Paleoproterozoic Copper System in the Zhongtiaoshan Region, Southern Margin of the North China Craton: Ore Geology, Fluid Inclusion, and Isotopic Investigation

Abstract

The Zhongtiaoshan region is located in the south segment of the North China Craton and hosts a number of significant Paleoproterozoic copper deposits with a total metal endowment of approximately 400 Mt of metal Cu. Among these Cu deposits, the Tongkuangyu and Hujiayu Cu deposits constitute approximately 80 % of the total reserves. The Tongkuangyu Cu deposit, the largest copper deposit in the Zhongtiaoshan region, is hosted in quartz-monzonite porphyry (~2.1 Ga) and its wall rocks of the meta-quartz crystal tuffs of the Jiangxian Group. In contrast, the Hujiayu Cu deposit is hosted within reduced marine sedimentary sequence of the mid-Paleoproterozoic Zhongtiao Group. For decades, as these ancient deposits underwent subsequent metamorphism, the metallogenesis models of the two deposits are highly controversial. More importantly, the nature and origins of the ore-forming fluids and the mechanism of multistage fluid mineralization are still unclear. Summarizing previous researches, we systematically investigate the ore geology, fluid inclusions, and stable isotopes of the typical Tongkuangyu and Hujiayu Cu deposits. The main conclusions are as follows: (1) The oxygen fugacity of hydrothermal system of the Tongkuangyu Cu deposit fluctuates near the Magnetite–Hematite (MH) buffer line, corresponding to the redox state of typical oxidized porphyry copper deposits. The main ore-forming fluids of this deposit consist of magmatic–hydrothermal fluids with middle-high temperature and high salinity. Orebody spatial structure, mineralization types, and alteration features support a porphyry copper deposit model. The formation age of the quartz-monzonite porphyry is consistent with Cu mineralization. We propose that the Tongkuangyu Cu deposit is a porphyry copper deposit formed in an arc-related extension environment. (2) Fluid inclusion studies on the Hujiayu Cu deposit show that the ore-forming fluids of the early mineralization stage are mainly characterized by high salinity and moderate temperature basinal brines. The ore-forming fluids of the late mineralization stage are characterized by CO2-rich and high salinity and high temperature metamorphic hydrothermal solutions, which obviously experience phase separation. Early stage mineralization of the Hujiayu Cu deposit may occur via interaction of oxidized Cu-bearing brines from the underlying red beds [formed after the Great Oxidation Event (GOE)] with the upper reducing carbonaceous shales. In contrast, late stage mineralization at the Hujiayu deposit is likely related to CO2 escaping from metamorphic hydrothermal solutions. The Hujiayu copper deposit is a typical sediment-hosted stratiform copper deposit. The “Hu-Bi” type copper deposits can be comparable with Central Africa Copper Belt.