In situ zircon U-Pb dating and O isotopes of the Neoarchean Hongtoushan VMS Cu-Zn deposit in the North China Craton: Implication for the ore genesis

摘要

The Hongtoushan volcanogenic massive sulfide (VMS) deposit is the largest Archean Cu-Zn deposit in China, located in the Qingyuan greenstone belt on the northern margin of the North China Craton. The Cu-Zn mineralization was stratigraphically controlled by the interbeds (similar to 100 m in thickness) of mafic-felsic volcanic sets and overlain by banded iron layers. However, the relationship between VMS deposits and associated volcanics has not been examined. This study ultimately clarifies the times and sources of the volcanics and mineralization. Based on in situ zircon U-Pb and O isotope on VMS-hosting mafic, felsic volcanic rocks, banded and massive sulfide ores and postmineralization pegmatite vein, we considered that there were two main formation stages for the Qingyuan Cu-Zn deposits; one was exhalative-hydrothermal sedimentation and another was further Cu-Zn enriched by later hydrothermal processes. The timing of the first stage occurred at 2571 +/- 6 Ma based on the magmatic zircons in the VMS-hosting mafic volcanic rocks, from which the inherited zircons also indicate the existence of 2.65-3.12 Ga ancient supercrustal rocks in the Qingyuan district. A modem mantle-like delta O-18(zircon) value of 5.5 +/- 0.1 parts per thousand (2SD) for this volcanism was well preserved in the inherited core domains of ore samples. It suggests that the mafic volcanics was most likely sourced from partial melting of juvenile crust, e.g., TTG granites. A large-scale metamorphic or hydrothermal event is documented by the recrystallized zircons in sulfide ores. The timing is tightly constrained by the hydrothermal zircon U-Pb ages. They are 2508 +/- 4 Ma for the banded ore, 2507 +/- 4 Ma for the massive ore and 2508 +/- 2 Ma for the postmineralization pegmatite vein. These indistinguishable ages indicate that the 2507 Ma hydrothermal systems played a significant role in the upgrading of the VMS Cu-Zn orebodies. The weighted delta O-18 values of hydrothermal zircons show a successively increasing trend from 6.0 +/- 0.1 parts per thousand (2 sigma) for the banded ore, 6.6 +/- 0.2 parts per thousand (2 sigma) for the massive ore to 7.3 +/- 0.2 parts per thousand (2 sigma) for the later pegmatite vein. This variation might be induced by gradual inputting of the delta O-18-rich oceanic crust and/or oceanic sediment during the hydrothermal cycling system. Considering its modern mantle-like oxygen isotope composition of 2571 Ma volcanism, a submarine volcanic hydrothermal system involving mantle plumes is a preferred setting for the Neoarchean VMS Cu-Zn deposits in the Qingyuan greenstone belt. (c) 2014 Elsevier B.V. All rights reserved.