LAUSR

The Kougarok prospect is situated in a Sn-W ( + Ta, Nb, Li, Be) metallogenic belt formed in a post-collisional to within-plate tectonic environment. Crystal fractionation of granitic magma, combined with its mixing/mingling with mantle-derived mafic magma, is proposed as the major process causing the formation of the complex B-rich to Li-F-type granitic suite and associated Sn-rare metal (Ta, Nb, Li) mineralization. An intrusion of alkalic (potassic lamprophyric) mafic magma into a crystallizing biotite-tourmaline granite magma reservoir may have supplied a geochemically distinct assemblage of volatiles (F) and associated metals (Li, Ta, Nb) into the boron- and Sn-rich granitic system. An alternative model considers differentiation-driven unmixing of mafic and granitic silicate melts sequestering different volatile and mineralizing species that could also trigger magma-fluid and fluid-fluid unmixing, with separation of gaseous fluid coexisting with immiscible granitic and mafic magmas in the form of magmatic melt-fluid-crystal “suspension.” Biotite-tourmaline granite was accompanied by early quartz-tourmaline-cassiterite greisen at > 430–380 °C followed by quartz-tourmaline-chlorite-cassiterite stockwork at < 350 °C. The immiscibility of F- and B-rich fluids was followed by preferred ascent of F-rich fluids upward in the granitic magma reservoir, with its strong enrichment in F, Li, Ta, and Nb and subsequent crystallization of zinnwaldite granite at a shallower level, with separation of homogenous high-salinity magmatic fluid at ~ 600 °C. Crystallization and fluid exsolution continued at lower temperatures, followed by formation of topaz-quartz stockscheider at 550–500 °C. Ta-Nb mineralization in the uppermost part of the zinnwaldite granite appears to be associated with final episodes of magmatic crystallization, particularly W-Nb-rutile and columbite-tantalite with a higher Nb content. Some Ta-Nb minerals, such as columbite-tantalite with a higher Ta content, appear to be also stable during the post-magmatic stage, in quartz-tourmaline + topaz greisen and higher-temperature quartz-albite-Li mica alteration that replace zinnwaldite granite at ~ 500 to 400 °C. Nearly contemporaneous, lower-temperature quartz-albite-Li mica alteration and quartz-muscovite, quartz-topaz, and quartz-fluorite greisens formed from boiling fluids at 390–350 °C and caused removal of Ta-Nb minerals. Instead, fluid cooling and neutralization of boiling fluids affected Sn solubility and promoted massive cassiterite deposition in the late greisens. The latest phyllic quartz-sericite-carbonate alteration assemblage, comprising arsenopyrite, pyrrhotite, chalcopyrite and other sulfides, native Bi, and Bi tellurides, formed from boiling fluids at < 310 °C.