Cryptic compositional variation of olivine between interlayered troctolite and net-textured Fe-Ti oxide ore in the Baima layered intrusion, SW China: Implications for the origin of banded Fe-Ti oxide ores

The Baima layered intrusion in the Panzhihua-Xichang (Panxi) region in SW China hosts thick Fe-Ti oxide ore layers in its lower zone, including net-textured ores, banded ores and disseminated ores. Particularly, the banded ores are composed of melanocratic layers of net-textured ores alternating with leucocratic layers of troctolite or olivine gabbro, forming a distinct type of layering in the intrusion. Although there are many geochemical and mineralogical studies have been carried out to constrain the origin of Fe-Ti oxide ores in the Baima intrusion, it still remains unclear how such a type of layering formed. In this study, we conducted detailed examination of the textures and compositions of minerals for the interlayered troctolite and net-textured ores, which occur in a scale of centimeters and have sharp, unconformable contacts. Plagioclase laths in the net-textured ores are commonly corroded and show preferred orientation of c-axis parallel to the layering, whereas those in troctolite show random orientation. Olivine grains in the net-textured ore layers are generally rounded, whereas those in the troctolite layers show both granular and poikilitic habits. Analytical results show that plagioclase has nearly consistent An content (55-58) along the profiles. In contrast, olivine shows cryptic variation of Fo content (71-74) across the profiles with multiple reversals of Fo content within the troctolite or plagioclase-rich layers. The olivine grains in the net-textured ore layers have complex P zoning, indicating a two-stage growth process of olivine. Combining with the results of previous studies, we propose that such a layered structure may have formed by slumping of immiscible Fe-rich melts with a troctolitic mush along the inclined floor of the magma chamber. This study provides new evidence for late-stage crystallization of Fe-Ti oxides in a crystal mush, which may have broad implications for the origin of Fe-Ti oxide ores in the layered intrusions elsewhere in the Panxi region.

Automated summary

The Baima layered intrusion in the Panxi region of southwest China contains thick Fe-Ti oxide ore layers, including net-textured ores, banded ores, and disseminated ores. This study examines the textures and compositions of minerals in the interlayered troctolite and net-textured ores to understand the formation of such layering. Results suggest that the layered structure may have formed through the slumping of immiscible Fe-rich melts with a troctolitic mush along the inclined floor of the magma chamber. This research provides new evidence for the late-stage crystallization of Fe-Ti oxides in a crystal mush and has implications for the origin of Fe-Ti oxide ores in other layered intrusions.