A PERMIAN MAGMATIC-HYDROTHERMAL SYSTEM AND ASSOCIATED TUNGSTEN DEPOSIT IN THE ALLEGHANIAN FORELAND OF SOUTHWESTERN CONNECTICUT

New work in southwestern Connecticut has recognized a northwest-trending Permian magmatic-hydrothermal metallogenic system in high-grade Acadian metamorphic rocks. The northwest-trending zone about 10 km in length is defined by a small central granite pluton, dikes of rhyolite and dacite porphyry, and NW-trending hydrothermal veins in the granite pluton and the Acadian metamorphic rocks. The northernmost reach of this trend is the location of the first tungsten mine established east of the Mississippi River (1899), which is the type locality for tungstite. Mineralization at the site also includes scheelite, hosted by metasomatized mafic rocks, as well as hydrothermal veins composed of quartz, albite, and muscovite with topaz and/or fluorite. Fluorite-bearing veins also cut the Acadian metamorphic rocks between the tungsten mine and the central granite pluton, the Pinewood Adamellite. This central granite pluton contains fluorite as a magmatic and later hydrothermal phase, as well as small amounts of a magmatic tin-bearing phase. Fluorite is included in plagioclase crystals themselves included in megacrystic Carlsbad-twinned K-feldspar phenocrysts, implying early crystallization of fluorite. Fluorite is also present as interstitial grains in the granitic matrix as well as in quartz veins cutting the granite. Magmatic muscovite grains in the granite and late muscovite on the borders of quartz veins are also enriched in fluorine. The NW trend of the three intrusions (granite, rhyolite porphyry, and dacite porphyry) and the tungsten mine parallels the trend of the cross-cutting hydrothermal fluorite-bearing veins. The Permian age of the intrusions defined by U-Pb isotope ages of zircon and monazite establishes a temporal link with the final convergent assembly of peri-Gondwana with Laurentia. A peri-Gondwanan (lower plate) source for the Pinewood Adamellite is suggested by Pb isotopic analysis (Sevigny and Hanson, 1993) as well as by ~600 Ma cores in xenocrystic zircons. The NW trend of Permian magmatism and hydrothermal mineralization in the high-grade Acadian metamorphic rocks of southwestern Connecticut identifies this region as the last and westernmost extent of magmatic-hydrothermal activity in the Alleghanian foreland.