Laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) is used to compare the suitability of four cassiterite (SnO2) materials (SPG, Yankee, AY‐4 and Jian‐1), and three matrix‐mismatched reference materials (NIST SRM 612, NIST SRM 614 and 91500 zircon) for normalisation of U‐Pb and Pb‐Pb isotope ratios in cassiterite. The excess variance of ages determined by LA‐ICP‐MS is estimated to be ±0.33% for 207Pb/206Pb vs. 208Pb/206Pb isochron ages and ± 1.8% and for U‐Pb ages. Incorporation of this excess variance in cassiterite ages is necessary for realistic uncertainties. 207Pb‐206Pb ages are advantageous for dating Precambrian cassiterite such as SPG compared with U‐Pb ages as matrix effect on instrumental mass fractionation of Pb isotopes are generally considered to be minor. We note minor bias in 207Pb/206Pb vs. 208Pb/206Pb isochron ages (~ 0.6%) when using either the NIST SRM 614 or 91500 zircon reference materials and emphasise the requirement for uncertainty propagation of all sources of error and reference materials with comparable U and Pb mass fraction to the cassiterite. The 238U/206Pb isotopic ratios from normalisation to matrix‐mismatched reference materials show varied results, which emphasises the need to use matrix‐matched reference materials for calculating U‐Pb ages. When cross‐calibrated against each other, LA‐ICP‐MS U‐Pb ages of the ca. 1535 Ma SPG, ca. 245 Ma Yankee and ca. 155 Ma Jian‐1 cassiterites are all consistent with their ID‐TIMS values.

The Coles Hill uranium deposit, with an indicated resource of about 130 Mlb of U3O8, is the largest unmined uranium deposit in the United States. The deposit is hosted in the Taconian (approx. 480–450 Ma) Martinsville igneous complex, which consists of the Ordovician Leatherwood Granite (granodiorite) and the Silurian Rich Acres Formation (diorite). The host rock was metamorphosed to orthogneiss during the Alleghanian orogeny (approx. 325–260 Ma), when it also underwent dextral strike-slip movement along the Brookneal shear zone. During the Triassic, extensional tectonics led to the development of the Dan River Basin that lies east of Coles Hill. The mineralized zone is hosted in brittle structures in the footwall of the Triassic Chatham fault that forms the western edge of the basin. Within brittle fracture zones, uranium silicate and uranium-bearing fluorapatite with traces of brannerite form veins and breccia-fill with chlorite, quartz, titanium oxide, pyrite, and calcite. Uranium silicates also coat and replace primary titanite, zircon, ilmenite, and sulfides. Sodium metasomatism preceded and accompanied uranium mineralization, pervasively altering host rock and forming albite from primary feldspar, depositing limpid albite rims on igneous feldspar, altering titanite to titanium oxide and calcite, and forming riebeckite. Various geothermometers indicate temperatures of less than ~200°C during mineralization. In situ U-Pb analyses of titanite, Ti-oxide, and apatite, along with Rb/Sr and U/Pb isotope systematics of whole-rock samples, resolve the timing of geologic processes affecting Coles Hill. The host Leatherwood Granite containing primary euhedral titanite is dated at 450 to 445 Ma, in agreement with previously obtained ages from zircon in the Martinsville igneous complex. A regional metamorphic event at 330 to 310 Ma formed anhedral titanite and some apatite, reequilibrated whole-rock Rb/Sr and U-Pb isotopes, and is interpreted to have coincided with movement along the Brookneal shear zone. During shearing and metamorphism, primary refractory uranium-bearing minerals including titanite, zircon, and uranothorite were recrystallized, and uranium was liberated and mixed locally with hematite, clay, and other fine-grained minerals. Uranium mineralization was accompanied by a metasomatic episode between 250 and 200 Ma that reset the Rb-Sr and U-Pb isotope systems and formed titanium oxide and apatite that are associated and, in places, intimately intergrown with uranium silicate dating mineralization. This event coincides with rifting that formed the Dan River Basin and was a precursor to the breakup of Pangea. The orientation of late-stage tectonic stylolites is compatible with their formation during Late Triassic to Early Jurassic basin inversion, postdating the main stage of uranium mineralization and effectively dating mineralization as Mesozoic. Based on the close spatial and temporal association of uranium with apatite, we propose that uranium was carried as a uranyl-phosphate complex. Uranium was locally reduced by coupled redox reactions with ferrous iron and sulfide minerals in the host rock, forming uranium silicates. The release of calcium during sodium metasomatic alteration of primary calcic feldspar and titanite in the host rock initiated successive reactions in which uranium and phosphate in mineralizing fluids combined with calcium to form U-enriched fluorapatite. Based on the deposit mineralogy, oxygen isotope geochemistry, and trace element characteristics of uranium silicate and gangue minerals, the primary mineralizing fluids likely included connate and/or meteoric water sourced from the adjacent Dan River Basin. High heat flow related to Mesozoic rifting may have driven these (P-Na-F-rich) fluids through local aquifers and into basin margin faults, transporting uranium from the basin or mobilizing uranium from previously formed U minerals in the Brookneal shear zone, or from U-enriched older basement rock.

