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Uraninite from the metamorphic deposit investigated is divided into two groups on thebasis of its occurrence and physical and chemical properties.Uraninite of the first group occursas distorted crystals in the form of elongated cubes a{100};the second group occurs mainlyin the form of rhombo-dodecahedra d{100),all belonging to the GaF_2 type in the isometricsystem.The chemical formulae are 8UO_2·UO_3·PbO for the first group and 5UO_2·UO_3·PbO forthe second.Isotopic dating yields 1,900 and 300 m.y.for the formation time of uraninite inthis deposit.The former value corresponds to the main mineralization period and the latterrepresents the reformation event. 1.The major economic uranium mineral in metamorphic uranium deposit No.3075is a typical uraninite.On account of the low content of Th,it is also called“Th-free’ uranini-te of pure uranium type.It is obviously different from the eguivalent with high contentsof Th and rare earths present as accessory minerals in granite and pegmatite elsewhere inChina.2.The crystal form of uraninite No.1 is simple,whereas that of No.2 is complex.The UO_2/UO_3 ratio for No.1 is larger than that of uraninite No.2.Uraninite No 1 hashigh values for some physical parameters,such as reflectance,cell size,weight gain and har-dness as-compared with uraninite No.2.3.These two types of uraninite are formed in different geological environments anddifferent in genesis.Uraninite No.1 occurring in two-mica quartz-schist in the Langzishan.Formation at the base of the Lower Proterozoiic Liaohe Group is of dynamo-thermal meta-morphic origin,as evidenced by:(1)The uraninite occurs as elongated platy crystals andis arranged in a direction paralled with schistosity.It is due to directional stress that the crystals have been elongated to become distorted cubic crystals during crystallization;(2)The ore-bearing rocks are generally similar in lithology to the barren rocks,without the in-troduction of extraneous substances.This is due to the fact that the formation of new mi-nerals during dynamo-metamorphism is based on the recombination of various chemical ele-ments in the original rocks and,therefore,the assemblage of metamorphic minerals is simi-lar in composition to the original rocks;(3)H_2O is low in uraninite No.1.Uraninite No.2 related to migmatization is of ultrametamorphic hydrothermal origin,as justified by:(a)coarse-grained rock-forming minerals in the ore-bearing rocks and remarkable recrystalliza-tion;(b)the addition of vein material and replacement by K and Na;(c)much higherwater content.4.Extensive metamorphism and migmatite,granitization in response to regional te-ctonics have resulted in faults,folds and fractures which serve as channels and traps formetamorphic hydrothermal solutions(ore fluids)and also provide vast space for crystalliza-tion of uraninite,thus making it possible for the two types of uraninites to have crystallizedinto perfect euhedral crystals.5.Isotopic dating of uraninites No.1 and No.2 gave the mineralization ages of 1,900m.y.and 300 m.y.,respectively.The former is corresponding to the main period of minera-lization and the later to the late reformation time.Although uraninites No.1 and No.2are formed in different environments,the continuity is evident in their formation time,i.e.,the products potdating the white migmatite granite.Particularly uraninite No.2 has aclose genetic relationship with the white migmatite granite.6.The identification of uraninite of economic importance gives proof of the occur-rence of uraninite ore deposits in Precambrian metamorphic rocks of China.The study ofuraninite is of theoretical and practical significance in understanding the genesis of sucha type of uraninite deposits as well as in geological survey and exploration.Since this type of deposits are closely related to metamorphic hydrothermal solutions,favourable target region and locations should be outlined in our further exploration philosop-hy.
Uraninite from the metamorphic deposit investigated is divided into two groups on thebasis of its occurrence and physical and chemical properties. Uraninite of the first group occursas distorted crystals in the form of elongated cubes a {100}; the second group occurs mainly in the form of rhombo -dodecahedra d {100), all belonging to the GaF 2 type in the isometricsystem. The chemical formula is 8UO 2 .UO 3 .PbO for the first group and 5UO 2 .UO 3 .PbO forthe second. Isotopic referations 1,900 and 300 myfor the formation time of uraninite inthis deposit. The former value corresponds to the main mineralization period and the latterrepresents the reformation event. 1.The major economic uranium mineral in metamorphic uranium deposit No.3075is a typical uraninite. Account account of the low content of Th, it is also called “Th-free” uranini-te of pure uranium type. It is obviously from the eguivalent with high contents of Th and rare earths present as accessory minerals in granite and pegmat ite elsewhere inChina.2. The crystal form of uraninite No.1 is simple, yet that of No. 2 is complex. UO_2 / UO_3 ratio for No.1 is larger than that of uraninite No. 2.Uraninite No 1 hashigh values for some physical parameters, such as reflectance, cell size, weight gain and har-dness as-compared with uraninite No.2.3. These two types of uraninite are formed in different geological environments and different in genesis. Uraninite No.1 occurring in two- mica quartz-schist in the Langzishan. Formation at the base of the Lower Proterozoic Liaohe Group is of dynamo-thermal meta-morphic origin, as evidenced by: (1) The uraninite occurs as elongated platy crystals andis arranged in a direction paralled with schistosity .It is due to directional stress that the crystals have been elongated to become distorted cubic crystals during crystallization; (2) The ore-bearing rocks are generally similar in lithology to the barren rocks, without the in-troduction of extraneous substances.This is due to the fact that theformation of new mi-nerals during dynamo-metamorphism is based on the recombination of various chemical ele-ments in the original rocks and, therefore, the assemblage of metamorphic minerals is simi-lar in composition to the original rocks; (3) H_2O is low in uraninite No.1. Tribune No. 2 related to migmatization is of ultrametamorphic hydrothermal origin, as justified by: (a) coarse-grained rock-forming minerals in the ore-bearing rocks and remarkable recrystallization; (b) the addition of vein material and replacement by K and Na; (c) much higher water content 4. Extensible metamorphism and migmatite, granitization in response to regional te-ctonics have response in faults, folds and fractures which serve as channels and traps for metamorphic hydrothermal solutions (ore fluids) and also provide vast space for crystalliza- tion of uraninite, thus making it possible for the two types of uraninites to have crystallizedinto perfect euhedral crystals. 5.Isotopic dating of uraninites No.1 and No.2 gave the min eralization ages of 1,900 my and 300 my, respectively. The former is corresponding to the main period of mine-lization and the later to the late reformation time. Although uraninites No. 1 and No. 2 formed in different environments, the continuity is evident in their formation time, ie, the products potdating the white migmatite granite. Particularly uraninite No.2 has aclose genetic relationship with the white migmatite granite.6. the identification of uraninite of economic importance gives proof of the occur-rence of uraninite ore deposits in Precambrian metamorphic rocks of China. The study ofuraninite is of theoretical and practical significance in understanding the genesis of sucha type of uraninite deposits as well as in geological survey and exploration.Since this type of deposits are closely related to metamorphic hydrothermal solutions, favourable target region and locations should be outlined in our further exploration philosop-hy.