Redox state of the Baogutu reduced porphyry Cu deposit in the Central Asian Orogenic belt

Abstract

Reduced porphyry Cu deposits are scarce and their origins remain enigmatic. The Late Carboniferous Baogutu reduced porphyry Cu deposit in the western Junggar Terrane, NW China, provides an opportunity to address this issue. In this study, we conducted systematic analyses in terms of petrography, mineral chemistry, major elements, trace elements and Fe isotopes to investigate the redox state of its primary magma and further constrain the origin of Baogutu copper deposit. Based on the petrological observation and mineral chemistry, two magmatic stages have been recognized during the formation of ore-forming diorite. The early stage is characterized by the crystallization of magnetite and high Mg# [Mg/(Mg + Fe2+)] hornblende under oxidized condition (ΔNNO > 2.36, calculated by the equation given by Scaillet and Evans (1999). The late stage is characterized by the crystallization of ilmenite and low Mg# biotite under reduced condition (ΔNNO <−0.6). The large decrease of oxygen fugacity (log fO2, more than 3 units) between the two stages may result from the contamination of organic carbon during the emplacement of magma. In addition, major and trace element compositions of the Baogutu ore-forming diorites show the affinity of arc magmas which are like those that host oxidized porphyry Cu deposits.In this study, we first report the Fe isotopic compositions of reduced porphyry copper deposit and discuss its application for the origin of ore deposit. The δ56Fe values of less altered diorites and granodiorites (+0.083‰ and +0.054‰) fall into the field of arc magmas, consistent with their major and trace elements results. The δ56Fe values of sulfides (chalcopyrite and pyrite) show a large span (−0.228 to +0.614‰) and the early formed disseminated sulfides are enriched in light Fe isotopes relative to the late formed vein-hosted sulfides, which indicates that hydrothermal fluid evolves toward heavy Fe isotope enrichment. The average δ56Fe of all hydrothermal sulfides (chalcopyrite and pyrite) is much higher than that of Baogutu silicate rocks, which demonstrates that the hydrothermal fluid are Fe isotopically heavy and it well explains the positive correlations between δ56Fe and indices of hydrothermal alteration for altered plutonic rocks and wall rocks.In summary, major elements, trace elements and Fe isotopes of the Baogutu porphyries show the affinity of arc magmas, same as the typical oxidized porphyry Cu deposits. Both petrological observations and Fe isotope features of the Baogutu porphyry Cu deposit indicate that the primary magma of Baogutu porphyries is oxidized and its reduced features are secondary.