A high-efficiency gold precipitation model associated with Fe carbonates: Example from the Jiudian deposit of the world-class Jiaodong gold province

Pyrite (FeS2) is a common host for gold in various hydrothermal gold deposits, but the close relationship between siderite (FeCO3) and gold is rarely reported. Furthermore, the gold precipitation mechanism is also poorly understood. In the Jiudian gold deposit of the world-class Jiaodong gold province (eastern China), two gold ore stages were identified: pyrite-smoky grey quartz (the first ore stage), quartz-siderite-polymetallic sulfide (the second ore stage). Gold occurs as visible electrum with minor native gold in both ore stages. The latter stage ores have higher gold grade, indicating gold precipitation associated with second-stage siderite is more efficient than that associated with first-stage pyrite. An integration of mineral texture and composition and fluid inclusion evidence demonstrates that gold deposition in the first ore stage was formed by fluid immiscibility, which destabilized the Au (HS)2− complex in the ore-forming fluid. Fluid immiscibility (boiling) also occurred in the second ore stage, resulted from the abrupt drop of pressure and temperature due to the fracturing of the ore-controlling faults during seismic activity and mixing with cold oxidized meteoric water, and then the polymetallic sulfide precipitation was triggered. The presence of CH4 in the hydrothermal fluid of the second ore stage may have also promoted the fluid unmixing. Subsequently, the metasomatic reactions of pre-existing pyrite into marcasite and siderite occurred successively, and further facilitated with fluid immiscibility mutually by building a positive chain circuit. These complex processes may have formed a highly drastic and disequilibrated fluid system, in which the crystallized pyrite is featured by the decoupling of As and Au, which is different from the common early-ore Au-As association. The abnormal hydrothermal physicochemical perturbance may have triggered high-efficient gold precipitation associated with siderite.