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Uranium-Thorium Silicates, with Specific Reference to the Species in the Witwatersrand Reefs

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Uranium-Thorium Silicates, with Specific Reference to the Species in the Witwatersrand Reefs REPORT No. M278 URANIUM-THORIUM SILICATES, WITH SPECIFIC REFERENCE TO THE SPECIES IN THE WITWATERSRAND REEFS by G. Smits 15th January, 1987 COUNCIL FOR MINERAL TECHNOLOGY 200 Hans Stnjdom Road RANDBURG South Africa DSffiïuïJK (MINERALOGY DIVISION) REPORT No. M278 URANIUM-THORIUM SILICATES, WITH SPECIFIC REFERENCE TO THE SPECIES IN THE WITWATERSRAND REEFS by Georgette Smits 15th January, 1987 Printed and published by the Council for Mineral Technology ISBN 0 86999 772 6 Investigator G. Smits Director of Division S.A. Hiemstra Editor A. Cowey Programme No. 018 Project No. 03385 Project Report No. 1 All correspondence to Mintek, Private Bag X3015, Randburg, 2125 South Africa SYNOPSIS (U,Th)-silicates form two complete series of anhydrous and hydrated species with general formulae (U,Th)Si()4 and (U,Th)S!04«H;0 respectively. The end-members of the anhydrous series an- anhydrous coffinite and thorite, and those of the hydrated series, coffinite and thoiogummitc. Although the silicates are relatively rare in nature, coffinite is a common ore mineral in uranium deposits of the sandstone type. In the Witwater.'rand reefs, (U.Th)-silicates are extremely rare in most reefs, except for the Elsburg Reefs on the West Rand Goldfield and the Dominion Reef. In these reefs deirital uraninite has been partly or entirely transformed to (U.Th)-silicates of coffinite composition, but thorite and thorogummite of deti ital origin are also four d in (he Dominion Reef. In leaching tests on polished sections of rock samples containing (U,Th)-silicates, a dilute sulphuric acid solution, to which ferric iron had been added, was used as the lixi.iant. It appeared that the dissolution of coffinite is less rapid than that of uraniniit ind uraniferous leucoxene. However, the reaction of silicates of high thorium con;?r:*. is .nuch slower, and was not completed during the tt.t.. SAMEVATTING (U.Th)-silikate vorm twee volledige reeksc waiervrye en gchidrateerde spcsics waarvan die algemene foimules onderskeidelik (L ,Th)Si04 en (U,Th)Si04 rtH;0 is. Die einddele van die watervryc serie is walervrye coffiniet en toriet, en die van die gehidrateerdc scrie, coffiniet en torogummiet. Hocwel die silikate betreklik skaars is in die natuur, is coffiniet 'n algemene ertsmineraai in die sandsteentipe uraanafsettitig.. (U.Th)-silikate is niters skaars in die mecste riwwe van die VViivvatcrsrand bchalwc in die f Isburg-riwwe van die Wesrandie goudveJd en die Dominion-rif. In hierdie riwwe is detritnlc uraniniet gedeeltelik of geheel en al omvorm in (U.Th) silikate met 'n coffinietsamestelling, maar toriet en torogummiel met 'n detri'ale oorsprong kom ook in die Dominion-rif voor. 