Discovery of Authigenic Rare Earth Mineral-Monazite in Precambrian Sedimentary Rock of Dalian Area and its Significance

SONG Tian-rui

The rare earth mineral monazite ((Ce,La,Th)P0₄)is usually found in magmatic rocks or related meta-morphosed rocks as one of seldom associated minerals. Sometimes the monazite can be found in elastic rocks or placer. Although the authigenic monazite has been found in bauxite deposits or karstification sediments,but it is the first time that discovery of authigenic monazite in Precambrian sedimentary rock of Dalian area China. The authigenic monazite is found in the Shisanlitai Formation of Precambrian Sinian System of the Jin-shitan Beach,Dalian area. This formation is composed of abundant reddish stromatolite limestone. The authigenic monazite has been found in the intercalated green mudstone between limestone beds. It was supposed that Cynobacteria biomineralization promoted formation of the authigenic monazite. The authigenic monazite-bearing mudstone is consisted entirely of illite (sericitized) by X-ray diffraction determination. Exactly the authigenic monazite is involved in the hematite veinlets of the illite mudstone. The electron scanning image analysis of monazite shows subhedral crystal form in l0um diameter. The chemical composition of the authigenic monazite is determined by electron-probe analysis (Wave Spectrum Analysis) as follows(%): Ce₂O₃ 33.28,La₂O₃ 9.52, Nd₂O₃ 17.26,P₂O₅ 30.04,ThO₂ 2.73,Pr₂O₃ 3.12,Sm₂O₃ 2.01, Gd₂O₃ 0.59,UO₂ 0.19,Fe₂O₃ 0.34,SiO₂ 0.56,CaO 0.14,Al₂O₃ 0.08,TiO₂ 0.11,Na₂O 0.01,total 99.98. Therefore,the authigenic monazite belongs to Ce,La,Nd, monazite varities. The significance of the discovery of autaigenic monazite in Precambrian sedimentary rock are discussed in this paper,e. g.,the favourable sedimentary environment of authigenic monazite, the poesibie origin of rare earth elements of the famous Baiyun Obo rare earth minerral deposits and so nn. Besides,the discovery of auhi-genic monazite shows the possibility of isotopic age determination by U,Th-Pb method for the non-fossil Precambrian strata.

Discovery of Authigenic Rare Earth Mineral-Monazite in Precambrian Sedimentary Rock of Dalian Area and its Significance

Institute of Geology, Chinese Academy of Geological Sciences Beijing 100037

Received Date: 1999-03-30

Publish Date: 1999-12-10

Key words:

Abstract: The rare earth mineral monazite ((Ce,La,Th)P0₄)is usually found in magmatic rocks or related meta-morphosed rocks as one of seldom associated minerals. Sometimes the monazite can be found in elastic rocks or placer. Although the authigenic monazite has been found in bauxite deposits or karstification sediments,but it is the first time that discovery of authigenic monazite in Precambrian sedimentary rock of Dalian area China. The authigenic monazite is found in the Shisanlitai Formation of Precambrian Sinian System of the Jin-shitan Beach,Dalian area. This formation is composed of abundant reddish stromatolite limestone. The authigenic monazite has been found in the intercalated green mudstone between limestone beds. It was supposed that Cynobacteria biomineralization promoted formation of the authigenic monazite. The authigenic monazite-bearing mudstone is consisted entirely of illite (sericitized) by X-ray diffraction determination. Exactly the authigenic monazite is involved in the hematite veinlets of the illite mudstone. The electron scanning image analysis of monazite shows subhedral crystal form in l0um diameter. The chemical composition of the authigenic monazite is determined by electron-probe analysis (Wave Spectrum Analysis) as follows(%): Ce₂O₃ 33.28,La₂O₃ 9.52, Nd₂O₃ 17.26,P₂O₅ 30.04,ThO₂ 2.73,Pr₂O₃ 3.12,Sm₂O₃ 2.01, Gd₂O₃ 0.59,UO₂ 0.19,Fe₂O₃ 0.34,SiO₂ 0.56,CaO 0.14,Al₂O₃ 0.08,TiO₂ 0.11,Na₂O 0.01,total 99.98. Therefore,the authigenic monazite belongs to Ce,La,Nd, monazite varities. The significance of the discovery of autaigenic monazite in Precambrian sedimentary rock are discussed in this paper,e. g.,the favourable sedimentary environment of authigenic monazite, the poesibie origin of rare earth elements of the famous Baiyun Obo rare earth minerral deposits and so nn. Besides,the discovery of auhi-genic monazite shows the possibility of isotopic age determination by U,Th-Pb method for the non-fossil Precambrian strata.

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Discovery of Authigenic Rare Earth Mineral-Monazite in Precambrian Sedimentary Rock of Dalian Area and its Significance

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