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 $$ 9{\mathrm{Fe}}_2^{2+}\mathrm{Ti}{\mathrm{O}}_4+0.75{\mathrm{CO}}_2+1.5{\mathrm{H}}_2\mathrm{O}\leftrightharpoons 9{\mathrm{Fe}}^{2+}\mathrm{Ti}{\mathrm{O}}_3+3{\mathrm{Fe}}_2^{3+}{\mathrm{Fe}}^{2+}{\mathrm{O}}_4+0.75\mathrm{C}{\mathrm{H}}_4 $$ The applicability of this model is supported by the common occurrence of CO 2 and CH 4 in fluid inclusions in mafic rocks.

中文翻译：

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流体诱导氧化对加拿大 Coldwell Complex 东部辉长岩中 Fe-Ti 氧化物成分的影响：对 Fe-Ti 氧化物在岩石形成和矿物勘探中的应用的影响

在镁铁质-超镁铁质 Ni-Cu-PGE 系统中，磁铁矿 (mag)-钛铁矿 (ilm) 共生比 mag-ulvöspinel (usp) 共生更常见，但前者没有已知的固溶体。在陆地环境中形成 mag-ilm 共生的最被接受的模型是 mag-usp 组合被水中的氧气流体诱导氧化。在这项研究中，我们根据地壳流体具有非常低的 p O 2 并且岩浆共生通常发生在几乎没有或没有热液蚀变迹象的岩石中这一事实重新检查该模型。我们还描述了在岩浆-薄膜共生体形成过程中发生的化学变化，以及它们如何影响 Fe-Ti 氧化物化学在岩石形成和矿物勘探中的应用。在东部辉长岩，Coldwell Complex，Fe-Ti 氧化物共生的连续体发生在从布 (mag-usp) 到格子 (mag-ilm) 类型的范围内。格状结构共生体具有更高的体积 Fe 3+ :Fe 2+ 比率，并且不仅主要富含一些多价（Ge、Mo、W、Sn）元素，而且还富含 Cu 和 Ga，这与它们通过氧化形成的一致富含金属的流体。这些成分变化相对于 Fe-Ti 氧化物中典型的元素丰度来说是显着的，如果不考虑它们，可能会导致对原生岩浆过程的错误解释。整个东部辉长岩的共生体的不规则分布表明不同的岩石系列和矿化带经历了不同程度的流体诱导氧化。