'n Verdunde swaelsuuroplossing vaarhv ferriyster gevoeg is, is as die loogmiddel gebrnik in loogtoetse op gepoleerdc snittc van rotsmonstcrs «at <U,'I'h)-silikale beval. Dil wil voorkom of coffiniet sladiger as uraniniel of tiraanhondende lenkokseen oplos. Die reaksie van silikale met 'n hoc (oritiminhoud is egter haic stadiger en is nic tydens die toetse heclteinal voltooi nie. CONTENTS 1. Introduction I 2. Review oi the Available Data on (U.Th)-silicates I 2.1. Modes of Occurrence 1 2.2. Minera!ogical Aspects 3 ï.2.1. Physical and Optical Properties 3 2.2.2. X-ray Diffraction 3 2.3. Chemical Aspects 3 2.3.1. Synthesis of (U.Th)-sili'cates 6 2.3.2. Thermal Beha,;our 8 2.3.3. Infrared Absorption Spectra 9 3. (U.Thi-silicates in the Witw«aersrand Reefs 12 3.1. Microscopic Description 12 3.2. Composition of Witwatersrand (U.Th)-silicaies 13 3.3. Leaching Behaviour of (U.Th)-siMcates 14 4. Discussion 15 5. Summary 31 6. References 31 LIST OF TABLES Table 1. Localities where coffinite has been idertified I Table 2. Stratigraphy of the Witwatersrand Supergroup 1 Table 3. Optical and physical properties of thorium-bearing silicates 4 Table 4. Optical and physical properties of coffinite 4 Table 5. X-ray-diffraction data for thorium and uranium silicates 5 Table 6. Compositions of thorium silicates from the Witwatersrand and e'sewhere 6 Table 7. Compositions of coffinites from deposits other than the Witwatersrand 7 Table 8. Electron-microprobc analyses of coffinites from the Witwatersrand 8 Table 9. Average elec'ron-microprobe analyses of radioactive minerals from the Dominion Reef 9 Table 10. X-ray-diffraction data for coffinite from mushy anatase 24 Table 11. Electron-microprobe analyses of coffinite associated with mushy anatase 26 Table 12. Chemical analyses of uranoan thorites from the Dominion Reef 26 Table 13. Compositions of minerals of the thorite-zircon group 26 Table 14. Electson-microprobe analyses of uraninite and coffinitc of primary and secondary origin from the Monarch Reef, West Rand Goldfield 27 Table 15. Electron-microprobe aralyses ot uraninitc and coltinite trom the Dominion Keel, before and after leaching 27 Table 56. Eiectron-microprobe analyses of yttrium phosphates 30 LIST OF ILLUSTRATIONS Figure I. location of gold mines from which rock samples were taken r~; microscopic studies. 2 Figure 2. Compositions of (U,Th)-silicates plotted on the basis of atoms per cent for the (U + Th)- Si-H20 system 10 Figure 3. Composition of (IJ.Th)-silicates plotted on the basis of atoms per cent for the L'-Th-Si system 11 Figure 4. Photomicrographs of coffinitc in the Elsburg Reefs of Cooke Section 16 Figure 5. Photomicrographs of uraninitc-bcaring kerogen and altered dctrital uraninitc from the Wit watcrsrand 18 Figure 6. Photomicrographs of (U.Th)-bearing minerals 20 Figure 7. Photomicrographs of uranium-bearing minerals 22 Figure 8. Back-scattered electron image and X-ray distribution images for uranium, titanium, and silicon in mushy anatase 25 Figure 9. Photomicrographs illustrating the leaching tests 28 Figure 10. Eh-pH diagram in the U-O2-CO2-H2O system at 25 C, a typical groundwater carbon dioxide pressure of 10 : aim, and a total dissolved-uranium concentration of 10 *'M (0,24 mg 1) 30 1. Introduction TABLET (U.Th)-silicates occur in two series, comprising the / oci'tities where coffinite has been identified anhydrous and the hydrated species. On the basis of ex­ isting names, the end members are respectively: anhydrous coffinite and thorite, and Location Near Author coffinite and thorogummitc. Colorado Plateau Province !'»55 i Stictt ,7 ul." C olorad * Pla'cau Province i'*55 ! Crimer and Smith" Uranium-bearing thorite species are referred to as ura- ( olorado Vlaleau Province I ¥56 j Kock-tra and luchs'1 1 noan thorites and, in analog) with this nn;ne, the I olor.ulo Plat at1 P'ovii.ce I*: \io.