EVOLUTION OF LI- AND B-BEARING FLUIDS IN GRANITIC PEGMATITE SYSTEMS

The Late Cretaceous Sierra Ranges batholith hosts many syn- to post-emplacement, gem-bearing pegmatites, characteristically with high Li and B content. These two fluxing elements, together with water, promote large crystal growth in undercooled granite dikes. Microthermometry of fluid inclusions in quartz and cathodoluminescence (CL) were used to determine the behavior of Li and B during fractional crystallization and subsolidus conditions in three pegmatite suites: La Posta, Himalaya, and Blue Lady. These pegmatites consist of a layered aplite (line rock) along the lower contact (absent in the upper contact), a coarse-grained intermediate zone with inward-pointing crystals from the top and bottom contacts, and a core zone with crystals that terminate into frequent pockets, in which feldspars are often altered into clays. Fluid inclusions demonstrate progressive evolution of fluid compositions during crystallization and subsolidus alteration of the Li- and B-rich pegmatite dikes.

Fluid inclusion populations include primary (PI) and two types of secondary inclusions (SI1 and SI2)—all two-phase aqueous that homogenized to liquid. PI’s exhibit depressed eutectic temperatures (relative to the NaCl-H₂O system) near wall rock contacts (–49.2 to –44.2°C) suggesting high LiCl content. PI’s with both depressed (~–36.8°C) and elevated (~–15.2°C) eutectics occur in cores, indicating coexistence of immiscible chlorine-rich and boron-hydroxyl-rich fluids, respectively. Homogenization temperatures for the PI’s near contacts range from 278.2 to 315.9°C and in cores from 123.2 to 156.4°C for both primary types of inclusions. The lower temperatures in the cores may stem from Li and B accumulation by fractional crystallization of the dikes.

Secondary inclusions and CL reveal evolution of fluids during subsolidus conditions. Eutectic temperatures of S1 inclusions range from –17.9 to –12.1°C, again suggesting high B-hydroxyl concentrations. Eutectics of SI2 range from –21.8 to –21.6°C, indicating NaCl-dominated fluids. CL presents an anomalous pink color along fractures in quartz and feldspars that LA-ICP-MS analysis indicates is lepidolite and/or petalite. The NaCl-rich fluids may have been produced by removal of Li from Li-Na-Cl fluid during crystallization of these minerals.