Reactivation of metal-fertilized lower continental crust: Origin of intrusion-related Asiha gold deposit in Eastern Kunlun orogenic Belt, China

Abstract

Gold deposits in the East Kunlun Orogen Belt (EKOB) are generally identified as orogenic gold deposits. However, recently, a new magmatic-hydrothermal model has been proposed to explain the genesis of these deposits that show intimate spatial and temporal relationships with abundant Triassic granitic intrusions, implying an intrusion-related gold system. The petrogenesis of these granites and their relation to gold mineralization are still unclear, owing to the lack of direct evidence of the connection between gold mineralization and granitic rocks. The gold-bearing Asiha intrusions (GBAI) from the Gouli area of the eastern segment of the EKOB are not only contemporary and located with mineralization but also have abnormally high gold content and sulfide accessory mineral. The GBAI are mainly composed of quartz diorite and granite porphyry. An integrated investigation involving mineral composition, zircon U-Pb dating and Hf isotopes, whole-rock elemental (including Au) and Sr-Nd-Pb isotopic geochemistry of the GBAI, as well as the in-situ Pb isotopic geochemistry of pyrites from the Asiha gold deposit, was conducted to constrain magmatic sources and petrogenesis of the GBAI, and gold mineralization mechanism. Zircon U-Pb dating reveals that the Asiha quartz diorite was formed at 230 Ma. The mineralized granite porphyries with high SiO2 contents (71.7–74.3 wt%) are peraluminous high-K calc-alkalic rocks, while the unmineralized quartz diorite with low SiO2 contents (58.1–60.0 wt%) are metaluminous high-K calc-alkalic rocks. Both rocks are potassic, strongly enriched in LILEs and LREEs with high LILE/HFSE ratios, depleted in Nb, Ta, Ba and Sr, and have weakly negative Eu anomalies and high zircon saturation temperature. Petrographic and geochemical features suggest that the quartz diorite is typical of I-type, while the granite porphyry is highly fractionated I-type granite. Their variable concentrations of major and trace elements are due to fractional crystallization of plagioclase, hornblende, and accessory minerals (apatite, zircon and monazite). Notably, the Asiha intrusions all contain high Au contents (Au = 0.02–0.05 ppm) compared to the primitive mantle estimated from chondrite values (0.88 ppb), indicating the existence of gold enrichment. All these granites are characterized by limited ranges of initial 87Sr/86Sr (0.7087–0.7133), εNd(t) (−6.0 to −5.0), 206Pb/204Pb (18.1–19.0),207Pb/204Pb (15.6–15.7), and 208Pb/204Pb (38.2–38.9), as well as zircon εHf(t) with values of −4.4 to −1.7. Their lead isotopes are consistent with that of pyrite in the ores (206Pb/204Pb = 18.41–18.45, 207Pb/204Pb = 15.61–15.65, 208Pb/204Pb = 38.47–38.56). Based on our results, we suggest that the GBAI were originally generated by partially melting of a previously Au-fertilized juvenile lower crust, which had been formed by metasomatized SCLM-derived alkaline basaltic underplating, and underwent AFC processes during emplacement. Subsequently, the post-magmatic fluids concentrated and precipitated Au to form the Au-bearing quartz veins.