Geochemical exploration for buried sandstone-hosted uranium mineralisation using mobile U and Pb isotopes: case study of the REB deposit, Great Divide Basin, Wyoming

Mobile radiogenic lead isotopes (206Pb, 207Pb, 208Pb, and 210Pb) represent products of radioactive decay of their parental uranium and thorium isotopes (238U, 235U, 232Th), and are considered potential geochemical pathfinders of the buried sandstone-type uranium deposits. Soil samples collected along a geochemical traverse intersecting buried uranium roll front mineralisation at the REB deposit in the Great Divide Basin, Wyoming, USA were studied. Mineralisation of this deposit is hosted in weakly lithified arkosic sands, at a depth of 120 - 200 metres, without a strong surficial expression of its presence at depth which makes discovery of this deposit type difficult, slow, and expensive. All soil samples have been analysed for ratios of the mobile long-lived Pb isotopes and their parental U and Th isotopes, determined from partial leach products obtained using a weak acid leaching technique. The samples were also analysed for trace elements, assayed both in the partial leach products and using conventional whole soil sample assays. Ratios of the mobile radiogenic Pb isotopes to their parental U and Th isotopes (206Pb/238U, 207Pb/235U and 208Pb/232Th) determined in the partial leach products exhibit anomalous contents in the soil samples collected above the uranium rolls. The anomalous values are several times greater than background values, to lateral distances of 350-400m outside of the roll fronts. Notably, conventional whole soil assays have failed to detect the anomalies which were detected using mobile Pb isotopes. Supplementary material:https://doi.org/10.6084/m9.figshare.c.5610980

Lead Isotopes in North American Precipitation record the presence of Saharan Dust

Atmospheric dust is an important mass transfer and nutrient supply process in Earth surface ecosystems. For decades, Saharan Dust has been hypothesized as a supplier of nutrients to the Amazon Rain Forest and Eastern North America. However, isotope studies aimed at detecting Saharan dust in the American sedimentary record have been ambiguous. A large Saharan dust storm emerged off the coast of Africa in June 2020 and extended into southeastern United States. This storm provided a means to evaluate the influence of Saharan dust in North America confirmed by independent satellite and ground observations. Precipitation samples from 17 sites within the National Atmospheric Deposition Program (NADP) were obtained from throughout the southeastern United States prior to, during, and after the arrival of Saharan dust. Precipitation samples were measured for their lead (Pb) isotopic composition, total Pb content, and 210Pb activity using multi-collector inductively coupled plasma mass spectrometry. We measured a significant isotopic shift (approximately 0.7 % in the 208Pb/206Pb relative to the 207Pb/206Pb) in precipitation that peaked in late June 2020 when the dust blanketed the southeastern US. However, the magnitude and short time period of the isotopic shift would make it difficult to detect in sedimentary records.

Provenance of Zhou Dynasty bronze vessels unearthed from Zongyang County, Anhui Province, China: determined by lead isotopes and trace elements

Thirteen Zhou Dynasty bronze vessels and two slags from Zongyang County along the north bank of the Yangtze River in Anhui were analyzed by LA-MC-ICP-MS and EDXRF. The results of lead isotope analysis showed that there were two kinds of lead materials in the Zongyang bronzes. Vessels in Class I, thought to have originated from the Wannan region were primarily used during the Western Zhou and the following Spring and Autumn period; while those in Class II, possibly from the local mines in Zongyang County, were mainly present during the Warring States period. Such a shift in the ore material sources is also revealed by the trace element analysis of the Zongyang bronzes. With reference to the relevant historical documents, we suggest that the transformation of the bronze material sources might have been associated with the changes of the political situation during the Zhou Dynasty.

Strong evidence for the continued contribution of lead deposited during the 20th century to the atmospheric environment in London of today

Although leaded gasoline was banned at the end of the last century, lead (Pb) remains significantly enriched in airborne particles in large cities. The remobilization of historical Pb deposited in soils from atmospheric removal has been suggested as an important source providing evidence for the hypothetical long-term persistency of lead, and possibly other pollutants, in the urban environment. Here, we present data on Pb isotopic composition in airborne particles collected in London (2014 to 2018), which provide strong support that lead deposited via gasoline combustion still contributes significantly to the lead burden in present-day London. Lead concentration and isotopic signature of airborne particles collected at a heavily trafficked site did not vary significantly over the last decade, suggesting that sources remained unchanged. Lead isotopic composition of airborne particles matches that of road dust and topsoils and can only be explained with a significant contribution (estimate of 32 ± 10 to 43 ± 9% based on a binary mixing model) of Pb from leaded gasoline. The lead isotopes furthermore suggest significant contributions from nonexhaust traffic emissions, even though isotopic signatures of anthropogenic sources are increasingly overlapping. Lead isotopic composition of airborne particles collected at building height shows a similar signature to that collected at street level, suggesting effective mixing of lead within the urban street canyon. Our results have important implications on the persistence of Pb in urban environments and suggest that atmospheric Pb reached a baseline in London that is difficult to decrease further with present policy measures.

