Apatite trace elements and O-Sr isotopes reveal different magmatic sources of Fe-Ti oxide deposits in the eastern Tianshan, NW China

Magmatic Fe-Ti oxide deposits are one of the most important iron deposit in eastern Tianshan, NW China, and their ore-forming mechanism remains obscure. Here, we use trace elements and O-Sr isotopes of apatite from two representative magmatic Fe-Ti oxide deposits (Niumaoquan and Weiya) in the eastern Tianshan to constrain the genesis of ore formation. The apatites from the Niumaoquan Fe-Ti oxide ores exhibit higher Eu/Eu* values, lower Mn contents, and larger REE variations than those from the Weiya Fe-Ti oxide ores, indicating a protracted evolution process of the Niumaoquan ore-forming magmas under a relatively reduced condition. Specifically, the apatites from the Niumaoquan Fe-Ti oxide ores can be divided into two groups: Group 1 with relatively low REE contents, low 87Sr/86Sr ratios (0.70574–0.70641) and high δ18O values (5.1–5.6‰), and Group 2 with relatively high REE contents, high 87Sr/86Sr ratios (0.70703–0.70792) and low δ18O values (2.8–3.3‰). The Group 1 apatites formed by early crystallization in the mantle, while the Group 2 apatites formed by assimilation of low δ18O-high 87Sr/86Sr altered rocks during magmas emplacement en route to the continental crust. Unlike the Niumaoquan apatites, the apatites from the Weiya Fe-Ti oxide ores have relatively homogeneous, high δ18O values (8.3‰-9.1‰) and 87Sr/86Sr ratios (0.70798–0.70848), indicating involvement of continental crustal materials in their mantle sources. Combined with previous studies, we suggest that the Niumaoquan and the Weiya Fe-Ti oxide deposits formed in a non-plume tectonic setting but they originated from different magmatic sources. Our study highlights that trace elements and O-Sr isotopes of apatite can be used as powerful tools to constrain the source characteristics and ore-forming processes of magmatic Fe-Ti oxide deposits.