湖南常宁县康家湾铅锌金矿硅化角砾岩岩石地球化学特征

PETROCHEMICAL CHARACTERS OF SILICIFIED BRECCIA IN THE KANGJIAWAN POLY-METALLIC DEPOSIT, CHANGNING, HUNAN

湖南常宁县康家湾铅锌金矿床硅化角砾岩带由未硅化角砾岩—极弱硅化角砾岩—硅化角砾岩—强硅化角砾岩 (似硅质岩 )组成 ,产于侏罗系与下伏二叠系间的不整合面附近 ,角砾成份复杂 ,充填物和胶结物类型多样。随着硅化程度的加强 ,硅化角砾岩带SiO2 含量变化大 ,最高可达 95 34% ,而Al2 O3 、MgO、FeO、K2 O、Na2 O、CaO、CO2 和P2 O5含量特别是MgO、CaO、CO2 和P2 O5含量明显降低 ,且K2 O >Na2 O ,TiO2 的含量显著偏低。硅化角砾岩带LREE HREE比值为 1 95～ 4 93 ,Ce(δCe =0 44～ 0 81 )和Eu(δEu =0 58～ 0 89)均为弱负异常 ,属轻稀土富集型 ,但随硅化程度增高稀土元素含量显著减少 :未硅化和弱硅化角砾岩稀土元素总量较高 ,为 (1 76 82～ 31 8 93)× 1 0 -1 0 ,与当冲组泥质岩配分曲线相似 ;硅化强烈的角砾岩稀土元素总量低 ,为 (7 71～ 65 95)× 1 0 -1 0 ,与下伏栖霞组灰岩稀土元素配分曲线极为相似。结合微量元素F、Ba、Cl、Cr、Ni、Sr、V研究结果及硅化角砾岩带自底部至顶部特有的下粗上细的韵律性层理构造 ,认为康家湾铅锌金矿床硅化角砾岩带是在地台体制向地洼体制转变期的大地构造环境下 ,由于地壳快速隆升 ,二叠纪灰岩、泥质岩、石英砂岩等岩石剥蚀 ,在古河流环境下搬运、沉积形成的。

The poly genetic breccias are distributed in the Kangjiawan poly metallic deposit, among which the ore deposit\|related one is silicified breccia with complicated pebble compositions,and composed of a series of unsilicified,weakly silicified,well silicified and intensely silicified breccias.The breccia occurred near the unconformity plane between the Jurassic and Permian, with the pebbles being mainly constituted by siliceous rocks, quartzite and calcareous rocks, the fillings consisting of clay and siltstone, and the cements being siliceous, calcic and ferric matters. The geochemical characters of bulk elements for the silicified breccia show the content of SiO 2 increasing with one number level and reaching to 95.34% mostly, while the contents of Al 2O 3、MgO、FeO、K 2O、Na 2O、CaO、CO 2 and P 2O 5,especially MgO、CaO、CO 2 and P 2O 5 , decreasing clearly. The content of K 2O is higher than that of Na 2O, and the TiO 2 is very low. The REE of silicified breccia is characterized by the ratio of LREE/HREE being 1 95～4 93, both Ce( δ Ce=0 44～0 81) and Eu ( δ Eu=0 58～0 89) being weakly negative anomaly. All of the REE patterns are LREE enrichment type. With the increasing silicification of the breccia, the total REE contents of breccia rocks decrease obviously. The non silicified and weakly silicified breccias have much higher REE amount (176 82×10 -10 ～318 93×10 -10 ) than that of silicified breccia (7 71×10 -10 ～65 95×10 -10 ), the REE pattern of the former being identical to that of argillaceous rocks of the Dangcong Formation, and the latter to limestone of the Qixia Formation beneath the unconformity. Combined with the petrochemical characters of trace elements (such as F、Ba、Cl、Cr、Ni、Sr and V) and the rhythmical layer structure in the silicified breccias (the top finer and the bottom coarser),it is suggested that the silicified breccias in the Kangjiawan poly metallic deposit were formed in an ancient fluvial environment, namely, the crust uplifted quickly, the Permian eroded, the pebbles and clasts transported by river and redeposited in the river, then silicified by the magmatism during the Diwa stage. So the breccias should be formed in a tectonic transforming period from the platform system to geodepression (activated tectonics or Diwa) system.