ILMENITE-PYROPHANITE AND NIOBIAN RUTILE IN THE SOUTH MOUNTAIN BATHOLITH, NOVA SCOTIA

Ilmenite and rutile are ubiquitous accessory minerals in granitoid rocks of the differentiated peraluminous South Mountain Batholith (SMB). Ilmenite occurs as blocky <0.90 mm grains in biotite, and as discrete larger anhedral grains along silicate grain boundaries, and shows compositional zoning toward the pyrophanite (MnTiO₃) end-member (cores 3-15 wt% MnO; rims 5-23 wt% MnO). With increasing fractionation in the SMB, the MnO contents of the ilmenites tend to decrease. Texturally and chemically, ilmenite appears to be a primary magmatic mineral of the SMB throughout its crystallization history, although some anhedral grains resemble those occurring in Meguma Supergroup country rocks and may be xenocrystic. Rutile occurs as three texturally and chemically distinct types. Type 1 rutile occurs as large discrete grains with the highest concentrations of Nb (0.20-8.11 wt%) and Ta (0.02-2.60 wt%), and appears to be primary magmatic in origin. Type 2 rutile grains occur within chloritized biotite, are smaller than Type 1 rutiles, have moderate concentrations of Nb (0.07-2.00 wt%) and Ta (0.01-1.25 wt%), and appear to be the product of the hydrothermal alteration of biotite. Type 3 rutile grains occur within grains of ilmenite, are the smallest of the three types, have the lowest concentrations of Nb (0.25-0.91 wt%) and Ta (0.03-0.24 wt%), and appear to be the product of ilmenite breakdown during hydrothermal alteration. In the early crystallization history of the SMB, ilmenite sequesters most of the titanium from the silicate melt, whereas in the later stages of evolution, rutile sequesters most of the titanium and acts as a host to niobium and tantalum. During chemical evolution of the SMB, whole-rock Nb+Ta remains roughly constant at about 10-15 ppm, but the latest and most evolved rocks show a wide variation from 5-50 ppm Nb+Ta. With this differentiation, the whole-rock Nb/Ta ratio decreases from ~15 to 3, whereas the rutile Nb/Ta ratio increases from ~5 to ~20. Niobium-tantalum fractionation, as indicated by the variation in whole-rock and rutile Nb/Ta ratios, has implications for the formation of tantalum mineral deposits in the late stages of differentiation of the batholith.