Silicification in Wongfu Phosphorus Deposits in Guizhou,China
Chen Doufu Chen xianpei (Guangzhou Branch of Institute of Geochemistry ,Chinese Acacdemy of Sciences)
Wongfu phosphorus deposits is a kind of sedimentary phosphorus deposits formed in Doushantou formation with era of Sinian,widely associated with silicified rocks which were formed by means of replacing original rock(such as phosphorate ,dolomite.etc. )below sedimentary level during the period of penecontemporaneous sedimentation. The characteristics of the silicified rocks in fields are : (1)Spreaded widely .they could be seen in almost all the mined columns. (2)Formed in many formations and along them, the silicified rocks mainly distributed on the top and bottom of the phospharate formation,and a little less in the phosphorate formation. ( 3) Silicification had the characteristics of selected replacement. which were showed by both of selecting formation and lithology. The former means that the top and bottom formations of phosphorate were firstly silicified. The later indicates that some type of rocks were silicified easily. The lithologic sequence in order of silicification is as follows:stromatolithic dolomite-dolomite-stromatolithic phosphorate-clastic phosphorate-black banded laminated phosphorate with colloform structure. (4) The silicified clasts in clastic phosphorate showed that the silicification took place during the period of penecontem-poraneous sedimentation. Under microscope, the silicified rocks mainly consist of coarse, fine,micro-,muddy crystal quartz and chalcedony. However,they were not a diagenetic series. The silicified rocks were divided into slight,strong and crystalized silicified rocks respectively, according to the level of replacement and their texture and structure. The three groups of rocks varied transitionally in space,which shows the characteristics from strong to weak silicification. U/Th is 4. 6-102. 5,∑REE is ranged from 6. 12 to 18. 537ppm.in the silicified rocks LREE/HREE is 7. 283-94. 5,δCe = 0. 251 - 0. 413. There were both positive and negative Eu abnormality (δEu=1. 36 in silicified rocks, δEu = 0. 37 in intense ones). δ18 O of 18. 83 and 20. 80-23. 67 in crystalized and strong silicified rocks respectively. The forming temperature of crystalized and intense silicified rocks is 101 - 105℃ and 66-98℃ respectively .through the isotopic equilibrium fractionation temperature function of quatz-wa-ter.