Abstract

The gem- and specimen-bearing, complex granitic pegmatite and aplite dikes and bodies of Cretaceous age emplaced into the Southern California batholith locally contain fractures and cavities (“pockets”) that are usually clay filled. Some pockets, however, contain only a thin coating of cookeite or sericite, deposited as “snow on the roof.” Other pockets contain pseudomorphs of lepidolite after elbaite, bavenite after beryl, and clays and micas after spodumene. Pseudomorphism took place under nearly closed system conditions at temperatures similar to those at the time of pocket formation. Most pockets contain fragmented primary minerals enclosed in a matrix of Ca-Na zeolites; white, pink, or red beidellite; Li-tosudite; Ca-Mg montmorillonite; and rare calcite. Paragenetic relationships indicate a decrease in Na relative to Ca during the crystallization of zeolites and a decrease in Li and Al–concomitant with an enrichment of Ca, Mg, and Si–for the layer silicates. The most prevalent minerals found are stilbite, laumontite, cookeite, Li-tosudite, beidellite, Mg-Ca montmorillonite, palygorskite, and calcite. Sparse amounts of nontronite, heulandite, and todorokite have also been identified. Deposition of zeolites, clays, and carbonates in open pockets took place subsequent to crystallization of the primary pocket minerals, under hydrothermal conditions with temperatures ranging from approximately 400 to 150°C. The major amounts of Ca and Mg, represented by as much as 8.4 wt% MgO in palygorskite and more than 10 wt% CaO in minerals such as laumontite and bavenite, were provided by hydrothermal alteration of gabbro-norite and/or tonalite host rocks. Redbrown (iron-stained) kaolinite is locally present as a final phase of deposition in some pockets and is probably indicative of a transition from an alkaline to an acidic environment.

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