广西德保矽卡岩型铜锡矿床成矿流体演化特征

Characteristics of Mineralizing Fluids Forming the Debao Skarn-type Deposit in Guangxi Province

位于右江盆地的德保铜锡矿床是广西最大的铜矿床,目前仅有少量年代学和矿物学等方面的研究,对于该矿床流体特征、流体环境、矿床元素组成等所知甚少。通过矿石矿物的探针分析、流体包裹体岩相学和显微测温、平衡热力学计算约束了该矿床成矿流体的地球化学参数,分析了成矿阶段的流体环境。与典型矽卡岩型铜锡矿床不同的是,德保铜锡矿的石榴子石以镁铝榴石和钙铝榴石为主,含少量钙铁榴石;锡主要以类质同象富集在黄铜矿、黄铁矿等硫化物中,含量达96×10^(-6)~627×10^(-6);石榴子石的流体包裹体均一温度182~249℃,平均值219℃;方解石包裹体均一温度109~215℃,平均值157℃;平衡热力学计算模拟表明,德保铜锡矿床硫逸度和氧逸度范围分别是:logf_(S_(2))>-20,-44<logf_(O_(2))<-25。早期成矿流体温度的降低为黄铜矿沉淀提供了有利条件,锡元素伴随黄铜矿沉淀;成矿流体的氧逸度高于世界典型锡矿的氧逸度,这种高氧逸度的成矿流体环境不利于锡元素的运移及再富集,这可能是德保铜锡矿中锡较少以锡石存在的重要原因之一。

The Debao skarn-type copper-tin deposit in the Youjiang Basin is the largest copper deposit in Guangxi province.Little is known about the geochemical characteristics of ore-forming fluids and the occurrence state of tin beyond its chronology and mineralogy.We constrain the characteristics and parameters of mineralizing fluids using microprobe analysis,fluid inclusion petrography and microthermometry,and geochemical modeling.Different from typical skarn-type copper deposits,the garnet in Debao belongs to the end-member of pyrope and grossular and contains a small amount of andradite.Tin occurs mainly in chalcopyrite and pyrite,and its abundance can reach up to 0.11%.The homogenization temperatures of fluid inclusions in garnet are 182-249℃ with an average of 219℃,and those of fluid inclusions in calcite ranges from 109℃ to 215℃ with an average of 157℃.Therefore,the mineralizing fluids in the early stage of the Debao deposit were interpreted to be of low temperature and mediumlow salinity,while those in the late quartz-carbonate stage are characterized by low temperature and low salinity.Temperature decrease favors precipitation of chalcopyrite.The sulfur fugacity and oxygen fugacity of the Debao deposit constrained from equilibrium phase diagrams of Cu-S-O and Fe-S-O systems under mineralization temperatures are log f_(S_(2))>-20,and-44<logf_(O_(2))<-25,respectively.The oxygen fugacity levels of the Debao deposits were higher than those of typical tin deposits and the high logf_(O_(2)) fluid environment prevents tin transport in fluids.This is probably one of reasons why cassiterite is rare in the Debao deposit.