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A geochemical survey of the Sherbrooke and Lewiston 1?? ?? 2?? quadrangles was conducted from 1979 to 1982 by the U.S. Geological Survey as part of the Conterminous United States Mineral Assessment Program. The area covers about 23,000 km2. The region is characterized by a temperate climate, abundant rainfall, and dense forests. The topography ranges from rolling and subdued to mountainous. Bedrock is generally covered by glacial deposits, lakes, and bogs. The bedrock includes metamorphic and plutonic rocks ranging in age from Precambrian to Cretaceous. Known mineral deposits within the area include massive-sulfide deposits, Cu-Mo porphyry deposits, minor base-metal vein deposits, minor Sn-bearing vein deposits, and minor Be-bearing skarn deposits. Stream sediments were the primary sample medium because: (1) drainage systems are extensively developed; (2) bedrock is poorly exposed; and (3) previous studies had established stream-sediment sampling as a viable procedure for reconnaissance geochemical studies in this region. The data from about 1,700 stream-sediment samples collected in earlier U.S. Geological Survey programs were included to give a total of 6,935 samples of stream sediment. At 1,420 sites, samples of heavy-mineral concentrate from stream sediment were also collected. Both sample media were analyzed for as many as 32 elements. The distributions for Cr, Cu, Zn, Mo, and U in stream-sediment samples and for Sn and W in nonmagnetic heavy-mineral-concentrate samples are discussed here. Some significant findings are: (1) nonmagnetic heavy-mineral concentrates are a valuable sample medium for the region; (2) certain Jurassic granites are potential hosts to Sn deposits; (3) a prominent linear trend of stream-sediment samples containing high Cr may be caused by glacial transport of mafic or ultramafic rock material for distances as great as 160 km; (4) glacial action must be considered in the interpretation of (a) elements, such as Cr, W, and Sn, that form resistate minerals and (b) elements, such as Cu, that are associated with chromite or other resistate minerals; (5) glacial action seems to be a minor factor in the regional dispersion of elements from sulfide deposits; and (6) the geochemical expression of some known massive-sulfide deposits is subtle; thus, weak anomalies in areas of no known deposits may be significant. ?? 1987.
Additional Publication Details
Regional geochemical studies in parts of Maine, New Hampshire and Vermont, U.S.A.