TEXTURAL RELATIONSHIPS AND COMPOSITIONAL VARIATIONS OF FLUORAPATITE IN SOME GRENVILLE-AGE MAGNETITE ORES FROM THE ADIRONDACK MOUNTAINS, NY

Apatite with low Cl (up to 0.5 wt. %) and high F (2.85 wt % from EMPA) crystals from some Precambrian magnetite-apatite and magnetite deposits from the eastern side of the Adirondack Mountains were investigated for textural relationships and compositional variations by transmitted light microscopy, BSE, EMPA, ICP-MS and CL. In both massive magnetite-apatite and magnetite ore the fluorapatite grains are enriched in LREE (47,903 ppm), Y (11,825 ppm) as well as in Si (up to 5. 22 wt. %). The chondrite-normalized plot for the bulk analysis of apatite shows strong enrichment in LREE (higher than the fluorapatite from some Kiruna-type deposits), strong Eu anomaly, and similar pattern with the host-rock. Four major textural types were analyzed: a) fractured fluorapatite with tiny secondary monazite-Ce and kainosite-Y developed along fractures; b) apatite grains mantled by monazite-Ce, monazite-Ce intergrown with magnetite or alanite-Ce; c) tiny patches of zoned apatite more or less with the same crystallographic orientation in larger apatite, and d) areas with relatively low BSE intensities within brighter apatite or along crystal rims. The brighter areas of the apatite in all the analyzed samples have more REE, Y, Na, and Si than the darker ones. This is considered a covariation between Na, Si and LREE that is balanced by the increase in Ca and P in darker areas (Harlov et al. 2002).The dark areas in apatite are interpreted as the result of the selective leaching of the LREE, Y and Si by late fluid phases along the grain boundaries. Monazite-Ce (33. 17 wt. % Ce2O3, 14.50 wt. % Nd2O3, 13.44 wt. % La2O3) contains 1.61 wt. % Th2O3 and 1.19 wt. % Y2O3 and shows low xenotime and huttonite but large amounts of La and Nd for Ce substitutions. The CL study reflects variations in the distributions of the REE within fluorapatite of the c) and d) textural types and coupled with EMPA indicates very low substitution of Ca by Fe2+ in the fluorapatite structure.

The mobilization of the LREE, Y, and Si and their deposition as secondary monazite, kainosite, and quartz is correlated with the late development of the low metamorphic grade assemblage containing chlorite, hematite, V-bearing titanite, ferro-actinolite and rutile as the result of the interaction between the primary minerals (magnetite, ilmenite, hedenbergite, REE-bearing apatite) and a crustal fluid.