Genesis of the Xinqiao stratiform siderite orebodies and its implication for stratabound skarn Cu mineralization,Tongling region,Lower Yangtze River metallogenic belt:Constrains from ore mineralogical characteristics
Stratabound sulfide deposits,predominantly hosted in Carboniferous carbonate are widespread along the Middle-Lower Yangtze Metallogenic Belt(MLYMB).Two contrasting genetic models including a Carboniferous like SEDEX formation overprinted by magmatic-hydrothermal system and an Early Cretaceous carbonate replacement deposit has been well established,while the mineralization mechanism of stratabound deposits remains ambiguous.The siderite is widely distributed in the Carboniferous-Triassic carbonate units with occurrences of nodular,lenticle or like stratiform,which is dominated in the Upper Carboniferous-Lower Permian carbonate units with similar occurrence of the wallrock carbonate,and its genesis remains controversial which contrarily limits the establishment of a metallogenic model for the stratabound sulfide deposits and depict of mineralization mechanism of stratabound deposit deposits.The Xinqiao deposit in the Tongling mineralization cluster,is the typical stratabound deposit.In this study,mineral composition,morphological characteristics,microstructure of typical siderite ore from the Xinqiao deposit were studied through powder X-ray diffraction(XRD)and field emission scanning electron microscopy(FE-SEM).Results reveal that siderite ore mainly consist of siderite,quartz,illite and organic materials,with few bearing-Ti minerals and microbe microfossil,which is a mineral assemblage formation at room temperature and surficial environment.Siderite crystals length vary between n nm and～5 μm,and the fillings of nanometer siderite particles or clay mineral grains is a common feature in intergrain spaces of coarse siderite crystals.The authigenic euhedral quartz mold is a common microtexture on the siderite crystals surfaces.In the studied siderite samples,mega-quartz is present as crystalline overgrowths on detrital quartz grains,individual crystals as well as clusters crystals.The size of these quartz crystals is within less than 5 μm.The isolated single crystals are mainly of euhedral,subhedral and globule shape,which is authigenic quartz formation during early diagenesis of siderite.Many subhedral quartz overgrowths show siderite mold impression on their crystal surfaces.The common occurrences of siderite molds within quartz crystals and quartz molds on the siderite surface indicate that precipitation of the quartz took place concurrently with host bearing iron carbonates.Mineral association and microtextures of siderite findings demonstrate that these authigenic quartz crystals,as well as overgrowth on the detrital quartz can originate under near room temperature and pressure,and microbially mediated Fe2+ and CO32-may be one of an important pathways of siderite formation.Organic material-Fe2+ rich bearing-iron carbonate stratum together with contemporary colloform pyrite can have a key role on formation of stratabound massive sulfide deposits in the Tongling cluster as well as MLYMB.These reductive carbonaceous-ferrous in the carbonate strata is an important geochemical reduction agent,which transforms high oxidizing sulfur of SO42-to S2-in the ore-forming magmatic-hydrothermal fluid and triggered the copper polymetallic mineralization.
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