A three stage fluid flow model for Xiaoqinling lode gold metallogenesis in the He''nan and Shaanxi provinces, central China.
豫陕小秦岭脉状金矿床三期流体运移成矿作用

The Xiaoqinling lode gold field is located in the boundary of He'nan and Shaanxi provinces in central China. More than 1200 gold-bearing quartz veins have been found in the field and it is the second largest gold concentration region of China. All lode gold deposits possess similar mineral assemblages, deformational state, fluid flow characteristics, ore fluid composition and have comparable P - T conditions. Fluid inclusion studies show that large number fluid inclusions, including CO2-rich inclusions, CO2-H2O inclusions and aqueous inclusions, are hosted in vein quartz and carbonate in various mineralization stages. Three successive crystallisation stages are recorded during the formation of gold-bearing veins. Stage 1 - the development of milky quartz veins that formed main parts of the quartz veins. The fluids were H2O-rich and no clear evidence was found for gold deposition during this stage. Stage 2-intense microfissuring of the earlier quartz vein (Stage 1) infillings occurred, associated with the main episode of gold deposition. Massive pyrite-clear quartz ores and network polymetallic sulfide minerals-grey colour quartz ores formed. The fluids were middle-low salinity CO2-H2O-NaCl type. Changes of temperature, pressure and fluid compositions in ore-forming fluids were main causes for gold deposition. Stage 3 fluids changed to lower salinity H2O-rich type. Quartz-carbonate mineral veins were formed and weak gold mineralization happened.