白银厂酸性火山岩地球化学特征及成因分析

甘肃白银厂矿田是我国著名的火山成因块状硫化物矿床产地,赋矿岩石主要为酸性火山岩(石英角斑岩和石英角斑凝灰岩)。文章通过对白银厂矿区酸性火山岩的岩相学和岩石地球化学特征研究,结果表明:1)岩石发生了低绿片岩相的变质作用,镜下可见明显的绢云母化、绿泥石化等蚀变作用,造成了岩石K、Ca、Na、Rb等活泼元素的迁移;2)岩石具有高硅、低钛、弱铝、贫镁,亏损高场强元素Nb、Ta,以及明显的Sr、P、Ti、Eu的负异常和Th、U、Pb正异常特征,表明本区酸性火山岩可能为下地壳基性岩重熔的产物,源区存在斜长石、磷灰石以及钛铁矿等矿物的残留,并且岩浆在演化过程中经历过围岩的混染与弱的分离结晶作用;3)结合北祁连的区域构造演化、岩浆活动以及本区火山岩的年代学资料,认为该酸性火山岩可能处于由岛弧向弧后盆地过渡的环境中。

Geochemical Characteristics and Genetic Analysis of the Acidic Volcanic Rocks in Baiyinchang

Baiyinchang ore field in Gansu province is the place where occurs the famous volcanogenic massive sulfide deposit in China. The deposit is mainly controlled by the bi-model volcanics.The host rocks are acidic volcanics, including quartz keratophyre and quartz-keratophyre tuff. This article deals with petrography and geochemistry of the acidic rocks. The Result obtained 1):they were exposed to low grade green schist faies metamorphism with obvious alteration of sericitization and chloritization etc. Which can be seen under microscope and lead to migration of K, Ca, Na, Rb and other active elements. 2) The rocks are characteristics of high silicon, low titanium, poor aluminum and magnesium, depleted in high filed intensity elements of Nb, Ta, obvious negative anomalies of Sr, P, Ti, Eu, and positive anomalies of Th, U as well as Pb, suggesting that they were may be generated by partial remelting of the lower crust, with plagioclase, apatite, ilmenite and other minerals remaining in the source area, and the initial magma had undergone assimilation and weak fractional crystallization in the process of evolution, 3)Combining the magmatism and regional tectonic evolution of North Qilian and the chronological data of volcanic in this area, it is concluded that the acidic volcanics were formed in the transitional environment of the arc to the back-arc basin.