Analysis On Oxygen Fugacity And Water Content Of Gangdese Magmatic In Tibet

The Gangdese metallogenic belt is currently the largest porphyry copper molybdenum?gold?metallogenic belt in China,and the difference of oxygen fugacity and water content of magma affects the formation of porphyry copper deposits.Based on previous studies,petrology,mineralogy and elemental geochemistry of the Eocene and Miocene magmatic rocks in Gangdese are studied in this paper,and we comprehensively analyze the oxygen fugacity and water bearing characteristics of the magma in the early and Neocene epoch of Gangdese to evaluate the metallogenic potential of the Miocene magmatic rocks in the Western Gangdese.The Ce⁴⁺/Ce³⁺and Eu/Eu*ratios of zircons in western Eocene magmatic of the Gangdese are generally higher than those of the eastern Gangdese,but both in eastern and western parts,the average of Ce⁴⁺/Ce³⁺are less than 300 and the average of Eu/Eu*are less than 0.4,it suggests that the Eocene magmatic of Gangdese is characterized by low oxygen fugacity,and the intrusions are not ore-bearing porphyry.The average of the oxygen fugacity of the Eocene Gangdise is?NNO=+1.3,logfO₂=-12.4 and magmatic water is 4.8%,it shows that the Gangdis Eocene magma has low oxygen fugacity and high water content.The average of zircon Ce⁴⁺/Ce³⁺in the eastern part of the Miocene magmatic is423,while the average in the western is 314.The Miocene magmatic oxygen fugacity is much higher than the Eocene magmatic.Identically,the eastern Ce⁴⁺/Ce³⁺ratios are higher than the western ones,so the eastern Miocene magmatic of Gangdese has higher oxygen fugacity.Both eastern and western parts of Miocene granite havesimilar temperature,pressure and water content,but the oxygen fugacity of magma in the western is slightly higher.On the whole,the mean temperature is781?,the pressure is 96MPa,it's corresponding depth is 3.8km.The average value of the oxygen fugacity of magma is NNO=+1.8,logfO₂ is-11.5 and the average of water content is 3.7%.The Miocene magmatic of Gangdese has high oxygen fugacity and low water content.The oxygen fugacity and water content of magma restricts the formation of porphyry deposits.The higher the oxygen fugacity of magma,the more favorable for the enrichment of minerals,and the ore body is larger,too.From paleocene-Eocene to Miocene,the Th/Yb and La/Yb ratios are increasing.The water content is not related to ore bearing.The ore-bearing magma is not ore-forming magma,and the higher the water content of magmatic,the better the formation of porphyry deposit.