Magmatic-hydrothermal Sn deposits are commonly associated with high silica magmas,but why most global high silica granites do not bear economic Sn ore grades remains unclear.Two crucial factors controlling magmatic-hy...Magmatic-hydrothermal Sn deposits are commonly associated with high silica magmas,but why most global high silica granites do not bear economic Sn ore grades remains unclear.Two crucial factors controlling magmatic-hydrothermal Sn mineralization,including advanced fractionation and depressurization-induced rapid cooling,were revealed in the case study of the Guyong granitic pluton linked with the Xiaolonghe Sn deposit,in the Tengchong block,SW China.The Guyong granitic pluton comprises three petrological facies:less evolved biotite syenogranite,evolved alkali granite and leucogranite,and highly evolved facies(the protolith of greisenized granite).Similar crystallization ages(~77 Ma)and gradual contact between different petrological facies indicate the Guyong granitic pluton records a continuous fractionation process.Monte Carlo-revised Rayleigh fractionation model suggests the fractionation degree of the Guyong pluton is markedly high(>87 wt.%)that can only be achieved by a high initial water(≥4 wt.%)content in the parent granitic magma revealed by rhyolite-MELTS calculation.Advanced degree fractionation causes the first Sn enrichment but it also significantly increases the viscosity of evolved magmas,suppressing the exsolution and transport of hydrothermal fluids.Hence,it must be compensated by the second critical factor:depressurization-induced rapid cooling,reflected by the occurrence of highly metamict zircons in the greisenized granite.The highly metamict feature,indicated by the large full width at half maximum(FWHM)values of zirconν3(SiO_(4)) peak(>19.5 cm^(-1)),suggests these zircons do not experience thermal annealing but rapidly ascend into a shallow cooling environment.Depressurization-induced rapid cooling facilitates exsolution and transport of hydrothermal fluids,interacting with wall rocks and resulting in Sn mineralization.展开更多
The damage in zircon structure is caused by α-decay which is associated with radiation of uranium and thorium in zircon. Over the narrow dose range from 0.3×10<sup>16</sup> to 0.8×10<sup>1...The damage in zircon structure is caused by α-decay which is associated with radiation of uranium and thorium in zircon. Over the narrow dose range from 0.3×10<sup>16</sup> to 0.8×10<sup>16</sup> α-events mg<sup>-1</sup>, natural crystalline zircon begins to convert to metamict state. When α-decay dose reaches 0.8×10<sup>16</sup> α-events mg<sup>-1</sup>, the crystalline zircon would be changed to completely metamict state through several million geological years, and the metamict state zircon ap-展开更多
基金financially supported by the Major Research Project of National Natural Science Foundation of China(NSFC ProjectGrant Nos.92162215 and 92155305)+2 种基金National Key Basic Research Development Program of China(973 ProgramGrant Nos.2015CB452602 and 2015CB452606)111 Plan under the Ministry of Education of China and the State Administration of Foreign Experts Affairs,China(Grant No.B07011)。
文摘Magmatic-hydrothermal Sn deposits are commonly associated with high silica magmas,but why most global high silica granites do not bear economic Sn ore grades remains unclear.Two crucial factors controlling magmatic-hydrothermal Sn mineralization,including advanced fractionation and depressurization-induced rapid cooling,were revealed in the case study of the Guyong granitic pluton linked with the Xiaolonghe Sn deposit,in the Tengchong block,SW China.The Guyong granitic pluton comprises three petrological facies:less evolved biotite syenogranite,evolved alkali granite and leucogranite,and highly evolved facies(the protolith of greisenized granite).Similar crystallization ages(~77 Ma)and gradual contact between different petrological facies indicate the Guyong granitic pluton records a continuous fractionation process.Monte Carlo-revised Rayleigh fractionation model suggests the fractionation degree of the Guyong pluton is markedly high(>87 wt.%)that can only be achieved by a high initial water(≥4 wt.%)content in the parent granitic magma revealed by rhyolite-MELTS calculation.Advanced degree fractionation causes the first Sn enrichment but it also significantly increases the viscosity of evolved magmas,suppressing the exsolution and transport of hydrothermal fluids.Hence,it must be compensated by the second critical factor:depressurization-induced rapid cooling,reflected by the occurrence of highly metamict zircons in the greisenized granite.The highly metamict feature,indicated by the large full width at half maximum(FWHM)values of zirconν3(SiO_(4)) peak(>19.5 cm^(-1)),suggests these zircons do not experience thermal annealing but rapidly ascend into a shallow cooling environment.Depressurization-induced rapid cooling facilitates exsolution and transport of hydrothermal fluids,interacting with wall rocks and resulting in Sn mineralization.
基金Project supported by the National Natural Science Foundation of China.
文摘The damage in zircon structure is caused by α-decay which is associated with radiation of uranium and thorium in zircon. Over the narrow dose range from 0.3×10<sup>16</sup> to 0.8×10<sup>16</sup> α-events mg<sup>-1</sup>, natural crystalline zircon begins to convert to metamict state. When α-decay dose reaches 0.8×10<sup>16</sup> α-events mg<sup>-1</sup>, the crystalline zircon would be changed to completely metamict state through several million geological years, and the metamict state zircon ap-
