Fluid alteration and magnetite-apatite mineralization of the Lyon Mountain granite: Adirondack Mountains, New York State.

摘要

The Lyon Mountain granite (LMG) is exposed in the Adirondack Mountains of northern New York State and is the host to numerous magnetite-apatite (Kiruna type) deposits. The results in this study suggest that the LMG was emplaced in the middle to late Ottawan orogeny between 1065 and 1045 Ma, along extensional shear zones during the onset of orogenic collapse. The original protolith, perthitic granite, has been extensively altered by subsequent Na- and K-rich fluids. Early-formed, zircon-poor clinopyroxene-magnetite-apatite deposits related to granite emplacement were remobilized and chemically modified by Na- and F-bearing fluids to form new deposits that crystallized hydrothermal zircon. Age determinations using U-Th-Pb geochronology of these hydrothermal zircon indicate that Na alteration and second generation mineralization occurred between 1040 Ma and 1000 Ma, 20 to 60 million years after crystallization of the host granite. Hafnium isotopes in zircon suggest that Lu-rich apatite and clinopyroxene associated with early-formed ore deposits broke down during Na and F alteration releasing radiogenic Hf from apatite and Zr from clinopyroxene. Granitic dikes and pegmatites which crosscut the fabric of the LMG, are dated between 1040 Ma and 1015 Ma and are contemporaneous with zircon growth in the iron ore deposits. This implies that Na alteration and Fe-mineralization were the result of externally derived fluids that penetrated the Lyon Mountain granite along extensional shear zones during orogenic collapse or by fluids related to granitic dike and pegmatite emplacement.;The presence of U-Th-Pb zircon ages that are 20 to 60 m.y. younger than the host granites of the magnetite ores, and the highly variable mineralogy of the ore deposits, suggests that multiple fluid and mineralization processes were responsible for ore formation and periodic modification of preexisting ores. However, the presence of magnetite-apatite ore deposits almost exclusively within the LMG implies a "fertility" requirement of the host granite and hence that mineralization was in part related to magmatism.