Effects of fluid-induced oxidation on the composition of Fe–Ti oxides in the Eastern Gabbro, Coldwell Complex, Canada: implications for the application of Fe–Ti oxides to petrogenesis and mineral exploration

Magnetite (mag)–ilmenite (ilm) intergrowths are more common than mag–ulvöspinel (usp) intergrowths in mafic–ultramafic Ni–Cu–PGE systems, yet the former has no known solid solution. The most accepted model for the formation of mag–ilm intergrowths in terrestrial environments is fluid-induced oxidation of mag–usp assemblages by oxygen in water. In this study, we re-examine this model in light of the fact that crustal fluids have very low p O 2 and that mag–ilm intergrowths commonly occur in rocks that show little or no evidence of hydrothermal alteration. We also characterize the chemical changes that occurred during the formation of mag–ilm intergrowths and how they affect the use of Fe–Ti oxide chemistry for petrogenesis and mineral exploration. In the Eastern Gabbro, Coldwell Complex, a continuum of Fe–Ti oxide intergrowths occur ranging from cloth (mag–usp) to trellis (mag–ilm) types. Trellis-textured intergrowths have higher bulk Fe 3+ :Fe 2+ ratios and are predominantly enriched not only in some multivalent (Ge, Mo, W, Sn) elements, but also in Cu and Ga, consistent with their formation via oxidation by a metal-rich fluid. These compositional changes are significant relative to typical elemental abundances in Fe–Ti oxides and could potentially lead to erroneous interpretations regarding primary magmatic processes if they are not taken into consideration. The irregular distribution of the intergrowths throughout the Eastern Gabbro suggests that different rock series and mineralized zones experienced variable degrees of fluid-induced oxidation. It is proposed that C in CO 2 rather than O 2 in water could potentially be an important oxidizing agent in mafic systems: 9 Fe 2 2 + Ti O 4 + 0.75 CO 2 + 1.5 H 2 O ⇋ 9 Fe 2 + Ti O 3 + 3 Fe 2 3 + Fe 2 + O 4 + 0.75 C H 4 The applicability of this model is supported by the common occurrence of CO 2 and CH 4 in fluid inclusions in mafic rocks.