青海野马泉铁多金属矿床两期成岩成矿事件——来自锆石、石榴子石U-Pb定年和全岩地球化学的证据OA

[Objective]The Qimantag area in Qinghai Province is located on the southern margin of the Qaidam Basin and belongs to the East Kunlun metallogenic belt.Extensive Late Paleozoic to Mesozoic magmatic activities are developed in this region,producing numerous Fe-Cu-Pb-Zn polymetallic deposits,such as Hutouya,Kaerqueka,and Niukutou.Previous studies have confirmed that most deposits in this belt record two episodes of magmatism and mineralization.The Yemaquan deposit is a typical skarn-type Fe-polymetallic systems in the Qimantag area.Previous studies have focused only on the Late Triassic granite-skarn metallogenic system of this deposit.Triassic magmatism has generally been considered the sole controlling factor for mineralization.The metallogenic potential of Devonian intrusive rocks lacks chronological and geochemical constraints.Whether two independent magmatic-mineralization episodes occurred in the Yemaquan deposit has long remained controversial.To resolve this scientific issue,this study systematically constrains the timing,magma source,and geodynamic setting of two episodes of intrusions and associated skarn mineralization.[Methods]This study focused on the Late Triassic intrusive rocks in the western Yemaquan deposit,the Devonian granodiorite from deep drill cores in the M13 magnetic anomaly zone in the southeast,and garnet from skarn in the contact zone.Field geological mapping and petrographic microscopic identification were conducted.Zircon and garnet LA-ICP-MS U-Pb dating,whole-rock major-and trace-element analysis,and in situ Lu-Hf isotope analysis of zircon were also conducted to systematically constrain the formation ages,magma sources,and tectonic settings of the two episodes of intrusive rocks.[Results]Zircon 206Pb/238U U-Pb dating yielded consistent weighted average ages of 225.0-223.4 Ma(MSWD=1.7-2.4)for diorite,monzogranite,and K-feldspar granite from the southwestern part of the deposit.The ages were highly consistent with the mica Ar-Ar mineralization age of(222±1.3)Ma from the deposit,constraining the timing of Late Triassic magmatism and associated skarn mineralization.Deep granodiorite samples from the M13 zone yielded zircon ages of 392.8-392.0 Ma,whereas garnet from the skarn yielded a Tera-Wasserburg isochron age of(407.7±5.1)Ma.The two sets of ages were basically consistent within the error range,demonstrating the synchronous development of magmatic activity and Fe-polymetallic mineralization during the Early-Middle Devonian.Whole-rock geochemistry analyses indicate that both intrusive suites belonged to the high-K calc-alkaline series.The Late Triassic rocks were metaluminous to weakly peraluminous,whereas the Devonian granodiorite underwent intense hydrothermal alteration and severe Na loss,resulting in strongly peraluminous characteristics.Both intrusive had crust-mantle mixed magma sources:The primary magmas originated from partial melting of lithospheric mantle and experienced crustal material assimilation during upward migration.The two magmatic episodes formed during different tectonic stages.The Devonian intrusions formed in a post-collisional extensional setting after the closure of Proto-Tethys Ocean and continental block collision,whereas the Late Triassic intrusions formed in an intraplate extensional setting after the termination of Paleo-Tethys Ocean subduction.Integrated geochronological and geochemical data demonstrated that the Yemaquan Fe-polymetallic deposit recorded two episodes of magmatic intrusion in the Early-Middle Devonian and Late Triassic,respectively,which triggered two independent episodes of skarn-type Fe-Cu-Pb-Zn mineralization.This study first identified the Devonian skarn mineralization event in the Yemaquan ore field and revised the conventional understanding that only Triassic mineralization occurred in the deposit.[Conclusion]The findings improve the metallogenic evolution theory of the Qimantag belt,providing important theoretical foundations and exploration directions for targeting two phases of intrusive rock-related polymetallic deposits in this region.