A PETROGRAPHIC COMPARISON OF THE LYON MOUNTAIN GRANITE FROM DANNEMORA MOUNTAIN AND THE ARNOLD HILL, PALMER HILL AND CHATEAUGUAY MINES IN THE NORTHEASTERN ADIRONDACK MOUNTAINS, NEW YORK

The Lyon Mountain Granite (LMG), which gives a U-Pb zircon age of ~1060 Ma (Valley et al., 2009), crops out extensively in the northeastern Adirondack Mountains of New York state and contains numerous magnetite-apatite deposits. Because the Lyon Mountain Granite varies in composition throughout the study area and shows evidence of periods of both Na and K metasomatism, samples were collected for a comparative petrographic modal analysis from four mineralized localities, Dannemora Mountain, and the Arnold Hill, Palmer Hill and Chateauguay mines. Thin sections were made and examined for all LMG samples with the following petrographic results. At Dannemora Mountain, the LMG was a pristine microcline granite (Valley et al., 2011) with microcline and microcline perthite being the dominant phases at 70% with lesser amounts of quartz (13%), abundant magnetite (10%) and minor amounts of biotite and zircon (4 and 3%, respectively). The Arnold Hill and Palmer Hill mines are located ~ 5 km apart but differ in LMG mineralogy. At Arnold Hill, the LMG contained equal amounts of quartz and microcline perthite (35%), abundant magnetite (15%), albite (An₉) at 3% and lesser amounts of biotite (3%), titanite (2%) and zircon (1%). The Palmer Hill LMG contained a greater percentage of quartz (50%) and equal amounts of microcline and microcline perthite (30%) compared to Arnold Hill. Albite (An₇) was also present (7%). Magnetite was less abundant (2%) and biotite (3%) and zircon (2%) were present in minor amounts. At the Chateauguay Mine in Lyon Mountain, the LMG showed the greatest mineral variation. Quartz was the dominant phase at 48% with microcline and microcline perthite (combined) at 30%. Clinopyroxene was present for the first time in any of the LMG samples from this study (8%). Titanite was abundant at 6% and albite (An₈) at 4%, magnetite (2%), biotite and zircon (1% each) were also present. The LMG samples from this study in the northern Adirondacks showed the following: 1) a range of composition from microcline-rich to quartz-rich, 2) abundant magnetite but no apatite, 3) generally albite was present which indicates Na-rich fluid metamorphism (Valley et al., 2011), and 4) minor amounts of titanite, biotite, and zircon.