APATITE GEOCHEMISTRY RECORDS MAGMATIC AND METASOMATIC PROCESSES IN THE MOUNTAIN PASS INTRUSIVE SUITE, CALIFORNIA

Carbonatite intrusive bodies host some of the largest rare earth element (REE) deposits in the world, and are often spatially and temporally associated with alkaline silicate rocks. Formation models for carbonatite and associated alkaline silicate rocks remain debated, as does the role of fluids in concentrating REE to form economically significant deposits. The Mountain Pass intrusive suite, located in the Mojave Desert, CA, consists of a shonkinite-syenite-granite series of alkaline stocks and dikes, as well as carbonatite dikes and one carbonatite stock. The carbonatite stock is the only actively producing REE deposit in the U.S.A. and has historically been considered an example of a magmatic REE deposit, although some recent work has suggested fluids played a role in forming the deposit.

Apatite [Ca₅(PO₄)₃(F,Cl,OH)] is a ubiquitous accessory mineral in silicate rocks and most carbonatites. At Mountain Pass, apatite occurs as a trace mineral in the carbonatite, and as an accessory mineral in the alkaline silicate suite. In this study, the texture and geochemistry of apatite are examined to constrain both its magmatic history and the role of fluids in its formation. Three compositional and textural apatite groups were identified in the alkaline silicate rocks. A primary magmatic group is typically homogenous or concentrically zoned, while a metasomatized group is typically patchy and complexly zoned. The metasomatized apatite are generally closer to pure end-member apatite, consistent with removal of impurities (e.g., REE, Si, Sr) by a fluid. The final group, which occurs as rounded or partially resorbed cores of grains, is interpreted as inherited from the crystalline basement based on key differences in composition, particularly low Sr and negative Eu anomalies. Two groups of carbonatite apatite are identified: a primary magmatic group with high REE contents, and one heavily modified by fluids with low REE contents. Results of the study reemphasize the role of crustal assimilation in forming the silicate intrusions and indicate fluids were present in both the carbonatite and alkaline silicate systems. Importantly, lower REE contents in metasomatized apatite suggest these fluids remobilized REE. This study indicates that the Mountain Pass carbonatite should not be classified as a purely magmatic REE deposit.