Geology and ore-forming fluid of silver-lead-zinc lode deposit of Shagou, western Henan Province

Abstract Shagou vein-type Ag-Pb-Zn ore deposit in the south margin of North China Craton is mainly composed of polymetallic sulphide ore type and quartz-carbonate-sericite replacement ore type. The main Shagou vein presents a paragenesis that can be divided into four stages: An early quartz-pyrite stage (I) prior to polymetallic mineralization, a sphalerite-quartz-galena-minor silver minerals polymetallic stage (II 1), a galena-quartz-sphalerite- silver minerals polymetallic stage (N 2), and a calcite-(quartz) stage (III) after polymetallic mineralization. The three phase (LVH) fluid inclusions (I 1) have been interpreted as the product of direct exsolution of a crystallizing magma or sudden decompression of the fluid exsolved by the magma, which indicates a possible magmatic fluid endpoint in the study area. Combined with melting temperature and inclusions composition determined by Quadrupole Mass Spectrometer, liquidrich inclusions and minor 3-phase H 2O- CO 2 rich inclusions trapped in polymetallic stages (I 1&II 2) show that the ore forming fluid is a CO 2 bearing H 2O-NaCl system with a lower temperature. The fluid inclusion homogenization temperatures (145-288°C, 194°C in average) of stage (II 2) are lower 46°C than the homogenization temperatures (185-357°C, 240°C in average) of stage (II 1), while the salinities (1.91%-10.86%, 6.38% in average) of stage (II 2) is slightly lower than the salinities (4.65% -10.11%, 7.77% in average) of stage (II 1). The systematic decreases in temperatures and salinities suggest a possible mixing with meteoric water, which is further supported by continuing cooling (homogenize at temperatures between 129-208°C, with an average of 165°C) and dilution (with a salinities of 1.40% -4.03%, 2.50% in average) of calcite-(quartz) stage (III). Decreases in salinity and temperature as result of the potential fluid mixing are supposed to have played an important role in triggering the precipitation of ore minerals from the hydrothermal solution.