Microthermometry and Synchrotron Radiation X‐ray Fluorescence Analysis of Fluid Inclusions in the Dongping Gold Deposit, Northern Margin of the North China Craton

Abstract

AbstractTo shed light on the genesis of the Dongping deposit and reveal the behaviour of CO2, Au and other ore elements (e.g., Cu, Fe, Zn, As, Sb, Co etc.) in ore‐forming fluids, petrographic, microthermometric and synchrotron radiation X‐ray fluorescence (SRXRF) analyses of fluid inclusions were conducted. The ore‐forming fluid is characterized as an H2O‐CO2‐NaCl system with medium‐high temperatures and low salinity. Four mineralization stages are identified, namely, feldspar‐quartz (stage 1); pyrite‐white quartz (stage 2); sulfide‐smoky grey quartz (stage 3); and carbonate‐quartz (stage 4). Three types of inclusions were distinguished, based on fluid composition, phase assemblages and mode of homogenization. Type A: H2O‐rich fluid inclusions show 2 phases (liquid H2O (LH2O) + vapour H2O (VH2O)) at room temperature and homogenize to the liquid phase. Type B: H2O‐CO2 fluid inclusions contain 2–3 phases (liquid H2O (LH2O) + vapour CO2 (VCO2); liquid H2O (LH2O) + liquid CO2 (LCO2)); liquid H2O (LH2O) + liquid CO2 (LCO2) + vapour CO2 (VCO2)) at room temperature and homogenized to the liquid H2O phase. Type C: H2O‐CO2 fluid inclusions show 2 phases (liquid H2O (LH2O) + liquid CO2 (LCO2) at room temperature and homogenize to a critical state. CO2 is clearly more enriched in the fluid inclusions of stages 2 and 3 than in stage 1. Stage 1 is dominated by type A (H2O‐rich) inclusions with homogenization temperatures (Th) of 220–359°C and salinities of 1.1–3.1 wt% NaCl equivalent. Type B (CO2‐rich) inclusions gradually increase in stages 2 and 3. The Th range of type B inclusions in stage 2 is 241–397°C with salinities of 2.2–6.9 wt% NaCl equivalent; the Th range of type A inclusions is 217–373°C with salinities of 1.2–8.1 wt% NaCl equivalent. The Th range of type B inclusions in stage 3 is 215–361°C with salinities of 2.9–6.1 wt% NaCl equivalent; the range of type A inclusions is 158–351°C with a salinity of 0.7–5.5 wt% NaCl equivalent. Stage 4 is characterized by type A with Th of 151–250°C and salinities of 0.9–8.3 wt% NaCl equivalent. The mapping results show that elements As, Te and Sb are more concentrated in vapour CO2 than in liquid H2O at room temperature, which suggests that vapour components are more able to transport elements when phase separation occurs. The SRXRF quantitative results show that Au, Cl, S and some other metals are obviously more enriched in the fluid inclusions of stages 2 and 3 than those in stage 1. Additionally, the contents of S in stages 1–3 are much lower than those of Cl, which suggests that gold might migrate mainly in the form of a gold‐chloride complex. Au is more enriched in type B fluid inclusions than in type A fluid inclusions, which suggests that the enrichment and migration of gold are closely related to CO2 and CO2 plays a critical role in the migration and enrichment of gold. The trace elements in the fluid have a similar change trend to those in the Yanshanian syenogranite distributed in the southeastern part of the mining area, which may provide some evidence for an intrusion‐related genesis for the Dongping gold deposit.