GEOLOGY OF HISTORIC MINERAL COLLECTING LOCALITIES IN ST. LAWRENCE COUNTY, NEW YORK

St. Lawrence County has produced spectacular mineral specimens for museums and collectors for nearly two centuries. In addition, five mineral species new to science have been identified. While specimens from known and forgotten localities exist in museums and collections world-wide, relatively little is known of their geological significance. Therefore, we have begun a study to determine the field relations, age, and origin of these enigmatic occurrences. Nearly three dozen known, and many forgotten or lost, localities recognized for their well-formed crystals (up to 50 cm or more) occur within a 30-km long sinuous belt of metasedimentary rocks proximal, or adjacent, to the Carthage Colton Shear Zone. The assemblages are dominated by pale-to-dark green to grey to white tremolite and/or diopside, typically associated with one or more additional minerals including calcite, quartz, fluorapatite, the tourmaline group, scapolite, albite, micas, and minor sulfides. The rocks hosting the mineral localities include an assemblage of calc-silicate gneisses with relatively pure layers of dolomitic marble, quartzite, and magnetite and/or biotite-bearing quartzofeldspathic gneisses. All of these rocks can be found near or in structural contact with weak to strongly layered and folded leucogneisses and granites (foot wall). At many locations a diopside-bearing, white perthite-quartz pegmatite cross-cuts the host rocks. The crystal-bearing mineralized seams also cross-cut the foliation in the host rocks indicating a post-tectonic, intrusive origin. Pegmatitic veins associated with the Powers Farm tourmaline deposit in Pierrepont, NY yield a late to post-Shawinigan age of 1158.3 ± 3 Ma; currently samples from several of these locations are under geochronological investigation to confirm or refute this age and the synchronicity of these occurrences.