Geology, fluid inclusions, isotope geochemistry, and geochronology of the Paishanlou shear zone-hosted gold deposit, North China Craton

The Paishanlou gold deposit lies along the northern margin of North China Craton gold province, the third largest An province in China. The deposit is mainly hosted in Archean metamorphic rocks of the Jianping Group and is structurally controlled by two sets of ductile shear zones. Gold mineralization is closely associated with intense hydrothermal alteration along the ductile shear zones, with a typical greenschist facies alteration assemblage of sericite+chlorite+calcite+biotite+quartz and a distinct alteration zoning of inner pyrite-sericite zone, middle carbonate zone, and outer epidote-biotite-chlorite zone from orebody to wall rock. Fluid inclusion petrography and microthermometric results suggest that four types of fluid inclusions are present at the deposit. Type 1 CO2-rich inclusions have homogenization temperatures of 22.4-27.9 degrees C. Type 2 H2O-CO2 inclusions show salinities of 4.6-12.3 wt.% NaCl equivalent and Th-tot of 242 to 337 degrees C. Type 3a primary aqueous inclusions have salinities of 5.9-11.2 wt.% NaCl equivalent and Th-tot of 187-289 degrees C, while Type 3b secondary aqueous inclusions have salinities of 3.1-8.3 wt.% NaCl equivalent and Th-tot of 128-239 degrees C. Type 4 multiphase inclusions have salinities ranging from 31.4 to 35.2 wt.% NaCl equivalent and Th-tot of 370-447 degrees C. The close association Of CO2-rich inclusions and H2O-rich inclusions in groups and along the same trail suggests the presence of fluid immiscibility. Estimated P-T values range from 1.4 to 1.9 kbar at 322-417 degrees C. The delta(18)O values of the ore-forming fluids are calculated to range from 3.8 parts per thousand to 7.4 parts per thousand and the delta D values from -87.3 to -116.2 parts per thousand, indicating a mixed source of deep-seated magmatic water and shallower meteoric water. The delta(34)S values (0.3-7.5 parts per thousand) of sulfide ore minerals are similar to those of both pyrite from metamorphic country rocks and whole rock sulfur of metamorphic country rocks. The Pb-206/Pb-214, Pb-207/Pb-204 and Pb-208/Pb-204 of sulfide ores range within 16.40-17.0, 15.21-15.37, and 36.69-37.38, respectively, with the model ages of 397-727 Ma, consistent with those of Late Mesozoic granites and lying between Archean metamorphic rock lead and the Proterozoic dolomitic marble lead. These sulfur and lead features reflect the fact that the ore-forming materials originated mainly from multiple wall-rock sources. The well-defined Ar/Ar plateau age (126.6 +/- 1.1 Ma) of biotite from the NNE ductile shear zone provides a possible temporal link between Au mineralization, magmatism and deformation at the Paishanlou deposit. The concordance of the biotite age with other well-constrained mineralization ages for the deposit, and the intrusion age of the granite, suggests that Au mineralization was essentially contemporaneous with late Mesozoic granitic magmatism and the second stage of mylonitization. In terms of its genetic linkage with the subduction process at the leading northern edge of the North China Craton, the Paishanlou deposit can be favored as an orogenic gold deposit. Its formation benefited from the combined action of protracted activity of ductile fault zones, hydrothennal circulation associated with granitoid intrusion and preferred structural setting favoring gold deposition. The major geodynamic switch towards generalized extensional tectonics that occurred during uplift, and thinning of the upper crust after the collisional stages in the northern edge of the North China Craton, may be the key factors controlling Au concentration and enrichment in the Paishanlou deposit.