In situ crystallization of Fe-Ti oxides in the Panzhihua layered intrusion, SW China: Constraints from crystal size distribution and high-resolution X-ray computed tomography analyses

The Panzhihua gabbroic layered intrusion in SW China hosts a giant magmatic Fe-Ti-(V) oxide deposit. The intrusion can be divided into marginal zone (MGZ), lower zone (LZ), middle zone (MZ), and upper zone (UZ) from the base upwards. The Fe-Ti oxide ore layers mainly occur in the LZ and MZ of the intrusion and are mainly composed of massive and net-textured ores. It is highly debated in earlier studies that the Fe-Ti oxide ore layers were formed by in situ crystallization or settling and sorting of Fe-Ti oxides. To examine the accumulation mechanism of Fe-Ti oxides, we investigate the crystal size distributions (CSDs) and composition of clinopyroxene in the Fe-Ti oxide ore samples from the LZ and MZ of the intrusion, and the three-dimensional (3D) morphology of Fe-Ti oxides in the ores using the high-resolution X-ray computed tomography technique. The concave-up CSD curves for clinopyroxene grains in the ores indicate that they consist of two crystal populations (denoted as Cpx1 and Cpx2) with different origins. Cpx1 (>0.9 mm in size) has characteristic length and L-max (the average of the four largest intersection lengths of clinopyroxene) unrelated to the mode of Fe-Ti oxides in the samples, and is likely primocryst crystal that crystallized earlier than interstitial Fe-Ti oxides. Cpx2 (<0.9 mm in size) has higher CaO but lower FeO and TiO2 contents than those of Cpx1, indicating that it may have crystallized simultaneously with or later than interstitial Fe-Ti oxides. Building on the silicate liquid immiscibility model proposed for the Panzhihua intrusion in previous studies, we consider that both Cpx2 and Fe-Ti oxides may have crystallized from an interstitial Fe-rich melt within the crystal mush. The 3D morphology of the ores shows that the Fe-Ti oxides with equivalent spherical diameter (ESD) less than 1000 mu m are discrete and commonly spherical to elongated in shape, whereas those with an ESD higher than 1000 mu m form interconnected networks among silicates. This is consistent with a scenario that the Fe-rich melt percolated down through the crystal mush and crystallized Fe-Ti oxides in situ, forming the Fe-Ti oxide ore layers in the LZ and MZ of the Panzhihua intrusion. This study also demonstrates that the combined analysis of the CSDs for silicate minerals and the 3D morphology for Fe-Ti oxide ores is a powerful approach for examining the accumulation mechanism of Fe-Ti oxides in layered intrusions.