Pegmatites of the Middletown area, Connecticut

Abstract

The pegmatites of the Middletown area in Connecticut have been mined almost continuously for feldspar and muscovite mica since about 1865. Pegmatites in this and other areas have recently become the subject of renewed interest because they are the potential source of beryl, the ore mineral of beryllium. From 1948 to 1950 the Geological Survey studied and mapped the pegmatites in an area of about 58 square miles near Middletown. The primary objectives of the mapping were to determine beryllium resources of the Middletown area and to ascertain relations of beryl-bearing pegmatites to non-beryl-bearing pegmatites and to type of wall rock.More than 330 concordant and discordant pegmatites were examined within the mapped area. The pegmatites cut the metasediments of the Bolton schist of pre-Mississippian(?) age and, in ascending order of sequence, mafic gneisses, the Glastonbury granite gneiss, Maromas granite gneiss, and Monson gneiss. The position of these formations in the stratigraphic column cannot be determined with accuracy; their maximum age is indicated by their relation to the pegmatites, which have been dated by uranium-and thorium-lead ratios as about 260 million years old, presumably Mississippian.The Bolton schist includes middle-to high-rank mica-quartz schist of various compositions interbedded with minor quantities of quartzite, diopside-and hornblende-bearing marble, and hornblende-bearing schist and gneiss. The mafic-.gneiss is predominantly hornblendic gneiss and schist with subordinate mica schist and light-colored feldspathic gneiss. These metasediments are cut by the Glastonbury granite gneiss, a gray to pink medium-to coarse-grained porphyritic gneiss; by the Maromas granite gneiss, a gray medium-grained biotite orthogneiss; and by the Monson gneiss, a banded to massive mediumgrained biotite granodiorite gneiss.. The metamorphic rocks in the northern part of the Middletown area dip westward, but in the southern part the Bolton schist is bowed up by the Monson gneiss into a dome. Between these areas the rocks show evidence of intense folding and overturning. The western part of the area is covered by fanglomerate, conglomerate, sandstone, and siltstone of Triassic age, bounded by normal faults. An east-trending fault of indeterminate age probably offset the meta? morphic and .igneous.rock units in the area near Great Hill..The pegmatites are composed essentially of perthite, quartz, plagioclase, and muscovite. The common accessories are tourmaline, beryl, garnet, and biotite. The pegmatites have been divided into four groups based on the content of 614 CONTRIBUTIONS TO 'EOONOMDC GEOLOGY plagioclase feldspar: those with less than 40 percent of plagioclase, with and without beryl; and those with more than 40 percent of plagioclase, with and without beryl. About 42 percent of the pegmatites contain more than 40 percent plagioclase, and 58 percent of them contain less than 40 percent of plagioclase.Most of the pegmatites are nearly homogeneous bodies with a grain size from about 1 to 4 inch...