Sericite 40Ar/39Ar and zircon U-Pb dating of the Liziyuan gold deposit, West Qinling orogen, central China: Implications for ore genesis and tectonic setting

Abstract The West Qinling orogen in central China contains several dozens of gold deposits. However, absolute ages of gold mineralization and its relations to regional tectonism, magmatism, and metamorphism remain not tightly constrained for most of these gold deposits. Here we integrate 40Ar/39Ar dating of sericite from various hydrothermal alteration assemblages and U-Pb dating of zircon grains from magmatic rocks within and surrounding the Liziyuan gold deposit in the northern belt of the West Qinling orogen to provide new insights into the age and tectonic setting of gold mineralization. The Liziyuan gold deposit consists of six ore camps (Yuzigou, Liushagou, Jiancaowan, Kuangou, Suishizi, and Yingfang) that are hosted in Ordovician greenschist facies metamorphosed sedimentary-volcanic rocks and, less significantly, the Middle Triassic Tianzishan monzogranite pluton intruding the metamorphic rocks. Gold mineralization consists of auriferous quartz-pyrite veins and subordinate sulfide disseminations in hydrothermally altered wall rocks, with minor calcite-polymetallic sulfide veins in the monzogranite-hosted orebodies. Ore-related alteration assemblages are dominated by quartz, sericite, pyrite, chlorite, and calcite, with gold occurring mostly as native gold grains enclosed in or filling microfractures of pyrite and quartz. Barren alteration assemblages, dominated by K-feldspar, quartz, sericite, and pyrite, are locally developed along eastern margin of the Tianzishan monzogranite. Two sericite aggregates extracted from gold ores in the Liushagou camp yield well-defined 40Ar/39Ar plateau ages of 200.1 ± 1.2 and 198.8 ± 1.1 Ma (2σ), whereas those from barren alteration assemblages in the Tianzishan monzogranite, which hosts the Suishizi ore camp, give rise to older 40Ar/39Ar plateau ages of 211.9 ± 1.2 and 211.3 ± 1.3 Ma (2σ). The Tianzishan monzogranite pluton and several dioritic to granitic stocks and dykes in and surrounding the gold mines have zircon U-Pb ages ranging from 241.3 ± 1.2 to 212.4 ± 0.9 Ma (2σ). These new isotopic ages suggest that the Liziyuan gold deposit formed at the Triassic-Jurassic boundary, arguably representing the youngest orogenic-type gold mineralization event over the northern belt of the West Qinling orogen. Gold mineralization postdates magmatism in the Liziyuan gold deposit at least by about 10 million years, precluding a possible genetic relation between the two. Rather, the ore genesis was likely related to metamorphic fluids released from Paleozoic sedimentary and volcanic rocks in relation to the Qinling orogenesis. In contrast, the barren hydrothermal alteration assemblages were likely caused by fluids exsolved from a magma presumably represented by the ca. 212 Ma diorite dyke. Results presented here, when combined with independent geological and geochemical data, suggest that the Liziyuan gold deposit formed most likely under a post-collisional extensional setting related to the West Qinling orogeny.