The Globe pegmatite, Rio Arriba County, New Mexico

Abstract

The Globe pegmatite, in the Petaca district of northern New Mexico, is a steeply dipping, tadpole-shaped dike about 650 feet long. Exposures developed during extensive mining of mica shoots above the 100-foot level reveal a symmetrical internal structure outlined by four microcline-quartz zones, and by a quartz core that is exposed near one end of the dike. In general the grain size and the proportion of quartz in the pegmatite increase from the walls inward. Albite and associated green muscovite vein and corrode microcline, quartz, green fluorite, and beryl of the zones, and are concentrated in replacement units that are best developed between the walls of the dike and its innermost zones. Spheroidal aggregates of radially oriented cleavelandite tablets are abundant, and featherlike clusters of columbite and moμazite occur in many of them. Purple fluorite, spessartite, and sericite also are associated with the cleavelandite, but such minor late constituents as samarskite, sulfide minerals, bismutite, some purple fluorite, and lepidolite seemingly are unrelated to it. The following paragenesis is indicated by rather clear-cut megascopic relations: (1) early development of quartz, microcline, green fluorite, and beryl, (2) formation of albite, green muscovite, ,and associated accessory minerals, chiefly along fractures in the earlier minerals, and (3) continued deposition of fracture-filling quartz, in part associated with earlier accessory minerals and in part with distinctly later and rarer minerals.