Mobile Pb-isotopes in Proterozoic sedimentary basins as guides for exploration of uranium deposits

Lead isotope ratios and associated trace element concentrations (U, Th and Pb) extracted by partial-leaching with 2% nitric acid from Proterozoic sandstones and basement rocks reveal much about the fluid evolution of sedimentary basins hosting unconformity-type uranium deposits. In addition, these techniques have great potential as a guide for exploration of uranium and other types of deposits in basins of any age. Isotope ratios of Pb in Proterozoic sandstones from basins known to contain high-grade uranium deposits are radiogenic at key geological localities and settings distal to known mineralization and particularly in altered zones proximal to mineralization. Sandstones completely cemented by quartz overgrowths typically have non-radiogenic Pb isotope ratios, indicating early closure of porosity and isolation of these rocks from later fluid events. Alternatively, the unconformity served as both a source of uranium and radiogenic Pb as well as an avenue for late-stage (<250–900 Ma) fluid flow. The mafic volcanic units, which are relatively reducing lithologies and therefore have removed uranium from basinal brines, have uranium-supported radiogenic Pb isotope ratios. Comparison of 238U/206Pb and 206Pb/204Pb ratios is useful in determining the timing and nature of U and Pb migration before, during and after mineralization in these basins. This comparison can be used to delineate the presence of radiogenic Pb isotope ratios that are not internally supported by uranium and thorium in rocks, eventually providing the explorationist with geochemical vectors that point toward sites of high potential for economic uranium mineralization.

[1]  V. Růžička.  Unconformity-associated uranium , 1995 .

[2]  L. B. Gustafson,et al.  Post-Kombolgie metasomatism at Jabiluka, Northern Territory, Australia, and its significance in the formation of high-grade uranium mineralization in lower Proterozoic rocks , 1983 .

[3]  A. Miller.  A Progress Report: Uranium-Phosphorous Association in the Helikian Thelon Formation and Sub-Thelon Saprolite, Central District of Keewatin , 1983 .

[4]  J. Kramers,et al.  Approximation of terrestrial lead isotope evolution by a two-stage model , 1975 .

[5]  M. Pagel,et al.  Sm-Nd, Pb-Pb, and Rb-Sr systematics of the basement in the Cigar Lake area, Saskatchewan, Canada , 1993 .

[6]  A. Mann,et al.  Application of the mobile metal ion technique to routine geochemical exploration , 1998 .

[7]  A. Wilde,et al.  Fluid inclusion evidence on the origin of some Australian unconformity-related uranium deposits , 1989 .

[8]  Gwendy E.M. Hall,et al.  Analytical perspective on trace element species of interest in exploration , 1998 .

[9]  T. Kotzer,et al.  O, U, and Pb isotopic and chemical variations in uraninite; implications for determining the temporal and fluid history of ancient terrains , 1993 .

[10]  Issai S Goldberg,et al.  Vertical migration of elements from mineral deposits , 1998 .

[11]  S. Short,et al.  Distribution of uranium- and thorium series radionuclides in mineral phases of a weathered lateritic transect of a uranium ore body , 1999 .

[12]  C. Nutt.  Chloritization and associated alteration at the Jabiluka unconformity-type uranium deposit, Northern Territory, Australia , 1989 .

[13]  R. Maas.  Nd-Sr isotope constraints on the age and origin of unconformity-type uranium deposits in the Alligator Rivers uranium field, northern territory, Australia , 1989 .

[14]  T. Kotzer,et al.  Petrogenesis of the Proterozoic Athabasca Basin, northern Saskatchewan, Canada, and its relation to diagenesis, hydrothermal uranium mineralization and paleohydrogeology , 1995 .

[15]  M. Fayek,et al.  Characterization of multiple fluid-flow events and rare-earth-element mobility associated with formation of unconformity-type uranium deposits in the Athabasca Basin, Saskatchewan , 1997 .

[16]  S. Sun,et al.  Origin of uranium mineralization at Jabiluka , 1980 .

[17]  D. Sverjensky,et al.  Geochemical modeling of the formation of an unconformity-type uranium deposit , 1996 .

[18]  H. H. Mehnert,et al.  Changes in the HOAr isotope composition of clays during retrograde alteration , 1987 .

[19]  R. Armstrong,et al.  Sr isotopic study of Helikian sediment and diabase dikes in the Athabasca Basin, northern Saskatchewan , 1985 .

[20]  T. K. Kyser,et al.  Comparison of diagenetic fluids in the Proterozoic Thelon and Athabasca Basins, Canada: implications for protracted fluid histories in stable intracratonic basins , 2002 .

[21]  I. Williams,et al.  Age of the barramundi orogeny in northern Australia by means of ion microprobe and conventional U-Pb zircon studies , 1988 .

[22]  A. Miller,et al.  U–Pb, Pb–Pb, and K–Ar isotopic study and petrography of uraniferous phosphate-bearing rocks in the Thelon Formation, Dubawnt Group, Northwest Territories, Canada , 1989 .

[23]  K. Ludwig,et al.  Age of uranium mineralization at the Jabiluka and Ranger deposits, Northern Territory, Australia; new U-Pb isotope evidence , 1987 .

[24]  J. Lewry,et al.  Thermotectonic evolution of the churchill province in Northern Saskatchewan , 1980 .

[25]  J. Banar,et al.  Ages of major uranium mineralization and lead loss in the Key Lake uranium deposit, northern Saskatchewan, Canada , 1984 .

[26]  D. J. Rawlings,et al.  Geology, geochronology and emplacement structures associated with the Jimbu Microgranite, McArthur Basin, Northern Territory , 1999 .

[27]  H. Baadsgaard,et al.  U–Pb geochronology of minerals from the Midwest uranium deposit, northern Saskatchewan , 1984 .

[28]  R. Needham,et al.  Tectonic evolution of the Pine Creek Inlier, Northern Territory , 1988 .

[29]  P. Bruneton.  Geological environment of the Cigar Lake uranium deposit , 1993 .

[30]  Q. Gall.  The Proterozoic Thelon paleosol, Northwest Territories, Canada , 1994 .

[31]  J. Richards,et al.  Pitchblende and galena ages in the Alligator Rivers region, Northern Territory, Australia , 1976 .

[32]  P. Ramaekers.  Hudsonian and Helikian Basins of the Athabasca Region, northern Saskatchewan , 1981 .

[33]  N. Clauer,et al.  Formation and evolution of the Cigar Lake uranium deposit based on U–Pb and K–Ar isotope systematics , 1993 .

[34]  T. Kotzer,et al.  RETROGRADE ALTERATION OF CLAY MINERALS IN URANIUM DEPOSITS : RADIATION CATALYZED OR SIMPLY LOW-TEMPERATURE EXCHANGE ? , 1991 .