Ore-Forming Fluids Of The Pb-Zn Deposits Hosted In The Sinian System, Margin Of The Up-Middle Yangtze Block

The margin of Up-Middle Yangtze Block, where developed a large number of lead, zinc, manganese, gold, antimony, silver, vanadium and other large deposits, is a giant polymetallic metallogenic belt with great exploration potential. In this area, the mineralization of lead and zinc resources that developed in the Sinian System is an excellent target to recover the regional fluid migration events and tectonic evolution of the Up-Middle Yangtze Block.These lead-zinc deposits from the margin of the Up-Middle Yangtze Block are mainly distributed in Kangdian Axis in the west (i.e. Huize, Wusihe, Daliangzi, Tianbaoshan and Chipu lead-zinc deposits), Beiba Uplift in the north (i.e. Nanmushu lead-zinc deposit) and Jiangxi to Western Hubei in the east (i.e. Dongjiahe lead-zinc deposit, Huanyuan lead-zinc deposit, Bingdongshan, Baijihe and Aozigang Zinc deposits).The representative Aozigang zinc deposit, Wusihe lead-zinc deposit and Nanmushu lead-zinc deposit was chosen to study the characteristics of ore-forming fluids, lead-zinc mineralization from different geologic setting and metallogenic mechanism. Through study of these lead-zinc deposits hosted in the stratum of the Sinian, we test the data of the fluid inclusions (FIs) to trace the nature of the fluid system combined with a detailed geological survey. Then we analyze regional comprehensive information to trace the mineralization mechanism. In addition, Zhangshiba lead-zinc deposit in Jiangxi Provience and Huangshaping lead-zinc deposit in Hunan Provience from the Up-Middle Yangtze Block were studied for comparable. This study is aim to explore the migration and accumulation of ore-forming fluids, and discuss characteristics of ore-forming fluids and ore-controlling factors of typical deposits. Ore-forming fluids of lead-zinc deposits are not only useful for tracing the ore-forming process and the evolution of ancient fluid systems, but also record reservoir diagenesis, flowed fluid history, components and nature. This study can provide a clear understanding for the large-scale metallogeny and metallogenic specialization. In premise of full understanding geologic background, the analysis of FI features by using microthermometry and laser Raman microspectroscopy and estimating the pressure of mineralization, then the results yielded as follow:(1) The Aozigang zinc depositThe Aozigang deposit, which is located in the eastern margin of the Up-Middle Yangtze Block, is a zinc deposit hosted in the Dengying Formation of the Sinian system. The mineralization process can be divided into depositional stage and hydrothermal stage in terms of two kinds of sphalerite with different petrographic characteristics. In depositional stage formed strip-lamellar ore, and alga played a role in adsorbing metallogenic materials. In the hydrothermal stage, the organic fluid activities that influenced on breccia and disseminated ores accounted for mineralization. There are two stages of bitumen, and the earlier one was associated with sphalerite.Fluid inclusion (FI) petrographic study shows that there are mainly three types of FIs, including Aqueous (W)-type, Hydrocarbon (G)-type and S-type. Aqueous (W)-type FIs commonly occur in mineral, and have liquid-vapor phases at room temperature. The FI sizes cluster between 4 and 15 μm. Hydrocarbon (G)-type FIs commonly occur in quartz veins formed at dolomite and coarse grain sphalerite. The FI sizes vary from 6 to 20 μm with irregularity and oval shapes. The hydrocarbon materials in G-type FIs show black or brownness under polarizing microscope and yellow fluorescence under fluorescence microscope. S-type FIs, which commonly coexist with W-type and G-type inclusions, are multiphase inclusions, containing liquid, vapor, and crystalline phases. The daughter minerals are mainly halite with cube form with size ranging from 8 to 16μm in diameter.Microthermometric data show that the first melting temperatures range from-34.3 to-31.0℃ in calcite, and -34.8℃ in quartz. It is suggested that the ore-forming fluids are multicomponent system, including organic and inorganic materials (i.e. NaCl, KCl, CaCl2, MgCl2 and H2O).The liquid-rich W-type FIs in calcite yield final ice melting temperatures of-14.0～-5.0℃, salinities of 7.9 to 17.8 wt.% NaCl equiv, and the calculated densities of 0.91～1.03 g/cm3. The W-type FIs in dolomite have been homogenized to liquid at temperatures of 139 to 237 ℃, with ice-melting temperatures of -12.9～-1.9℃, salinities of 3.2～16.8 wt.% NaCl equiv. The calculated densities are between 0.91 and 1.04 g/cm3. In quartz, the W-type FIs have been homogenized at 122～221℃, with ice-melting temperatures of -12.9～-3.8℃, salinities of 6.2-16.8 wt.