Abstract T his D ay of the field trip visits the large Chateaugay open pit of the Lyon Mountain group of iron mines, in the northern Adirondack Mountains, following by visits to roadcuts in the Adirondack Lowlands to the west, in the vicinity of the Balmat zinc mine. The Chateaugay open pit has superb exposures of iron ore remaining on walls and in the entrances to stopes, which clearly establish the geologic and paragenetic relations of the magnetite ores and yield compelling evidence for their high-temperature, intramagmatic origin via hydrothermal fluids (McLelland et al., 2001b, 2001c). A wide variety of rock types ranging from leucogranite to mineralized skarns are present. In the afternoon this day features stops at five roadcuts between the town of Gouverneur, New York, and the Balmat zinc mine to the east. These roadcuts provide a good overview of the regional geologic setting of the Balmat deposit, including stratigraphy, structure, deformation, and metamorphism. Whereas the geochronology in the Adirondack Highlands to the east is well established (e.g., McLelland et al., 2001c), the Lowlands is underlain mostly by metasedimentary rocks hence its precise age range is difficult to determine. One constraint is provided by U-Pb zircon ages of 1284 ±7 Ma, 1236 ± 6 Ma, and 1230 ± 33 Ma for the Gouverneur, Fish Creek, and Hyde School alaskite bodies, respectively, that occur in the Lowlands (Grant et. al., 1986; McLelland and Chiarenzelli, 1990; McLelland et al., 1992). Although these alaskite bodies were interpreted by Carl and Van Diver (1975) to

Abstract Adirondack iron ores consist primarily of massive magnetite that generally occurs in layers or in elongated shoots parallel to fold axes. The magnetite is commonly accompanied by REE-enriched apatite and aegerine-augite. Hand lens inspection of ore reveals numerous octahedra indicating that the ore is either undeformed or recrystallized. The ore is invariably associated with various subunits of Lyon Mt. Granitic Gneiss (LMG), which are either in close proximity or serve as the host rock. A strong correlation exists between occurrences of ore and the presence of albitic subunits of LMG. Detailed investigations of these subunits demonstrate that they result from replacement of LMG microperthite by almost pure albite (˜Ab 98 ). The replacement process can be followed through initial stages of checkerboard albite and finally to quartz-albite rocks. Magnetite concentrations commonly occur in proximity to these rocks and almost every magnetite deposit hosted by granitoids shows evidence of checkerboard albite in the wall rocks. Thin section examination of several layered magnetite deposits demonstrate that they are intimately interleaved with highly deformed feldspathic wall rock that is grain size reduced and contains rolled feldspars reflecting shear strain. Present in the magnetite are crystals of bright green aegerine-augite interpreted to be coeval with the ore and undeformed even at their edges. This observation indicates that the magnetite is also postdeformational, an interpretation supported by narrow veinlets of magnetite that cut deformed feldspar tails at high angles but remain undeformed. It is proposed that ore constituents were transported through, and deposited within, fracture systems that localized iron-bearing fluids. An origin of this sort is consistent with either hydrothermal or immiscible magmatic processes. The latter possibility is considered to be unlikely, since the magnetite appears to be in equilibrium with the several silicate phases that it is in contact with. Moreover a hydrothermal origin is supported by the ubiquitous presence of checkerboard albite. Oxygen isotope studies indicate that the sodic, ore-bearing hydrothermal fluids equilibrated with wall rocks and ore at temperatures of 600°–700°C. The large scale of sodic alteration and magnetite deposition are best explained by a regional fluid of surface origin that evolved into saline brines either by evaporation or by interaction with evaporitic deposits. Such fluids are commonly associated with rare-earth-enriched, Kiruna-type, low Ti, Fe oxide deposits similar to those present in the Adirondacks.