Age, petrogenesis, and tectonic setting of granitic gneiss and amphibolite from the Weizigou gold deposit, Heilongjiang Province, NE China: Constraints from geochronology and geochemistry

The Weizigou gold deposit is located in the western Jiamusi Massif, Northeast China. Gold mineralization is hosted in the amphibolite, which intruded the granitic gneiss. Although the deposit shows similarities to iron-oxide-copper-gold deposits, the detailed ore-forming process remains uncertain. To determine the formation age, petrogenesis, and tectonic setting of the granitic gneiss and amphibolite, LA-ICP-MS zircon, titanite, and monazite U-Pb dating, whole-rock major-and trace-element analyses, and LA-ICP-MS in situ zircon Hf isotope analyses were conducted on samples from these rocks. The granitic gneiss yielded two age populations of 951-882 Ma, and ca. 500 Ma, with a monazite U-Pb concordia age of 501.5 +/- 5.1 Ma. The amphibolite yielded a crystallization age of 292 Ma, consistent with the results for magmatic titanite U-Pb dating, and a meta-morphic age of 272-258 Ma. The granitic gneiss contains typical aluminum-rich minerals, such as garnet and muscovite, mean SiO2 = 73.31 wt%, and molar ratio Al2O3/(CaO + K2O + Na2O) values of 1.02-1.07, indicating an S-type granite protolith. The amphibolite belongs to the tholeiitic basalt series and has low SiO2 and high MnO contents. These results, together with epsilon Hf(t) values and two-stage model ages ranging from-9.5 to 2.3 and -0.3 to 5.7, and from 2010 to 1659 Ma and from 1331 to 947 Ma, respectively, allow us to infer that the parental magmas of the granitic gneiss and amphibolite were derived from the partial melting of Paleoproterozoic lower crust and the partial melting of metasomatized depleted mantle, respectively. The granitic gneiss is characterized by positive Th and Hf anomalies, and negative Nb, Ta, Sr and Ti anomalies, whereas the amphibolite is enriched in K, Rb, and depleted in Ba, Nb, Ti, and Zr. These geochemical features suggest that the S-type granite was formed in an active continental margin during the Neoproterozoic and underwent granulite-facies meta-morphism during the early Paleozoic. The protolith of the amphibolite was gabbro that formed in an extensional setting (e.g., a backarc basin) associated with westward subduction of the Paleo-Pacific oceanic plate beneath the eastern Jiamusi Massif during the early Permian. The gold mineralization can most likely be attributed to contact metasomatic metamorphism of gabbro during the middle-late Permian.