Fe-Ti oxides as petrogenetic indicators in carbonatite from the Caotan iron deposit in the Qinling orogenic belt of central China

Abstract

FeTi oxides such as magnetite and their chemical compositions are useful petrogenetic indicators. High Ti content in magnetite has traditionally been considered a definitive criterion for an igneous origin. Many recent studies have reported the occurrence of titanomagnetite (magnetite with high Ti content) in hydrothermal deposits, thereby challenging the reliability of Ti content in magnetite as a provenance indicator. The geochemistry of titanomagnetite from igneous rocks, magmatic metallic deposits, and specific hydrothermal systems has been studied, upon which various discrimination diagrams were proposed. Carbonatites are one of the most prospective rocks on Earth, hosting metal reserves such as Nb and rare earth elements. Titanomagnetites in carbonatites have been reported but have rarely been concerned in previous provenance studies. This study reports the geochemistry of titanomagnetite from Caotan carbonatite (Qinling orogenic belt, Central China), which is unusual due to the intimate association of dolomite and calcite carbonatites. First, the mineral chemistry of the intergrown magnetite-ilmenite pairs was obtained using an electron probe microanalyzer (EPMA), from which the equilibrium temperature (T) and oxygen fugacity (fO₂) were estimated. The results show a decreasing trend in oxygen fugacity (Δlog fO₂ = −22 to −36) coupled with cooling with temperatures from 529 to 351 °C, which are attributed to changes in melt conditions caused by interactions with country rocks. The trace element compositions of the titanomagnetites were determined through laser ablation ICP–MS analysis. Based on detailed comparisons with existing elemental data of titanomagnetite from other carbonatite locations and those of other origins, we find that Al depletion and relative Sn enrichment are distinctive features for titanomagnetite from carbonatites. A new discrimination diagram of V/(Ni × Co) versus Sn/Al × 100 is proposed in this study to distinguish high-Ti magnetite from carbonatites to those of other origins.