| 内蒙古狼山北侧中元古代变基性岩特征及其成矿意义 |
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| 引用本文：.1992.Characteristics And Metallogenic Significance Of Middle Proterozoic Metavolcanites On The Northern Side Of Langshan Mountain, Inner Mongolia[J].Mineral Deposits,11(2):142~153 |
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| 中文摘要:内蒙古狼山北侧霍各乞矿区及外围中元古代地层中存在顺层产出的绿片岩、角闪石岩等变基性岩。长期以来，该类岩石的原岩时代不清，对其性质和成矿意义缺乏研究。文章分析了变基性岩产出的大地构造环境，根据Sm-Nd同位素年龄及野外产状，认为其原岩是中元古代同沉积期形成的火山岩。论述了其岩石化学特征、稀土元素组成特征，初步分析了其在中元古代喷气沉积成矿过程中所起的作用。 |
| 中文关键词:霍各乞铜矿 中元古代 变基性岩 内蒙古狼山 |
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| Characteristics And Metallogenic Significance Of Middle Proterozoic Metavolcanites On The Northern Side Of Langshan Mountain, Inner Mongolia |
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| Abstract:The northern side of Langshan mountain in central Inner Mongolia, located in the transitional zone between north China platform and Hercynian Xingmeng geosyncline, is one of the important metallogenic belts in China. It covers the Hegeqi copper-lead-zinc deposit and other polymetallic mineralized spots which all occur in Middle Proterozoic Langshan Group and bear some resemblance to the group of sedimentary-hosted massive base metal sulfide deposits proposed by Large(1980). Langskan Group is dominated by epimetamorphic clastic and carbonate rocks intercalated with metavolcanites. Metavolcanites consist of amphibolite, albite chlorite schist, actinolitite and actinolite schist. Stratigraphically, they are restricted within the middle part of Langshan Group. The foliations in them are commonly parallel to those in country rocks, and they as a whole have been folded conformably with metasedimentary rocks. Petrographic evidence such as blastoamygdaloidal structure indicates a volcanic origin of these rocks. In Niggli diagram, all plotted points fall into the magmatic rock region; in the La/Yb-ΣREE diagram, plotted points are predominantly within the basalt region. However, ambiguity of field relationship between these metavolcanites and metasedimentary rocks: combined with the K-Ar isotopic ages published has brought about a dispute about the age of metavolcanites. This paper presents a Sm-Nd isotopic age of 1491×106 a, indicating that they are Middle Proterozoic symgenetic volcanic rocks. Geochemically, SiO2 content of amphibolite and that of greenschist range from 47.50 to 52.05 wt％and from 45.00 to 50.75 wt％ respectively, suggesting that metavolcanites are mainly basic volcanic rocks. Plotted points in the(Na2O+K2O)-SiO2 diagram and the F-A-M diagram show that they are largely tholeiite. Total REE, ΣLREE/ΣHREE and chondrite-normalized REE pattern of metavolcanites are similarto those of tholeiite. Mineralizations on the northern side of Langshan mountain occur in metamorphic exhalite overlying metavolcanites or along the contact zone between metavoieanites and metamorphic sedimentary rocks. Close spatial relationship between mineralizations and metavolcanites implies the existence of a connection between volcanic activity and mineralization. Co/Ni ratio of pyrite from ores is larger than 1, suggesting that volcanic rocks provided part of metals for mineralization. Nevertheless, sizes, ore-forming element contents of metavolcanites and geochemical features of ore show that not all metals came from them. The more important significance of metavolcanites consists in the contribution of high geothermal gradient, thus facilitating the migration of metals from the underlying, basement into the basin. The nonlinear relationship between solidification indices or differentiation indices and oxide contents demonstrates that assimilation and contamination once took place between magma and basement rocks rich in copper, lead and zinc. |
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