COMPARISON OF APATITE FISSION TRACK AGES AND TRACE ELEMENT CONCENTRATIONS IN APATITES FROM IRON ORE DEPOSITS IN THE ADIRONDACK MOUNTAINS IN NEW YORK STATE AND CARBONATITES IN SOUTHERN ONTARIO

Apatite fission-track (AFT) ages and U concentrations have been determined for apatite samples from the following types of apatite-rich deposits: 1) the Cheney Pond nelsonite from Tahawus, NY, 2) the Lyon Mountain gneiss from the Rutgers and Palmer Hill Mines north of Ausable Forks, NY, and 3) the Silver Crater mine and Bear Lake Diggings carbonatite deposits from near Bancroft, Ontario. The Cheney Pond nelsonite yielded an AFT age of 143 Ma which is comparable to other AFT ages from the High Peaks region (168-135 Ma) and to an AFT age for the Port Leyden nelsonite (136 Ma) from the Adirondack Lowlands. Both nelsonites had very low mean U concentrations of 4 ppm and 16 ppm, respectively. In contrast, samples from the Palmer Hill and Rutgers Mines in the Lyon Mountain gneiss yielded significantly higher U concentrations of 83 and 49 ppm, respectively, comparable to a Lyon Mountain gneiss sample from Mineville (68 ppm) in the eastern Adirondacks. The Palmer Hill sample yielded an AFT age of 140 Ma; however, the AFT age measured for the Rutgers Mine sample was anomalously low, 7.4 ± 2 Ma. Field observations do not show any sign human activity that could have heated the rocks above the closure temperature for fission-track retention in apatite (~100°C) near where the sample was collected, such as a blast furnace. Additional samples have been collected to verify this unrealistic AFT age.

The Palmer Hill and Rutgers Mine apatites are fluorapatites having Cl/Cl+F ratios of 0.4 which is less than the Cl/Cl+F ratio of Durango apatite (0.10). These apatites also yielded high Ce and Nd concentrations (1.41 and 0.69 weight % and 0.40 and 0.15 weight %, respectively). The Cheney Pond nelsonite yielded a lower Cl/Cl+F ratio of 0.02 and Ce and Nd concentrations (0.067 and 0.032 weight %) than the Ausable Forks area apatites.

Samples from apatite-rich carbonatites at Silver Crater mine and Bear Lake Diggings near Bancroft, Ontario yielded comparable AFT ages of 146 and 156 Ma, and U concentrations of 36 and 19 ppm, respectively. Apatites from both locations are fluorapatites giving a very low Cl/Cl+F ratio of 0.0005. Compared to iron ore deposit associated apatites, Silver Crater and Bear Lake apatites are enriched in Sr content (0.10 and 0.38 weight %) and are comparable to the Rutgers Mine apatites in Ce (0.30 weight %) and Nd (0.09 weight %) concentrations.