HYDROTHERMAL HALOGEN-RICH POTASSIC ALTERATION IN SOME IRON DEPOSITS FROM HUDSON HIGHLANDS, NY

Three types of deposits, based on their mineral and geochemical association, can be recognized in the late Grenville shear zone-hosted Precambrian iron deposits from Hudson Highlands: a. Fe (oxide); b. Fe (oxide) + Fe + Cu+/-Ni, Co, Mo (sulfides); c. Fe (oxide) + Fe +/- Cu (sulfides) + U (oxide). These deposits, containing two main oxide and sulfide sequences, display igneous, metamorphic and metasomatic-hydrothermal features that reveal a complex evolution through time.

The iron oxide sequence, probably igneous in origin, is hosted by amphibolites and contains magnetite (+/- ilmenite in places) as compact or disseminated bodies, accompanied by serpentinized olivine (hortonolite-hyalosiderite), and pyroxene or pyroxene, amphibole (more common hastingsite, magnesiohastingsite, pargasite), plagioclase and biotite. Complex, crystallographically oriented spinel (spinel, hercynite, ulvospinel) exsolutions in magnetite or between magnetite and ilmenite are common. The ore bodies are concordant with the metamorphic fabrics. Some of the iron, mobilized by the former partial melts in the granulite facies, occurs as slightly discordant bodies.

The metasomatic-hydrothermal sulfide sequence, hosted by amphibolites or quartzofeldspatic gneisses, and containing pyrrhotite, chalcopyrite and very rare cobaltite, pentlandite and molybdenite, crosscuts the magnetite ore. It is accompanied by potassic alteration that generated K- and Cl- or F-rich amphiboles. The new-formed products develop between pyroxene, magnetite grains, or on the former amphiboles, probably from two compositionally distinct, fluids. The formation of the K, Cl or F-rich amphiboles can be modeled assuming that initially halogen bearing crustal fluids infiltrated the rocks and ore, and subsequently, by hydration reactions with pyroxenes and Fe - Ti oxides, formed K and halogen-rich amphiboles. The mineral composition, the absence of martite, and the Fe3+/Fe2+ ratio suggest a relatively low f(O2) for the fluids. The preferential incorporation of H2O compared to Cl or F into the amphiboles at lower temperature and pressure can explain the compositional variations and the lack of a Cl dominant amphibole. The halogen source is not known, but the best option seems to be a high saline, crustal brine.