华北中—新元古代火山岩及其对成矿作用的制约——以河北洞子沟银多金属矿床为例

华北地区中—新元古代发育有大红峪期、串岭沟期等多期火山岩,产出的主要岩石类型为碱性基性岩,包括响质碱玄岩、碱玄质响岩、响岩、副长岩、粗面岩、粗面玄武岩和粗面安山岩。该岩石为碱性,Ti含量高,微量元素表现为富集大离子亲石元素而贫高场强元素,显示其构造环境属拉张环境;微量元素比值和同位素分析显示岩浆为幔源,但受到大陆地壳的强烈影响,属裂谷环境。在该区发育有洞子沟、将军关、银冶岭、罗选厂、翟庄等一系列银多金属矿床(点)。这些矿床(点)在时间上和火山岩的形成时代基本一致,在空间上相互关联,许多矿体直接产于火山岩或火山机构中,矿石中发育与火山活动关系密切的低温硫化物矿物黝铜矿、自然铜、碲银矿等,从而证明中—新元古代火山作用对该区银多金属矿床的形成具有重要的作用。

THE PETROLOGY AND ITS CONSTRAINTS ON MINERALIZATION OF THE MESO—NEOPROTEROZOIC VOLCANIC ROCKS:AN EXAMPLE FROM THE DONGZIGOU SILVER-POLYMETAL DEPOSITS OF NORTH CHINA

There are three episodes volcanic eruptions in the North China at Meso-Neoproterozic, the volcanic rocks named Dahongyu Group, Chuanlinggou Group and Tuanshanzi Group. They are mostly alkaline basic rocks except for some dykes. The main rocks consist of phonotephrite, tephriphonolite, phonolite, foidite, trachyte, trachyandesite. The geochemical characteristic of the volcanic rocks indicate the major contents are very rich for alkaline especially in K2O and TiO2; the trace elements are rich in LILE,but they are depleted in HFSE, especially in Yb and Y. It is usually interpreted that these volcanic rocks were formed in the expand rift, since the incompatible element ratios such as Ba/Nb, Th/Nb, Ba/La, Zr/Hf, Nb/Ta and Sm-Nd.Rb-Sr isotopic data showed that magma may be from the mantle. However, they also were influenced by the continental crust. Therefore, it is implicated the volcanic rocks were formed in a continental rift. There also distributed many silver-polymetal deposits such as Dongzigou, Jiangjunguan, Yinyeling, Louxuanchang and Zhaizhuang deposits along the volcanic rocks. The mineralization epoch of these deposits and forming ages of the volcanic rocks are both about 1,521 Ma to( 1,700) Ma.Most ore bodies are distributed within the volcanic rocks ,sometimes volcanic rocks are the wall rocks of the ore bodies, and sometimes ore bodies are occurred in volcanic edifice. The dominant ore mineral assemblages are tetrahedrite, native copper, hessite and other sulfides, which implicated that they formed on low-middle temperature with volcanism. Therefore,it can be proved that the volcanism have taken a pivotal action for the forming of deposits in the Meso-Neoproterozic.