PERMIAN HIGH-TEMPERATURE HYDROTHERMAL VEINS AND AMPHIBOLITE METASOMATISM IN SOUTHWESTERN CONNECTICUT – EVIDENCE FOR THEIR ASSOCIATION WITH ALLEGHENIAN INTRUSIVES

The Old Mine Park area (OMP), Trumbull, Connecticut hosts famous topaz-bearing veins, and one of only two tungsten mines east of the Mississippi River. These mineralized veins and metasomatic deposits follow an Acadian metamorphic evolution peaking near ~390 Ma and are much more complicated than previously understood. With our new mapping, terrific exposures of amphibolite, marble and quartzite, mapped as basal Straits by Rodgers (1985), are reassigned to the regionally significant Russell Mountain Fm. (Sr) as mapped in MA, a unit that underlies the pelitic The Straits Schist (Dst) (Goshen Fm in MA). Although Sr underlies DSt in the northern OMP map area, to the south Sr was found to be thrust over DSt along a syn-tectonic migmatite zone, a complex interval demarcated by granitic pegmatite with included intercalated screens of amphibolite and schist.

Approximately 100 m.y. later, undeformed fluorite-bearing Permian granitoids, including Trumbull’s Pinewood Adamellite (291 ± 4 Ma, Sevigny & Hanson, 1993), were intruded along a 320º trend (Crowley, 1968) that intersects the OMP. The mineralization at the park takes two forms: (1) the veins themselves and (2) alteration of rocks adjacent to the veins. Undeformed, 1 m thick topaz veins are coarse-grained, compositionally zoned, and contain primarily quartz in cores, and intermediate and border zones of muscovite, topaz and/or albite and fluorite (a rare few contain primarily calcite). They probably crystallized above 400°C, well before a new 267 Ma ⁴⁰Ar/³⁹Ar closure age of muscovite from one vein. Alteration around the veins includes metasomatic replacement of host amphibolite to phlogopite, scapolite and albite, with minor scheelite. This mineralization is similar to, and apparently related to, discontinuous zones of metasomatically altered amphibolite characterized by very coarse-grained quartz, clinozoisite, scapolite, albite ± scheelite. The undeformed nature of the mineralized rocks, the occurrence of fluorite in the granitoids and the veins, and the 320º trend of the steep veins shared with the trend of the granitoids suggests that metasomatism is associated with another Permian intrusive a few m below the surface. Fluids rising from it invaded fractures associated with its 320º emplacement trend, bringing late mineralizing fluids to the park area.