TRACE ELEMENTS IN BOROSILICATES FROM SOME NEW YORK PEGMATITES BY LA-ICP-MS

Tourmaline and dumortierite are the two most common borosilicates in pegmatites. New York pegmatites cutting the Grenville-age metamorphic rocks of the Adirondack Mountains and Fordham gneiss (Bedford) and Taconic-age rocks of the Manhattan Schist were sampled for LA-ICP-MS analysis of trace elements in these two minerals.

Schorl is the dominant tourmaline species in the Adirondack and Bedford pegmatites. A sample from the Kinkel Quarry in Bedford, NY has foitite formed in cavities or as an extension of the schorl crystals. Trace element abundances are similar in all the schorl samples investigated. The REEs (LREE>HREE), Ba, Y, Nb, Ta, Zr, Ag, Cd, Sb, Hf, Bi and the actinides are low. Some schorls display color zonation in plane polarized light from a blue core to a brown rim. The rim is enriched in Li, Be, Sc, V, Cr, Ge, Sr, Y, and Sn while the core contains more Zn and Hg. In addition, the rim is also enriched in REEs compared to the core. The schorl from Bedford contains more Li, Be, V, Zn, Ga, Ge, Sr, LREEs, and Pb than associated foitite, while Co, Ni, Zn, Zr, In, Cd, and Sb are of similar concentration, and the HREEs were not detected in either tourmaline species.

Dumortierite is an important mineral of the abyssal class of pegmatites. It is considered a source of Nb required to form columbite in many pegmatites and has variable substitutions of Fe, Mg, and Ti for Al, and P for Si. Dumortierite from Benson Mines, NY shows the highest amount of Fe, Mg, As, and Sb. The trace element composition suggests that the solid solution between dumortierite and the possible isostructural holtite is not significant, some samples showing only low to moderate amount of regular minor components such as As (0.7 - 2362 ppm), Sb (0.21 - 4070 ppm), Nb (9 - 209 ppm), and Ta (0.4 - 2873 ppm). Dumortierite from pegmatites intruding the Manhattan Schist shows the highest content of Nb and Ta. Ti and Mn are low and vary from 8 to 224 and 28 to 669 ppm, respectively. Bi has also low content of 2 to 132 ppm. Additional data is forthcoming.