Rb-Sr Geochronology of Chalcopyrite from the Chehugou Porphyry Mo-Cu Deposit (Northeast China) and Geochemical Constraints on the Origin of Hosting Granites

摘要

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 (2 including 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.