Genesis And Geological Significance Of Hydrothermal Sedimentary Rock In Jiangjiawan, Gansu

Jiangjiawan area belongs to Mianlue zone according to geotectonic background.This a-rticle will re-formulate the rock association and formation connections in the area, anal-yse petrogenesis,tectonic environment,mineralization problem in the area and pu t fo-rward the geological signification to the region through wild observation,petrology, mine-ralogy,geochemistry,etc.The main understanding we get are as follows:1.The original rock of phllite in the area should be mudstone.It is similar to North American Shales in trace elements and rare earth elements.It has same characteristics of mudstone,so it is sure that they are terrigenous mud sedimentary rocks.Sheet minerals in A₃ layer mostly are sericites.So,we can guess that the mud components are very single when sediment happened.It should mainly be kaolinite clay which absorbed a certain mount of K⁺.2.According to occurrence,mineral composition,texture,structure and geochemistry characteristics of siliceous rocks,the Cherts(quartz schists) of this area are hydrothermal sedimentary rocks.There has interfusion of terrigenous(mudstone) sedimetary rocks in the sedimentary process.The sou-rce area which hydrothermal solution circulated through is continental crust.It maybe is crystalline basement in this area-Archean-Mesoproterozoic felsic metamorphic body(Yuzidong Group) and middle-late Proterozoic era epi-metamorphic volcanic rocks(Bikou Group).3.Through the analysis of phllites and quartz schists,we consider:A₃ layer,A₂ layer and B layer are productions which were interfused hydrothermal sediments and terrigenous sendiments.A₃ layer and Part of B layer mainly are hydrothermal sediments,but A₃ layer and most B layer are terrigenous sediments which formed in deep water body.The continous variation of trace elements and rare earth elements in A₃ layer and A₂ layer maybe reflection of duality interfusion endmember variation.The difference between them and B layer is the reflection of difference between material source and sedimentary medium.The characteristics of geochemistry,trace elements and rare earth elements show composition commonness and sedimentary enviroment commonness of A₃ layer,A₂ layer and B layer,and we make sure that the sedimentary environment is passive continental margin.4.The study of this area indicate that:the hydrothermal sedimentary environment is Rift trough;the material source area that hydrothermal solution circulated is continental crust but not oceanic crust.It indicates that there was not ocean basin in Middle Devonian in this area,which was consistent with tectonic environment of Mianlue zone.Mineral source layer or mineralization layer is a series of sericite quartz schists- sericite K-feldspar quartz schistssericite quartz K-feldspar rocks,which has the characteristics of stratabound deposit.The probability of which original rocks are sandstone is excluded,so was the smash alteration model of gold mineralization in siliceous rocks.Bikou-jide maybe the original mineral source layer of gold deposit,the awareness has a certain direction significance to exploration and exploitation of gold deposit in this area.In addition,the study of this article has definite effect in more study in strata of Mianlue zone in the same period.