MINERAL ASSEMBLAGES, FLUID INCLUSIONS, PYRITE TRACE ELEMENTS, AND S-O ISOTOPES OF GOLD ORES FROM THE CENOZOIC DAPING DEPOSIT, SW CHINA: IMPLICATIONS FOR THE GENESIS OF COMPLEX OROGENIC LODE GOLD SYSTEMS

Abstract

Abstract 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 (VA) and galena-dominated polymetallic sulfide veins (VB) hosted by Neoproterozoic diorite are associated with quartz-sericite-chlorite ± epidote (± barite in VB veins) alteration. Pyrite-dominated (VC) and pyrite-pyrrhotite–dominated veins (VD) hosted by Silurian dolostone (intercalated with carbon-bearing argillaceous limestone in VD 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 VA and VB veins are controlled by steeply SW-dipping ductile-brittle shear zones with jigsaw wall-rock breccias in VB veins, whereas gently SW-dipping faults host VC and VD veins. There are some significant differences between different veins: (1) there were more acidic mineralization conditions for VA and VB veins relative to VC and VD veins, and more oxidized conditions for VB veins relative to other veins; (2) pyrite is rich in Co-Ni in VA and VB veins, compared to enrichment in As-Au for VC and VD veins; (3) sulfide δ34S values of 3.2 to 11.8‰ (median 8.2‰) for VA, VC, and VD veins contrast with –4.6 to +0.9‰ (median 0‰) for VB 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‰) VB 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.