Apatite as a magma redox indicator and its application in metallogenic research

As an early crystallizing phase in the magma, apatite can accommodate a wide range of multivalent cations and has a capacity resistant to weathering and hydrothermal alteration. These features enable apatite to record the primary redox state of precursor magma. Here, we present new electron probe microanalysis (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) geochemical data on apatite and zircon grains from the epithermal gold ore-hosting Dahalajunshan Formation volcanics in the Tulasu basin (Western Tianshan Orogen, Central Asia). Calculated zircon Ce-/Ti-based oxybarometric results and sulfur content of apatite suggest that the parental magma of the volcanics was relatively oxidized. Furthermore, we compiled a regional apatite chemical dataset to assess whether other elements (Mn, V, Eu, Ce, As and Ga) in apatite can serve as a magma redox proxy. Negative correlation between apatite Mn and whole-rock MgO is evident, implying the control of magma fractionation on apatite Mn contents. Also, oxygen fugacity (fO2) calculated by Mn-in-apatite oxybarometer correlates strongly with magma fractionation, which indicates that fractional crystallization (instead of fO2) controlled the Mn-uptake in apatite. The apatite As budget is sensitive to fO2 variation but does not show any correlation with the ratio of nonbridging oxygens to tetrahedral cations (NBO/T), whole-rock SiO2, or aluminum saturation index. In contrast, apatite V and Ga contents correlate obviously with whole-rock SiO2 and NBO/T but not fO2, and apatite Eu and Ce anomalies show no correlation with fO2. Therefore, we conclude that only the apatite As and S contents can reflect the magma redox state. The calculated fO2 of the Dahalajunshan Formation volcanic rocks is ΔFMQ+0.53 ± 0.52 (based on the fayalite+magnetite+quartz buffer), which is markedly lower than those of typical porphyry Cu ore-forming magmas worldwide. Thus, from the perspective of magma fO2, regional magma is inferred to have little potential to form the porphyry Cu deposit.