MINERALOGY OF GRANITIC PEGMATITES FROM THE SISCOWIT GRANITE, STAMFORD AND POUND RIDGE 7.5 MINUTE QUADRANGLES, SOUTHWESTERN CONNECTICUT

The impetus for new bedrock mapping in the Stamford and Pound Ridge 7.5 minute quadrangles in southwestern CT was the discovery of high U in well water in Stamford by the Stamford Health Department. Unpublished data from the Stamford Health Department show that out of 1,245 wells tested, 72% contained uranium, 12.5% contained more uranium than the EPA standard of 30 µg/L, and 35 wells (2.8%) contained over 100 µg/L uranium. There does not appear to be a clear spatial correlation between the location of high U well water and surface bedrock. In Stamford, there is an on-going need to develop an understanding of the relationship between bedrock geology and groundwater chemistry. One possible U source is pegmatitic granite that occurs as sills and dikes throughout the Siscowit Granite in the Stamford quadrangle in CT and the Pound Ridge quadrangle in both CT and NY.

Much of the Stamford and Pound Ridge quadrangles are underlain by the Siscowit Granite, named by Scott (GSA Bull. 67 155) for granite, granitic gneiss, and granitic pegmatite in and around the Siscowit Reservoir in Pound Ridge, NY. In the Pound Ridge and adjacent Stamford quads in CT, the Siscowit has four main varieties: (1) whitish-gray to gray, fine- to medium grained bt-mcl-pl-qtz granite with a ropey texture on natural outcrops related to variation in grain size; (2) distinctive white to light gray, medium-grained bt granite containing layers with conspicuous small (less than 3 mm but as large as 1 cm), rounded red garnet; (3) whitish-pink to pink, medium- to coarse-grained bt-ms-mcl-pl-qtz with conspicuous pink colored K-feldspar megacrysts; and (4) coarse, pegmatitic granite that can occur both as undeformed dikes and sills in the Siscowit and other map units and as deformed layers with a well-defined foliation parallel to the regional fabric. Very coarse-grained variety (3) is gradational into pegmatitic variety (4) and where strongly foliated, has a distinctive gneissosity with augen-like K-feldspar.

We have identified three major types of pegmatitic granite that mimic the mineralogy of the varieties of Siscowit: one has the assemblage [kfs+ab+qtz+bt] with minor pyrite; the second has the assemblage [kfs+ab+qtz+ms+bt±grt] with minor magnetite; and the third has the assemblage [kfs+ab+qtz] with medium- to coarse-grained secondary muscovite covering foliation surfaces where the pegmatite is strongly flattened.