Rare earth element mineralization during prolonged hydrothermal alteration of the Siwana peralkaline granite, western India

Abstract:

The peralkaline Siwana granite in western India has undergone extensive post-magmatic hydrothermal alteration and remobilization of rare earth and other high field strength elements (REE and HFSE). The suite can be subdivided into hypersolvus and transsolvus granites and granitic pegmatites. The granitoids have undergone five-stage evolutionary history comprising an early stage of fractional crystallization followed by late-magmatic stage involving fluid exsolution, as seen from δ⁷Li of clinopyroxene. Three stages of hydrothermal alteration can be recognized. An early hydrothermal stage involved interaction of the granitoids with highly saline fluids followed by two later hydrothermal alteration events (II and III) during which the rocks interacted with evolved acidic and low-temperature oxidizing fluids, respectively. The mobilization of REEs and other HFSEs took place during the hydrothermal stages. The early hydrothermal stage led to the alteration of arfvedsonite to aegirine, creating low f_O2 and high pH conditions. The low f_O2 conditions resulted in the production of higher hydrocarbons instead of CO_2, and the high pH stabilized REE-hydroxyl complexes. It triggered the precipitation of hydroxyl-bearing REE silicate minerals. Stage II hydrothermal alteration occurred after the complete transformation of arfvedsonite to aegirine, which decreased the pH. Further lowering of the pH increased the CO₂ significantly and stabilized REE fluoride species and promoted bastnäsite-(Ce) crystallization. In the third hydrothermal stage (III), Zr was mobilized by complexation with F at low pH. During this stage, the fluid was highly oxidized due to the reduction of carbonic species. The high f_O2 led to the precipitation of REE oxides such as yttrobetafide-(Y) and nioboaeschynite-(Ce) during the waning stages of fluid-rock interaction.