Mineralogy, chalcopyrite ReOs geochronology and sulfur isotope of the Hujiayu Cu deposit in the Zhongtiao Mountains, North China Craton: Implications for a Paleoproterozoic metamorphogenic copper mineralization

• The Hujiayu Cu deposit is a Paleoproterozoic metamorphogenic Cu deposit.
• The host rocks underwent ~ 550 °C metamorphism revealed by graphite geothermometry.
• The discordant thick vein type Cu mineralization formed during retrograde cooling.
• The selenium fugacity markedly increased after major chalcopyrite precipitation.
• Chalcopyrite ReOs dating showed that Cu mineralization occurred at ~ 1952 Ma.

The Hujiayu Cu deposit, located in the Zhongtiao Mountains district of southern North China Craton, is hosted by graphite schist and dolomitic marble with disseminated to veinlet (stage I) and thick vein (stage II) mineralization. Stage I mineralization, characterized by stratabound, disseminated pyrite and chalcopyrite within the graphite schist host rock, formed at the syn-metamorphic stage. Graphite geothermometry showed that the host rock was subjected to an upper-greenschist to lower amphibolite metamorphism at a temperature range of 486 to 596 °C, averaging of 546 ± 35 °C (1 σ, n = 19). Stage II mineralization, consisting of brecciated dolomitic thick veins cemented by quartz-sulfide assemblages, was a product of metamorphic hydrothermal activity. This thick vein was subdivided into an early hydrothermal dolomitic alteration composing of dolomitic breccia with some cobaltiferous pyrite (stage IIa) and a late siliceous-copper mineralization consisting of quartz-sulfide assemblages (stage IIb). A clausthalite–siegenite–sphalerite–chalcopyrite mineral assemblage was observed in stage IIb, constraining the sulfur fugacity and selenium fugacity within − 18.7 to − 11.7 and − 21.7 to − 14.7, respectively. It was inferred stage IIb was marked by a drop in sulfur fugacity and a substantial increase in selenium fugacity after the major chalcopyrite precipitation. Calculations based on the compositions of cobaliterous pyrite and sphalerite reveal that the mineral assemblage at stage IIa formed at an approximately temperature range of 400 to 300 °C, whereas the minerals in stage IIb occurred at temperature of 256 ± 9 °C (1 σ, n = 7). Sulfides from stage I have δ34S value ranging from 10.1 to 22.2‰ with an average value of 16.9 ± 3.4‰ (1 σ, n = 27), supporting the model that sulfides precipitated through thermochemical reduction of sulfate at sedimentary stage followed by metamorphic homogenization of δ34S isotopic signatures. Sulfides from the stage II have δ34S values in highly variable ranging from 3.4 to 19.2‰, indicating a rather complex source. Four chalcopyrite samples yielded a weighted model age of 1952 ± 39 Ma (1 σ, MSWD = 1.5), suggesting that the copper mineralization formed synchronously with regional metamorphism (1970–1850 Ma) and hence a Paleoproterozoic metamorphogenic copper mineralization is implicated. Therefore, we envisaged disseminated-veinlet mineralization formed during a metamorphic peak and the major hydrothermal copper mineralization occurred during the retrograde cooling.

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