Mineralization of the Shuangwang Gold Deposit in the Western Qinling Orogen, China: Relations to Crust–Mantle Interactions and Tectonic Evolution

Abstract

The Shuangwang Au deposit in the western Qinling Orogen is hosted by a WNW–ESE-trending breccia belt that is structurally controlled by the northern limb of the Yindonggou fold. Igneous rocks area in the deposit are part of the Xiba pluton, which comprises granodiorite and monzogranite that contains mafic microgranular envlaves (MMEs), and later mineralized granitic porphyry dikes. The mineralized granitic porphyry dikes were controlled by the same structures that controlled the ore bodies. Zircon LA-ICP-MS U-Pb dating yields ages of 220.0 ± 1.9 Ma for the granodiorite, and 217.9 ± 1.9 Ma for the granitic porphyry, which is consistent with the mineralization ages reported in previous studies (220–218 Ma). Together with the similarity of alteration mineral assemblages between ore and mineralized granitic porphyry, we suggest that the mineralization was controlled by structure and Xiba pluton. The geochemical data show that the granodiorite and granitic porphyry are subalkaline and the MMEs are alkaline in composition. All samples have similar chondrite-normalized rare earth element patterns with enrichment of light rare earth elements. The granodiorite and MMEs are depleted in Nb, Ta, Sr, P, and Ti and enriched in U, K, Pb, Zr, and Hf. The granitic porphyry is enriched in large-ion lithophile elements but depleted in high-field-strength elements. The granodiorite and MMEs have low whole-rock ε_Nd(t) values (–10.90 to –2.32) and (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7000–0.7285), similar to coeval Triassic granites in the western Qinling Orogen. The (⁸⁷Sr/⁸⁶Sr)_i ratios of the granitic porphyry have been affected by fluid metasomatism that results in higher (⁸⁷Sr/⁸⁶Sr)_i values. The geochronological, geochemical, and isotopic evidence suggest that the Xiba pluton formed by partial melting of thickened lower crust that had been intruded by alkaline mafic magma, as documented by the MMEs, which were derived from a source metasomatized by subduction-related fluids. The granodioritic and granitic porphyry magmas were relatively oxidized (fayalite–magnetite–quartz [FMQ] to magnetite–hematite (MH) buffer conditions; zircon Ce⁴⁺/Ce³⁺ = 72–813; log(fO₂) = –22 to –8). We propose that magma mixing between lower crust and mantle-derived mafic magma was triggered by the tectonic transition from a collisional to post-collision setting, which provided the metals, S, fluids, and increase in magma oxygen fugacity that enabled the formation of the Shuangwang Au deposit. Since the Late Triassic, the western Qinling Orogen evolved from a syn-collisional compressional to post-collisional extensional environment. The mineralization of the Shuangwang Au deposit involved early formation of a tectonic breccia in the compressional stage. Subsequently, hydrothermal fluids derived from a magma ascended, migrated, mixed, and precipitated ores in the tectonic breccia during the later extensional stage to form the Shuangwang Au deposit.