LOW-TI MAGNETITE DEPOSITS OF THE LYON MOUNTAIN GRANITE, ADIRONDACK MOUNTAINS, NEW YORK: AN OVERVIEW

A series of low-Ti magnetite ore bodies are hosted by the Lyon Mountain granite (LMG) in the northeastern Adirondack Mountains of New York. These deposits are highly variable mineralogically and chemically. Ores are comprised almost exclusively of low-Ti magnetite with varying amounts of clinopyroxene, amphibole, quartz, feldspar, biotite, and apatite. Rarely psuedomorphic hematite replaced magnetite. Crosscutting pegmatite dikes remobilized magnetite forming a subset of deposits. Deposits can be subdivided into low-phosphorus and phosphorus bearing ores. Phosphorus content is controlled by apatite, which may show extreme enrichment in rare earth elements (REE). Contacts between the ore and the host LMG may be diffuse or razor sharp. Ore bodies form lenses or elongate “cigar-shaped” shoots that pinch and swell and are separated from each other by lean ore or zones of diffuse magnetite enrichment. This suggests that the ore bodies may have originally been “sheet like” and were subsequently deformed and sheared. Ore bodies that reside in the hinge region of folds are connected by weak mineralization through the fold limbs.

Magnetite mineralization shows evidence for both a magmatic and hydrothermal origin. Individual ore deposits commonly contain evidence of both magmatic and hydrothermal genesis. Blocks of magnetite ore can appear to “float” in a matrix of LMG or contain magmatic textures, but yield U/Pb zircon ages that are 20 to 60 million years younger (1040 Ma to 1000 Ma) than the host granite (~1060-1050 Ma). Zircon ages, together with extremely radiogenic Hf isotope ratios in zircon, suggest hydrothermal growth of the ore zircon grains. Additionally, albitization of the host LMG forms halos around many of the ore bodies. Sub-solidus shear zones are located at or near the ore-granite contact. This suggests that fluid migration used these shear zones as a pathway and altered both the magnetite ore and the granite. Despite being discovered nearly 200 years ago, these deposits are enigmatic and a litany of questions remain.