Abstract

Paragerietic relationships among lithiophilite Li(Mn, Fe)2+ PO4 and its alteration products are described for one of the complex granitic pegmatites in the Pala pegmatite district, San Diego County, California. At the Stewart pegmatite, lithiophilite occurs in the upper-intermediate, microcline–quartz zone while spodumene and amblygonite are found in the quartz core. The sequence of primary mineral formation within the pegmatite reflects an increase in the activities of both lithium species and volatile components (phosphorus and fluorine). Extensive alteration of lithiophilite involved oxidation, hydration, and cation leaching, but suprisingly little metasomatism. Secondary minerals present include sicklerite, hureaulite, purpurite, stewartite, phosphosiderite, several incompletely identified phases, and various manganese oxides. Secondary phosphates formed during the initial stages of alteration pseudomorphously replaced lithiophilite as a result of its limited hydrothermal interaction with late-stage pegmatitic fluids. Later members of the alteration sequence represent supergene weathering products. The Stewart pegmatite crystallized from a highly-differentiated granitic magma at shallow crustal depths (about 3–5 km). The lack of extensive metasomatic replacement, which is so evident among the phosphate mineral assemblages of other granitic pegmatites, is thought to primarily be a result of the relatively rapid cooling and volatile fluid loss at the shallow depths of formation of the Stewart pegmatite. Under these conditions, there was little opportunity for metasomatic reaction to take place between lithiophilite and residual pegmatitic fluids.

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