Isotopic composition of lead in galena and Ni-arsenides of the Midwest deposit, northern Saskatchewan

Abstract

Pb-isotope ratios from galenas and Ni-arsenides of the Midwest deposit indicate that there are three classes of Pb present. The first, with 2 0 6~b / 2 0 4~b ratios of about 22.5 and 2"XPb/2MPb ratios of about 43.5, occurs only in galenas and is restricted to the basal Athabasca Formation. The second occurs from about 35 m above the unconformity to the deepest samples available to us (60 m into the basement rocks). Characteristically the 2 0 6~b / 2 "~b ratios are 30-35 and the 20?b/204Pb ratios are about 50, and this Pb occurs in both galenas and Ni-arsenides. The third class has elevated 2MPb/2"Pb ratios ranging up to 350 and occurs within about 10 m of the unconformity in close association with U minerals.A model is developed in which radiogenic additions to galena have occurred during at least two different events. The first added U and Th daughters to "sandstone-type" Pb at 265 ? 50 Ma, and the second added U daughter only to the "basement-type" Pb in the past 50 Ma or less. These events are consistent with Pb loss recorded in the U minerals and indicate that the lead-loss mechanism from these U minerals is catastrophic rather than continuous diffusion.Some trace Pb isotopic compositions in Ni-arsenides suggest to us the occurrence of disequilibrium migration of 2 7 8~ daughter products, but there is no clear evidence that this occurred on a macroscopic scale in the galena.The model implies that some galena predates the U mineralization, and thus that there is a potential for Pb-(Zn?) mineralization in the basal Athabasca Formation.Les rapports des isotopes du Pb dans la galene et les arskniures de Ni du gisement Midwest rCvblent qu'il existe trois catCgories de Pb present. La premiere ayant des rapports 2 0 6~b / 2 "~b d'environ 22,5 et des rapports Z0"b/2MPb d'environ 43,5, apparait seulement dans la galbne et est restreinte a la formation basale d'Athabasca. La seconde se manifeste a environ 35 m au-dessus de la discordance et jusque dans les echantillons rtcoltts 5 la plus grande profondeur par nous (60 m dans les roches du socle). Les rapports caractCristiques de '"'~b/~"Pb varient de 30 a 35 et les rapports 20XPb/204Pb sont environ 50, et ce Pb reside dans les galenes et les arskniures de Ni. La troisieme possede des rapports 2"hPb/2"4Pb ClevCs, jusqu'a 350, et elle apparait en dedans de 10 m de la discordance et intimement associee avec les minCraux d'U.Le modele Clabork fait intemenir des additions radiogkniques dans la galene par l'intermediaire d'au moins deux CvCnements diffkrents. Le premier comporte des additions d'U-et de Th-filles au Pb du "type gres" il y a 265 ? 50 Ma, et le second a enrichi en U-fille uniquement le Pb "du type socle" au cours des derniers 50 Ma ou moins. Ces CvCnements sont en accord avec la perte de Pb enregistree dans les mintraux d'U et rkvblent que le mecanisme de la perte en Pb de ces mineraux d'U est de nature soudaine plut6t qu'une diffusion continue.Certaines compositio~s isotopiques du Pb en trace dans les arskniures de Ni tkmoignent d'une migration sans Cquilibrat...

Radiogenic lead mobility in the Shea Creek unconformity-related uranium deposit (Saskatchewan, Canada): migration pathways and Pb loss quantification

“…Radiogenic Pb has however been trapped within the orebody, at least partly, as shown by the extremely uranogenic Pb composition of the three galenas collected in the mineralised sample. Their compositions compare well with that of the galenas from the mineralised zone of the Midwest deposit [5]. However, the oldest possible deposition age of the uranium source (983 ± 46 Ma) does not correspond to the age of the U oxides present in the same thin section (1281 ± 6 Ma, [7]).…”

“…Les galènes (ii) sont moins radiogéniques (Tableau 1), avec une contribution thorigène non négligeable. Ces galènes témoignent d'une redistribution de plomb, par suite du lessivage de minéraux riches en Th de la base du bassin ou des granitoïdes du socle [4,5] et, dans une moindre mesure, d'oxydes d'uranium.…”

“…Hence, common and thorogenic lead from these galenas may derive either from Th-rich sandstone layers (heavy-mineral-rich layers of the basal MF formations) or from Th-rich high K-calcalkaline granitoids in the basement [4]. Such Pb isotopic compositions correspond to those of the 'basement-type' galena around the Midwest orebody [5], which have been interpreted as resulting from the alteration of the regolith below the unconformity.…”

“…Previous mineralogical studies on U deposits reveal that part of Pb did not necessarily migrate over long distances, as radiogenic galena is usually found within or adjacent to uraninite grains or in coffinite resulting from uraninite alteration [13,19]. However, radiogenic galenas have also been observed further away from the U orebodies, e.g., up to 35 m above the Midwest unconformitytype deposit [5], in the clay mantle surrounding the Oklo natural reactors, or 3 to 45 m below them in the conglomerate [11,13]. Excess radiogenic Pb in Uand Th-poor sandstone samples has been reported as far as several kilometres from some U deposits in the Athabasca and Thelon Basins [12].…”

“…Les oxydes d'uranium tendent à perdre leur plomb radiogénique du fait de la large différence de rayon ionique entre Pb 2+ et U 4+ . Si le plomb peut s'accumuler à proximité des oxydes d'U sous forme de galène [13,19], une partie généralement migre en dehors des zones minéralisées [5,[11][12][13]. Dans cette étude, nous essayons de quantifier la proportion de plomb radiogénique qui a pu migrer en dehors d'un gisement d'uranium de type discordance, le gisement de Shea Creek (Ouest Athabasca, Saskatchewan, Canada, Fig.…”