llmenite-Pyrophanite and Niobian RuHle in tbe South Mountafn Batholith, Nova Scotia
Pelrine, Karla M.
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Ilmenite and rutile are ubiquitous, but modally scarce (<0.5%), minerals in gr~nitoid rocks of the differentiated peraluminous South Mountain Batholith (SMB). Ilmenite occurs as blocky 0.05-0.90 mm grains in biotite, and as discrete larger anhedral grains along silicate grain boundaries. Ilmenite grains show compositional zoning toward the pyrophanite (MnTi03) end-member, ranging from 3-15 wt.% MnO in the cores to 5-23 wt.% MnO on the rims. Rim-core differences range from 2-12 wt.% MnO, generally with larger variations in the more fractionated rocks. With increasing fractionation in the batholith as a whole, the MnO contents of the ilmenites tend to decrease, albeit with considerable s<;atter. Several anhedral ilmenite grains appear to exhibit the same characteristics as those that occur in the neighbouring Meguma Supergroup. Texturally and chemically, ilmenite appears to be a primary magmatic mineral of the SMB througho11t its crystallization history, although some grains may be xenocrystic. Rutile occurs as 0.03-0.70 mm, euhedral to anhedral grains, mainly as inclusions in biotite. Compositionally, most rutiles contain Nb20s (up to 4 wt. %) and Ta20 5 (up to 2 wt. %), both elements becoming more highly concentrated in rutiles from the more fractionated granitic rocks (from 0.5-1.5 wt.% Nb20s+Ta20s in the early rocks to 0.1-3.5 wt.% Nb20s+Taz0s in the most evolved rocks). Rutile occurs as three texturally and chemically distinct types. Type 1 rutile occurs as large discrete grains with the highest concentrations ofNb and Ta, 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 ofNb and Ta, 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 ofNb and Ta, and appear to be the product of ilmenite breakdown during hydrothermal alteration. In the early history of the batholith, ilmenite sequesters most of the titanium available in 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 evolution of the batholith, whole-rock Nb+ Ta remains roughly constant at about 1 0-15 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