Initial gold enrichment within a Neoarchean granite-greenstone belt: Evidence from ore-bearing and ore-barren samples in the Jiapigou deposits, NE China

- The basement gneisses were formed at 2512 ± 9 Ma.
- They are mainly derived from a juvenile mafic LCC.
- Zircon reverse discordance is presented.
- The mechanism of initial Au preenrichment is presented.

The Jiapigou gold mineralization hosted in a Neoarchean granite-greenstone belt in the southern Jilin Province, at the easternmost part of the northern margin of the North China Craton (NCC), is one of the important gold deposits in China. However, its evolution, in particular the initial Au source and enrichment, have not been well constrained. The initial gold enrichment defined by Au concentrations of Neoarchean unaltered gneisses of this region, range from < 0.005 ppm to 0.001 ppm, increasing to 0.006-0.031 ppm Au for the ore-hosting Neoarchean altered mylonite gneisses. A combined study of zircon trace element, UPb dating, Hf isotope compositions, as well as whole rock chemical compositions was performed for dioritic gneisses from both ore-bearing (close to the ore body) and ore-barren zones (within the nearby rock body) with a view to constrain the gold initial enrichment. The dioritic gneisses have varied SiO2 contents (54.19-61.34 wt.%), Al2O3 (13.06-16.75 wt.%) with moderate to high K2O/Na2O ratios (0.7-1.3) and MgO (2.20-3.77%). Based on their chemical compositions, positive zircon εHf(t) values, and their formation ages of 2512 ± 9 Ma, we infer that these dioritic rocks were generated by partial melting of a juvenile source, most likely thickened mafic lower continental crust (LCC) with residual plagioclase associated with minor amphibole. The ore-bearing sample (H5) from a ductile shear zone has been extensively altered by fluids and is the sample containing zircons with blurred zoning and reverse discordance. Zircons from H5 have much higher radiogenic Pb but lower U and Pb contents than those in the ore-barren sample (H3), which contains zircons with oscillatory zoning and represents the basement rock. Based on the available petrological, geochronological and geochemical constraints, we propose the following model for Au initial enrichment within the Neoarchean granite-greenstone belt. At ca. 2.54 Ga, associated with the accretionary processes in the proto-NCC, subduction-related fluids infiltrated the metasomatised subcontinental lithospheric mantle (SCLM), scavenging Au and Pb (mainly radiogenic Pb, henceforth Pb*) and triggering partial melting. The resulting basaltic magmas underplated the LCC and were re-melted at 2512 ± 9 Ma, when the onset of an extensional regime permitted the advection of heat and melts/fluids into the LCC. This generated voluminous dioritic magmas with juvenile Hf isotope signatures with little evidence of interaction with the mantle. Subsequent ca. 2505 Ma re-melting of some granodioritic rocks led to the formation of potassic granite and residual fluids further enriched in Au and Pb*, both focused into ductile shear zones and other lithospheric weak zones. The fluids with anomalous UPb fractionation resulted in the rarely reported zircon reverse discordance, alteration and also enrichment in Au, as observed in sample H5 and H4, representing the initial stage of Au enrichment within the Neoarchean granite-greenstone belt.