云南都龙锡锌多金属矿床矽卡岩矿物学和矿物化学特征及其地质意义

都龙锡锌多金属矿床位于华夏地块西缘,是中国最大的锡石硫化物矿床之一。矿体主要呈层状、似层状产于晚白垩纪花岗岩体上覆新元古代新寨岩组的矽卡岩带内。矽卡岩蚀变矿化可划分为5个阶段:石榴子石-辉石阶段、阳起石-绿泥石阶段、磁铁矿-锡石阶段、锡石-磁黄铁矿-闪锌矿阶段、闪锌矿-黄铜矿阶段。以都龙矿床中的矽卡岩为研究对象,在识别矿物组合与结构的基础上,采用电子探针与LA-ICP-MS技术对矽卡岩中的石榴子石、辉石进行了主量、微量元素分析。结果表明:都龙矿床矽卡岩中的石榴子石可分为3类,且具有一定的世代特征。Grt Ⅰ世代为早阶段形成的钙铁榴石(And_95～99Gr_1～5),多位于核部,呈残余状,与透辉石共生; Grt Ⅱ世代为形成时间稍晚的钙铁-钙铝榴石(And_50～80Gr_20～50),多围绕Grt Ⅰ世代形成,与钙铁辉石共生; Grt Ⅲ世代为最晚阶段形成的细粒钙铝榴石(And_30～40Gr_60～70),量少,多呈不规则状。石榴子石与辉石的矿物组合变化暗示流体氧化性具有逐渐降低趋势。微量元素特征显示Grt Ⅰ世代呈轻稀土元素富集、重稀土元素亏损的右倾型特征,Grt Ⅱ世代呈重稀土元素富集、轻稀土元素亏损的左倾型特征,表明稀土元素分配可能受石榴子石晶体化学结构和固溶体端元组分共同影响。不同世代石榴子石成矿元素In和Sn含量差异也较大。Grt Ⅰ世代In和Sn含量分别为(63.0～264.0)×10^-6(平均值为159.0×10^-6)和(2 790～13 100)×10^-6(平均值为7 441×10^-6); Grt Ⅱ世代In和Sn含量分别为(1.6～81.0)×10^-6(平均值为20.0×10^-6)和(436～4 740)×10^-6(平均值为2 518×10^-6)。石榴子石中In与Sn含量的正相关关系暗示了两者的同时性、同源性,并且在矽卡岩形成早期已经具有一定的选择性富集趋势。因此,系统地研究矽卡岩中的矿物化学特征对有效约束成矿流体在矽卡岩蚀变与矿化过程的演化具有重要意义。

Skarn Mineralogy and Mineral Chemistry Characteristics of Dulong Sn-Zn Polymetallic Deposit in Yunnan,China and Their Geological Significances

The Dulong Sn-Zn polymetallic deposit is located in the western margin of Cathaysia and is one of the largest cassiterite-sulfide deposits in China. Ore bodies mainly occur as strata-bound in skarns within Neoproterozoic Xinzhaiyan Formation, overlying Late Cretaceous granite. The skarn alteration and mineralization can be divided into five stages: garnet-pyroxene stage, actinolite-chlorite stage, magnetite-cassiterite stage, cassiterite-pyrrhotite-sphalerite stage, sphalerite-chalcopyrite stage. The skarn in Dulong deposit was chosen as research object, and the major and trace elements of garnet and pyroxene were analyzed by EPMA and LA-ICP-MS based on the identification of mineral assemblage and structure. The results show that garnets can be divided into three types and show certain generation characteristics. In the early stage(Grt Ⅰ), garnet is andradite(And_90-99Gr_1-5), most of which is located in the nuclear part, and symbiosis with diopside; Grt Ⅱ belongs to andradite-grossular solid solution series(And_50-80Gr_20-50), it is formed late around Grt Ⅰ and symbiosis with hedenbergite; Grt Ⅲ is fine-grained grossular(And_30-40Gr_60-70)formed in the latest stage, and most of which is irregular. The change of mineral assemblages of garnet and pyroxene indicates that the oxidization of fluid decreases gradually. Trace element characteristics show that Grt Ⅰ displays LREE-enrichment and HREE-depletion, whereas Grt Ⅱ displays HREE-enrichment and LREE-depletion. It is suggested that REE distribution may be influenced by the crystal chemical structure of garnet and the solid solution endmember components. Contents of In and Sn in different generations of garnet are also divergent significantly. Contents of In and Sn in andradite(Grt Ⅰ)are(63.0-264.0)×10^-6 with the average of 159.0×10^-6, and(2 790-13 100)×10^-6 with the average of 7 441×10^-6, respectively. Contents of In and Sn in grossular(Grt Ⅱ)are(1.6-81.0)×10^-6 with the average of 20.0×10^-6, and(436-4 740)×10^-6 with the average of 2 518×10^-6, respectively. The positive correlation of contents of In and Sn in garnet implies the simultaneous and homologous relationship between them. There is a certain tendency of selective enrichment in the early stage of skarn formation. Thus, the systematic research of skarn mineralogy and mineral chemistry characteristics is of great significance to effectively constrain the evolution of ore-forming fluid in skarn alteration and mineralization process.