Beryl as indicator of metasomatic processes in the California Blue Mine topaz-beryl pegmatite and associated miarolitic pockets

Abstract

Beryl occurs in a wide variety of rare-element pegmatites and has provided significant insights into their evolution. The geochemical signatures of beryl can record igneous and metasomatic processes but rarely could they be studied at the same location in both, the pegmatite body and associated miarolitic pockets. Further, the link between beryl chemistry and the petrogenesis of pegmatites has not yet been established convincingly. This study focuses on the recently discovered topaz-beryl granitic pegmatite deposit at the California Blue Mine in San Bernardino County, California, USA. This pegmatite deposit provides an exceptional opportunity to study microtextural (SEM and WDS-element maps) and major EMPA and trace element LA-ICP-MS concentrations in beryl in the pegmatite body and associated miarolitic pockets. The mineralogy of the pegmatites consists of: quartz + K-feldspar + albite (i.e., cleavelandite variety) + biotite + muscovite ± spessartine ± beryl (i.e., aquamarine variety) ± topaz, with a mineralogic zoning that includes border, wall, intermediate, and core zone. The miarolitic pockets contain gem quality beryl, topaz, and quartz crystals. Two major beryl types were distinguished including a primary (Brl I) and a metasomatic beryl (Brl II). Brl I occurs in the intermediate pegmatite zone and displays a blocky concentric crystal zoning characterized by a core enriched in Al and rims enriched in Na, Rb, K, and Sc. In contrast, Brl II is found mostly in the pegmatite core zone and at the contacts to, or within miarolitic pockets, and is characterized by a distinct enrichment in Cs and Li correlating with an enrichment in Na, B, and As and a depletion in Rb, K, Sc, and Fe. Brl II occurs as two variants including as replacement of Brl I (i.e., dissolution-precipitation microtextures in the pegmatites) and as single hydrothermal crystals (i.e., precipitation in the miarolitic pockets). The contrasting trends between Brl I and Brl II are interpreted to reflect two distinct processes affecting trace element variations in beryl including fractionation from a magma with systematic crystal core to rim zoning (decreasing Li and enrichment in Na, K, Rb, Sc, and Fe), and metasomatism with enrichment in Li, Cs, Na, B and As. An extensive beryl trace element dataset was generated in this study and further compared to beryl of the NYF (Nb-Y-F) and LCT (Li-Cs-Ta) pegmatite families worldwide. A significant link was found between beryl chemistry and the petrogenetic relationships of the pegmatites, which were used to distinguish four major geochemical beryl groups: (i) a low and a high Fe–Mg — alkali group, both displaying a positive alkali vs. Fe + Mg correlation; (ii) a low Fe–Mg — high alkali group displaying a negative alkali vs. Fe + Mg correlation; (iii) a low Li–Cs group; (iv) a high Li–Cs group. The more evolved LCT-type pegmatites generally overlap with the low Fe–Mg — high alkali and high Li–Cs groups (ii and iv). In contrast, NYF pegmatites coincide better with the trends observed for the low Li–Cs group as well as the low/high Fe–Mg — alkali beryl groups displaying a positive Fe + Mg vs. alkali correlation (i and iii). The beryl signatures from the California Blue Mine coincide with the NYF pegmatite classification but a partial overlap with the LCT pegmatites indicates a possible mixed origin or crustal contamination (i.e., hybrid pegmatites). These new insights reveal that beryl signatures provide a significant tool for interpreting the evolution of pegmatites from different geologic settings and permits distinguishing between igneous and metasomatic origins.