The relationship between gold mineralization, exhumation of metamorphic core complex and magma cooling: Formation of the Anjiayingzi Au deposit, northern North China Craton

Abstract

The Anjiayingzi gold deposit is hosted by the Jiguanzi Quartz Monzonite in the footwall of the Kalaqin Metamorphic Core Complex (MCC) at Chifeng in the northern part of the North China Craton (NCC). An earlier (ca. 135–125Ma) top-to-the-NE ductile–brittle shear and a later (ca. 121Ma) top-to-the-NW and SE brittle detachment normal faulting are recognized around the Kalaqin MCC. The Jiguanzi Quartz Monzonite intruded the footwall of the Kalaqin MCC at ca. 133±1Ma with crystallization temperatures of 640–691°C, pressures of 167–271MPa and an average depth of 8.6km. Gold veins in the Anjiayingzi gold deposit are hosted by the monzonite and controlled by the NE-trending brittle–ductile and brittle fault zones. These ore-bearing faults are products of a regional sinistral shearing event coinciding with the earlier top-to-the-NE ductile–brittle shear at the Kalaqin MCC. The ore of the Anjiayingzi deposit is characterised by its low Au/Ag ratio (<1) and high sulphide content (4–30vol.%). The assemblage associated with gold mineralization consists of: (1) quartz–coarse-grained pyrite; (2) quartz–fine-grained pyrite; (3) quartz–polymetallic sulphide; and (4) quartz–carbonate. The ore formed at 245–358°C from a H2O–CO2–NaCl fluid system with salinities of 1.3–15.6wt.% NaCl equiv. Trapping pressures of ore fluids were 50–110MPa, corresponding to an average depth of ~2.5km under a lithostatic load and ~7.5km under a hydrostatic load. Fluid mixing and immiscibility are potential causes for the rapid precipitation of sulphides carrying Au. The stable isotope δD (−80.3 to −96.5‰) and δ18OH2O (3.7 to 5.5‰) of the ore fluids, the consistent Pb isotope compositions between the ore-bearing sulphides and K-feldspar from the Jiguanzi Quartz Monzonite, and a >2billionyear time gap between the gold mineralization and metamorphism of the basement all indicate that the mineralizing fluids and metals are derived from a concealed intrusion near the major eastern Louzidian Fault in the Kalaqin MCC. This proposal is supported by the presence of mafic magmatic enclaves and rhyolite dykes in the Jiguanzi Quartz Monzonite, and higher average Thtotal (322°C) and δ34S (2.4‰) of Eastern veins than those of Western veins (with a Thtotal of 287°C and δ34S value of 0.3‰). The ca. 133–126Ma gold mineralization at Anjiayingzi was formed during rapid cooling of ca. 133–126Ma Jiguanzi Quartz Monzonite at a cooling rate of 42–71°C/Ma, and fast exhumation and cooling of the Kalaqin MCC between ca. 135 and 121Ma at a cooling rate of 30°C/Ma. This process was accompanied by fluid immiscibility and mixing within the structural transition from ductile–brittle to brittle deformation with resultant fluid-pressure fluctuations between the supra-hydrostatic and hydrostatic regimes.