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Magmatic sources of porphyry deposits in postcollisional settings remain controversial. We have used new and published petrological and geochemical data for the Eocene–Oligocene porphyry Cu ± Mo ± Au deposits in the Sanjiang region, SW China, to address this outstanding issue. New data for three deposits (Machangqing, Tongchang, and Beiya) in the Ailaoshan–Red River porphyry Au-Cu-Mo belt (southern part of the Sanjiang region) suggest that ore-forming porphyries were emplaced at ~ 35 Ma, have high (87Sr/86Sr)i (0.7068–0.7071) and negative εNd(t) (− 6.9 to − 5.0), low zircon εHf(t) (− 5.3 to 4.5), and relatively high δ18O (5.9–9.0‰). Magmatic amphibole phenocryst compositions indicate that the parental magmas are all relatively oxidized (ΔFMQ = 1.7 ± 0.6), and H2O-rich (3.8 ± 0.3 wt% H2O). These results are consistent with those estimated from zircon compositions (ΔFMQ = 1.8 ± 0.8) and high whole-rock Sr/Y ratios (75 ± 31), respectively. Based on the new and published data, we suggest that the parental magmas for the Ailaoshan–Red River porphyry Au-Cu-Mo belt were derived from a preserved juvenile arc lower-crust and the underlying metasomatized subcontinental lithospheric mantle (SCLM) attributed to a Neoproterozoic subduction event, whereas the parental magmas for the Yulong porphyry Cu-Mo belt (northern part of the Sanjiang region) originated from the Permian–Triassic juvenile arc lower-crust and metasomatized SCLM. Additionally, parental magmas for these porphyry deposits are all oxidized and H2O-rich, and we attribute such characteristics to inheritance from mixed mantle-crust sources that were modified by previous oceanic slab subduction. [ABSTRACT FROM AUTHOR]

The Paleo-Tethyan arc belt is host to a small number of porphyry Cu deposits the key controls on which remains poorly understood. It has been hypothesized that the Paleo-Tethyan arc magmas were reduced and therefore less fertile for porphyry Cu formation but few studies have been conducted to directly assess the magmatic oxygen fugacity. We investigate the magma fertility of eastern Paleo-Tethyan arc igneous rocks in the Sanjiang region, SW China, represented by the Maoding (268.3 ± 2.3 Ma), Suwalong (275.6 ± 1.4 Ma to 264.7 ± 1.4 Ma) and Baimaxueshan (255.7 ± 1.5 Ma to 249.3 ± 1.1 Ma) granitoids. They have geochemical features consistent with typical continental arc rocks, formed from partial melting of the mantle wedge, with increasing contamination of crust-derived melts over time. The presence of amphibole, high Sr/Y ratios (35 ± 20, n = 17), and lack of negative Eu anomalies (Eu N /Eu N ⁎ = 1.04 ± 0.3, n = 17) are consistent with derivation from hydrous magmas. Zircons from the intrusive rocks have relatively low Eu N /Eu N ⁎ (mostly <0.4) ratios and calculated ΔFMQ values (−2.1 ± 1.2, n = 86). Combined with apatite microphenocrysts with extremely low sulfur contents (< 0.12 wt% SO 3), it suggests that the intrusions originated from water-rich but reduced arc magmas, that were less fertile for porphyry Cu formation. Our study highlights that magmatic f O 2 is probably a critical control on the prospectivity of arc systems and confirms that zircon and apatite compositions can be used to evaluate porphyry Cu fertility. • The Paleo-Tethyan arc igneous rocks were dated from ~275 to 249 Ma. • They were derived from partial melting of mantle wedge during MASH processes. • These arc magmas were reduced and less fertile for porphyry Cu formation. • Zircon and apatite compositions can be used to evaluate porphyry Cu fertility. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrothermal zircons intergrown with gold-related minerals and containing primary fluids inclusions were identified from the Badu gold deposit in the Youjiang basin, SW China. • The hydrothermal zircons display high LREE contents and relatively flat light rare-earth element patterns, consistent with hydrothermal zircon reported worldwide. • In-situ SIMS and LA-ICP-MS U-Pb datings on the hydrothermal zircons reveal that the Badu deposit formed at ca. 140 Ma, probably in response to the Early Cretaceous large-scale lithospheric extension of South China. Selecting a robust chronometer is crucial to determine the primary mineralization age of the deposits that have witnessed multiple hydrothermal events. The Youjiang basin of South China hosts the world's second-largest Carlin-type gold province. However, the age of the Carlin-type gold mineralization remains highly debated, partly due to the presence of multiple hydrothermal events that may have disturbed the primary isotopic system with low durability. Hydrothermal zircon is a robust chronometer that can survive multiple hydrothermal events. To this end, in this study, we report in-situ secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb dating results of hydrothermal zircon identified in carbonate–sericite–sulfide–altered dolerite from the Badu Carlin-type gold deposit to constrain the timing of gold mineralization. The hydrothermal zircons exhibit subhedral to anhedral morphologies, sector cathodoluminescence zonings, and contain gold-related mineral assemblages of arsenian pyrite, quartz, sericite, and carbonate. Primary two-phase fluid inclusions also commonly occur in the hydrothermal zircons. Compositionally, these zircons feature relatively flat light rare-earth element (REE) patterns, high La contents, low (Sm/La) N values, and small Eu and Ce anomalies, which are different from the magmatic counterparts of the Badu pluton but consistent with those of hydrothermal zircons reported worldwide. These features indicate that the hydrothermal zircons are probably paragenetically associated with the gold mineralization. Consequently, the U–Pb ages of ca. 140 Ma of these zircons, which are much younger than those of the magmatic zircon in the fresh dolerite, can represent the timing of primary gold mineralization of Badu. Our new data suggests that there truly was an episode of gold mineralization during the Early Cretaceous in the Youjiang basin, which may have been driven by large-scale lithospheric extension in the interior of South China. This study demonstrates that hydrothermal zircon could be a robust clock for investigating the timing of magmatic rock-hosted Carlin-type gold deposits. [ABSTRACT FROM AUTHOR]