Geologische Rundschau

, Volume 77, Issue 2, pp 371–388 | Cite as

Fluvial facies patterns and chemical sediments. Their control on U, Ba, and base metal concentration in the »Burgsandstein« (Upper Triassic Keuper, SE Germany)

  • Harald Dill
Article

Abstract

The Upper Triassic clastic »Keuper« beds in southern Germany have attracted much attention during the last decades for their sandstone-hosted U deposits in the »Burgsandstein«, in particular. Paleogeographically this arenaceous-argillaceous sequence is compared with a fanplaya environment, showing a great variety of fluvial drainage patterns (distal braided stream, meandering streams displaying different degrees of sinuosity). Aside from this, duricrusts (dolcretes/calcretes, petrified wood/»silcretes«, phosphatized arkoses/phoscretes) are widespread in these arenaceous beds.

Both, clastic and chemical sediments, play a significant part for metal concentration. Ore enrichment is of two principal types, early and late diagenetic. They were deeply influenced by the basin subsidence.

Phosphatization concomitant with U precipitation took place near the fan head, whereas silicified wood and calcretes are more common within a transition from the fan towards the playa, where meandering rivers predominate. Uraniferous calcretes partly originated from paleosols, partly from ground water percolation. At a subsequent stage, these carbonate encrustations locally got reworked.

U black ores containing Cu, Pb, Fe, Zn, and Co-sulfides were precipitated from ground waters when entering less permeable lithologies. Ground water stagnated at the playafan boundary or, on a more local scale, between fluvial channels and overbank or crevasse splay depositions. Eh and pH changes are suggested to have been controlling the U and sulfide concentration.

Keywords

Uranium Parmi Chemical Sediment Basin Subsidence Arkose 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Zusammenfassung

In den letzten Jahren haben die Keupersandsteine des obertriassischen »Burgsandsteins« wiederholt die Aufmerksamkeit der Urangeologen auf sich gezogen. Dieser wird interpretiert als eine Playa-Schwemmfächermilieu-Assoziation mit »braided streams« und meandrierenden Flu\systemen. Daneben haben dolomitische und kalzitische Verwitterungskrusten, »verkieselte Hölzer« und phosphatführende Arkosen in diesen klastischen Sedimenten eine weite Verbreitung.

Die Faziesausbildung klastischer wie chemischer Sedimente spielt eine gewichtige Rolle bei der U- und Buntmetall-Konzentration. Die Erzkonzentration ist das Ergebnis früh- und spätdiagenetischer Prozesse, die auch durch die Subsidenz des Beckens beeinflu\t wurde. Die Phosphate mit ihren Urananreicherungen entwickelten sich im proximalen Teil des Schwemmfächers, wogegen die Verkieselungen und Kalkkrusten mehr beckenwärts angetroffen werden. Die U-führenden Kalkkrusten sind sowohl paläogeographisch erklärbar, als auch durch Grundwasserzirkulation. Durch jüngere fluviatile Prozesse werden diese Krusten aufgearbeitet.

Uran-Schwarzerze (Uranpecherz, Coffinit) und Cu-, Fe-, Zn- und Co-Sulfide wurden an Grundwasser stauenden, weniger permeablen Sedimenthorizonten gefällt. Diese »Barrieren« liegen am übergang von Rinnensedimenten zu überlagernden Absätzen von Schwemmebenen und »crevasse splay«-Ablagerungen. Eh- und pH-änderungen sind für die Fällung von Sulfiden und Uran die entscheidenden Faktoren.

Résumé

Les couches arénacées du Keuper (Triasique supérieur) du sud de l'Allemagne ont retenu l'attention au cours des dernières années en raison de la teneur élevée en uranium de certaines d'entre elles, particulièrement dans la formation du »Burgsandstein«. Au point de vue paléogéographique, cette série argilo-arénacée est rapportée à un milieu de cÔne fluviatile/playa présentant une grande diversité de configurations fluviatiles (cours anastomosés, méandres de divers degrés de sinousité). De plus, des croûtes indurées (cuirasses dolomitiques/calcaires, »bois silicifiés«, arkoses phosphatées) sont abondamment réparties parmi ces sédiments clastiques.

Les deux types de sédiments — clastiques et chimiques — jouent un rÔle significatif dans la concentration du métal. La minéralisation appartient aux types diagénétiques hatif et tardif; elle est également fort influencée par la subsidence du bassin.

Une phosphatisation, contemporaine de la précipitation de l'U s'est développée dans la région du sommet du cÔne, tandis que les bois silicifiés et les cuirasses calcaires sont plus fréquents dans la zone de transition du cÔne vers la playa, où prédominent les cours d'eau sinueux. Les cuirasses calcaires uranifères tirent leur origine en partie de paléosols, en partie de la percolation d'eaux souterraines. Ultérieurement, ces encroûtements carbonatés peuvent Être localement remaniés.

