Hydrothermal alteration, mass transfer and magnetite mineralization in dextral shear zones, western Hudson Highlands, NY

Massive magnetite ‘veins’ formed during hydrothermal mineralization within northeast trending dextral shear zones in the crystalline bedrock of the western Hudson Highlands, New York. The veins formed in right step-over dilational jogs during the late stages of movement, in an open fracture system. Acidic metamorphic fluids derived from metavolcanic country rock and saturated with iron, flushed through fractures, reacted with wall rock, and exchanged chemical species. Buffered by the composition of the local country rock, fluids migrated and mixed along the fault during seismically induced ‘pumping’ events. The fluids migrated and deposited varying gangue mineral assemblages in favorable conditions in the dilated fractures. These assemblages reflect the changing flux, fluid buffering source, and/or physical conditions. Three distinct zones formed through this process. The wall rock adjacent to the vein was altered to form a ‘bleached’ zone. The vein contains an outer ‘layered’ zone of ferromagnesian-rich bands, and a core of ‘massive’ magnetite ore and gangue minerals. Bleached zones are dominated by amphibole and/or pyroxene, including scapolite, biotite, and apatite, within metavolcanic and quartzofeldspathic gneiss, or phlogopite and calcite within calc-silicate country rock. Calc-silicate layered and massive assemblages contain clinopyroxene, calcite, amphibole, and/or biotite or phlogopite. Quartzofeldspathic and metavolcanic layered and massive assemblages are dominated by amphibole and/or orthopyroxene, with quartz and/or sulfides and calcite locally. Both assemblages contain magnetite, including the massive ore, central to the deposits. Geochemical modeling of the bleached zone shows overall gains in volume (2.5-20.3%) and mass (3.1-18.1g relative to 100g of wall rock). In all instances, iron (2.4-5.3g), magnesium (1.0-2.8g), and calcium (0.6-6.5g), were gained, more so in calc-silicate adjacent deposits. Deposits adjacent to quartzofeldspathic country rock had large gains in silica (4.4-7.4g), whereas deposits in/near mafic metavolcanic rock lost silica (1.4-3.8g). Based on the mobility of silica, fluid fluxes were calculated between 5.3 x 105 to 6.6 x 106 cm3/cm2 for bleached zone alteration, large enough for significant element transport. Elements in abundance in the country rock contributed to the composition of the deposits, dominantly silica, calcium and sulfides.M.S.Includes bibliographical referencesIncludes vitaby Michael J. Kalczynsk