Volatile and C-H-O isotopic compositions of giant Fe-Ti-V oxide deposits in the Panxi region and their implications for the sources of volatiles and the origin of Fe-Ti oxide ores

Abstract

The Panzhihua, Hongge, and Baima Fe-Ti-V oxide deposits in the Panzhihua-Xichang (Panxi) region are hosted in large layered mafic-ultramafic intrusions. The layered intrusions intrude either the Neoproterozoic Dengying Formation, composed mainly of limestone, or the Paleoproterozoic Hekou Formation, composed of meta-sedimentary-volcanic rocks. It remains unclear if the wall rocks have been involved during the fractionation of magmas and have affected the sequence of crystallization of Fe-Ti oxide. Volatiles and their C-H-O isotopic compositions of magnetite, apatite, clinopyroxene, and plagioclase of different types of ores from the three intrusions are analyzed using a technique of stepwise heating mass spectrometer to evaluate the role of wall rocks in the formation of Fe-Ti oxide ores. Volatiles released from magnetite are composed mainly of H2O and CO2, whereas the other minerals are composed mainly of H2O, CO2 and H2. At 800–1200°C temperature interval, the average δ13C values of CO2 of all the minerals from the three intrusions range from −7.7‰ to −13.5‰ and the average \(\delta ^{18} O_{CO_2 } \) values from 19.1‰ to 19.5‰, which are scattered in a mixed field with basalt and the two types of wall rocks as end-members, indicating that CO2 from the wall rocks may have been involved in the magmas from which the three intrusions formed. At 400–800°C temperature interval, both δ13C values (−13.7‰ to −17.9‰ on the average) and δ18O values (16.2‰ to 19.2‰ on the average) of CO2 of all the minerals are lower than those for 800–1200°C temperature interval, and much closer to the values of the wall rocks. Abundant H2O released at the 400–800°C temperature interval has relatively low δD values ranging from −90‰ to −115‰, also indicating the involvement of fluids from the wall rocks. The average bulk contents of volatiles released from magnetite of the Hongge, Baima, and Panzhihua intrusions are 4891, 2996, and 1568 mm3 STP/g, respectively, much higher than those released from other minerals in total, which are 382, 600, and 379 mm3 STP/g, respectively, indicating that magnetite crystallized from magmas with much more volatiles than other minerals. This can be interpreted as that crystallization of clinopyroxene and plagioclase in the early fractionation of magmas resulted in volatiles such as H2O that were eventually enriched in the residual magmas and, at the same time, fluids from the wall rocks may have been involved in the magmas and were trapped in magnetite, which crystallized later than clinopyroxene and plagioclase.