MINERALOGY AND MINERAL CHEMISTRY OF SHEAR ZONE-HOSTED IRON DEPOSITS FROM THE WESTERN HUDSON HIGHLANDS, NY

The Proterozoic iron deposits from the western Hudson Highlands, NY are hosted by late Grenville dextral strike-slip shear-zones within a sequence of metasedimentary, metaigneous and quartzofeldspatic gneiss. The shear-zones became dilatational during the terminal phase of activity and were infiltrated by hot, chemically complex fluids that selectively leached iron and other species out from a pre-Grenville Fe-rich protolith and deposited magnetite ore and gangue minerals as concordant/slightly discordant, tabular and lensoidal vein-like bodies within the faults. Dikes of basalt, camptonite and rhyolite cross cut the country rocks, shear-zones and iron ore.

Two general mineral sequences were identified within these zones: 1) Oxide sequence associated with amphibolites, calc-silicate and quartzofeldspathic gneiss carrying magnetite (no martite) with spinel (spinel, hercynite, ulvöspinel) exsolutions, as granulite facies features, ilmenite, diopside, augite, enstatite, ferrosilite, olivine of hortonolite-hyalosiderite composition (Fo 43 to 61), and almandine (Alm70) ± calcite or quartz in varying proportions depending upon host rock composition and 2) Sulfide sequence crosscutting the magnetite ore, with pyrrhotite (later replaced by marcasite), chalcopyrite, ± molybdenite, cobaltite, skutterudite, pentlandite, accompanied by K-Cl and/or F rich amphiboles (ferro-edenite, hastingsite, magnesiohastingsite, pargasite, potassicpargasite, K dominant hastingsite, K and F dominant hastingsite, F dominant tremolite), phlogopite, F dominant phlogopite and marialitic scapolite (Cl up to 0.49% and SO3 up to 1.24%). The K (up to 3.2%) and Cl (up to 3.66%)/F (up to 2.26%) rich amphiboles indicate positive relationship for AK and IVAl versus Cl and a negative one for A[1-(Na+K)] versus Cl. Calculated Fe3+/Fe2+ ratio and mineral composition suggest a crystallization under conditions of relatively low f(O2). The source of both Cl and F is not clear, but the best match is a high salinity, crustal brine.

Very coarse-grained pegmatites are primarily around the ore bodies, encapsulate or cross them and contain xenoliths of ore material. They are REE (monazite, allanite) – U (uraninite) alkali-feldspar pegmatites with augite and/or hastingsite and magnesiohastingsite.