Metamorphic P-T-t evolution of Paleoproterozoic schist-hosted Cu deposits in the Zhongtiao Mountains, North China Craton: Retrograde ore formation during sluggish exhumation

Abstract

The stratabound Cu deposits in the Zhongtiao Mountains district contain good records about the Precambrian tectonic evolution in the central North China Craton. Five stages of metamorphic and metallogenic processes are identified and the P-T conditions are constrained using mineral geothermobarometry and P-T pseudosection. Results revealed a prograde metamorphism (stage M−1) from 565 to 630°C, and peak metamorphism of 6.5±0.5kbar and 610±20°C (stage M−2). The post-peak metamorphism consisted of a continued retrogression (stage M−3) at 545±5°C and 6.0±0.1kbar, and Cu mineralizations (stages M−4 and M−5). Two distinct mineral assemblages of arsenopyrite-pyrrhotite-pyrite-chalcopyrite and bornite-chalcopyrite-parkerite- wittichenite-bismuth are observed in veinlets. The Cu mineralization occurred from 446 to 341°C at a pressure of ∼5.0 to 4.4kbar. We obtained laser ablation ICPMS monazite U-Pb ages of 1881±15Ma and 1863±17Ma for staurolite schist and mineralized veins, respectively. A Nano-secondary ion mass spectrometer zircon U-Pb age of 1815±32Ma for fluid-modified zircon rims from host schist. These ages are interpreted as the timing for the stages M−2, M−4 and M−5, respectively. These results define a clockwise P-T-t path that displays nearly parallel prograde and retrograde trajectory (Franciscan type). The retrograde portion is similar to near-isobaric cooling (IBC) stage, which supports a relatively slow exhumation following a continental collision. It is proposed that the ore materials were mainly derived from the host rocks, which might have been pre-enriched. Cu mineralization formed during the retrograde cooling stage at greenschist facies, and was likely related to a post-orogenic sluggish exhumation at a rate of 0.31–0.05mm/yr.