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. Border zones are fine-grained selvages rarely more than a few inches thick. Wall zones, next inside the border zones, generally are coarser and much thicker. One or more intermediate zones separate the wall zone and the core, and are typically coarser grained than the outer zones. Intermediate zones commonly are incomplete shells. Cores are irregularly lenticuiar units in a more or less central position in the wider parts of the pegmatite. Normally the core contains very coarse-grained minerals. Pegmatites in which zonal structure is poorly developed may contain small, unevenly distributed coarse-grained minerals in lenses or pods, which simulate cores. The sequence of mineral assemblages in the various districts studied indicates a general sequence of units, based on essential minerals, from the wall rock inward: (1) plagioclase, quartz, muscovite; (2) plagioclase, quartz; (3). quartz, perthite, plagioclase with or without muscovite and/or biotite; (4) perthite, quartz; (5) perthite, quartz, plagioclase, ambly-gonite, spodumene; (6) plagioclase, quartz, spodumene; (7) quartz, spodumene; (8) lepidolite, plagioclase, quartz; (9) quartz, microcline; (10). microcline, plagioclase, lithia-mica, quartz; and (11) quartz. Few, if any, pegmatites contain all eleven zones, but those that are present will be in this sequence. Many fracture-filling units are clearly contemporaneous with and have the same zonal sequence as the inner zones. These units rarely are economically or quantitatively important. Replacement bodies range from thin, discontinuous veinlets to units several tens of feet thick and more than 500 feet long They are irregular, tabular, podlike, or sheetlike in shape. Their distribution in general is controlled by fractures, differences in lithology, or a combination of these features. They are particularly common in pegmatites in New Mexico and New England, and less common in South Dakota, Colorado, and other areas. Zones, the most abundant type of pegmatite unit, appear to have developed from the walls inward, by crystallization of pegmatitic magma, essentially by fractional crystallization and incomplete reaction in a restricted system. Some fracture fillings are offshoots of zones and therefore formed by crystallization of pegmatitic magma; others formed from liquids that have given rise to replacement bodies. The application of techniques used during these investigations has been successfully tested in the prospecting, exploration, and development of several pegmatite mineral deposits. The close relation between structural features and the sequence of mineralogical units, particularly the relation of these units to the wall rock contact of the pegmatite, if properly used, should decrease the hazards of mining.

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.