LOW-TEMPERATURE THERMAL HISTORY OF THE GILMORE DOME AREA, FAIRBANKS MINING DISTRICT, ALASKA

Eight apatite and two zircon fission-track ages provide evidence of co mplex Tertiary thermal overprinting by hydrothermal fluids in the Gilm ore Dome area. Five ages on apatite from the Fort Knox gold deposit av erage 41 Ma, one from the Stepovich prospect is 80 Ma, and two from Pe dro Dome average 67 Ma. Elevations of these samples overlap but their ages do not, indicating that each area experienced a different thermal history. Ages of apatite from the Fort Knox gold deposit decrease wit h elevation from 42 to 36 Ma but have data trends indicative of comple x cooling. Two approximately 51 Ma ages on zircon indicate that maximu m temperatures approached or exceeded approximately 180-degrees-C. An alteration assemblage of chalcedony + zeolite + calcite + clay in the deposit resulted from deposition by a paleohydrothermal system. The da ta suggest that the system followed a complex cooling path from > 180 to < 110-degrees-C between 51 and 36 Ma, and that final cooling to bel ow 60-degrees-C occurred after approximately 25 Ma. The 80 Ma age from Stepovich prospect either resulted from cooling after intrusion of th e underlying pluton (approximately 90 Ma) or records postintrusion the rmal overprinting sometime after approximately 50 Ma. The 67 Ma sample s from Pedro Dome may also have experienced partial age reduction duri ng later heating. The differences in the data from the different areas and the presence of a late alteration assemblage at Fort Knox suggest that the fluids responsible for heating were largely confined to the highly fractured and porous Fort Knox pluton.