GEOCHRONOLOGY OF CLASSIC MINERAL DEPOSITS OF ST. LAWRENCE COUNTY, NEW YORK

Mineral localities in St. Lawrence County, New York have received attention from collectors and provided fine specimens to the world’s mineralogical museums for more than a century, yet little is known about their geological setting and age. Here we present U-Pb zircon data from three locations known for the size, abundance, and quality of their specimens, and a potentially related occurrence of geological interest. An undeformed pegmatitic dike consisting primarily of black tourmaline and quartz, cross-cutting the foliation in Grenville supracrustal gneisses on the Powers farm, in Pierrepont, yielded a population of large, euhedral, zoned zircon crystals whose age is 1158.3 ± 3 Ma, establishing a late Shawinigan intrusive age, consistent with its location in the Adirondack Lowlands. An undeformed intrusive mass of diopside-calcite at Edwards yielded an age of 1049 ± 15 Ma establishing an Ottawan age of intrusion consistent with its location in the Adirondack Highlands. Xenocrystic zircons ranging in age from 1255-1515 Ma were derived from the underlying country rocks. An undeformed pegmatitic occurrence of calcite-purple diopside-scapolite-k-spar pegmatite at Pitcairn has a complex zircon population yielding Ottawan (1045.6 ± 7.5 Ma), Shawinigan (1169.1 ± 10 Ma), and pre-Grenvillian (1373 ± 20 Ma) ages. This deposit occurs in the Highlands and is undeformed, supporting an Ottawan intrusive age. Diopside-calcite-k-spar pegmatitic pods between clasts in a mega-breccia exposed in the bed of the Raquette River within the Carthage Colton Shear Zone (CCSZ) contains two dominant zircon populations including cores with Pre-Grenville ages of 1262 ± 13 and 1332 ± 7.3 Ma and voluminous rims yielding an age of 1165.2 ± 3.1 Ma. A small population of zircons are of Ottawan age (1042.5 ± 7.2 Ma). Given the association with a post-orogenic tectonic breccia, the age of the pegmatite is considered Ottawan; however, xenocrystic zircon indicates the influence of pre-Grenville crust modified by Shawinigan orogenesis. The xenocrystic zircon documents previously unrecognized pre-Grenvillian crust in the vicinity of the CCSZ. The origin of late calcite-diopside Ottawan-aged pegmatites is poorly understood but likely incorporates post-orogenic mantle and crustal components intruded in the deep roots of the orogenic belt.