The formation of Luoboling porphyry Cu–Mo deposit: Constraints from zircon and apatite

Abstract

The Luobuling porphyry Cu–Mo deposit belongs to the Late Cretaceous Zijinshan Cu–Au–Mo mineralization field in southeastern China. Due to intensive hydrothermal alteration and weathering, it is very difficult to collect fresh whole rock samples for geochemical and isotopic studies in Luobuling. Zircon and apatite are accessory minerals that are resistant to hydrothermal alterations. In this study, we compared the trace element and isotope compositions of zircon and apatite from ore-bearing and barren samples to understand the formation of the Luoboling Cu–Mo deposit. Zircon U–Pb LA-ICP-MS dating shows that the Luoboling porphyries formed at ~100Ma (100.3±1.2Ma, 100.6±1.5Ma and 98.6±1.2Ma), which belongs to the late stage mineralization of the Zijinshan mineralization field. Zhongliao porphyritic granodiorite has the same age as the deposit (99.5±1.6Ma). The age of barren Sifang granodiorite is slightly older (109.7±0.8Ma). All these zircon grains have high Ce4+/Ce3+ ratios, indicating high oxygen fugacities. The ore-bearing samples show variable εHf(t) of −7.3 to 0.2, suggesting either heterogeneous sources or mixing of two different magmas. Interestingly, the Hf isotope composition of barren samples is systematically higher (εHf(t) of −3.6 to 5.5), implying a lower contribution of crustal materials. The OH mole percent of apatite grains from barren samples (LBL22-03 and SF09-05) is ~0.5, which is higher than that of apatite from the ore-bearing samples (LBL20-01 LBL20-02 and LBL22-02), indicating lower F, Cl contents or higher water contents in the magma. In apatite from the ore-bearing samples, Sr is high, indicating the absence of plagioclase crystallization. In contrast, barren samples have varied and lower Sr, indicating that apatite crystallization was accompanied by plagioclase. These patterns were controlled by water contents because the crystallization of plagioclase is suppressed by high water contents in magmas. It also suggests that the lower OH of ore-bearing apatite was resulted from higher F, Cl in the magma. High oxygen fugacity and water contents of Luoboling plutons are favorable for mineralization, which also implies their close relations to plate subduction.