Texture, geochemistry and U–Pb geochronology of monazite of different origins: Implications for the magmatic–hydrothermal evolution of the Guilingou porphyry Mo-W deposit, South Qinling Orogen, central China

Traditional methods for constraining the chronology of magmatic–hydrothermal processes in porphyry deposits involve the use of two or three isotopic systems (such as zircon U–Pb, molybdenite Re–Os, or K-rich mineral ⁴⁰Ar/³⁹Ar). However, the durations of such processes estimated using different chronometers with different closure temperatures could reflect fundamental methodological biases and have large uncertainties. The south Qinling Orogen in central China contains several potential porphyry Mo–W deposits. In the Guilingou deposit, the Mo-W mineralization is closely related to the emplacement of the Sihaiping two-mica monzogranite, which hosts the mineralization and whose emplacement drove hydrothermal processes. Orebodies hosted in the Sihaiping pluton include ore-bearing quartz (± sericite) veins and ore-bearing granitoid ores, which are locally rich in monazite. In this study, we provide reliable distinctions in terms of texture, mode of occurrence and geochemistry between magmatic and hydrothermal monazite, as follows: (1) Monazites in orebodies are relatively small and have patchy or unzoned structure, in contrast to magmatic monazites, which are generally large in size and have oscillatory zoning or core–rim structure, and are consistent with the features of typical hydrothermal monazites. (2) High concentrations of hydrothermal monazite grains are found within small regions, where they display paragenetic relationships with ores and other hydrothermal mineral assemblages. They generally occur as subhedral to anhedral, sub-rounded grains and commonly appear as irregular and discontinuous overgrowths. In contrast, magmatic monazites occur as single crystals with a euhedral pyramidal shape and are relatively sparse but uniformly distributed, filling small pore spaces between diagenetic biotites, subhedral feldspars, and quartz. (3) Compared to magmatic monazite, hydrothermal monazite shows steeper REE patterns, greater enrichment in LREEs and depletion in HREEs, with higher LREE/HREE ratios, and less intense and variable Eu anomalies. Magmatic monazites have concentrated Th/U and Gd/Yb_N ratios and show some correlations between the corresponding trace elements, whereas hydrothermal monazites have variable ratios and no correlation between the elements. (4) Elemental mapping by electron microprobe analysis shows homogeneous and high Ce contents in the hydrothermal monazite, together with extremely low Th, U, Y and SiO₂ contents in the whole grains, in contrast to magmatic monazites, which have high Y contents and are strongly oscillatory zoned with respect to Ce, Th and SiO₂. The magmatic monazite crystals obtained a weighted mean ²⁰⁶Pb/²³⁸U age of 205 ± 0.6 Ma, indicating a magmatic event prior to ore formation. Hydrothermal monazite crystals have weighted mean ²⁰⁶Pb/²³⁸U ages of 200 ± 0.9 Ma and 201 ± 1.5 Ma, which are interpreted to represent the timing of alteration/mineralization at Guilingou. This indicates a duration of ca. 4 Myr from magmatism to hydrothermal fluid flow for the Guilingou porphyry system. This time frame provides an additional tool for explorers to assess the metallogenic potential of the porphyry system.