Substantial in situ Ti isotope variations in rutile record source and fluid evolution of porphyry copper mineralization systems

Abstract

Titanium (Ti) and its stable isotopes have been widely used as tracers for magmatic processes. However, our understanding of Ti isotope behavior in magmatic-hydrothermal systems remains limited. Hence, the in situ Ti isotope composition (δ⁴⁹Ti) of magmatic titanite and hydrothermal rutile associated with magnetite and chalcopyrite mineralization was determined for the first time in four well-characterized porphyry copper deposits in southern Tibet. The rutile formed through the alteration of primary Ti-rich minerals during fluid-rock interaction in the early high-temperature magnetite and later moderate-temperature chalcopyrite stages of mineralization. Hydrothermal rutile, altered from magmatic titanite, exhibits δ⁴⁹Ti values similar to those of residual magmatic titanite. This suggests that hydrothermal rutile inherited the Ti isotope composition of magmatic titanite. The average δ⁴⁹Ti values of rutile are negatively correlated with whole-rock ε_Nd(t) and zircon ε_Hf(t) data, and positively correlated with whole-rock (⁸⁷Sr/⁸⁶Sr)i values, which suggests that the initial Ti isotope compositions of hydrothermal rutile in porphyry copper deposits primarily reflect their source. Rutile from the Qulong deposit sometimes exhibits fractionation of δ⁴⁹Ti at levels exceeding 0.5‰, displaying a negative correlation with Zr and FeO, which may be attributed to the formation of magnetite and rutile at an early potassic alteration stage. Isotopically light Ti is preferentially incorporated into magnetite and rutile. Thus, the rutile associated with sulfide mineralization that formed from the remaining fluids during a later stage of phyllic alteration is enriched in heavy δ⁴⁹Ti. These findings contribute to the understanding of how rutile fractionates Ti isotopes in hydrothermal systems related to porphyry copper deposits. In local contexts, the substantial crystallization of magnetite, along with the preferential incorporation of isotopically light Ti during the early stages, leads to a decrease in oxygen fugacity within the ore-bearing fluid. This, in turn, facilitates the formation of sulfides during later stages. The results of this study demonstrate the efficacy of in situ Ti isotope analysis as a powerful tool for tracking fluid and metal sources, and can be used to help interpret ore precipitation throughout different stages of magmatic-to-hydrothermal ore-forming processes.

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ORCID for Martin R. Palmer: orcid.org/0000-0002-3020-0914

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