Geological Characteristics And Mineralization Of The Meishan Fe-Au Polymetallic Deposit And The Tongjing Cu-Au Deposit,Ningwu Basin

The Meishan iron deposit is one of the representative porphyrite iron deposits,is located at the northern Ningwu basin of the Middle-Lower Yangtze River metallogenic belt,and occurs at the contact between the gabbro-diorite porphyry and pyroxene andesite of the Late Cretaceous Dawangshan Formation.The Au-Cu-Mo ore have been found at the surrounding alternation belt of the Meishan iron deposit.The compositions of garnets within the Meishan Fe-Au polymetallic deposit are similar to those of skarn deposits in end member,classified as andradite-grossular series.Magnetite and hematite display the characters of both Kiruna-type and porphyry copper deposits.Hematite and siderite were formed by hydrothermal process,and the hematite have at least two types.Gabbro-diorite porphyry(host rocks),magnetite and apatite share the similar chondrite-normalized REE distribution patterns,indicating the same source.But the gabbro-diorite porphyry does not show Eu anomalies,possibly resulting from magmatic fractional crystallization with high oxygen fugacity.Negative Eu anomalies of magnetites and apatites probably have resulted from the enrichment of Eu2+ in the fluids during the process of albitization.Iron was extracted from Fe2+-rich silicate minerals into the ore-forming fluid due to albitization of the gabbro-diorite porphyry.The Au-Cu-Mo ores is dominated by pyrite,magnetite,quartz,calcite and dolomite with minor chalcopyrite and galena.Four types of fluid inclusions,including liquid-rich two phase inclusions,gas-rich two phase inclusions,pure gas monophase inclusions and daughter mineral-bearing three-phase inclusions,have been recognized in quartz,calcite and dolomite.The homogenization temperatures of fluid inclusions range from 120? to 392?,with the majority between 150 and 230?.The salinities of fluid inclusions range from 2.07 to 28.33 wt.%NaCleq,with the majority between 2 and 8 wt.%NaCleq,indicating that the ore-forming fluid associated with gold mineralization were characterized by medium-low temperature and low salinity.The ?Dv-smow values of quartz,calcite and dolomite vary from-154.0‰ to-1 10.9‰,and the ?18OH2O values vary from-1.3‰ to 6.8‰,indicating the ore-forming fluids derived from the mixture of magmatic fluid and meteoric water.The ?34S values of pyrite during the stage of gold mineralization vary from-0.2‰ to+9.3‰,with mean value of 5.8‰.The sulfur in the The Au-Cu-Mo ore bodies of the Meishan deposits was possibly derived from magma sources.The formation of iron ore bodies is followed by gold-mineralization in the Meishan deposit.The iron and gold mineralization was associated with Cretaceous gabbro-diorite porphyry,and formed within a large-scale magmatic-hydrothermal system.The Tonging Cu-Au deposit is located in the middle part of the Ningwu basin,and hosted by Cretaceous volcanic rocks of the Dawangshan and alkaline volcanic rocks of the Niangniangshan Formations.The deposit host five ore belts containing veins and lenses of Cu-Au mineralization.Crosscutting relationships and mineral associations within the deposit have enabled the identification of two periods of mineralization:(1)hydrothermal and(2)supergene.The hydrothermal ore-forming period can be further divided into three stages,including a main quartzsiderite-sulfide stage(stage ?),a quartz-bornite stage(stage ?)and a quartz-carbonate stage(stage III).Using optical microscopy and scanining electron microscopy equipped with energy dispersive system,gold mineral is dominantly of native gold,which is distributed between quartz crystals,between quartz crystal and iron-oxide crystal,and in fissures of the quartz crystals.Four types of fluid inclusions have been found in quartz and siderite formed in stage ?,including liquid-rich two phase inclusions,gas-rich two phase inclusions,pure gas monophase inclusions and daughter mineral-bearing three-phase inclusions.The homogenization temperatures of two phase fluid inclusions in stage ? mostly range from 220 to 280?,with the majority salinities from 12 to 20 wt.%NaCleq.,indicating that the ore-forming fluids associated with Cu-Au mineralization were characterized by medium temperature and low salinity.The ?DV-SMOW values of quartz in stages ?vary from-97.1‰ to-49.9‰,and the ?18OH2O values vary from-5.1‰ to 3.5‰,indicating the ore-forming fluids derived from the mixture of magmatic fluid and meteoric water.The ?34S values of pyrite and chalcopyrite vary from 0.4 to 4.4‰,with a mean value of 3.0‰,suggesting that the sulfur in the Tongjing deposits possibly was derived from magma sources.The Tongjing Cu-Au deposits were likely formed in an extensional setting within the Yangtze Craton where partial melting of lithospheric mantle material was probably controlled by lithospheric thinning and upwelling of hot asthenosphere along major NE-SW-striking fault zones(e.g.,the Tanlu and Yangtze River fault zones).The Tongjing Cu-Au deposits is a volcanic-subvolcanic hydrothermal deposit that is associated with subvolcanic rocks and their corresponding volcanic rocks of the Lower Cretaceous Niangniangshan Formation.