% NaCl equiv. The densities of W-type FIs are 0.93 to 1.04 g/cm3. During heating, halite-bearing S-type FIs display that halite dissolution followed by homogenization, with total homogenization temperatures between 200 and 217℃ and salinities of 32 to 33 wt.% NaCl equiv.At the hydrothermal stage of the Aozigang zinc deposit, microthermometric data show that the FIs have totally homogenized concentrating at 160～180℃, suggesting that the ore-forming environment is a relatively stabilized. Salinities of W-type FIs are 6～18 wt.% NaCl equiv, densities of which are 0.91 to 1.04 g/cm3. S-type FIs have captured approximately with salinities of 32～33 wt.% NaCl equiv, with calculated densities are between 1.09 and 1.11 g/cm3.The laser Raman microspectroscopy results indicate that H2O is the dominated FIs in domolite and quartz. In addition, CH4, C3H8 and bitumen can be detected in G-type FIs from dolomite and reddish brown coarse-grained sphalerite. The isochors of S- and W-type FIs are defined on the P-T diagram using the FLINCOR software, and estimated the trapping minimum pressures are 22-84 MPa.In sum, the Aozigang zinc deposit forms a source bed in sedimentary stage and enriched in metallogenic materials by hydrothermal activity brought in hydrothermal stage. We considered that it is a sedimentary diagenesis-weak hydrothermal reform type.(2) The Wusihe lead-zinc depositWusihe lead-zinc deposit is a large deposit in western margin of the Up-Middle Yangtze Block, and its lead and zinc resources are estimated up to 3.7 Mt that developed along the Dadu River valley in Sichuan Province. The mineralization process of the Wusihe Lead-Zinc deposit can be divided into depositional stage and hydrothermal stage.Mineralized layers formed in the depositional-diagenetic stage, and the sedimentary characteristics are locally observed. The deposit is influenced by the hydrothermal ore-forming fluid and structure reformation. The hydrothermal stage is the main metallogenic stage with materials concentration. The whole hydrothermal ore-forming processes involve four stages, including Pyrrhotite-Pyrite-Sphalerite stage, Bitumen-Pyrite-Galena-Sphalerite stage, Sphalerite-Galena stage, Bitumen-Quartz-Calcite stage.Primary fluid inclusions (FIs) in dolomite, quartz, calcite, fluorite and sphalerite are composed of two phases (L+V type), however, the component are complex. The laser Raman spectroscopy study shows that the gas components have a certain amount of CH4, H2S, C2H6, C2H2, N2 and NH3. The first melting temperatures indicate it is NaCl-H2O fluid system with Mg2+ and Ca2+ at earlier stage.At the depositional stage, the microthermometric data show the average of homogenization temperatures is 183℃, with an average salinity of 9.5 wt.% NaCl equiv. The homogenization temperatures are from 132℃ to 223℃, with salinities of 6.5～14.8 wt.% NaCl equiv and calculated densities are between 0.9land 1.02 g/cm3.At the hydrothermal stage, microthermometric data show that the homogenization densities of 0.92～1.07 g/cm3. The mineralization pressures is estimated to be 32-68 MPa by using fluid inclusions isochoric and laser Raman spectroscopy study.The formation of Wusihe lead-zinc deposit is related to migration of majority metallogenic material in hydrothermal stage, and we consider it is a sedimentary diagenesis-strong hydrothermal reform type. Ore-forming fluids in this deposit are multicomponent system with middle-low temperature and salinity, and initial fluids contain Na+, Mg2+, Cl-, SO42- and organic matter. The mixing ore-forming fluids offer heat, SO42-, and Mg2+ to facilitate the thermochemical sulfate reduction (TSR). The reaction mechanisms of TSR could be a key factor controlling sulfur source and H+. The change of pH value further leads to the precipitation of metallogenic elements.The thermochemical sulfate reduction (TSR) was involved in the lead-zinc mineralization of the Wusihe deposit. Sulfur isotope fractionation and bitumen, CH4, N2, C2H6, C2H2 and H2S are witnessed.(3) The Nanmushu lead-zinc depositThe Nanmushu large lead-zinc deposit lies in the northern margin of the Yangtze Block. However, ore-forming fluids, especially feature of fluid inclusions from different mineralization stages, have not been well studied. This study made a systematic research works on fluid inclusions in Nanmushu lead-zinc deposit and the first time identified the organic fluid inclusions. The whole hydrothermal ore-forming processes involve three stages, including Pyrite-Sphalerite-Quartz-Dolomite stage, Polymetallic sulfide stage and Barite-Calcite stage. There were two stages bitumen, and the later one is closely associated with sphalerite.There are two kinds of sphalerite with different petrographic features. Sphalerite from metallogenic stage I exists as brownish red, fine grained, and hypidiomorphic to xenomorphic granular grain, which intergrows with idiomorphic-granular pyrite. Sphalerite from metallogenic stage II is coarse and idiomorphic granular in grain size and shows yellow-yellow white in color.FI petrographic study shows that all samples contain abundant primary inclusions, and the two phases (L+V type) are dominated. On the basis of different features, they can be further divided into aqueous inclusions, oil inclusions with fluorescence colors and high-CH4 inclusions. The organic fluids, especially the high-CH4 inclusions and oil inclusions with fluorescence colors from stage I and II, are contributed to mineralization.At Stage I, homogenization temperatures of FIs range from 180 to 340℃ with salinities of 4.0-13.3 wt.