HYDROTHERMAL ALTERATION, MASS TRANSFER AND MAGNETITE MINERALIZATION IN DEXTRAL SHEAR ZONES, WESTERN HUDSON HIGHLANDS, NY

Hydrothermally mineralized, massive magnetite veins within NE trending, dextral shear zones, formed in an open fracture system, during the late dilational stages of deformation (~924 Ma), in crystalline rocks of the western Hudson Highlands, NY. Metamorphic fluids flushed through fractures and reacted with wall rocks, mobilizing elements and buffering the composition of the fluids depending upon the composition of the local country rock. As these fluids encountered favorable physical or chemical conditions, they deposited gangue minerals, in cm-scale banding of different assemblages and compositions. The banding reflects the changing chemistries of the fluids from changes in flux, fluid source, and/or physical conditions. Massive, meter-scale magnetite deposits occur in the center of the veins and were extensively mined during the middle to late 19th century.

The vein-wall rock contact is sharp and semi-concordant to mylonitic foliation, however on the small-scale, it crosses foliation and erodes into the wall rock. Country rocks are primarily quartzofeldspathic, calc-silicate, and amphibole-pyroxene (metavolcanic) gneisses. A “bleached zone” in the wall rock at the vein boundary is characterized by mineral assemblages, where extensive rock/fluid interaction has led to alterations of local mineralogy, dominantly Pl to more Ca-, Si-, and S-rich phases. In quartzofeldspathic rock, the bleach zone is marked by retrogression of Fsp to Mca and Px to Am, also containing Scp, Ap, and Cal locally, followed by Mag, cemented by massive Qtz. In areas of calc-silicate gneisses, they contain skarn intergrowths of Scp, Di, and Phl followed by Mag and cemented by Cal. Zones that connect the magnetite deposits are thinner and typically composed of random to aligned Cpx with minor Mag, Phl, and/or Qtz, also where low Fe Px (Di) are replaced by high Fe Px (Hd). Cm-scale bulk chemistries, Am, Scp, and Phl yield information on the composition of the fluids involved in the evolution of the iron deposits. Preliminary isocon analysis reveal general trends with increases in CaO, Na₂0, MgO, Fe₂O₃, and MnO, with decreases in K₂O and possibly P₂O₅, between wall rock and the bleach zone, adjacent to the vein.