Paleoproterozoic skarn mineralization events and geodynamic background in the Jiao-Liao-Ji Belt: Geological, geochemical and chronological evidence from the Daqinggou Fe–Cu polymetallic deposit in Ji'an, NE China

Abstract

The Daqinggou Fe–Cu polymetallic deposit has been discovered in the Ji'an district, east of the Jiao-Liao-Ji Belt (JLJB). It consists of the Daqinggouli and Beigou ore sections, and the ore bodies are mainly magnetite ore bodies, followed by copper polymetallic ore bodies. Six metallogenic stages have been distinguished here: The dry skarn (I1), wet skarn (I2), iron oxide (I3), early quartz-sulfide (II1), late quartz-sulfide (II2), and quartz-carbonate stages (II3). There are also three types of fluid inclusions (FIs): gas-rich (W1-type), liquid-rich (W2-type), and daughter crystal (S-type) FIs. The ore-forming fluids here belong to a H2O–NaCl ± CO2 system with a medium-high temperature and medium-high salinity during the Stages I3 and II1, in addition to a H2O-NaCl system with a medium-low temperature and medium-low salinity in the Stage II2. In-situ S isotopes indicate that the Stage II1 sulfides were derived from magmatic sources, whilst the Stage II2 sulfides were derived from a mixture of magma and stratigraphic sources and Stage II3 was dominated by the sulfur source of the sedimentary strata. Additionally, the Pb isotope data reveal that the ore-forming minerals were of crustal or orogenic origin. By comparing the metallogenic elements of typical skarn deposits, we believe that the Daqinggou deposit is a calcareous skarn deposit, which originates from dimetasomatism between the microcline granite and marble of the Mayihe Formation in the Ji'an Group. The immiscibility and mixing of ore-forming fluids are the key mechanisms of metal precipitation within the deposit. Furthermore, the U-Pb ages of single zircons from microcline granites and hydrothermal apatite in the ores constrained the formation of rocks and deposit to the late Paleoproterozoic. Elemental geochemistry and zircon Hf isotope data further divided the microcline granite into I-type granite with high potassium and differentiation, originating from the partial melting of Neoarchean crustal materials in the region, which then underwent fractional crystallization. The diagenetic tectonic environment shows the characteristics of an active continental margin, which may have resulted from regional large-scale arc-continent collisions (Longgang Block and Nangrim Block), also implying the occurrence of ocean subduction during the evolution of the JLJB.