibrium, allowing for calculation of the bulk S isotope composition of carbonatite. The calculated bulk δ³⁴S of the carbonatite system (0.12‰) aligns with typical mantle-sourced S (0 ± 2‰). A correlation between high Sr in sulfate minerals and more fractionated δ³⁴S suggests the barite-celestine transition is related to fractional crystallization and oxidation of the carbonatite melt; greater sulfate-sulfide S isotope fractionation typically indicates an increase in melt fO₂. The silicate dikes contain δ³⁴S that overlaps the carbonatite range, but δ³⁴S in barite in a fenitized silicate stock is 2–4‰ higher than the carbonatite sulfate δ³⁴S range. This suggests a different or an additional sulfur source in the silicate rocks, possibly a crustal assimilant, whereas the high concentration of mantle-derived S buffered the carbonatite against δ³⁴S shifts caused by crustal assimilation. Alternatively, barite in this silicate stock may have been a late phase related to fractionated, fenitizing fluids.