The geology of the Houser Canyon pegmatite with an x-ray analysis of monazite

Abstract

A large granitic pegmatite near Campo, in southern San Diego County was found to contain several radioactive biotite-rich inclusions. The pegmatite is irregularly zoned, and in general comprises a well defined graphic granite border zone, an irregular wall zone of intergrown quartz and perthite, a discontinuous intermediate zone of blocky perthite, and a well developed but somewhat irregular quartz core. The included masses of biotite-rich rock are spatially related to segments of the quartz core and their content of rare-earth minerals may be due in part to this relationship. The inclusions are composed of interlayered biotite books and feldspar plates which form a series of folded and distorted layers that are randomly oriented throughout the inclusion. Separation and analysis of minerals from the inclusion were accomplished by means of cloud-chamber detection, x-ray powder diffraction analyses, and spectrographic analysis. Thorium-bearing monazite was identified as the source of the radio-activity. The rare-earth elements and thorium evidently were concentrated in the late residual fluids of the pegmatite. Pendants or inclusions of apatite-bearing schist are believed to have provided a source of phosphorous for the crystallization of monazite. As an additional result of reaction with the pegmatite solutions, the minerals of the schistose pendant were recrystallized and some mineralogic changes were made as well.