Subduction-related Cu-(Au) deposits which represent giant geochemical anomalies of metals and S in the upper crust are commonly associated with arc magmas. However, the fundamental differences between barren and ferti...Subduction-related Cu-(Au) deposits which represent giant geochemical anomalies of metals and S in the upper crust are commonly associated with arc magmas. However, the fundamental differences between barren and fertile magma producing these deposits still remain highly controversial. In this study, we report the chemical compositions of zircon and silicate melt inclusions(SMIs) from barren arc lavas at eastern Taiwan Island aiming to increase our knowledge on the factors that affect the mineralization potential of arc magma systems. The zircon U-Pb dating shows the magmatism occurred at ~0.7 Ma and the andesitic lava formed at ~900–950℃ with a reduced magmatic environment. The plagioclase-hosted SMIs show an andesitic melt composition and variable Cu content features. The calculated H_(2)O content of parental melt is ~3–4 wt.%. The evidence presented above suggests that oxidation state(f O_(2)) and H_(2)O content are probably the key controls of Cu-(Au) fertility of arc magmas. We interpret that a reduced and relatively dry magma may be a potential hinderance to cause the absence of porphyry Cu deposits at the eastern Taiwan Island.展开更多
Origin of garnet in skarn(magmatic vs.hydrothermal)and the prograde skarn fluid evolution are still controversial.Two generations of garnet(Grt1,Grt2)were identified at the Tongshankou deposit:Grt1 is anisotropic with...Origin of garnet in skarn(magmatic vs.hydrothermal)and the prograde skarn fluid evolution are still controversial.Two generations of garnet(Grt1,Grt2)were identified at the Tongshankou deposit:Grt1 is anisotropic with oscillatory zoning and resorbed boundary,whilst Grt2 grew around Grt1 and formed oscillatory rims.In-situ LA-ICP-MS U-Pb dating of Grt1 and Grt2 yielded a lower intercept^(206)Pb/^(238)U age of 142.4±2.8 Ma(n=57;MSWD=1.16)and 142.3±9.6 Ma(n=60;MSWD=1.06),respectively,coeval with the ore formation and ore-related granodiorite emplacement.Positive Eu anomaly,non-CHARAC Y/Ho value and low TiO_(2)content,together with the mineral assemblages indicate that both Grtl and Grt2 have a hydrothermal origin.The existence of melt and melt-fluid inclusions in Grt1,together with similar LREE-enriched patterns to the granodiorite,further indicate that Grt1 may have formed in the magmatic-hydrothermal transition.Higher U contents and LREE-enriched patterns of Grt1 indicate that fluid I is mildly acidic pH and low fO_(2).The inner gray Grt2 rims(Grt2A)is HREE-enriched with low U contents,indicating that fluid II has nearly neutral pH and high fO_(2).The wider Y/Ho range and LREE-enriched patterns of the outer light-gray Grt2 rims(Grt2B)show that the evolved magmatic fluidⅡhad mixed with an external fluid,characterized by being mildly acidic pH and with high fO_(2)·Our results suggest that the prograde skarn-forming fluids can be multistage at Tongshankou,and the mixing of meteoric water may have been prominent in the prograde skarn stage.展开更多
基金supported by the National Natural Science Foundation of China (Grant No. 41921003)the National Key R&D Program of China (Grant No. 2022YFC2903301)+1 种基金the National Natural Science Foundation of China (Grant No. 42003031)financial support from the China Scholarship Council program (CSC) during the stay at McGill University。
文摘Subduction-related Cu-(Au) deposits which represent giant geochemical anomalies of metals and S in the upper crust are commonly associated with arc magmas. However, the fundamental differences between barren and fertile magma producing these deposits still remain highly controversial. In this study, we report the chemical compositions of zircon and silicate melt inclusions(SMIs) from barren arc lavas at eastern Taiwan Island aiming to increase our knowledge on the factors that affect the mineralization potential of arc magma systems. The zircon U-Pb dating shows the magmatism occurred at ~0.7 Ma and the andesitic lava formed at ~900–950℃ with a reduced magmatic environment. The plagioclase-hosted SMIs show an andesitic melt composition and variable Cu content features. The calculated H_(2)O content of parental melt is ~3–4 wt.%. The evidence presented above suggests that oxidation state(f O_(2)) and H_(2)O content are probably the key controls of Cu-(Au) fertility of arc magmas. We interpret that a reduced and relatively dry magma may be a potential hinderance to cause the absence of porphyry Cu deposits at the eastern Taiwan Island.
基金the Guangdong Major Projects of Basic and Applied Research(2019B030302013)the National Natural Science Foundation of China(NSFC Projects Nos.42102064,42173065 and 41725009)the China Postdoctoral Science Foundation(2021M693188).
文摘Origin of garnet in skarn(magmatic vs.hydrothermal)and the prograde skarn fluid evolution are still controversial.Two generations of garnet(Grt1,Grt2)were identified at the Tongshankou deposit:Grt1 is anisotropic with oscillatory zoning and resorbed boundary,whilst Grt2 grew around Grt1 and formed oscillatory rims.In-situ LA-ICP-MS U-Pb dating of Grt1 and Grt2 yielded a lower intercept^(206)Pb/^(238)U age of 142.4±2.8 Ma(n=57;MSWD=1.16)and 142.3±9.6 Ma(n=60;MSWD=1.06),respectively,coeval with the ore formation and ore-related granodiorite emplacement.Positive Eu anomaly,non-CHARAC Y/Ho value and low TiO_(2)content,together with the mineral assemblages indicate that both Grtl and Grt2 have a hydrothermal origin.The existence of melt and melt-fluid inclusions in Grt1,together with similar LREE-enriched patterns to the granodiorite,further indicate that Grt1 may have formed in the magmatic-hydrothermal transition.Higher U contents and LREE-enriched patterns of Grt1 indicate that fluid I is mildly acidic pH and low fO_(2).The inner gray Grt2 rims(Grt2A)is HREE-enriched with low U contents,indicating that fluid II has nearly neutral pH and high fO_(2).The wider Y/Ho range and LREE-enriched patterns of the outer light-gray Grt2 rims(Grt2B)show that the evolved magmatic fluidⅡhad mixed with an external fluid,characterized by being mildly acidic pH and with high fO_(2)·Our results suggest that the prograde skarn-forming fluids can be multistage at Tongshankou,and the mixing of meteoric water may have been prominent in the prograde skarn stage.
