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。论述了其岩石化学待征、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。
The northern side of Langshan mountain in central Inner Mongolia, located in the ransitional zone between north China platform and Hercynian Xingmeng geosyncline, is one of he mportant metallogenic belts in China. It covers the Hegeqi copper-lead-zinc deposit and other olymetallic mineralized spots which all occur in Middle Proterozoic Langshan Group and bear ome resemblance to the group of sedimentary-hosted massive base metal sulfide deposits roposed y Large(1980). Langshan Group is dominated by epimetamorphic elastic and carbonate rocks ntercalated witb metavolcanites.Metavolcanites consist of amphibolite, albite chlorite schist, ctinolitite and actinolite schist. Stratigraphically, they are restricted within the middle art of Langshan Group. The foliations in them are commonly parallel to those in country rocks, nd they as a whole have been folded conformably with metasedimentary rocks.Petrographic vidence such as blastoamygdaloidal structure indicates a volcanic origin of these rocks. In iggli diagram, all plotted points fall into the magmatic rock region; in the La/Yb-EREE iagram, plotted points are predominantly within the basalt region. However, ambiguity of field elationship between these metavolcanites and metasedimentary rocks combined with the K-At sotopic ages published has brought about a dispute about the age of metavolcanites. This paper resents a Sm-Nd isotopic age of 1491×10~6a, indicating that they are Middle Proterozoic yngenetic volcanic rocks.Geochemically, SiO_2 content of amphibolite and that of greenschist ange from 47.50 to 52.05 wt％0 and from 45.00 to 50.76 wt％ respectively, suggesting that etavolcanites are mainly basic volcanic rocks. Plotted points in the(Na_2+OK_2O)-SiO_2 diagram nd the F-A-M diagram show that they are largely tholeiite. Total REE, ΣLREE/ΣHREE and hondrite-normalized REE pattern of metavolcanites are similar to those of tholeiite.Mineralizations on the orthern side of Langshan mountain occur in meta morphic exhalite overlying metavolcanites or along the ontact zone between metavoicanites and metamorphic sedimentary rocks. Close spatial relationship etween ineratizations and metavolcanites implies the existence of a connection between volcanic activity and ineralization. Co/Ni ratio of pyrite from ores is larger than 1, suggesting that volcanic rocks provided part f etals for mineralization. Nevertheless, sizes, ore-forming element contents of metavolcanites and eochemical features of ore show that not all metals came from them. The more important significance of etavolcanites consists in the contri bution of high geothermal gradient, thus facilitating the migration of etals from the underlying basement into the basin. The nonlinear relationship between solidification indices r differentiation indices and oxide contents demonstrates that assimilation and contamination once took lace between magma and basement rocks rich in copper, lead and zinc.
出处
《矿床地质》
CAS
CSCD
北大核心
1992年第2期142-153,共12页
Mineral Deposits
基金
国家教委博士基金研究工作总结
