Magmatic–hydrothermal zircons in syenite: A record of Nb[sbnd]Ta mineralization processes in the Emeishan large igneous province, SW China

Abstract

Zircon is a common accessory mineral in igneous rocks and can provide a record of radiation damage and hydrothermal alteration through changes in oxygen isotopic values, crystal structures, and geochemical and petrographic characteristics. Syenites with NbTa mineralization have been discovered recently in the Panxi region, which is part of the ca. 260 Ma Emeishan large igneous province in SW China. Four types of zircon were identified in samples of altered syenite from the Baicao and Luku deposits in the Panxi region. Type I zircons are fresh, similar to magmatic zircons from the Baima syenite. Type II zircons crystallized later than those of Type I and underwent weak hydrothermal alteration. Type III zircons were strongly affected by ore-forming fluids. Type IV zircons were precipitated from ore-forming fluids and occur as dipyramidal zircons or rims on Type III zircons. Type III zircons have higher Th, Hf, rare earth element (REE), Y, Pb, Nb, and Ta contents (24,261, 11978, 5597, 9080, 476, 1511, and 160 ppm, respectively) than Type I zircons (267, 918, 1344, 16.2, 5.66, and 2.16 ppm, respectively), Type II zircons (570, 8768, 1808, 3122, 42.7, 42.3, and 15.9 ppm, respectively), and Type IV zircons (2115, 9304, 3123, 5580, 61.5, 153, and 19.4 ppm, respectively). The measure DɑT is used to quantify the radiation damage undergone by zircon. A comparison of DɑT values, trace-element contents, and Raman characteristic peaks of the four types of zircon shows that chemical incorporation of trace elements into the zircon lattice has a greater impact than radiation damage on lattice distortion. Radiation damage and fluid alteration has affected the UPb isotopic system of the Type III zircons, meaning that the UPb ages of these zircons are unreliable. Similarly, UPb ages for the Type II and Type IV zircons are unreliable, due to their high U and high common Pb contents, respectively. Therefore, only the ages of Type I zircons are robust. UPb dating of Type I zircons yielded similar ages for mineralized and unaltered syenites (257 ± 0.8 and 259 ± 0.7 Ma, respectively). The NbTa mineralization of the Baicao and Luku deposits can be divided into four stages on the basis of the morphologies, element contents, and oxygen isotope values of the four types of zircon. Type I zircons represent the initial magmatic state of the syenites (stage I); Type II zircons represent the gradual evolution of magma and initial hydrothermal fluid activity (stage II); Type III zircons represent a fluid stage in which NbTa was enriched in large quantities but without mineralization (stage III); and Type IV zircons represent the mineralization stage, during which pyrochlores crystallized (stage IV). The geochemical characteristics of the Type III and IV zircons suggest that highly evolved ore-forming fluids became gradually more enriched in F, Ca, REE, Y, Th, Zr, Hf, Nb, Ta, and Pb. Furthermore, the lower δ18O values of Type II, III, and IV zircons compared with those of Type I show that meteoric water was added to the ore-forming fluid. Our findings may be more widely applicable to the development of other syenite-related deposits and their hydrothermal systems.