矿床地球化学国家重点实验室知识库

Titanite has been regarded as a potential petrogenetic and metallogenic indicator owing to its stability and enrichment of trace elements. To verify the availability of geochemical titanite proxies in the Cu-mineralized magma system and to reveal magmatic constraints on Cu mineralization related to magmatic hydrothermal fluid, in situ major and trace element abundances and Nd isotopes of igneous titanite were determined for three Mesozoic diorite intrusions in the Tongling ore district of eastern China. The investigated intrusions were the Tongguanshan, Fenghuangshan, and Hucun intrusions, all of which host porphyry-skarn Cu deposits. Our study shows that the titanite δEu value and Fe/Al ratio, rather than the δCe and Ga content, are more effective indicators of the magma oxidation state in all three intrusions. Based on the titanite δEu value and Fe/Al ratio, the causative magma of the Fenghuangshan intrusion (titanite δEu = 0.50 ± 0.08; Fe/Al = 0.21 ± 0.07) was identified as being less oxidized than those of the Tongguanshan (titanite δEu = 0.64 ± 0.09; Fe/Al = 0.93 ± 0.15) and Hucun (titanite δEu = 0.80 ± 0.03; Fe/Al = 0.91 ± 0.04) intrusions. This result was consistent with the zircon Ce⁴⁺/Ce³⁺ ratio. Moreover, owing to the limited effect of fluid exsolution and fractional crystallization on the F content of the investigated titanite, the higher titanite F content of the Fenghuangshan intrusion (titanite F = 0.40 ± 0.09 wt%) implies that the parental magma of this intrusion is more F-enriched than those of the Hucun (titanite F = 0.17 ± 0.09 wt%) and Tongguanshan (titanite F = 0.10 ± 0.07 wt%) intrusions; however, titanites from all three Cu-mineralized intrusions had similar or lower F content than those from intrusions without Cu mineralization. The above findings indicate that magmas with high oxygen fugacity have relatively high potential for porphyry-skarn Cu mineralization, while the high enrichment of F is not necessary for Cu mineralization. Additionally, we found that titanite ε_Nd(t) values can inherit whole-rock ε_Nd(t) values and thus can be used to investigate petrogenesis. This study confirms that titanite geochemistry is an efficient petrogenetic and metallogenic indicator for Cu-mineralized magma systems when the prerequisites are met.