甘肃阳坝铜多金属矿床流体包裹体及S、Pb同位素组成特征

Fluid inclusions and sulfur and lead isotopic composition of Yangba Cu-polymetallic deposit in Gansu Province

甘肃省阳坝铜多金属矿床位于碧口地体的东北部,矿体呈层状、似层状赋存于碧口群阳坝组细碧凝灰岩和凝灰质千枚岩的过渡部位,根据矿床地质特征,将成矿期划分为海底火山喷流沉积期和变质热液叠加改造期。基于对阳坝矿床详细的野外观察和矿相学的研究,通过对矿床流体包裹体和S、Pb同位素的研究,总结矿床的成矿流体性质和成矿物质来源,探讨成矿机制。研究表明,阳坝矿床海底火山喷流沉积期流体包裹体类型主要为水溶液包裹体,成矿流体均一温度为135～336℃,盐度w(NaCl_(eq))为0.70%～10.61%,密度为0.58～0.97g/cm^3;包裹体气相成分以H_2O为主,含少量的CO_2和N_2,属于中低温、低盐度的H_2O-NaCl流体体系,与典型VMS型矿床成矿流体特征相似;变质热液叠加改造期流体包裹体类型主要为水溶液包裹体、CO_2-H_2O包裹体和纯CO_2包裹体,成矿流体均一温度为179～384℃,盐度w(NaCl_(eq))为3.39%～14.78%,密度为0.61～0.99 g/cm^3,包裹体气相成分富含CO_2及少量N_2,属于中高温、低盐度的H_2O-CO_2-NaCl±N_2流体体系,与区域上造山型金矿成矿流体特征一致,均为来自深部的变质流体。喷流沉积期矿石硫化物的δ^(34)S为-7.5‰～3.4‰,均值为-0.46‰,成矿热液δ^(34)S_(∑S)值≈3.73‰,变质热液叠加改造期硫化物的δ^(34)S为-6.7‰～3.3‰,均值-0.575‰,均显示幔源硫的特征。喷流沉积期(^(206)Pb/^(204)Pb=17.505～18.008、^(207)Pb/^(204)Pb=15.521～15.558、^(208)Pb/^(204)Pb=37.494～37.851)与变质热液叠加改造期(^(206)Pb/^(204)Pb=17.293～17.947、^(207)Pb/^(204)Pb=15.498～15.542、^(208)Pb/^(204)Pb=37.388～37.640)的矿石硫化物的Pb同位素组成相近,认为2期矿石铅具有相同来源。通过与阳坝组火山岩、阳坝岩体的Pb同位素组成对比,并结合Pb同位素源区特征值、构造模式图解和△β-△γ成因分类图解分析,认为矿石铅来自上地壳和地幔的混合。阳坝铜多金属矿床属于海底火山喷流沉积-变质热液叠加改造型矿床。

The Yangba copper polymetallic deposit is located in the northeastern part of the Bikou Terrane, and the copper orebody occurs in bedded and para-bedded forms in the contact zone between spilite-porphyrite tuff and tuffaceous phyllite. According to the geological characteristics of the deposit, the metamorphic period is divided into the submarine volcanic sedimentary period and the metamorphic hydrothermal reformation period. Based on the field work and microscope study of Yangba deposit, the authors studied orebody features, fluid inclusions and S-Pb isotopes of minerals so as to determine the ore-forming fluid, ore-forming materials and metamorphic mechanisms. The results show that the fluid inclusions in the stage of submarine volcanic sedimentary exhalation mainly contain liquid-rich inclusions, and their homogenization temperature sand salinities vary from 135℃ to 336℃ and 0.70% to 10.61% NaCleq, and densities vary from 0.58 g/cm3 to 0.97 g/cm3. The gas phase components of inclusions are dominated by H2O and minor amounts of CO2, N2, so the ore-forming fluids are of low to medium temperature, low salinity H2O-NaCl type solutions, and are similar to those of typical VMS deposits. The fluid inclusions in the metamorphic superposed reforming period mainly contain liquid-rich H2O-CO2 inclusions and pure vapor inclusions, their homogenization temperatures and salinities vary from 179℃ to 384℃and 3.39% to 14.78% w(NaCleq), and densities vary from 0.61 g/cm3 to 0.99 g/cm3. The gas phase components of inclusions are dominated by H2O, CO2 and minor amounts of N2, so the ore-forming fluids are of medium to high temperature and low salinity H2O-CO2-NaCl±N2 type solutions, which are consistent with the metamorphic fluid characteristics of the orogenic gold deposits in this area and were probably derived from deep metamorphic fluids. The δ34S values of sulfides from the submarine volcanic sedimentary exhalation range from -7.5‰ to 3.4‰ with an average of -0.46‰ and the metamorphic fluid of δ34S∑S≈3.73‰, whereas the δ34S values of sulfides from the metamorphic superposed reforming period range from -6.7‰ to 3.3‰ with an average of -0.575‰,which all indicates the characteristics of deep source sulfur. There exist evident similarities in Pb isotopic composition of metal sulfides between the exhalative sedimentary epoch(206Pb/204Pb=17.505~18.008, 207Pb/204Pb=15.521~15.558、 208Pb/204Pb=37.494~37.851)and the superimposed-reform epoch(206Pb/204Pb=17.293~17.947, 207Pb/204Pb=15.498~15.542、 208Pb/204Pb=37.388~37.640), so the two periods of ore Pb sources are the same. By comparison with volcanic rocks of the Yangba Group and the Yangba pluton in combination with the source area features as well as the tectonic evolution diagram of Pb isotope with △β-△γ diagram of genetic classification of ore Pb, it can be concluded that the ore Pb came from the mixture of the upper crust and the mantle. The deposit should belong to the massive sulfide copper polymetallic deposit of the sea floor exhalation sedimentation- metamorphic hydrothermal reformation type.