Abstract

The Chehugou porphyry Mo-Cu deposit is located ca. 300 km north of Beijing in the boundary zone between the Archean North China craton and the Neoproterozoic-Mesozoic Central Asian orogenic belt. Using hydrothermal chalcopyrite from this deposit, we are able to show for the first time that the Rb-Sr isotope system in massive, nonfractured chalcopyrite grains can provide reliable age constraints for Mo-Cu deposits. We analyzed two chalcopyrite samples from slightly different petrogenetic “environments” showing contrasting texture and deformation history. Five 2- to 5-mg fragments of a single nonfractured chalcopyrite grain containing spatially isolated inclusions of K-mica, K-feldspar, and quartz yielded an Rb-Sr isochron age of 256 ± 7 Ma (2σ including λ uncertainty). For this sample, the dense chalcopyrite “matrix” apparently inhibited diffusion of Rb and Sr after chalcopyrite crystallization, and the age should therefore reflect the time of crystallization of chalcopyrite from the Mo-Cu mineralizing fluids. The evidence for Mo-Cu mineralization during the late Permian requires a revision of the previous notion that the Chehugou deposit was formed during the Yanshanian orogeny lasting from 200 to 60 Ma.

Fragments of a second chalcopyrite sample that, in contrast to the first, had experienced deformation as indicated by intragranular fractures and alignment of K-mica and K-feldspar inclusions along these fractures. This sample yielded a clearly younger Rb-Sr isochron age of 207 ± 15 Ma (2σ). This age obviously reflects an isotopically open system due to fluid movement and/or prolonged diffusion of Rb and Sr along the fractures. Single biotite crystals from an associated monzogranite yielded an Rb-Sr isochron age of 250 Ma, which is interpreted as a lower age limit for the Mo-Cu mineralization, and the time when the granite cooled below 400° to 300°C. Considering the field evidence for a broadly contemporaneous origin of the Mo-Cu mineralization and the hosting granitoids, we suggest that the granitoids are not much older than ca. 260 Ma.

The Mo-Cu–hosting granitoids range in composition from monzogranite to highly evolved syenogranite with high La/Yb and Sr/Y ratios. Initial εNd values of ca. −21 to −23 (Nd crustal residence times of the sources 1.9 to 2.7 Ga) indicate melting of the predominantly Archean North China craton, and a subordinate juvenile mantle-derived component. Much older U-depleted lower crustal sources for the granitoids are supported by high initial 208Pb/204Pb ratios. Lithospheric delamination and underplating of hot mantle-derived material may have facilitated crustal melting in a late orogenic to postcollision.

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