AGES AND ORIGINS OF LOW-TI MAGNETITE DEPOSITS OF THE EASTERN ADIRONDACKS: SHRIMP U-PB AND REE EVIDENCE FROM COMPLEXLY ZONED ZIRCON

The origins of Adirondack low-Ti iron deposits (magmatic vs. hydrothermal) remain unresolved despite decades of field and laboratory investigations. Also still in question are the number and duration of episodes responsible for Fe-ore formation. To provide temporal constraints on ore-forming processes in the eastern Adirondacks, we report detailed SHRIMP U-Pb geochronology and REE data for zircon from magnetite ores and adjacent quartz-albite host rocks (altered microperthite granite) at the Skiff Mtn. and Hammondville magnetite deposits.

Zircon in magnetite from the Schofield mine (Skiff Mtn.) is mostly euhedral, blocky, and colorless. High resolution CL imagery reveals an internal stratigraphy of three sector-zoned rims (mR1, mR2, and mR3) that occur in all grains. Zircon from altered granite has partially resorbed oscillatory-zoned cores (gC), plus 3 episodes of rims (gR1, gR2, and gR3). Ages of Fe-ore zircon rims are 1030 ± 12 (mR1), 1022 ± 10 (mR2), and 996 ± 8 Ma (mR3); ages for altered granite zircon are 1116 ± 9 Ma (gC), 1044 ± 8 (gR1), 1041 ± 12 (gR2), and 1016 ± 10 Ma (gR3). Th/U and REE data indicate that gC is igneous and dates the time of emplacement of the granite protolith, whereas mR1 and mR2 are approximately coeval with gR2, and gR3, and are of hydrothermal origin. Geochemical evidence suggests that gR1and mR3 formed during separate metamorphic events.

Zircon in magnetite from the Blacksmith mine (Hammondville) is equant to oblong, and brown; zircon from the altered granite is euhedral and dark brown. In CL, the Fe-ore zircon displays 3 sector-zoned rims; zircon from the altered granite host contains strongly resorbed oscillatory-zoned cores, and at least two episodes of rims. Ages for Fe-ore zircon rims are 1059 ± 7, 1028 ± 5, and 1007 ± 4 Ma; ages for altered granite zircon are 1117 ± 18 Ma (igneous cores), plus 1044 ± 6 and 1037 ± 7 Ma (rims). Th/U and REE data suggest that all rims from both samples are metamorphic in origin.

These results indicate that emplacement of the host granites (~1120-1110 Ma) occurred about 90-40 m.y. prior to the beginning of ore formation, which began during the Ottawan event, and persisted episodically until at least 1000 Ma (Rigolet event). Thus, these data support interpretations of hydrothermal and metamorphic (i.e. post-magmatic) origins for these two magnetite deposits.