内蒙东升庙矿区狼山群中石英角斑岩的发现及意义

THE DISCOVERY OF QUARTZ KERATOPHYRE IN LANGSHAN GROUP OF THE DONGSHENGMIAO ORE DISTRICT AND ITS SIGNIFICANCE

一系列证据表明,东升庙一带狼山群(Pt_2ls)中的石英钠长浅粒岩、百英钠长变粒岩和(绢云)石英钠长片岩的原岩是石英角斑岩。该类岩石多呈层稳定出现在块状白云石大理岩中,含有变余(钠长石和石英)斑晶;Na_2O=3.64％～5.31％、K_2O=1.62％～2.66％;具有与上下正常沉积地层相同的多期次构造特征。石英角斑岩的发现,既可直接佐证东升庙矿床形成于海底火山喷发的环境,也可为区域成岩成矿的研究对比提供有益信息。

The Dongshengmiao polymetallic sulfide deposit, located at the western end of the Inner Mongolian axis and on the southern limb of the langshan anticlinorium, is one of the representative ore deposits in the western part of the Proterozoic polymetallic belt on the northern margin of the North China platform. Ore-bearing strata are of Middle Proterozoic Langshan Group, whose degree of metamorphism is generally as high as greenschist facies and locally reaches amphibolite facies, with obvious characteristics of multiphase deformation. Langshan Group, dated isotopically from 1000×10~6 to 1600×10~6a, consists of fine-crystalline dolomite marble and marble, phyllite, schist(mica schist and quartz-albite-sericite schist), quartzite, rnigrnatite, quartz-albite leucogranulitite and quartz-albite granulite.In terms of rock association, it might be divided in upward succession into three formations composed of nine lithologic members with sulfur, zinc, lead and copper sulfides and siderite beds seen in the three members of the second formation. The protoliths of quartz-albite leucogranulitite and granulite as well as quartz-albite-sericite schist are believed to be quartz keratophyric rocks, as can be proved by the following evidence: (1) they occur as beds in concordance with the overlying and underlying strata? ( 2 )the rocks are enriched in Na and deficient in K:Na_2O=3.64%～5.31%>K_2O=1.62%～2.66%,K_2O+Na_2O=6.30%～ 7.0%, K_2O/(K_2O+Na_2O)=0.23～0.42,SiO_2=70.47%～72.44%,and Fe_2O_3/FeO= 0.07～0.31; (3)petrochemical diagrams by Niggli and others show the protoliths to be igneous rocks; (4) lead isotopic model ages are 1118×10~6～1187×10~6a, quite close to isotopic ages of Langshan Group. Quartz keratophyric rocks are seen in the same horizon as sulfur, zinc, lead and copper as well as siderite ore beds or even occur as intercalations in siderite beds. Based on the relationship between quartz keratophyric rocks and various orebodies combined with geological characteristics of the ore deposit and geological-structural settings for metallization, it is thought that the Dongshengmiao ore deposit was formed in an environment of middle Proterozoic submarine volcanic activity. The confirmation of the existence of quartz keratophyric rocks in Langshan Group of the Dongshengmiao ore district contributes to the regional correlation and also conducive to the ore prospecting work in outskirts of the ore district and other areas with similar stratigraphic and geological conditions.