THE LATE-STAGE MINIFLOOD OF Ca IN GRANITIC PEGMATITES: AN OPEN-SYSTEM ACID-REFLUX MODEL INVOLVING PLAGIOCLASE IN THE EXOCONTACT

As Richard H. Jahns used to say, granitic pegmatites are prone to stew in their own juice. The early juice acts as a catalyst in transforming the early-formed disordered K-feldspar to microcline below 450 degrees C. As it becomes progressively enriched in fluorine during the system's evolution, the exsolved fluid becomes increasingly aggressive, and can create cavities by dissolution. It eventually leaks into the country rock, mostly from the apical parts of zoned pegmatites. There, it interacts with plagioclase in any lithology it encounters. As the plagioclase is highly strained and kinetically stuck, it is consumed, and the fluid quickly becomes more aluminous, more acidic and Ca-bearing. We envision a convecting fluid phase cycling around the pegmatite body, and a reaction path that brings the acidified fluid back into the heart of the pegmatite. There, it is aggressive toward the microcline perthite, which is replaced by cleavelanditic albite + muscovite + quartz, mostly in the range 250-400 degrees C, a process that enlarges the original cavities and creates new ones. Miarolitic pegmatites of either NYF and LCT affiliation contain clear signs of the presence of cations brought in from outside the system, mostly Ca (causing a miniflood of Ca), but also Sr, Fe, Mg, Sc, Li and F. Other elements are efficiently recycled internally. The recurrent cycling of an aqueous fluid has major ramifications concerning the economic aspects of granitic pegmatites. In particular, the cycles of hydrothermal enrichment in a large LCT pegmatite can give rise to zones enriched in micas of the lepidolite series, pollucite, tantalite-(Fe) and tantalite-(Mn), i.e., super-LCT deposits.