In situ elemental and Sr-O isotopic studies on apatite from the Xu-Huai intrusion at the southern margin of the North China Craton: Implications for petrogenesis and metallogeny

Abstract

Apatite as an excellent tracer of the magmatic and metallogenic processes, can incorporate large amounts of trace elements sensitive to melting and/or ore-fluid evolution. This paper reports a combined study of in situ apatite geochemistry and whole-rock geochemistry for Early Cretaceous ore-bearing (Group I) and ore-barren porphyries (Group II) in the Liguo Fe-Cu-Au deposit for the deciphering of its petrogenesis and the control factors of mineralization. The apatite from both Group I and Group II is magmatic fluorapatite, characterized by Eu negative anomalies, an enrichment of LREEs, and a depletion of HREEs. Meanwhile, both groups have high Sr/Y and δEu, indicating porphyry adakitic characteristics in origin. Compared to isotopes of whole rocks, the variable 87Sr/86Sr (0.70250–0.71262) and δ18O (6.22–9.00) values of both groups of apatite imply contributions of mantle-, crust- and/or sediment-derived materials. Although Group I apatite and II apatite show some similar geochemical characteristics, Group I apatite precedes the crystallization of plagioclase with no Sr-(La/Yb)N/(La/Sm)N/(Sm/Yb)N correlations, whereas Group II apatite coincides with plagioclase crystallization, displaying positive correlations. The geochemistry of the elements (δEu, δCe, MnO and V) of these apatite groups, which are sensitive to a redox state, shows high oxygen fugacity (between HM and NNO), and Group I apatite has systematically higher oxygen fugacity than Group II apatite. More importantly, these different trace elements and oxygen fugacity characteristics between Group I apatite and Group II apatite can be used as an indicator of mineralization, and allow the skarn Fe-Cu-Au mineralization range to be drawn for the first time. Furthermore, the estimated F and Cl contents of the host parent magma (F = 1300–2446 ppm, Cl = 140–4780 ppm) are higher than those in a primitive mantle and average continental crust, suggesting an enrichment process of F and Cl. According to the above adakitic characteristics, high oxygen fugacity, and high F and Cl contents, the Pacific Plate subduction may be the main dynamic mechanism of the diagenesis and mineralization found in Liguo.