HIGH TEMPERATURE, HYDROTHERMAL ORIGIN OF LOW-TI, FE-OXIDE (REE-AU-U-CU) DEPOSITS AND SODIC ALTERATION, ADIRONDACK MTS., NEW YORK, U.S.A

The Adirondacks contain significant Kiruna-type,low-Ti, Fe-oxide(REE-Au-U-Cu) deposits. Lyon Mt. Leucogranitic Gneiss (LMG) either hosts, or is proximal to, these deposits. Precise U-Pb zircon dating demonstrates that LMG consists of late- to post-tectonic granites emplaced during the waning stages of the ca.1090-1035 Ma Ottawan Orogeny. Crosscutting relationships constrain most ore deposition to the interval~1050-1030 Ma. Ores are generally massive, undeformed, and contain apatite and/or aegerine-augite. They commonly occur as shoots or layers parallel to host rock structures and are broadly conformable but subtle truncations are present. Ores remain undeformed even in highly deformed host rock. LMG consists of ~50% quartz-perthite (QP),~20% quartz-albite (QA),~25% quartz-microantiperthite (QA).The remaining 5% includes fayalite granite. Ores are commonly associated with QA and QP, although these may not be the immediate host. Chemical and petrographic study demonstrates that QA results from metasomatic replacement of QP perthite by albite. QA, characterized by checkerboard albite, is an intermediate product. Accompanying phases are sphene, apatite, aegerine-augite, and magnetite. Even where end-member QA is absent from deposits, checkerboard albite is present in wall rocks. Ore layers in granitoids are commonly interleaved with grain-size-reduced,rolled feldspars. Also present, and generally restricted to the magnetite with which it is coeval, is undeformed aegerine-augite. This observation demonstrates that the magnetite is also undeformed and postdates deformation in wall rocks. The most plausible origin for the ore and pyroxene is via hydrothermal fluids passing through the fracture systems. Oxygen isotope thermometry indicates that hydrothermal fluids associated with ore deposition equilibrated with LMG at 600°C-675°C and that whole rock d18O is consistent with saline brines containing a significant component of surface-derived basinal waters. These fluids were active during delamination-related uplift of the Ottawan Orogen and were driven by LMG plutonic heat.