Mineralogical evidence for crystallization conditions and petrogenesis of ilmenite-series I-type granitoids at the Baogutu reduced porphyry Cu deposit (Western Junggar, NW China): Mössbauer spectroscopy, EPM and LA-(MC)-ICPMS analyses

Primary ore-forming minerals retain geochemical signatures of magmatic crystallization information and can reveal the petrochemical conditions prevalent at the time of their formation. The Baogutu deposit is a typical reduced porphyry Cu deposit. Amphibole and biotite Fe 3+ /ΣFe ratios, minerals (feldspar, biotite, amphibole, zircon and apatite), in situ elemental and apatite Nd isotopic compositions were determined by Mossbauer spectroscopy, electron probe microanalysis, and laser ablation multiple-collection inductively coupled plasma mass spectrometry, respectively, to investigate the magma oxidation state, petrogenesis, source features, and to constrain the carbon species at magmatic stages for the intrusive phases. The results show that the primary plagioclase and amphibole in the mineralized diorite to granodiorite porphyry and post ore hornblende diorite porphyry are distinct (An 26-55 versus An 60-69 ; Mg-hornblende versus tschermakite). In particular, the amphibole shows distinct major and trace element compositions with light rare earth element enrichments and negative Eu anomalies in Mg-hornblende and light rare earth element depletions and no Eu anomalies in tschermakite. All the analyzed biotites are primary igneous phases with a biotite phenocryst profile showing significant variations of Zn, Cr, Sc and Sr from core to rim. These results may indicate the occurrence of mixing between two distinct magmas during mineral formation. Titanium in zircon and Si ∗ in amphibole thermometries indicate that magma crystallized at >900 °C and continued to ∼650 °C. In situ apatite Nd isotope (eNd(t) = 5.6–7.6, T DM2 = 620–460 Ma), indicate absence of significant reduced sedimentary contamination and the source of juvenile lower crust. Slightly decreasing Fe 3+ /ΣFe ratios from biotite and amphibole to whole rock indicate decreasing oxygen fugacity during magma crystallization. Recalculated biotite compositions according to Fe 3+ /ΣFe ratios indicate f O 2 values of less than Ni-NiO buffer (NNO) which show slightly lower values than that estimated according to zircon/melt distribution coefficients Ce anomalies (∼ΔNNO + 0.6). These values are consistent with the features of reduced porphyry Cu deposits. Crystallization of other mineral phases significantly affects the reliability of oxybarometer of zircon/melt distribution coefficients Eu anomalies and Mn contents in apatite. This oxidation state suggests that only CO 2 was present at the magmatic stage, and implies that CH 4 formed during CO 2 reduction occurring later hydrothermal alteration. The alteration of primary amphibole to actinolite released Ti, Al, Fe, Mn, Na and K to the fluid with later precipitation of titanite, albite and minor ilmenite and magnetite during actinolite alteration.