ULTRAMAFIC/MAFIC ROCKS ASSOCIATED WITH IRON MINERALIZATION IN THE WESTERN ADIRONDACK HIGHLANDS

Ultramafic rocks are volumetrically rare in the Adirondacks but provide important clues to the region's tectonic origin. Recent work in the Lowlands has identified peridotitic rocks that are part of a dismembered ophiolite suite (Pyrites Complex). However, thus far, similar rocks are not known from the Adirondack Highlands. A sequence of ultramafic and mafic rocks associated with iron deposits are known from Star Lake and Jayville in St. Lawrence County, NY. Benson mines, outside of Star Lake, was discovered in 1810 along the route of the new military road from Albany to Ogdensburg. During its operation it was the largest open pit iron mine in the world. Unique among the Adirondack’s numerous iron deposits, much of the iron is hosted in sillimanite-rich gneisses and the ore includes both magnetite and martite. In addition, much of the western extent of the ore body is in contact with a unit previously denoted as ferromagnesian gneiss. Currently little is known of the age or origin of deposit however, U-Pb zircon analyses of the ore have yielded an age of 1056+/-12 Ma. The characteristics of the zircon, particularly the high U/Th ratios (ca. 45.5), are compatible with a metamorphic origin. Recent quarrying of the mine tailings for road metal has exposed numerous green to black mafic to ultramafic boulder-sized blocks at the mine whose geochemistry and petrography are the subject of this study. This variable suite of rocks has Mg numbers generally in excess of 0.5 and Cr and Ni contents as high as several hundred ppm. The rocks are generally coarse-grained, green to black, and composed of pyroxene and amphibole with minor phases including magnetite, annite, and titanite. One sample contains large (5-10 mm) serpentine-talc-dolomite pseudomorphs after olivine in a coarse-grained magnetite groundmass. High F concentrations (~1.0%) in some rocks suggest kinship with hornblendite known from the nearby iron ore at Jayville. Euhedral and strongly zoned zircons from the Jayville hornblendite yield an age of 1045.7+/-8.9 Ma. The ore at Jayville is also unique in that it contains abundant Vonsenite (Fe²⁺₂ Fe³⁺BO₅) as one of the iron ore minerals. Understanding the geochemistry and petrography of these ultramafic rocks will help constraint the origin of these iron deposits which differ from those in the eastern Adirondack iron district.