Multistage exhumation of the Anjiayingzi gold deposit, northern North China Block: Geodynamic settings and exploration implications

摘要

Post-mineralisation exhumation is pivotal because of its strong effects on the preservation of ore deposit. The Jiguanzi quartz monzonite hosts the Anjiayingzi Au deposit at the centre of the Kalaqin Antiform (or Kalaqin metamorphic core complex) in the northern North China Block (NCB). We can link the exhumation of the deposit with regionally geodynamic events and evaluate its exploration implications using low-temperature thermochronology from the quartz monzonite host. Sericite Ar-40/Ar-39 dating of auriferous quartz-sulphide veins indicates that gold mineralisation timing is 130 +/- 1 Ma. Zircon fission track ages of fresh samples from the host quartz monzonite vary from 118 +/- 6 to 99 +/- 4 Ma, and apatite fission track ages vary from 99 +/- 6 to 66 +/- 5 Ma. A four-stage post-mineralisation exhumation history is revealed by combining the binomial peak fitting of single-grain ages and low-temperature thermal history inverse modelling. The first exhumation event is dated at ca. 130-100 Ma, and characterised by the most rapid exhumation rate of 167 m/Ma and ca. 5 km rise. This movement is related to the top-to-NW and a SE ductile-brittle extensional deformation of the Kalaqin Antiform. Regionally, it is also contemporaneous with the formation of extensional metamorphic core complexes, development of basins, and increasing heat flow and decreasing thermal lithospheric thickness in the eastern NCB. All of these synchronous events can be linked to the back-arc setting of the subducting Izanagi Oceanic Plate and resulted geodynamic changes beneath the NCB. The relatively slow stage 2 exhumation, dated at ca. 100-65 Ma with an exhumation rate of 20 m/Ma, was triggered by the collision between the Izanagi Oceanic Plate and eastern China at ca. 100 Ma and following change in the drift direction of the Izanagi Oceanic Plate. Sedimentation of basins in the NCB was continuous during this period. Resumed stage 3 exhumation (ca. 65-45 Ma, 27 m/Ma) and synchronous sedimentary inversion are products of regional compression event related to the collision between India and the Eurasian plates. Change of the Pacific Oceanic Plate drifting direction at ca. 50 Ma finally induced the monotonic cooling and exhumation of stage 4 (19 m/Ma). The estimated postmineralisation exhumation is similar to 7.1 km, and the ore-forming depth of the Anjiayingzi Au deposit is between 5.1 and 10.1 km. In combination with previously published data, the differential exhumation history, exhumation degree and preservation potential of major gold provinces in NCB are confirmed, which further indicate that their gold prospectivity must be assessed individually.