Internal Structure of Granitic Pegmatites

Abstract

Studies of strategic mineral deposits in the major domestic pegmatite districts by the U. S. Geological Survey from 1939 to 1945, primarily for the purpose of increasing production of mica, beryllium, lithium, and tantalum minerals, resulted in the clarification of many problems related to the general. features, internal structure, mineralogy, and origin of granitic pegmatites. This report summarizes about 68 man years of work in the pegmatite districts of New England, the Southeastern States, South Dakota, Idaho, Montana, Wyoming, Colorado, and New Mexico. The pegmatites of eastern United States, of Paleozoic age, and those of the Rocky Mountain region, mostly of pre-Cambrian age, are localized in metamorphic terranes, and are closely associated with either granite, quartz monzonite, or granodiorite masses. In some districts where pegmatites are associated with all three types of igneous rocks the composition of the plagioclase feldspar in three separate groups of pegmatites suggests a genetic relationship to the three types of igneous rocks. In other districts the distribution of pegmatites suggests a genetic relationship to the adjacent igneous mass. Consequently the mineralogy, texture, and structure of the pegmatites varies in detail from district to district, though within any one district variations as great or greater may be found as the parent igneous rock is approached. Pegmatites range from a few inches to more than a mile in length, and from a fraction of an inch to more than 500 feet in width. Most are tabular or thinly lenticular; others are branching, irregular, or teardrop-shaped. Pipelike, arcuate, and troughlike forms are less common. The shapes of pegmatites commonly are closely controlled by the type and structure of the wall rock. Many pegmatites in schistose rocks are concordant with regional foliation, and plunge parallel to the linear structure of the schist. The plunge of discordant bodies is commonly controlled by the intersection of fractures. Secondary foliation and drag folds indicate deformation, of the wall rocks by the pegmatites during emplacement. Alteration of schistose country rocks to granulites is most common along the contacts of discordant bodies. The structural and lithologic units that differ in mineralogy, texture, or both have been designated as: (1) fracture filling– tabular units that fill fractures in previously consolidated pegmatite, (2) replacement bodies— units formed primarily by replacement of pre-existing pegmatite, and (3) zones —successive shells, complete or incomplete, around an innermost unit or core that reflect to varying degrees the shape and structure of the pegmatite body. Zones, quantitatively and economically the most important, have been classified as (1) border zones, (2) wall zones, (3) intermediate zones, and (4) cores.