This paper investigates applicability of cassiterite to dating ore deposits in a wide age range. We report in situ LA-ICPMS U-Pb and Pb-Pb dating results (n = 15) of cassiterite from six ore deposits in Russia ranging in age from ~1.85 Ga to 93 Ma. The two oldest deposits dated at ~1.83–1.86 Ga are rare metal Vishnyakovskoe located in the East Sayan pegmatite belt and tin deposits within the Tuyukan ore region in the Baikal folded region. Rare metal skarn deposits of Pitkäranta ore field in the Ladoga region, Fennoscandian Shield are dated at ~1.54 Ga. Cassiterite from the Mokhovoe porphyry tin deposit located in western Transbaikalia is 810 ± 20 Ma. The youngest cassiterite was dated from the deposits Valkumei (Russian North East, 108 ± 2 Ma) and Merek (Russian Far East, 93 ± 2 Ma). Three methods of age calculations, including 208Pb/206Pb-207Pb/206Pb inverse isochron age, Tera-Wasserburg Concordia lower intercept age, and 207Pb-corrected 206Pb*/238U age were used and the comparison of the results is discussed. In all cases, the dated cassiterite from the ore deposits agreed, within error, with the established period of magmatism of the associated granitic rock.

First posted February 19, 2021 For additional information, contact: Director, Geology, Geophysics, and Geochemistry Science CenterU.S. Geological SurveyBox 25046, MS-973Denver, CO 80225-0046 The igneous geology of the St. Francois Mountains terrane in southeast Missouri is dominated by the products of 1.48 to 1.45 billion year old volcanic and plutonic magmatism but also includes volumetrically minor, compositionally bimodal contributions added during plutonism between 1.34 and 1.27 billion years ago. The 1.48 to 1.45 billion year old igneous rocks in the St. Francois Mountains terrane are bimodally distributed between volumetrically dominant felsic rocks and volumetrically minor rocks with mafic to intermediate compositions. All of these rocks are ferroan, which like most of their trace element abundances, suggests a genesis associated with farfield intraplate extensional tectonism and decompression-related magmatism. The diversity of compositions among 1.48 to 1.45 billion year old igneous rocks in the St. Francois Mountains terrane probably reflects mixtures of mantle-derived mafic inputs and low-degree partial melting of more evolved crustal protoliths. Newly determined ages define essentially continuous magmatism during the 30-million-year period between 1.48 and 1.45 billion years ago. The products of this magmatism are essentially coeval, whether intrusive or extrusive or having mafic, intermediate, or felsic compositions. In addition, the iron oxide-apatite (for example, Pea Ridge) and likely the iron oxide-copper gold (Boss) deposits in the St. Francois Mountains terrane have ages coincident with this magmatic episode. Spatial and temporal relations between 1.48 to 1.45 billion year old igneous rocks in the St. Francois Mountains terrane and the mineral deposits they host suggest the associated magmatic and mineralization processes are also genetically related.Geochemical, petrographic, geochronologic, and terrane-wide physical characteristics of the 1.48 to 1.45 billion year old igneous rocks in the St. Francois Mountains terrane are consistent with an origin involving extension well inboard from the margin of the Laurentian craton, associated mantle upwelling, lower crustal melting in response to mantle-derived thermal inputs, and mixing of mantle- and juvenile lower crustal-derived melts. Significant major and trace element compositional dispersion characteristics of these rocks likely reflect midcrustal magma reservoir fractionation of their principal rock-forming minerals. The resultant magmas constitute a series of variably hybridized reservoirs, emplaced at upper levels in the crust, that form a series of plutonic and associated eruptive products.

