REE mineralization in Siwana peralkaline granite, western India-role of fractional crystallization, hydrothermal remobilization, and feldspar-fluid interaction

Abstract

Peralkaline granites from the Siwana complex in western India host mineralization of rare earth and other high field strength elements (REE/HFSE). In this study, we use textural relations, major-trace element chemistry and Li-isotope composition of minerals to constrain the magmatic evolution of the granites and the mechanism of hydrothermal REE mineralization. Three facies of peralkaline granites, viz., hypersolvus, transsolvus, and pegmatitic are identified in the Siwana complex. The hypersolvus granites constitute two mineralogical variants: clinopyroxene- and ferrorichterite-bearing. Transsolvus granites contain riebeckite-arfvedsonite and occur as patches/network of dikes in hypersolvus granites. Rayleigh crystal fractionation modelling using incompatible trace element abundances in clinopyroxene and amphibole indicates that the hypersolvus and the transsolvus granites formed after 50-85% and 98% melt crystallization, respectively. Early magmatic clinopyroxenes and ferrorichterites in the hypersolvus granites have fairly heavy δ7Li (+18.5‰ to +20.4‰) suggestive of crystallization from highly evolved magmas. Late magmatic arfvedsonites in the transsolvus granites have even heavier δ7Li (+29.7‰ to +33.8‰). Rayleigh distillation modelling using Li isotopes reveal that the hypersolvus and transsolvus granites crystallized from parental magmas that had undergone >90% fractional crystallization.The Siwana granitoids are hydrothermally altered as evidenced from pseudomorphic re-equilibration of clinopyroxene, alkali feldspar megacrysts, vein and patch perthite formation, replacement of ferrorichterite/clinopyroxene by hydrothermal aegirine, alteration of primary fluorapatite, monazite, aenigmatite, eudialyte, and vlasovite. Three stages of hydrothermal alteration are identified in the granites: stage-I alteration was brought about by highly-saline high-temperature fluid that scavenged REE, HFSE, Ti, Na, and Fe from clinopyroxene and aenigmatite. This was followed by extensive replacement of ferrorichterite and arfvedsonite by aegirine, which decreased the pH and salinity of the fluid. The third stage of alteration led to the replacement of vlasovite by calciocatapleiite, elpidite, and eudialyte and was accompanied by the replacement of alkali feldspar by aegirine, quartz, and secondary REE minerals. The bulk of the REE mineralization occurs in transsolvus granites as cerite associated with aegirine and quartz. Textural relations indicate that these minerals pseudomorph alkali feldspar. Distinct assemblages of secondary REE minerals are identified in the pyroxene- and amphibole-bearing granites-only hydroxyl-dominated REE minerals like cerite are identified in the former while late-stage replacement of cerite by bastnäsite and chevkinite is observed in the latter, the difference controlled by the extent of ferrorichterite to aegirine transformation. The precipitation of secondary REE minerals was largely triggered by pH increase due to the interaction of alkali feldspar with REE-bearing hydrothermal fluids, resulting in the replacement of alkali-feldspar with Fe-phyllosilicates.