A new view on the genesis of the Chiweigou gold deposit, Jilin, China: A carbonatite-type deposit

Abstract

The Chiweigou gold deposit is located in the eastern Jilin and Heilongjiang provinces at the junction of the Paleo-Asian Ocean and Circum-Pacific metallogenic domains, where was affected by the events that marked the transition between these two domains. The gold orebodies in the Chiweigou deposit are hosted in three calcite veins controlled by compressional-shearing faults, which cut across Early Cretaceous intermediate–acidic volcaniclastic rocks. The Au-bearing calcite veins have low metal sulfide contents (<1%), and the main ore deposits display disseminated, brecciated, comb, and miarolitic structures. The minerals vary from euhedral to anhedral. In addition to the main carriers of the Au and Ag are native gold, calaverite, hessite, and aurotellurite, and the gold-carrying minerals coexist closely with other common minerals in the carbonatite veins such as calcite, quartz, apatite, and magnetite, implying genetic relationships among them. The main type of wall-rock alteration is carbonatization, and alkali metasomatism, including fenitization, is widespread. The SiO2 contents of the Au-bearing calcite veins show very large variations (0.87–53.9 wt%), and the veins have high contents of CaO (24.4–54.9 wt%) and low contents of MgO (0.06–0.13 wt%) and (Na2O + K2O) (0.10–0.39 wt%). The main oxide contents show good linear relationships with SiO2 on Harker diagrams, and this indicates that differentiation took place between the siliceous and carbonate components during the formation of the calcite veins. The LREE contents of the calcite veins are higher than the HREE contents (LREE/HREE ratios range from 10.7 to 15.8), and the normalized distribution patterns slope towards the HREE end. Eu and Ce show positive anomalies. The calcite veins are enriched in incompatible elements (Rb, Sr, Hf, and Ta) and depleted in K. The trace element ratios of the samples plot in the CHARAC field, indicating an igneous origin. In the εNd(t) vs. (87Sr/86Sr)i diagram the Chiweigou carbonatite plots close to Mesozoic carbonatites from the adjacent NCC, and the evolutionary trend is from primary mantle to an enriched mantle I-type. Based on our analytical results, and taking into account previous research, we conclude that the Chiweigou calcite veins are carbonatites, that were derived from an end-member EMI-type mantle source that had been metasomatized by highly differentiated C–H–O supercritical fluids. The Au was probably dispersed by and precipitated from colloidal Au–Si complexes (AuH3SiO4). The geochemical characteristics of the contemporary andesitic tuffs and diorite porphyrite indicate that the formation of the Chiweigou Au-bearing calcite veins resulted from extensional tectonics related to Early Cretaceous subduction of the Pacific Plate. During the early stages of intense extension (140 Ma), the carbonatite magmas ascended rapidly along extensional faults, and when they arrived in the shallow crust they mixed with meteoric water to produce the alkali metasomatism (fenitization) and the precipitation of Au as temperatures and pressures fell, thus forming the Au-bearing calcite veins and related mineralization of the Chiweigou deposit.