Les minerais uranifères noirs, contenant des sulfures de Cu, Pb, Fe, Zn et Co ont été précipités à partir des eaux souterraines lors de leur pénétration dans des lithologies moins perméables. Ces eaux ont stagné à la limite cÔne-playa ou, à une échelle plus locale, entre les sédiments des chenaux fluviatiles et les dépÔts d'inondation ou de remplissage de crevasses. Les modifications de Eh et de ph sont considérées comme des facteurs déterminants dans la concentration de l'U et des sulfures.

кРАткОЕ сОДЕРжАНИЕ

жА пОслЕДНИЕ гОДы пЕс ЧАНИкИ кЕИпЕРА Иж ВЕРхНЕтРИАссОВОгО «БУРгжАНДштЕИНА» пР ИВлЕкАУт ВсЕ БОльшЕ ВНИМАНИЕ гЕОлОгОВ, жА НИМАУЩИхсь РАжВЕДкО И НА жАлЕжИ УРАНА. ЁтИ жАлЕ жИ РАссМАтРИВАУт, кАк АссОцИАцИУ плАИЕВОгО кОНУсА ВыН ОсА с РАжВЕтВлЕНИЕМ Р ЕкИ, ИлИ МЕАНДРИРУУЩИМИ с ИстЕМАМИ РЕк. В ЁтИх кл АстИЧЕскИх сЕДИМЕНтАх ОЧЕНь РАс пРОстРАНЕНы ДОлОМИт ОВыЕ И кАльцИтОВыЕ кОРы Вы ВЕтРИВАНИь, «ОкРЕМНЕ ННыЕ ДЕРЕВьь» И АРкОжы, БОг АтыЕ ФОсФАтОМ.

ОБРАжОВАНИЕ ФАцИь кл АстИЧЕскИх И хИМИЧЕс кИх ОтлОжЕНИИ ИгРАЕт ВАжНУУ РОль пР И НАкОплЕНИИ УРАНА И пОлИМЕтАллОВ. кОНцЕН тРАцИь РУД — РЕжУльтА т РАННЕ И пОжДНЕДИАгЕНЕтИЧЕс кИх пРОцЕссОВ, НА пРОт ЕкАНИЕ кОтОРых ВлИьЕт тАкжЕ И пРОсЕДАНИЕ сАМОгО Б АссЕИНА ОтлОжЕНИь. ФОсФАты, кА к И Их ОБОгАЩЕНИЕ УРАН ОМ ОБРАжУУтсь В БлИжАИш Их ЧАстьх кОНУсА ВыНО сА, А ОкРЕМНЕНИь И ИжВЕсткОВыЕ кОРы Вы ВЕтРИВАНИь ОтклАДыВ АУтсь В стОРОНЕ, ОБРАЩЕННОИ к БАссЕИНУ. ИсхОДь Иж ДАННых пАлЕОгЕОгРАФ ИИ И гИДРОДИНАМИкИ гР УНтОВых ВОД, сДЕлАлИ пОпыткУ О БЩьсНИть ОБРАжОВАНИ Е РЕЧНых ИжВЕсткОВых кОР ВыВЕ тРИВАНИь, ВМЕЩАУЩИх У РАН, тАк: пОжДНЕИшИЕ пРОцЕссы ВыжВАлИ пРЕОБРАжОВА НИь ЁтИх кОР ВыВЕтРИВАНИь.

ЧЕРНыЕ РУДы УРАНА / УРА НОВАь сМОльНАь РУДА, к ОФФИНИт /, кАк И сУльФАты МЕДИ, ж ЕлЕжА, цИНкА И кОБАльт А ОсАжДАУтсь Иж гРУНтО Вых ВОД В жАстОИНых, МА лО пЕРМЕАБЕльНых сЕДИМ ЕНтНых гОРИжОНтАх. Ёт И «БАРьЕРы» НАхОДьтсь НА пЕРЕхОД Е От ОтлОжЕНИИ жЕлОБО В к пЕРЕкРыВАУЩИМ ОсАДк АМ плОскОстИ кОНУсА В ыНОсА И ОтлОжЕНИьМИ кОсОИ РА ссЕлИНы. ИжМЕНЕНИь жН АЧЕНИИ Eh И pH ОкАжыВАУтсь ВЕДУЩ ИМИ ВЕлИЧИНАМИ, кОтОР ыЕ ОпРЕДЕльУт ОсАжДЕНИ Е сУльФИДОВ И УРАНА.

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Copyright information

© Ferdinand Enke Verlag Stuttgart 1988

Authors and Affiliations

  • Harald Dill
    • 1
  1. 1.Continental Deep Drilling Programme of the Federal Republic of Germany/Project ManagementD-Hannover 51

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