-neh'-' thorium-bearing coffinites are called thoroan coffinites. Colorado PIaiea>: Province ITS : Kmi" These two names are then preceded bv 'anhydrous' it Colorado Plateau PIOVIIK • IVSO 1 udwic and l.ramh ' water is absent, and 'hydrated' if it is present. The Colorado Plat-.-au IY>iii..c W i Harrison c; .;/." silicates belong to the tetragonal crystal system, and are B:i!U'r dace Recior, ; isostructural with zircon and \enotime. The monocimic New /calanu IV?" Reed and (laridec " (.oniwall. tnyland mineral of the saine composition as thorite has been i'>-•• S I.i>lor anj Harrison 1 I nuncio cd locainv. named huttonite . It is the stable phase of ThSi04 at the I S.S.R. !1>?S : I ili[-cn o " = liquidus temperature, I975 + 50 C\ Ocrma'iv I WO Strun/ .:.id SceliecC ' Of the natuial thorium-bearing silicates thorite W i:\*. aicrsr.'iid 4 Somh Mr:..i iWS Hiemstra'" (ThSi04) was first described by Berzelius m 182'). lndi^clo>ed lociluv l'>" Duhiriciuik cr a!. thorogummite by Hidden and Mackintosh' in 1889, and Saskatchewan. ( arada !V" RII:IN.III--" 6 huttonite in 1950 by Hutton . In 1953, Frondel , «ho \iistna aiivl ^ ueoslavia IV" Kurat <•:• .;/.'" studied hydrated /ireon species and tiiorogumi;iites from \orth Bohemia. vanous parts of the world, proposed the following Czechoslovakia ivso Scharm c ,// ': SUucnia a:uf \ueii>l.iu.. IVSl Sirno-.i" chemical formula tor thorogummite: lh(Si04)i ,(OH)4, l-ilioi I akc District. Uranium-bearing silicates have been overlooked in ( an.;da I'M: KohuiNOii and S;-.ooil,'i ' many depos'ts, including the Witwatetsrand, because Kaio". Soinh \trK.i their optical properties are identical to those of the W 't^altr-rand. ivsl •., -in .,nd. I iter-» it," thonum-bearir.g species. It was not untii 1955 thai the s.m;!i Viiu uranium silicate was recognized by Stieff et al"'' in la irancv. Portugal. IVs' Oivrthur"' Sal no. 2 Mint on the Colorado Plateau in the U.S.A.. W't Ocrnunv I'm /ncln and Dar.Vf ' where it is a major constituent of the rock. The authors \\ iiwatcrsrand. South Atrica named the mineral coffinite, after Coffin, who was th<- IVS4 Smitv " first to study these sandstone-type uranium deposits of Witwa.ersrai.d. Sout'.i Vrnca Phancrozoic age. From a study of its infrared absorp­ "W5 (il.ittha.ir and I cither' tion spectrum, Stieff et al.'' proposed a chemical formula, U(SiÓ4)i - ,(OH)4l, similar to that of thorogummite Since 1955, coffinite has been identified in many more T>rJIF 2 deposits of different genetic origins, the localities of which are listed in Table I. « Stratigraphy of the ]l itwutersrand Supergroup In the Witwatersrand, (U.Th) silicates arc rare in most of the reefs, except for the Dominion Reef west of I Supcrsroiip | Subgroup Klerksdorp and the F.lsburg Reefs on the West Rand (• -f- Coldficld (Figure I). In contrast to the uraninites, which [ furflontcin I are of detrital origin, the (U,Th)-silicates occurring in the Johannesburg Witwatersrand precipitated after the sedimen' was ^ !^nn,.'vr,>ttn Wiiwatcr-.rand J (iovcrnmcnl deposited, as a resuil of geochemieal interaction between ' Hospital «ill uraninitc and circulating solutions ( Syfcrfontcm In 1962, Ortlepp" reported the presence of mctamicl ( Rhcnostcrhock hydrated uranothorite in the Dominion Reef, which occurs in the lower portion of the Wituatersrand Super­ group (Table 2).
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