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Microscopic, non-gem quality, grains of blue sapphire (corundum) have been identified in a small (1–2 cm wide), discontinuous, dike of nelsonite hosted by aluminous feldspathic gneiss. The gneiss was excavated during the construction of a hydroelectric plant on the Black River at Port Leyden, NY (western Adirondack Highlands). The sapphire location is 250 m NE of the Port Leyden nelsonite deposit. The small dike may represent a separate intrusion of nelsonite or one sheared from the main nelsonite orebody during Ottawan (circa 1050 Ma) deformation and metamorphism. The sapphires range in size from 0.1 to 2.0 mm, and commonly show parting, pleochroism, and hexagonal oscillatory zoning (from deep blue to clear). Electron microprobe analysis shows comparable levels of Fe in both clear (0.71–0.75 wt. %) and blue (0.38–0.77 wt. %) portions of grains, but clear sections have significantly lower TiO₂ levels (0.002–0.011 wt.%) compared to blue sections (0.219–0.470 wt. %). Cr₂O₃ abundances range from 0.006 to 0.079 wt. % whereas V₂O₃ abundances range from 0.010 to 0.077 wt. % in blue sapphires. Small amounts of MgO were detected in one of the clear corundum grains (0.013 wt. %) and two of the six blue grains (0.001–0.015 wt. %), but the remaining five grains were below the limit of detection. Ga₂O₃, however, was detected in five out of six blue-colored grains (0.026–0.097 wt. %) but was below the limits of detection for clear grains. Optical spectroscopic data collected on the blue sapphire grains show broad absorbance in the yellow, orange, and red part of the spectrum (~565–740 nm) consistent with intervalence charge transfer between the next nearest neighbor Fe²⁺ and Ti⁴⁺. A magmatic origin of the sapphire grains is supported by petrologic and trace element data from the blue sapphires, but Cr abundances are inconsistent with this interpretation. Sapphire in a nelsonite host rock represents a new type of occurrence. Full article

The Kunene Intrusive Complex (KIC), in NW Namibia and SW Angola, is one of the largest Proterozoic anorthosite massif-type exposures in the world. A geochemical, mineralogical and petrological study of four Fe–Ti(–V) oxide bodies located in the understudied Angolan part of the KIC has been performed. The massive Fe–Ti(–V) oxide bodies, locally apatite-rich (nelsonites), are lenticular or dike-like. They consist mostly of titaniferous magnetite, ilmenite and minor aluminous spinel, apatite, olivine and graphite. Titaniferous magnetite displays a wide variety of subsolvus features, including aluminous spinel–magnetite–ulvöspinel exsolutions and ilmenite (Trellis) exsolutions. This work estimated the composition of the titaniferous magnetite prior to the exsolution, in order to calculate the temperature and oxygen fugacity of the different lithologies of each ore body. The thermo-oxybarometry results obtained range from 600 °C to 820 °C and fO₂ from 10^−24.7 to 10^−14.7. These values do not correspond to magmatic crystallization in equilibrium, but to a later re-equilibration. In addition, the mineralogical and geochemical results indicate that the studied ore bodies contain economic reserves of Ti, V, and possibly of P and REE. Full article