羊拉地区含矿矽卡岩成因的地球化学证据

GEOCHEMICAL EVIDENCE OF EXHALATIVE_SEDIMENTARY ORE_BEARING SKARNS IN YANGLA COPPER MINERALIZATION CONCENTRATED AREA, DEQIN COUNTY, NORTHWESTERN YUNNAN PROVINCE

滇西北羊拉铜矿化集中区现已发现４个矽卡岩体，按成因分为喷流－沉积和接触交代矽卡岩两大类，前者呈层状，是铜的主要赋矿围岩．不同成因的矽卡岩具有不同的岩石化学组成，其中喷流－沉积矽卡岩有贫Ａｌ２Ｏ３，ＴｉＯ２，ＲＥＥ和富铁（ＴＦｅＯ）等特征．在ｗ（ＴｉＯ２）－ｗ（Ａｌ２Ｏ３），ｗ（ＳｉＯ２）－ｗ（Ａｌ２Ｏ３），Ｎ（Ｆｅ）／Ｎ（Ｔｉ）－Ｎ（Ａｌ）／Ｎ（Ａｌ＋Ｆｅ＋Ｍｎ），ｗ（ＳｉＯ２）／２－ｗ（ＴＦｅＯ）－ｗ（ＣａＯ），ｗ（Ａｌ）－ｗ（Ｆｅ）－ｗ（Ｍｎ）和ｗ（Ａｌ２Ｏ３）－ｗ（ＴＦｅＯ）－ｗ（ＣａＯ）等多种岩石化学图解中喷流－沉积矽卡岩显示出与交代矽卡岩有较大的差别，前者的数据点多落在热水沉积区，而后者的数据点则多分布在大理岩与岩浆岩（中基性火山岩和花岗闪长岩）之间；喷流－沉积矽卡岩的稀土分布形式与现代大洋中脊热水沉积物相似，而交代矽卡岩则更接近于相关的侵入岩．

The Yangla copper mineralization concentrated area, located in Deqin County, northwestern Yunnan Province, belongs to the Jinshajiang suture. In this area, the following four skarn bodies are discovered: Linong stratified skarn (SK1), Lunong skarn (SK2), Linong granodiorite_hosted skarn (SK3) and Beiwu skarn (SK4). These four skarn bodies can be divided into two major categories in terms of their origins: exhalative_sedimentary skarn bodies (SK1 and SK2) and contact metasomatic skarn bodies (SK3 and SK4). The exhalative_sedimentary skarn bodies, stratified, are the most important host rocks of copper deposit in this region. Skarns with different origins vary with their different petrochemical compositions. Relative to metasomatic skarns at this area and other regions in China, exhalative skarns are poor in Al 2O 3, TiO 2, low in Na 2O, K 2O, MnO, MgO, and rich in TFeO (TFeO = FeO+0.9 Fe 2O 3). In diagrams of w (TiO 2) versus w (Al 2O 3), w (SiO 2) versus w (Al 2O 3), N (Fe)/ N (Ti) versus N (Al)/ N (Al+Fe+Mn), w (SiO 2)/2_ w (TFeO)_ w (CaO), w (Al)_ w (Fe)_ w (Mn) and w (Al 2O 3)_ w (TFeO)_ w (CaO), exhalative and metasomatic skarns scatter in different fields: the data of the former are plotted in the hydrothermal sedimentary field, while those of the latter between carbonate rocks (marble) and magmatic rocks (granodiorite and intermediate_basic volcanic rocks). The exhalative and metasomatic skarns have also quite different REE mass fractions and distribution patterns, in that the exhalative skarns are featured by the lower REE abundance, distinctively positive δ (Eu) anomalies and slightly negative δ (Ce) anomalies and that the metasomatic skarns are often characterized by the higher REE mass fractions and the REE distribution patterns similar to those of the associated intrusive rocks. In summary, the geochemical features of the skarn rocks can serve as important criteria for skarn origins.