Controls on the Origin and Evolution of Wall-Rock Alteration at the Big Whopper Li-Cs-Ta Pegmatite, Canada

Abstract A feature of some of the most important lithium deposits that has received limited attention is the alteration of hornblende in the amphibolite host to holmquistite. This study investigates the holmquistite-bearing alteration halo of amphibolite around the Big Whopper lithium pegmatite of the Separation Rapids rare element pegmatite group. The alteration developed as a continuous zone marginal to all intrusive contacts and is characterized by a biotite or more sporadically by a biotite-holmquistite alteration assemblage, both without tourmaline. Mass balance calculations, using whole-rock analytical data, indicate that the biotitization resulted in a significant addition of K, Sn, Rb, and Cs to the amphibolite. In contrast, the formation of holmquistite was manifested by an enrichment in Li, Si, Be, Ba, Cs, and Rb. Although visible evidence of alteration is restricted to the first few centimeters of the contact, the geochemical signature of the dispersion halo was traced in amphibolite adjacent to the neighboring Snowbank pegmatite for over ~180 m from the contact, with Li demonstrating far greater mobility than Rb and Cs. Based on the whole-rock element mass changes, the mineralogy, and the mineral chemistry, we deduce reactions and develop a metasomatic model that provides a systematic description of the progressive alteration of the wall rock. The first step involved the exsolution of an aqueous phase in response to quenching of the magma against the amphibolite. This fluid altered hornblende in the amphibolite to biotite. The failure of a lithium mineral to form suggests that fluid exsolution occurred prior to significant magma crystallization. As only a small portion of the magma was quenched, the newly formed border zone sealed in the remainder of the magma, which continued to evolve in response to crystallization in a closed system, accumulating volatiles until the resulting fluid overpressure fractured the border zone, allowing the more evolved fluid to escape. Owing to crystallization and the progressive enrichment of the residual magma in Li, the later fluid was also enriched in Li. This fluid reacted with the available hornblende and the newly formed biotite in the wall rock to crystallize holmquistite. The later escape of fluid probably also triggered the transition from crystallization of the petalite in the pegmatite to the crystallization of fine-grained albite-lepidolite. The final stages of pegmatite development saw the release of the last aliquots of extremely evolved fluid, which enriched the existing biotite in the exocontact in Rb and Cs. This study provides new insights into the processes producing holmquistite-bearing alteration halos around lithium-cesium-tantalum pegmatites and an important lithogeochemical tool for their exploration.