FLUID EVOLUTION IN A GEM-BEARING POCKET PEGMATITE AT THE CRYO-GENIE MINE, SAN DIEGO COUNTY, CALIFORNIA. A NOVEL METHOD OF GEMSTONE EXPLORATION?

We are proposing a new exploration method for gemstones in LCT-type, pocket pegmatites using microthermometry and crush-leach chemical analysis of fluid inclusions. Upon exsolution from the crystallizing magma, fluids accumulate in pockets. During further cooling and crystallization, fluids released from ruptured pockets may become trapped in surrounding pegmatite crystals as secondary fluid inclusions. We sampled quartz in and around mineralized and non-mineralized pockets from the Cryo-Genie pegmatite located in the Chihuahua district of the San Diego pegmatite field, in order to test whether fluid inclusion analysis can be used to fingerprint the gemstone-bearing pockets. The Cryo-Genie pegmatite is a zoned, 2-4 m wide, 200 m long dike that has produced gem tourmaline and beryl specimens. We also systematically sampled along a cross-section of the pegmatite from the wall zone to the core to understand the fluid evolution during magma differentiation.

Two-phase, aqueous inclusions of low apparent salinity (average ice melt temperature of -1.6 ±0.4 °C) are dominant at Cryo-Genie. Ion chromatography on bulk-fluid leachates indicated a complex Cl, F, Na, and Li-rich composition. We established the primary vs. secondary character of inclusions based on textural evidence. We separated three main fluid types: type 1 represents an early magmatic fluid trapped in the pegmatite wall and intermediate zones; type 2 is a late magmatic fluid in the pegmatite core; and type 3 is fluid from pockets. The F/Cl and Li/Na atomic ratios of the three fluid types vary systematically. For example, the Li/Na ratios ranged from 0.02 - 0.1 in type 1, to 0.1 - 0.3 in type 2, up to 0.2 - 2.6 in type 3 fluid from mineralized pockets, but remained below 0.2 in non-mineralized pockets. The average homogenization temperature of the three types decreased modestly from 189 ±3°C, 166 ±1°C, to 161±9 °C, respectively. As anticipated, pegmatite samples situated near mineralized pockets yielded intermediate compositions, because, in addition to the early fluid (type 1 or 2), they were “contaminated” by late, type 3 fluid released from the pockets. Further study is necessary to test the applicability of microthermometry and Li-Na-F-Cl fluid systematics on surface samples to the exploration of unexposed gem pockets, before costly mining operations.