New insights on the evolution of the Lyon Mountain Granite and associated Kiruna-type magnetite-apatite deposits, Adirondack Mountains, New York State

The integrated approach of field work, microscopy, whole-rock and mineral-scale geochemistry, and in situ U-Th-Pb zircon geochronology has proven to be useful for recognizing the type, timing, and sequence of complex Na and K fluid alteration related to the development of Kiruna-type magnetite-apatite deposits and the tectonic evolution of the granites that host these deposits. The Lyon Mountain Granite in the northeastern Adirondack Mountains of New York State has undergone multiple episodes of hydrothermal fluid alteration and Fe mineralization. Perthite granite containing ubiquitous 1060–1050 Ma zircon grains was overprinted by potassic alteration, which in turn was overprinted by pervasive Na alteration. During the Na alteration, preexisting orebodies, consisting of magnetite, clinopyroxene, and apatite, were overprinted and remobilized to form new deposits that contain magnetite, apatite, quartz, and zircon. The U-Th-Pb zircon geochronology data suggest that the Lyon Mountain Granite intruded the Adirondack Highlands during the Ottawan orogeny between ca. 1060 and 1050 Ma. However, subsequent alteration obscured much of the prehistory of the LMG. Amphibolite layers within the Lyon Mountain Granite and granitic dikes and pegmatites that crosscut the foliation in the Lyon Mountain Granite have been dated between 1045 and 1016 Ma. These ages coincide with previous published zircon age data from second-generation orebodies associated with Na alteration.