Structural processes and dike emplacement mechanism in the Wulong gold field, eastern Liaoning

Abstract

The Wulong gold deposits are typical representative of deposits in the Liaodong ore concentration area in the eastern North China Craton. The gold deposits occur as the quartz-vein type. However, gold mineralization time of the Wulong gold deposits was uncertain, and structural controlling mechanism for them remains unclear. We present structural and geochronological data to constrain structural evolution and dike emplacement mechanism before and during the gold mineralization. The Wulong monzonitic granite pluton, the host intrusion for the gold deposits, was dated at 173−163 Ma (Middle Jurassic). The pluton suffered WNW-ESE shortening probably at the beginning of Late Jurassic, as shown by penetrative, ductile deformation and local ductile reverse shear belts in the pluton. The Wulong pluton was then subjected to nearly N-S compression that led to formation of several NE-striking sinistral faults and associated conjugate fractures at the beginning of Early Cretaceous. The compression was followed by WNW-ESE extension during the Early Cretaceous (ca. 131−113 Ma). The Early Cretaceous extension caused a series of NE- to NNE-striking normal faults and reactivated previous faults or fractures with varied orientations. Our fault-slip data gave reliable constraints on the stress fields during the compression and extension. The extension was also associated with emplacement of the Sangouliu pluton (131−120 Ma) in the south, the Wulongbei pluton (127−126 Ma) in the south and a series of diorite, granite, granitic pegmatite, lamprophyre and diabase dikes in the gold mine field. Previous work demonstrates that the diorite and granite dikes often coexist with the auriferous quartz veins. Our zircon U-Pb dating gave 127−113 Ma intrusion ages for the dated diorite dikes, 126−113 Ma ages for the granite dikes and a 121 Ma age for the granitic pegmatite dike. These demonstrate that the diorite and granite dikes in the Wulong gold field are synchronous and their emplacement of 127−113 Ma happened during the extensional stage. These Early Cretaceous dikes were emplaced along both the newly- formed, NE- to NNE-striking normal faults and the previous faults or fractures reactivated during the extension. The dike emplacement along the new and previous structures resulted in varied orientations in the study area. We propose that the auriferous quartz veins share the similar emplacement mechanism with the synchronous dikes in the Wulong gold field. This inference is supported by the common coexistence of the both. It is suggested therefore that the gold mineralization in the Wulong gold field took place during the period of 127−113 Ma when the North China Craton experienced the peak destruction. However, direct dating results from ore minerals are needed to constrain the accurate gold mineralization time in the Wulong gold field.