REE-RICH FLUORAPATITE TEXTURES FROM EASTERN ADIRONDACK IRON ORE DEPOSITS

Although apatites were relegated to the tailings pile, the apatite story tells of igneous and hydrothermal events during the evolution of these ore deposits. Iron ore deposits of the eastern Adirondacks bear zoned and altered apatites, which are rich in REE and Y. We present a combined approach including EPMA, LA-ICP-MS, and centimeter-scale BSE imaging. We compare apatites from the Cheever mine (Port Henry, NY) and one of the Hammondville openings (Crown Point, NY) to other apatites from the region.

Cheever mine apatites are concentrically zoned with high-REE, high-Si cores. Cerium is the most abundant REE (Ce>La>Nd>Y) for Cheever apatites. Sum of REE and Si are roughly equal (up to 1.2 apfu REE as compared to 8.8 apfu Ca on a basis of 25 oxygen). REE and Si concentrations decrease outward toward the rim in a manner consistent with diffusive loss or leaching. Distinct alteration products are crack filling including Ce-rich, Y-rich, and Si-rich precipitates. Cheever site also bears an unzoned, REE-free apatite having large quartz inclusions. These are spatially and texturally dissimilar to the diffusely zoned apatites.

Hammondville apatite shows sharply defined zoning of REE and Si, suggestive of a primary magmatic zoning. Hammondville apatites are more Y-rich relative to Cheever apatite (Y>Ce>Nd>La). Precipitation of REE-rich alteration products occurs within the center of the apatite grains rather than outward leaching.

On the chondrite-normalized REE diagram the fluorapatites are extremely enriched in all the REE, with concentrations between 10,000 and 100,000 times chondrite. Profiles are relatively flat, with a slight enrichment in the LREE and a pronounced negative europium anomaly. Eps Nd (CHUR) is -2.63 (Cheever ore), -1.45 (Cheever host rock), and -2.99 (Mineville).

The new field, geochemical, and textural evidence support the idea of an igneous origin and subsequent long-lived, fluid-rock interaction for magnetite ores of the eastern Adirondack Highlands.