HYDROTHERMAL MASS TRANSFER AND MAGNETITE MINERALIZATION IN DILATIONAL 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 local country rock, fluids migrated and mixed along the fault during seismically induced ‘pumping’ events. The fluids deposited varying gangue mineral assemblages and massive magnetite in the dilated fractures. The 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 mineralized 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 Amp and/or Px, including Scp, Bt, and Ap, within metavolcanic and quartzofeldspathic gneiss, or Bt and Cal within calc-silicate country rock. Calc-silicate layered and massive vein assemblages contain Cpx, Cal, Amp, and Bt. Quartzofeldspathic and metavolcanic layered and massive vein assemblages are dominated by Amp and/or Opx, with Qtz and Cal or sulfides locally.

Petrological and geochemical evidence reveals that fluids were derived, saturated in chemical species and transported to the dilational zones, where they deposited the gangue assemblages and magnetite ore. The bleached zone shows overall gains in volume (2.5-20.3%) and mass (3.1-18.1g/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, especially in calc-silicate rock. Deposits adjacent to quartzofeldspathic country rock had large gains in silca (4.4-7.4g), whereas deposits in metavolcanic rock lost silica (1.4-3.8g). Based on silica, fluid fluxes were calculated between 5.3 x 10⁵ to 6.6 x 10⁶ cm³/cm² for bleached zone alteration. Elements in abundance in the country rock contributed to the composition of the late cementing minerals, dominantly silica, calcium and sulfides.