Geology, geochronology, and tectonic setting of the Jiapigou gold deposits, southern Jilin Province, China

摘要

The Jiapigou area, at the northeastern corner of the North China Craton (NCC), is one of the most important gold-producing areas in China. It was responsible for nearly half of China's gold production in the 1960s and at present has >100 t Au reserves. Although numerous studies, mostly not published in international literature, have been undertaken on these deposits, their genesis, age, and tectonic setting are still controversial, mainly because of the lack of reliable geochronological data. Gold deposits at Jiapigou are hosted by several ductile shear zones within late Archean gneissic tonalite, trondhjemite, and granodiorite (TTG) and amphibolite (2523 + - 6 Ma), which have been retrogressed to greenschist facies. The deposits occur as massive auriferous quartz veins that contain sulfides (<10 vol. percent) and less common disseminated- and stockwork-style mineralization. The lodes are characterized by high Au/Ag ratios and relatively higher Cu (up to 1.3 percent) and Pb (as much as 10.3 percent) contents than most other gold deposits in the NCC. Although phyllic alteration is most common, potassic and chloritic alterations characterize felsic and mafic host rocks, respectively. Compiled stable isotope and fluid inclusion data show delta~(34)S, delta~(13)C, delta~(18)O and delta D values ranging from -0.2 per thousand to 12.6 per thousand, -4.2 per thousand to -5.0 per thousand, 5 per thousand to 12 per thousand, and -70 per thousand to -94 per thousand, respectively. These data likely indicate consistent sources of sulfur, carbon, and hydrothermal fluids, but do not actually indicate any specific reservoir. Fluid inclusions from the deposits show variable homogenization teniperatures between 150 and 350 deg C, and are characterized by low salinity (0.7 to 6.5 wt. percent NaCl equiv.) and H_2O-CO_2 + - CH_4, N_2 solutions. The initial lead isotope compositions of the deposits have considerable variations, even within individual deposits, possibly indicating multiple lead sources for the deposits. In the Jiapigou gold orefield, there are no major intrusions, but felsic, alkaline, and mafic dikes are common. These dikes usually occupy the same fault system as the gold lodes, with dikes locally hosting gold mineralization. Zircon SHRIMP U-Pb dates on pre-ore dikes yield ages of ca. 220 Ma, which constrains the maximum age of gold mineralization, whereas ~(40)Ar/~(39)Ar dates on hydrothermal sericite give an age of ca. 204 Ma. The timing of major gold mineralization in the Jiapigou area is therefore constrained between 220 and 204 Ma; this suggests a late Triassic gold-forming event. This episode of gold mineralization is coeval with, or slightly later than, the post-orogenic magmatism that followed the final orogenesis of the Inner Mongolian-Daxinganling erogenic belt (IMDOB), suggesting that a post-orogenic tectonic setting is appropriate for the late Trjassic gold mineralization. Zircon SHRIMP dating of a granitic batholith adjacent to the Jiapigou region and also hosting gold lodes that have been previously constrained-by Ai-Ar geoehronology between 144 and 120 Ma yielded an age of 166 + - 2 Ma, indicating that a mid-Jurassic (Yanshanian) gold-forming episode occurred at Jiapigou and in neighboring regions. This Yanshanian gold mineralization was contemporaneous "with subduction of the Pacific plate. On the basis of the nature, timing, and tectonic setting, we favor a classification of 'erogenic gold' for the Jiapigou deposits.