The Cenozoic Daping orogenic gold deposit, on the southeastern margin of the Tibetan Plateau, China, contains four lode types that contain a total of 55 t gold. Pyrite-chalcopyrite–dominated (V_A) and galena-dominated polymetallic sulfide veins (V_B) hosted by Neoproterozoic diorite are associated with quartz-sericite-chlorite ± epidote (± barite in V_B veins) alteration. Pyrite-dominated (V_C) and pyrite-pyrrhotite–dominated veins (V_D) hosted by Silurian dolostone (intercalated with carbon-bearing argillaceous limestone in V_D veins) are related to ankerite-siderite-quartz ± sericite alteration. All have free gold spatially and temporally associated with pyrite, chalcopyrite, galena, pyrrhotite, or quartz. Most V_A and V_B veins are controlled by steeply SW-dipping ductile-brittle shear zones with jigsaw wall-rock breccias in V_B veins, whereas gently SW-dipping faults host V_C and V_D veins.

There are some significant differences between different veins: (1) there were more acidic mineralization conditions for V_A and V_B veins relative to V_C and V_D veins, and more oxidized conditions for V_B veins relative to other veins; (2) pyrite is rich in Co-Ni in V_A and V_B veins, compared to enrichment in As-Au for V_C and V_D veins; (3) sulfide δ³⁴S values of 3.2 to 11.8‰ (median 8.2‰) for V_A, V_C, and V_D veins contrast with -4.6 to +0.9‰ (median 0‰) for V_B veins. The contrasting mineral parageneses, pH values, and pyrite geochemistry can be attributed to fluid-rock interaction as evidenced by replacements of amphibole by sericite in diorite and dolomite by ankerite and siderite in dolostone. The lower (~8‰) V_B sulfide δ³⁴S values and interpreted fluid oxidation relative to other veins, together with the presence of breccias and barite, can be explained by phase separation due to flash vaporization triggered by extreme hydrofracturing. The consistent NW-trending vein sets, similar median S-O isotope ratios of original ore fluids, and lack of multistage overlap of gold mineralization and alteration zones support a single-source fluid for gold mineralization at Daping. This study is diagnostic rather than just indicative in that it systematically and quantitatively portrays the mineralization diversity in an orogenic gold system formed by a single-source fluid regulated by the external fluid-rock interactions and internal hydrofracturing.

Article link: https://doi.org/10.5382/econgeo.4995