Textural and geochemical characteristics of gold-associated scheelites: Implications for the metallogenesis of the Xiaoqinling gold province, north China Craton

Abstract

Gold deposits in the Xiaoqinling province, southern margin of the North China Craton, are hosted by Neoarchean amphibolite-facies rocks of a metamorphic core complex. Over 630 t of gold resource was concentrated in the narrow-uplifted basement, which was classified into three (northern, middle, and southern) major ore belts. Gold-related scheelite grains from the Qinnan and Qiaoshangzhai (northern belt), Tonggou (middle belt), and Yangzhaiyu (southern belt) deposits were analyzed using a combination of cathodoluminescence (CL) imaging and LA-ICP-MS trace element analysis to decipher their metallogenesis. Our new results indicate that: (1) the Xiaoqinling scheelites are characterized by high Sr content and varying internal texture, replicating fluid-rock reactions. Oscillatory zoning and inhomogeneous trace element compositions suggest a dynamic hydrothermal system with fluctuating chemical properties; (2) the REEs of Xiaoqinling scheelites were mainly controlled by the substitution mechanism 2Ca2+ = REE3+ + Na+. All scheelite grains yielded typical bell-shaped REEN patterns, with negative and positive Eu anomalies occurring in samples from the Qinnan-Qiaoshangzhai and Tonggou-Yangzhaiyu deposits, respectively. Meanwhile, the scheelite Mo concentration in the northern belt (>100 ppm) is much higher than that in the middle and southern belts (<20 ppm), suggestive of significant variations in the oxygen fugacity (fO2). The spatial association of the northern ore belt with the Taiyao detachment fault may imply an increase in the fluid fO2 by percolating meteoric waters; (3) the obtained scheelite 87Sr/86Sr ratios show a broadly increasing trend from north to south (are 0.71480 to 0.71626 for Qinnan, 0.72784 to 0.72920 for Qiaoshangzhai, 0.71864 to 0.76251 for Tonggou, and 0.72760 to 0.74781 for Yangzhaiyu). This trend may depict the ore-fluid flow direction, i.e., the Sr isotopic signature shifted towards a more radiogenic composition with increasing distance from north to south due to fluid reaction with the Taihua Group (amphibolite- to granulite-facies metamorphic rock pile).