Provenance of the Zhou Dynasty Bronze Vessels Unearthed from Zongyang County, Anhui Province, China: Determined by Lead Isotopes and Trace Elements

Thirteen Zhou Dynasty bronze vessels and two slags from Zongyang County along the north bank of the Yangtze River in Anhui were analyzed by LA-MC-ICP-MS and EDXRF. The results of the lead isotope analysis showed that there were two kinds of lead materials in the Zongyang bronzes. Class Ⅰ which could have originated from the Wannan region were mainly used in the Western Zhou and the following Spring and Autumn periods; while Class Ⅱ, possibly from the local mines in Zongyang County, were mainly present in the Warring States period. Such a shift in the ore material sources is also revealed by the analysis of the trace elements of the Zongyang bronzes. With reference to relevant historical documents, it can be inferred that the transformation of the bronze material sources could be related to the changes of the political situation during the Zhou Dynasty.

Lead Isotopes to Identify Underwater Ceramic Contamination: The Example of the Kyrenia Shipwreck (Cyprus)

We present the lead isotopic composition of ceramic fragments from the Kyrenia shipwreck (Cyprus), selected from three chemical groups related to the Rhodes and Alimos regions (Greece). Fragments of the lead sheathing covering the ship’s hull and biogenic material formed on some of the ceramic sherds, were analyzed along with sherds from the Rhodes and Alimos source areas for comparison. The objective of this paper was to evaluate the impact of the seawater environment on the isotopic signature of the ceramics, and on our ability to use lead isotopes to source ceramics recovered from seawater. The results showed that the lead isotopic composition of the shipwreck ceramics was modified by its prolonged proximity to the lead hull sheathing. The lead signature of filtering marine organisms encrusted on the ceramics provided support for this hypothesis.

Plumbotectonics of Moroccan ore deposits

The synthesis of 240 lead isotopes analyzes, measured on Moroccan ore deposits of Ediacarian to Neogene ages located in all geotectonic domains of Morocco allows a global reflection on the metallogeny of Morocco. The isotopic compositions vary widely, from 17.738 (Bou Skour) to 18.905 (Draa Sfar) for the 206Pb/204Pb ratio, and from 15.521 to 15.706 for the 207Pb/204Pb ratio. The source of lead in the studied deposits is located in the upper continental crust, except for those in the Anti-Atlas (Bou Skour, Imiter, etc.) and some in the High Atlas (Azegour) with a clear mantellic contribution. Isotopic variations noted at the scale of a district result either from the presence of several superimposed hydrothermal events calling upon different local sources as at Tighza, or from a single event disturbed by the segmentation of a volcanosedimentary basin, as for the Jebilet and Guemassa ore deposits. At the scale of the deposit (Draa Sfar, Bou Skour), isotopic variations result from the superposition of several hydrothermal events each with their own lead and associated metals. Overall, we can distinguish three generations of lead incorporated successively into the Moroccan geological base by magmatism and / or hydrothermalism, characterized by their 206Pb/204Pb ratios: 17.74-17.90 (Ediacarian), 18.10-18.40 (Hercynian) and 18.75-18.90 (Alpine). Ediacarian lead is present in the Anti-Atlas, and very locally in the meseta (Bouznika), and feeds in part on the mafic magmatism of Gondwana. Hercynian lead is the most represented and displays a definitive rupture in the source of metals, which is now exclusively crustal. It invades all Moroccan areas, including the Anti-Atlas, where it re-mobilizes and mixes with the Ediacaran lead. Alpine lead, more discreet, marks out the large scarf going from Agadir to Nador which traces on the surface the mantle plume of the Canaries and accompanies a Neogene magmatism which may also have acted as a simple engine remobilizing Hercynian lead, in particular to form MVT deposits from Touissit. The Hercynian and Alpine lead influxes are partly responsible for resetting the mineralizations, as at Bou Azzer or Imiter. In the Sawkins’s model, lead isotopic results support successive remobilisations of lead stored in primary and secondary tanks, as well as inheritance phenomena. Finally, the good transfer of the isotopic signature of lead from deposits to surface gossans shows that the isotopic geochemistry of lead is a useful tool for mineral exploration in Morocco.