Geology and genesis of the Cihai mafic intrusions in Beishan Terrane, Xinjiang, Northwest China: Implication for iron mineralization and tectonic setting

Abstract

The Cihai Fe deposit (100 Mt @ 48% Fe) represents an economically important iron deposit in Beishan Terrane, on the southern margin of the Central Asian Orogenic Belt, Xinjiang, northwest China. Many Late Carboniferous to Permian Fe(-Cu) deposits (e.g., Yamansu, Shaquanzi, and Weiya) are also present in adjacent Eastern Tianshan orogenic belt. The Cihai deposit is hosted in mafic dykes (i.e., diabase) and in the contact zone between the mafic intrusion and Mesoproterozoic schist. New zircon U–Pb isotope dating in this study gives a crystallization age of 306.9 ± 3.2 Ma for the ore-bearing diabase, which is the first age-constrained Late Carboniferous mafic magmatic activity at Cihai district. In combination with previous Ar–Ar amphibole dating (281.9 ± 2.2 Ma) for the massive ores, it is suggested that the main iron mineralization and associated mafic magmatic activities occurred at ca. 306–282 Ma, coeval with the large-scale iron mineralization epoch (ca. 324–280 Ma) in Eastern Tianshan. The Cihai diabase samples are LILEs-rich (e.g., K, Rb, and Sr) and HFSEs-depleted (e.g., Nb, Ta, and Ti) calc-alkaline series, and exhibit high Mg# values (51–63), as well as low compatible element concentrations (Ni = 14–53 ppm; Co = 18–43 ppm; and Cr = 26–219 ppm), indicating a mantle wedge source. They have elevated Ba/La and Sr/Nd values with limited Th/Yb ratios (<1), and display slightly Eu anomalies (0.90–1.19). Isotopically, they are characterized by positive εHf(t) values (+13.0 to + 16.1) and young Hf model ages (300 to 420 Ma), which are broadly similar to the isotopic compositions of the depleted mantle and within the ranges of most other mafic–ultramafic intrusions in Beishan-Tianshan orogen. These features suggest that the Cihai diabase intrusions were most likely formed by partial melting of a mantle wedge with some degree of metasomatism by slab-derived fluids. Combined our new geochemical and isotopic results with previous regional investigations, we suggest that the earliest generation of mafic dykes formed in an arc setting related to southward subduction of the paleo-Tianshan Ocean at ca. 306 Ma, and the main iron mineralization and intensive mafic magmatism occurred in a subsequent post-collisional regime during the Early Permian.