Oxidation of the LaBlache Lake Titaniferous Magnetite Deposit, Quebec

The LaBlache Lake titaniferous-magnetite deposit was studied to determine, first, the cause of oxidation-exsolution of ilmenite from titaniferous magnetite in plutonio rocks and, second, the effect of oxidation on oxygen-isotope exchange, application of the magnetite-ilmenite geothermometer-oxygen barometer, and the distribution of manganese between titaniferous magnetite and ilmenite. The compositions and modal proportions of coexisting titaniferous magnetite and ilmenite (determined by electron-probe X-ray microanalysis and by weighing mineral separates) correlate closely with that predicted according to progressive oxidation-exsolution of ilmenite from titaniferous magnetite in a system open only to oxygen. The igneous titaniferous magnetite probably contained up to about the equivalent of 5 weight per cent of R₂O₃ as excess oxygen. This R₂O₃ probably exsolved as ilmenite above about 800° C. An additional 7 per cent of ilmenite may have resulted from oxidation-exsolution by trapped water during cooling to about 800° C. in a closed system. Ilmenite exceeding about 10 weight per cent was caused by the action of either an introduced oxidizing agent or by hydrogen loss from trapped water. Oxidation-exsolution of ilmenite from titaniferous magnetite proceeded to more than 95 per cent completion and a temperature of about 500° C. without any significant change in the oxygen-isotope ratio of the total oxides. Two rocks whose oxides do have a different oxygen-isotope ratio also contain a higher proportion of ilmenite than could be produced by oxidation of titaniferous magnetite. The same metasomatic event probably produced both the anomalous ilmenite abundance and the oxygen-isotope exchange. The molecular per cent Fe₂O₃ of the ilmenite is nearly constant and independent of oxidation except in rocks where more than 95 per cent of the original Fe₂TiO₄ has been oxidized. The compositional uniformity of the ilmenite probably results from cation exchange between ilmenite and the exsolved magnetite and ulvospinel phases. Thus the magnetite-ilmenite geothermometer-oxygen barometer gives minimum temperatures and oxygen fugacities for the cessation of oxidation of the LaBlache titaniferous magnetites. Fractional oxidation of titaniferous magnetite accounts for the distribution of manganese between titaniferous magnetite and ilmenite better than does temperature control.