%NaCl equiv. Brownish red sphalerite of Stage I captured the W-type FIs, which have totally homogenized at 225～314℃. These FIs from sphalerite yield final ice melting temperatures of -7.6～-3.8℃, salinities of 6.2 to 11.2 wt.% NaCl equiv and densities of 0.75-0.93g/cm3.At Stage II, microthermometric data show that the FIs have totally homogenized concentrating at 180～260℃ with salinities of 2.1～14.3 wt.% NaCl equiv. The FIs captured in yellow white sphalerite have been homogenized totally to a liquid phase at temperatures between 183 and 276℃. Their final ice melting temperatures range from -9.4 to-3.6℃, with salinities of 5.9-13.3 wt.% NaCl equiv. The densities of FIs are from 0.84 to 0.95g/cm3.At Stage Ⅲ, the primary minerals are calcite, quartz and barite. The FIs that were captured have totally homogenized concentrating at 140-180 ℃, with ice-melting temperatures of-15.4--1.2℃, salinities of 2.1～19.0 wt.% NaCl equiv. The densities of FIs are from 0.90 to 1.04 g/cm3.Due to the intensive fracture and hydrothermal activities, the Nanmushu lead-zinc deposit only shows the characteristics of hydrothermal filling type. The estimated trapping pressure is 55-80 MPa from main stages. Oil cracking could be a key factor to sulphide precipitation and providing the hydrogen sulphide and ore-forming fluids mixing must have played an important role in ore genesis.In conclusion, the three typical deposits were named "Yangtze type" lead-zinc deposits in terms of their geological position and their similarities. According to the differences between geological characteristics and mineralization, the "Yangtze type" lead-zinc deposits can be further divied into sedimentary diagenesis-weak hydrothermal reform type (i.e. the Aozigang zinc deposit), sedimentary diagenesis-strong hydrothermal reform type (i.e. the Wusihe lead-zinc deposit) and hydrothermal filling type (i.e. Nanmushu lead-zinc deposit). The formation of "Yangtze type" lead-zinc deposits are related to the interaction of Sinian sedimentary metallogenic system with Indosinian-Yanshanian fluid system, which have the following characteristics:(1) Ore-forming fluids in "Yangtze type" lead-zinc deposits are multicomponent system with NaCl, KCl, CaCl2, MgCl2, and H2O. The liquid components in fluid inclusions have Na+, K+, Ca2+, Mg2+, Cl-, and SO42-, and K+<Na+, Mg2+<Ca2+;(2) Petrographic study has shown abundant FIs in quartz, calcite, dolomite, barite and fluorite. Furthermore, there are plentiful FIs in the sphalerite. Primary fluid inclusions in "Yangtze type" lead-zinc deposits include two phases (L+V type);(3) Laser Raman spectroscopy study show that the "Yangtze type" lead-zinc deposits have high-CH4 inclusions. The Wusihe lead-zinc deposit and Aozigang zinc deposit contain component of CH4, N2, H2S, NH3, C2H6, C3H8, and C2H2;(4) The FIs that were captured have totally homogenized concentrating at 150～280℃, with salinities of 8-16 wt.%NaCl equiv; they are characterized by middle-low temperatures and salinities, and pressures of 20～80 MPa;(5) Fluid system is characterized by organic matters (e.g. bitumen and FIs) and related to mineralization. The liquid components in fluid inclusions may contain hydrocarbon on the basis of the fluorescence colours under fluorescence microscope (i.e. the Aozigang zinc deposit and Nanmushu lead-zinc deposit);(6) Oil cracking and thermochemical sulfate reduction (TSR) could be factors controlling sulfur source and related to mechanisms. The fluids content organic matter and other ore-forming fluids mixing have played an important role in ore genesis.The selected three typical deposits (i.e. the Aozigang zinc deposit, the Wusihe lead-zinc deposit, the Nanmushu lead-zinc deposit) are hosted in the Sinian stratum, and controlled by Indosinian-Yanshanian fluids system. They are different from the lead-zinc deposits (i.e. the Huangshaping and Zhangshiba lead-zinc deposit), which are related to magmatic hydrothermal ore-forming fluid system. The main differences include fluid system, fluid inclusion assemblages, components, temperatures, salinities and fluid metallogenic mechanism.