THE NORWALK LEAD-SILVER MINE PROSPECT: EVIDENCE FOR STRUCTURAL CONTROL OF HYDROTHERMAL SOLUTIONS IN WESTERN CONNECTICUT

A historical mine adit was discovered in a wooded area near Norwalk, CT during 1987. A vertical concrete slab plastered against a bedrock outcrop was removed to reveal the hidden adit. A 1-m thick sulphide-bearing quartz vein was exposed near the entrance. A mine tunnel plunged at an incline of approximately 30^o for several meters where it became submerged below the water table. Sulphides include galena, chalcopyrite, and pyrite. The vein has an attitude of 325^o-26^o and transgresses the Ordovician Trap Falls Fm. Prospecting for Pb in the Norwalk area was as early as 1765 but is not known to have been economic.

The Norwalk Pb prospect occurs along a NW-trending lineament, here named "the Silvermine River lineament (SRL)." The SRL is a presumed fracture zone and has a length of ~20 km. The Silvermine River and straight-line stream segments follow the lineament trace. The village of Silvermine and the historical "Wilton Lead Mine" also lie on the SRL. Numerous NW-trending lineaments that are parallel to the SRL occur elsewhere in western CT. Some have associated qtz-Pb-Zn mineralization (e.g., Sandy Hook and Trumbull localities). Near Trumbull the Permian Pinewood Adamellite pluton and associated dacite and rhyolite porphyry bodies occur along a regional line that is parallel to the SRL and other NW-trending lineaments (~325^o). Numerous NW-trending mesoscale quartz veins also occur along this line. And near Southbury, a Permian syenite was emplaced along a NW-trending fault. Regionally NE-trending fractures with calcite and zeolites cut the country rocks and the Jurassic basalt flows in the nearby Pomperaug basin. Qtz-Pb-Zn and calcite-zeolite assemblages coexist at an intersection of NW- and NE-trending lineaments near Thomaston. The paragenetic sequence is consistent with early qtz-Pb-Zn mineralization followed by calcite and zeolites.

Pb isotope analyses of galena from the Norwalk site yield Pb²⁰⁶/Pb²⁰⁴ ratios from 18.572 to 20.120. Data yielded a Pb²⁰⁷/Pb²⁰⁶ age of 1.29 Ga (±0.3) which is interpreted to represent the age of the source rocks. This implies that Grenville basement rocks underlie the area. Since the galena-bearing veins cut an Ordovician unit, the Pb in the galena must have been remobilized at least once since the Grenville, probably during the Permian. This timing is consistent with the age of the Pinewood Adamellite.