Mineral chemistry evolution of magmatic Li-bearing micas and Li-loss during postmagmatic phengitization in an A-type sieno-monzogranite: A mass balance approach at La Chinchilla pluton, Sierra de Velasco, La Rioja, Argentina

Abstract

La Chinchilla granite (∼3.75 km2) is an epizonal pluton intruded during the Lower Carboniferous in Sierra de Velasco, Sierras Pampeanas, northwestern Argentina. Three facies have been distinguished in the pluton (equigranular, porphyritic and fine-grained border zone facies), being equigranular and porphyritic the main two granite types, which host abundant millimeter to < 2 m sized miarolitic pegmatites and pockets of simple granitic mineralogy (quartz + albite + microcline + micas) ± beryl. Micrometer-sized accessory magmatic species are monazite-(Ce), several high field strength element oxide species, ilmenite, cassiterite (1), fluorapatite and fluorite. Primary Li-bearing micas with variable degrees of late to postmagmatic replacement occur in abundances that range from ∼2 to 7 %. Textural evidence and mineral chemistry allow to distinguish between early granitic and late bladed miarolitic micas. The mineral chemistry composition of the primary micas shows members of the siderophyllite-polylithionite series. Replacing phases are Li-muscovite (phengite). Electron probe microanalyses of primary micas of both main granite types suggest that these are compositionally distinct intrusives. According to mica chemistry, the porphyritic unit is more enriched in Li than the equigranular granite. The equigranular unit with interconnected miarolitic texture was more intensely affected by fluid-rock interaction processes and is significantly richer in U and Be. An F–Na rich fluid phase caused strong albitization during late miarolitic stages, along with crystallization of polylithionite, fluorite, pyrochlore group species and cassiterite (2). In terms of mass balance, the phengite replacement process is characterized by the conservation of Si, Al and K, a slight gain of Na and Ba, and a systematic loss of orderly decreasing Fe > F > Li > Mn > Ti > Mg > Zn. Likely, the muscovite-forming fluid was the same simultaneously involved in the hydrothermal alteration of high field strength element accessory species, largely included in primary micas, which generated secondary U- and Nb-rich species such as carlosbarbosaite. The hydrothermal late miarolitic to postmagmatic fluid-mineral replacement of primary magmatic micas released significant amounts of Li, of which about 59 wt % of the Li2O content of protholithic micas (i.e., 1 to 2.4 wt % Li2O) was inherited by replacing Li-bearing muscovite and the remaining 41 wt % was released from the system. It is calculated that ∼1.5 km3 of La Chinchilla body would have expelled ∼0.38 million tons of Li2O, just considering than only 30 % of primary siderophyllite-polylithionite was muscovitized. Such a Li2O tonnage was likely transferred to the paleohydrological cycle during Carboniferous times, when the La Chinchilla stock was still undergoing its cooling down path. Currently, evidence of the final fate of mobilized Li is lacking.