Melt volatile budgets and magma evolution revealed by diverse apatite halogen and trace elements compositions: A case study at Pulang porphyry Cu-Au deposit, China

Abstract

Apatites those shielded as inclusions in igneous minerals are avoided of diffusion effects associated with evolving melts and can record the original volatile and trace element evolutions of the magma. Volatile and trace element compositions of apatites enclosed in high-Al, medium-Al, low-Al amphibole, magmatic biotite, plagioclase, and groundmass from ores, quartz monzonite porphyry (QMP) and its mafic microgranular enclaves (MMEs), and pre-ore coarse-grained quartz diorite porphyry (CQD) were present to evaluate the magmatic processes in the formation of Pulang porphyry Cu-Au deposit. All apatite inclusions are magmatic fluorapatites and occur as early-crystallization phases (940–980 °C), followed by the crystallization sequence of high-Al amphibole, medium-Al amphibole, magmatic biotite, plagioclase, low-Al amphibole (altered medium-Al amphibole) with fluctuating ƒO2 conditions. Compared to the low SO3 (<0.27 wt%) concentrations of other apatites inclusions in fertile QMP, the biotite-hosted apatites retain an abnormal SO3 (0.15–1.01 wt%) and Cl (0.08–0.71 wt%) enrichment. Estimates of sulfur and chlorine concentrations using partitioning models for apatite in QMP return similar features, where the biotite-hosted apatites yield highly variable melt S (315–2025 ppm) and Cl (0.1–1.2 wt%) contents. The presence of biotite-hosted S-Cl enriched apatites with overgrowth texture, together with the resorbed medium-Al amphiboles in QMP, both suggest that they have crystallized from an extra sulfur supplement process caused by the injection of a series of S-Cl enriched magma, and even subsequent remelting of the preexisting dioritic batholith and its interior apatites. Such magma processes further account for the fluid exsolution and alteration from medium-Al amphiboles to low-Al ones. Whereas, those from the barren CQD systematically returns normal SO3 (0.18–0.70 wt%) and Cl (0.03–0.51 wt%) concentrations with estimates of melt S (335–1321 ppm) and Cl (0.1–0.5 wt%) contents exhibiting features of typical arc basalts (300–1000 ppm; 0.01–0.85 wt%, respectively). Although the acicular apatite of MMEs has normal SO3 contents (0.19–0.37 wt%), its abundance also confirm an extra sulfur supplement process. The altered apatites of both QMP and CQD showing pitted surface with visible voids and LREE-rich mineral inclusions return low SO3 (<0.06 wt%) and Cl (<0.22 wt%) concentrations, indicating that these elements were depleted during potassic and phyllic alteration.Given that Sr/Y ratios (2.21–3.62) of apatite at Pulang do not vary significantly with changing melt composition (Mg = 119.1–528.8 ppm), the Sr/Y and Eu/Eu* ratios of apatite from QMP have confirmed a water-rich, oxidized magma origin, whereas the CQD are originated from a water-poor, reduced source. Besides, the hydrothermal alteration possibly accounts for the abnormally increased Ce/Ce* ratios of altered apatites from CQD and decreased Sr/Y ratios of the low-Al amphibole-hosted apatite from QMP. Therefore, using trace elements as indicators for petrogenetic studies would be more robust for those apatites enclosed in igneous minerals. Our studies demonstrate that apatite inclusions can be linked to discrete periods in the crystallization history of its host phases, thus providing insight into the magma evolution process.