Recognition of the Xiayu intermediate-sulfidation epithermal Ag-Pb-Zn-Au(-Cu) mineralization in the East Qinling polymetallic ore belt, China: Constraints from geology and geochronology

Abstract

Porphyry Cu deposits are often temporal-spatial-genetically associated with epithermal polymetallic (Ag-Pb-Zn-Au-Cu) veins in some important metallogenic belts, forming a porphyry–epithermal ore systems. However, the genetic type of the large-scale vein-type Ag, Pb, Zn and Au mineralization in the world-class East Qinling porphyry Mo ore belt (Central China) has not been delineated. Here, we provide a case study of the Xiayu orefield as an example for a typical intermediate-sulfidation epithermal Ag-Pb-Zn-Au(-Cu) deposit. The Xiayu Ag-Pb-Zn-Au(-Cu) orefield, the largest Ag producer in the East Qinling ore belt, contains the Haopinggou Ag-Pb-Zn-Au, Shagou Ag-Pb-Zn, Tieluping Ag-Pb(-Cu), and Houzhanggou Ag(-Cu) deposits, which share similar geological characteristics. The Ag-Pb-Zn-Au(-Cu) mineralization occurs as open-space fillings, veins and local hydrothermal breccias and stockworks, infilled with carbonates (siderite, ankerite, and calcite), quartz, pyrite, sphalerite, galena, chalcopyrite, tetrahedrite, electrum, silver minerals (argentiferous tetrahedrite, stromeyerite, pyrargyrite, polybasite, argentite and native silver). The vein crosscutting and overprinting relationships indicate four hydrothermal stages: (I) siderite-pyrite-quartz-electrum ± magnetite, (II) dark-brown sphalerite-siderite-quartz ± galena, (III) galena-tetrahedrite-chalcopyrite-silver minerals-ankerite-quartz ± light-brown sphalerite, and (IV) ankerite-calcite-chalcedony-fluorite. Mineral zoning at the orefield-scale is recognized from north to south: pyrite, sphalerite-galena, and tetrahedrite-native silver, which corresponds to ore metal zoning from Au ± Zn to Zn-Pb-Ag and Cu-Ag.The Haopinggou granite porphyry consists of the megacrystic granite porphyry (P1) and granite porphyry (P2), which were emplaced at 134.6±1.2 Ma and 125.9±0.7 Ma, respectively. Anhedral monazite (Mon1) encapsulated in pyrite and siderite from stage I of the Haopinggou and Shagou deposits constrains the early hydrothermal activity at 133.3±1.0 Ma. U-Pb dating of syn-ore monazite (Mon2) and xenotime (intergrown with electrum, galena, sphalerite and quartz) from the Shagou, Haopinggou, and Tieluping deposits constrains the Ag-Pb-Zn-Au(-Cu) mineralization age to be 125.7±1.8 Ma to 123.3±1.7 Ma. The consistency of the magmatic and mineralization ages indicates that the magmatic-hydrothermal process may have played an important role in the Ag-Pb-Zn-Au(-Cu) mineralization. Gold distribution, mineral association, and geochronological data, all suggest that the Au and Zn-Pb-Ag ores were formed from the same mineralization event rather than two discrete mineralization events as previously suggested. Considering the geological characteristics, metal zoning, mineralization ages, mineral chemical composition and published data of fluid inclusion, we conclude that the Xiayu Ag-Pb-Zn-Au(-Cu) veins belong to typical intermediate-sulfidation epithermal mineralization and further propose a porphyry–breccia–intermediate-sulfidation epithermal Ag-Pb-Zn-Au(-Cu) mineralization system. It is the first reported intermediate-sulfidation epithermal mineralization in the East Qinling ore belt, which implies that the epithermal mineralization may have been developed in the world-class East Qinling porphyry Mo ore belt.