FIELD AND GEOCHRONOLOGIC CONSTRAINTS ON THE IRON OXIDE-APATITE (IOA) ORES OF THE EASTERN ADIRONDACKS

In conjunction with field studies of key localities, U-Pb zircon analyses have been carried out on the major iron oxide-apatite (IOA) deposits of the eastern Adirondack region, their host rocks, and cross-cutting late pegmatites. At the Cheever Mine, above Lake Champlain near Port Henry, the planar ore body trends north-south and dips moderately to the west. The workings can be traced for several kilometers along strike. The ore has a knife sharp contact with the host rocks and, in contrast to the wall rocks, is undeformed. The ore gave a U-Pb zircon age of 1035 Ma, while three samples of leucogranite yield ages of ca. 1045-1030 Ma, indicating a close temporal relationship between the ore and Lyon Mountain Granite. Granitic and syenitic gneisses cut by the ore give ages of ca. 1135 Ma and the zircons analyzed have well-defined rims which yield an age of ca. 1060 Ma. At Mineville, the ore consists of several parallel, inclined, intrusions which were worked along strike for nearly 10 kilometers underground. A variety of rocks exposed in the walls of the No. 21, Barton, and Old Bed mines have been dated. Three samples of granitic to syenitic host rocks yield ages between 1140-1130 Ma. Pegmatites cross-cutting the ore at Barton Hill and the Old Bed mines yield ages of ca. 1040-1020 Ma. Three samples of the ore were dated and yield ages of ca. 1040 Ma and 980-970 Ma. Multiple ages for the ore may indicate various pulses of hydrothermal alteration, zircon growth, and resetting of isotopic systems as has been previously documented for monazite, apatite, and allanite-monazite symplectite (Cheever Mine). A sample of gabbro yielded an age of ca. 1030 Ma, although a metamorphic origin is suspected for the zircons it contains. Compilation of the U-Pb zircon data available from the eastern Adirondack iron mines suggests a close temporal and, likely, genetic association between the Lyon Mountain Granite and ore intrusion. A linkage between post-orogenic collapse, extensional structures, leucogranite and ore intrusion, and hydrothermal alteration is proposed.