Hydrothermal alteration, mass transfer and magnetite mineralization in dextral shear zones, western Hudson Highlands, New York, United States

• Seismic pumping and fault-valve control asynchronous deformation and mineralization.
• Massive magnetite and gangue assemblages mineralized from metamorphic derived fluids.
• Fluids chemically buffered to wall rock, contributing to vein assemblage compositions.
• Local country rock geochemistry controlled mineralized vein assemblages.

Massive magnetite veins formed during hydrothermal mineralization within northeast striking dextral shear zones in Proterozoic-age crystalline bedrock of the western Hudson Highlands. The veins formed in an open fracture system in right step-over dilational jogs during the late stages of movement. 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 ‘seismic pumping’ events. The fluids deposited mineral assemblages in the fractures that reflect the changing flux, fluid buffering, and/or physical conditions.This process produced three zones, outward from the unaltered country rock which are: 1) a bleached zone of altered wall rock adjacent to the vein, 2) an outer banded zone in the vein, of ferromagnesian-rich bands, and 3) a core of massive magnetite ore and gangue minerals. Bleached zones are dominated by amphibole and/or pyroxene, with scapolite, biotite, and apatite, within metavolcanic and quartzofeldspathic gneiss, or phlogopite and calcite within calc-silicate country rock. Calc-silicate banded and massive assemblages contain clinopyroxene, calcite, amphibole, and/or biotite or phlogopite. Quartzofeldspathic and metavolcanic banded and massive assemblages are dominated by amphibole and/or orthopyroxene, with quartz and/or sulfides locally. Both assemblages contain magnetite central to the deposits.Geochemical modeling of the bleached zone shows overall gains in volume (2.5–20.3%) and mass (3.1–18.1 g relative to 100 g of wall rock). In all cases, iron (2.4–5.3 g), magnesium (1.0–2.8 g), and calcium (0.6–6.5 g) were gained, especially adjacent to calc-silicates. Deposits adjacent to quartzofeldspathic country rock had large gains in silica (4.4–7.4 g), whereas deposits in mafic metavolcanic rock lost silica (1.4–3.8 g). Based on the mobility of silica, fluid fluxes were calculated between 5.3 × 105 and 6.6 × 106 cm3/cm2 for bleached zone alteration. Elements in abundance in the country rock contributed to the composition of the deposits, dominantly silica, carbonate, and sulfides.