Fluid evolution characteristics and ore genesis in the Jinqu Au deposit, Qinling Orogen, China: Implications for ore genesis

The Jinqu Au deposit is located in the central Xiaoqinling gold field along the southern margin of the North China Craton. Ore-bearing auriferous quartz veins in the deposit are controlled by E-W trending structures within amphibolite facies metamorphic rocks of the Archean Taihua Group. The ore-forming process in this deposit can be divided into four stages represented by feldspar-quartz, pyrite-quartz, polymetallic sulfide-quartz, and carbonate-quartz veins and veinlets. Here we present fluid inclusion and isotopic data of the Jinqu Au deposit. Four types of fluid inclusions in quartz and calcite are identified, including carbonic-aqueous inclusions (type-1), aqueous inclusions (type-2), solid-bearing inclusions (type-3) and pure carbonic inclusions (type-4) varieties, based on petrography and laser Raman spectroscopy. Quartz in stage I contains type-1 fluid inclusions (FIs) which homogenized to aqueous or carbonic phase at temperature 361 to 406 degrees C and 385 to 400 degrees C respectively, and have salinities of 10.3 to 17.9 wt% NaCl equiv. Type-2 FIs show homogenization temperatures of 347 to 410 degrees C into the liquid phase, with high salinities of 11.6 to 18.6 wt% NaCl equiv. Quartz in stage II is the major host of type-1 FIs, with minor type-4 FIs, the former showing homogenization at temperatures of 305 to 390 degrees C into the carbonic phase and 280 to 410 degrees C into the aqueous phase, with variable salinities of 4.5 to 19.8 wt% NaCl equiv. The stage III quartz contains all the four types of FIs (homogenizing to carbonic or aqueous phase at temperature 309 to 360 degrees C and 295 to 400 degrees C, with salinities of 6.6 to 19.9 wt% NaCl equiv for type-1, and homogenizing to liquid at temperature 308 to 326 degrees C with salinities of 12.5 to 13.2 wt% for type-2 FIs). In stage IV, type-2 FIs are the major type in quartz and calcite with total homogenization temperatures of 213 to 280 degrees C into the liquid phase and variable salinities of 5.3 to 18.6 wt% NaCl equiv. Our data show that the ore-forming fluids of the Jinqu Au deposit are generally characterized by moderate to high homogenization temperatures and variable salinities, belonging to the CO2-H2O-NaCl +/- CH4 system. Based on fluid inclusion data, we infer that fluid immiscibility occurred in stages II and III, and the estimated trapping pressures for stages II and III fluids are between 81 and 275 MPa. The C-H-O isotopic values of fluids (delta O-18(water) = 1.2 to 6.4 parts per thousand; delta DV-SMOW = 68.5 to 56.0 parts per thousand; delta C-13(CO2) = 12.4 to - 3.4 parts per thousand) in stage I to IV and delta C-13(V-PDB) values for calcite (delta C-13(V-PDB) = - 7.6 to - 4.3 parts per thousand) in stage IV suggest that the ore-forming fluids were composed of a mixture of metamorphic water and mantle-derived fluids with minor involvement of meteoric water. Fluid inclusions in pyrite of stage II and III yield He-3/He-4 ratios of 0.88 to 1.39 Ra, and considerable Ar-40/Ar-36 ratios range from 2287.6 to 5669.9, suggesting the mixing of mantle and crustal components. The delta S-34(V-CDT) values of sulfide samples in stages II and III range from - 5.8 to 4.5 parts per thousand (except for a pyrite grain within calcite that shows - 28.5 parts per thousand). The values of Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/Pb-204 of pyrite samples in stages II and III are from 16.919 to 17.321, 15.340 to 15.539, and 37.303 to 37.971 respectively. Our data suggest that the Taihua Group was possibly a major source for the ore-forming metals of the Jinqu Au deposit. Integrating the data from ore geology, fluid inclusions, isotope geochemistry and regional setting, we propose that the Jinqu Au deposit is an orogenic-type deposit which formed within the continental collision setting of the North China and Yangtze Cratons.