THE ROLE OF LI, B, F, AND CL BEARING FLUIDS IN THE CRYSTALLIZATION AND MINERALOGY OF GRANITIC PEGMATITES

The San Diego County, CA, LCT-type pegmatites occur in the Mesozoic Peninsular Ranges Batholith. Emplacement of the dikes occurred at a low pressure (2–3 kbar) that promoted the formation of large miarolitic cavities (pockets). Pockets occasionally contain gem-quality, Li-bearing minerals. This study includes two of these pegmatite suites: the gem-bearing San Diego (SD) and the more barren La Posta (LP). Results show why some pegmatites in the San Diego County pegmatite district contain gem-quality minerals, such as elbaite and spodumene, whereas others do not. Fluid inclusions in quartz are used to describe the role that fluid-soluble elements, Li, B, Cl, and F, have in controlling pegmatite mineralogy and subsolidus alteration. The fluxing properties of Li and B in the melt may be responsible for increasing water solubility in melts, and therefore the development of large pockets that result from accumulation of aqueous fluids near centers of dikes.

Microthermometric measurements of homogenization temperatures <300 ºC (not pressure corrected) in primary fluid inclusions support low temperature crystallization of pegmatites. LA-ICP-MS and ion chromotography analyses of fluid inclusions reveal that Li, B, Na, Cl, and F are components in both systems. Li concentration is much higher in the SD pegmatite (up to 51 atom % of all cations) than in the La Posta where most inclusions contain no Li except a few primary inclusions with <25 at.%. B content is high in both systems (up to 65 at.% in SD and 50 at.% in LP). In both pegmatites, Na tends to be more concentrated in secondary inclusions that were also locally sourced.

The fluid inclusion data show that Li is concentrated in residual liquids and separated aqueous fluids, leading to crystallization of Li rich minerals in pegmatite cores. Furthermore, Li is an essential component in the formation of acidic fluids, as are F and Cl. Reactions such as:

3 LiF (aq) + 3 H₂O + B³⁺ (l) → 3 Li⁺ + 3 HF (aq) + B(OH)₃ (aq)

2 HF (aq) + H₂O + 2 NaAlSi₃O₈ (s) → Al₂Si₂O₅(OH)₄ (s) + 4 SiO₂ (s) + 2 NaF (aq)

show that as the activities of HCl and HF increase in the pegmatites, feldspars alter to form kaolinite and quartz in pockets and produce Na-bearing secondary fluids. Reactions between these fluids and minerals are revealed by lepidolite mineralization in fractures and adjacent mineral alteration.