Three carbonate ocelli-bearing lamprophyre dykes have been found in the Laowangzhai and Beiya gold orefields in the northern sector of the Ailaoshan gold deposit zone, Yunnan Province. Ocelli in the lamprophyre dykes ...Three carbonate ocelli-bearing lamprophyre dykes have been found in the Laowangzhai and Beiya gold orefields in the northern sector of the Ailaoshan gold deposit zone, Yunnan Province. Ocelli in the lamprophyre dykes are carbonates composed mainly of dolomite and calcite. Their trace elements, REE and C isotopic compositions are characteristic of carbonatite and the main mineral assemblages, major elements, trace elements and REE in the matrix are similar to those in the carbonate ocelli-barren lamprophyre dykes in the orefields, which are calc-alkaline lamprophyres that derived from the fertile mantle. The results indicate that the carbonate ocelli-bearing lamprophyre dykes in this area were produced at the time when the Himalayan lamprophyre magma evolved to a relatively late stage of silicate-carbonate liquid immiscibility. In the process of magmatic evolution there took place magmatic degassing with CO2 and H2O as the dominant released gases.展开更多
The ocellar lamprophyre dyke(ENE-WSW) is recorded at Wadi Nugrus,Eastern Desert,Egypt.It cuts porphyritic biotite granites and varies in thickness from 0.5 to 1.5 m and up to 3 km in length.The lamprophyre dyke has be...The ocellar lamprophyre dyke(ENE-WSW) is recorded at Wadi Nugrus,Eastern Desert,Egypt.It cuts porphyritic biotite granites and varies in thickness from 0.5 to 1.5 m and up to 3 km in length.The lamprophyre dyke has been altered,and it is characterized by porphyritic and panidiomorphic textures with plagioclase,olivine,and augite constituting the porphyritic phase in a fine groundmass of the same composition.Rutile,titanite,apatite,fluorite,graphite,calcite,allanite,autunite and Fe-Ti oxides are accessory minerals.Kaolinite,chlorite and epidote are secondary minerals.Carbonitization and hematitization are common.Rounded to sub-rounded porphyritic and zoned ocelli with radiate or brush-like shapes are generally common and represent physical traps for mineralization.The ocellar features are interpreted to represent the late stage of magmatic segregation or magmatic crystallization involving two immiscible magmatic liquids.Mineralogical studies of altered lamprophyre samples,based on X-ray diffraction(XRD) and environmental scanning electron microscopy(ESEM),indicate the presence of secondary uranium minerals(autunite and uranophane),associated with gold,silver,nickel,atacamite,molybdenum,pyrite and zincite.The presence of kaolinite indicates a high temperature environment.The mixing of volatile fluids with meteoric water and fluid-wall rock interaction result in changes of pH and oxygen activity and deposition of base metals in reducing environs(graphite and pyrite).Precipitation of hematite probably decreased the pH of the solution and rose acidic fluids.The sudden change in pH and temperature of the fluids would lead to destabilization of base metal complexes favoring their deposition.The magma source of the Nugrus lamprophyre is determined to be between calc-alkaline and alkaline and it has HKCAB and shoshonitic nature and may be derived from decomposition melting of such a metasomatised lithospheric mantle.Olivine is a major fractional phase to account for a rapid decrease in Ni and Fe2O3 and an increase in SiO2.Also,clinopyroxene fractionation is important in accounting for the positive correlations of CaO and Cr vs.MgO,whereas plagioclase differentiation is insignificant according to the negative correlations between MgO and Al2O3 and Sr.Crystal fractionation of accessory minerals such as apatite and Fe-Ti oxides is insignificant because of the less variable P2O5 and TiO2 contents.展开更多
基金This work was supported by the State Basic Research and Development Program (Grant No.1999043203) the State Climbing Program (Grant No. 95-yu-39) and the Innovational Program of the Chinese Academy of Sciences (Grant No. KZCX 2-101). Geological fiel
文摘Three carbonate ocelli-bearing lamprophyre dykes have been found in the Laowangzhai and Beiya gold orefields in the northern sector of the Ailaoshan gold deposit zone, Yunnan Province. Ocelli in the lamprophyre dykes are carbonates composed mainly of dolomite and calcite. Their trace elements, REE and C isotopic compositions are characteristic of carbonatite and the main mineral assemblages, major elements, trace elements and REE in the matrix are similar to those in the carbonate ocelli-barren lamprophyre dykes in the orefields, which are calc-alkaline lamprophyres that derived from the fertile mantle. The results indicate that the carbonate ocelli-bearing lamprophyre dykes in this area were produced at the time when the Himalayan lamprophyre magma evolved to a relatively late stage of silicate-carbonate liquid immiscibility. In the process of magmatic evolution there took place magmatic degassing with CO2 and H2O as the dominant released gases.
文摘The ocellar lamprophyre dyke(ENE-WSW) is recorded at Wadi Nugrus,Eastern Desert,Egypt.It cuts porphyritic biotite granites and varies in thickness from 0.5 to 1.5 m and up to 3 km in length.The lamprophyre dyke has been altered,and it is characterized by porphyritic and panidiomorphic textures with plagioclase,olivine,and augite constituting the porphyritic phase in a fine groundmass of the same composition.Rutile,titanite,apatite,fluorite,graphite,calcite,allanite,autunite and Fe-Ti oxides are accessory minerals.Kaolinite,chlorite and epidote are secondary minerals.Carbonitization and hematitization are common.Rounded to sub-rounded porphyritic and zoned ocelli with radiate or brush-like shapes are generally common and represent physical traps for mineralization.The ocellar features are interpreted to represent the late stage of magmatic segregation or magmatic crystallization involving two immiscible magmatic liquids.Mineralogical studies of altered lamprophyre samples,based on X-ray diffraction(XRD) and environmental scanning electron microscopy(ESEM),indicate the presence of secondary uranium minerals(autunite and uranophane),associated with gold,silver,nickel,atacamite,molybdenum,pyrite and zincite.The presence of kaolinite indicates a high temperature environment.The mixing of volatile fluids with meteoric water and fluid-wall rock interaction result in changes of pH and oxygen activity and deposition of base metals in reducing environs(graphite and pyrite).Precipitation of hematite probably decreased the pH of the solution and rose acidic fluids.The sudden change in pH and temperature of the fluids would lead to destabilization of base metal complexes favoring their deposition.The magma source of the Nugrus lamprophyre is determined to be between calc-alkaline and alkaline and it has HKCAB and shoshonitic nature and may be derived from decomposition melting of such a metasomatised lithospheric mantle.Olivine is a major fractional phase to account for a rapid decrease in Ni and Fe2O3 and an increase in SiO2.Also,clinopyroxene fractionation is important in accounting for the positive correlations of CaO and Cr vs.MgO,whereas plagioclase differentiation is insignificant according to the negative correlations between MgO and Al2O3 and Sr.Crystal fractionation of accessory minerals such as apatite and Fe-Ti oxides is insignificant because of the less variable P2O5 and TiO2 contents.