U-Pb dating of cassiterite and zircon from the Yazov granite (Transbaikalia region, Eastern Siberia, Russia) and cassiterite from spatially associated tin mineralization in the Tuyukan ore district in the Tonod uplift was conducted using in situ laser ablation inductively coupled plasma mass spectrometry. These analyses allow comparison of isotopic systematics for both minerals, especially related to transport in granitic magma. These data are also useful for understanding possible genetic links between the granite and the tin mineralization. Most of the U-Pb zircon analyses define a 206Pb/238U age of 719 ± 15 Ma for the granite; in addition, several zircon cores define an inheritance age of 1839 ± 21 Ma. U-Pb data for 10 nearly concordant analyses of disseminated cassiterite from the same samples yield a 206Pb/238U age of 1838 ± 34 Ma. This is the first documented evidence of cassiterite inheritance in granitic magma. These data indicate the robust character of U-Pb isotope systematics in cassiterite, comparable to that in zircon. The presence of numerous inclusions of cassiterite in zircon from the Yazov granite (revealed by nanotomography) supports the interpretation of inherited cassiterite included during Neoproterozoic zircon crystallization. The data indicate that high tin concentrations in the Yazov granite are due to the incorporation of older cassiterite crystals from country rock, not coeval cassiterite crystallization. Cassiterite samples from two ore occurrences spatially associated with the Yazov granite yield Pb-Pb isochron ages of 1.86–1.82 Ga, indicating that tin mineralization occurred in the Paleoproterozoic, nearly 1 Ga before emplacement of the Yazov granite. Tin mineralization of the ore region is probably related to ~ 1.85 Ga Chuya-Kodar tin-bearing granitic rocks that host tin deposits. These results have broad implications for understanding how critical elements, such as tin, may become enriched in rare-metal granites and how they are related to regional to global geodynamic processes.

High-uranium apatite samples from iron oxide-apatite (IOA) deposits of the eastern Adirondack Mountains, New York, are evaluated as potential standards for in situ U-Pb dating of high-U apatite. Age data for these minerals also have implications for better understanding the geological evolution in the region. Secondary ion mass spectrometry (SIMS), and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analyses of these samples show that they have high and variable uranium contents of 5-485 ppm. High-precision isotope-dilution thermal ionization mass spectrometry (ID-TIMS) dating yielded U-Pb errorchron ages of 907 +/- 14 Ma (MSWD = 20) and 924 +/- 13 Ma (MSWD = 547), showed disturbance in the U-Pb systems, and revealed radiogenic compositions of initial Pb (Pb-206/Pb-20(4) ratios of 339 +/- 48 and 162 +/- 39). Nominal age calculations using assumed initial Pb isotopic compositions close to average terrestrial crustal Pb yield biased and older ages 206 despite the radiogenic Pb isotopic composition of the apatites (Pb-206/Pb-20(4) from 1300 to >4000). The apatite ages calculated using the measured initial Pb isotopic compositions are younger than the U-Pb ages of 1008.0 +/- 3.2 Ma and 992.4 +/- 7.7 Ma determined by SIMS for zircon cores and rims, respectively, separated from one of these ore samples. It indicates protracted fluid activity in the deposit that lasted for similar to 100 Ma after the ore formation, or later disturbance of the apatite. The time of U enrichment that we inferred from radiogenic composition of initial Pb is within uncertainties with primary ore crystallization, indicating that high Pb-206/Pb-20(4) ratios of initial Pb were caused by dissolution-reprecipitation of primary apatite. Despite the variability in U contents and disturbance of the U-Pb system in the apatites, our SIMS and LA-ICPMS analyses show that these samples can be used, with due caution, as reference materials (RMs) for dating apatite with similarly high-U concentration. They potentially provide more precise and accurate results compared to the usage of better behaved but lower-U apatite RMs due to improved counting statistics and reduced matrix effects, especially important for LA-ICPMS with less sensitive quadrupole mass analysers. The McClure Mountain apatite, a widely used reference for in situ dating, was used in the present study as a matrix-matched reference material for in situ LA-ICPMS analyses. We also analysed it by ID-TIMS and obtained more radiogenic Pb-isotopic data and more precise ages that are in agreement within uncertainties with the previously published result (Schoene and Bowring, 2006); combined data yielded a U-Pb errorchron age of 525.3 +/- 1.7 Ma (MSWD = 70). The data corrected for initial Pb using the total U/Pb isochron yield the single Concordia age of 525.10 +/- 0.75 Ma (MSWD = 1.1). Two independent ID-TIMS U-Pb studies support the use of this apatite as a primary RM for dating of low-U apatite with a normal composition of initial Pb. However, its small grain size and low U concentration (11-29 ppm) can cause elevated uncertainty in LA-ICPMS age determinations for unknowns. Better precision in our LA-ICPMS analyses was achieved using Madagascar (MAD1 and MAD2) apatite RM with U contents in the range of 28-58 ppm.

High-U hydrothermal apatite with complex U-Pb systematics is closely spatially associated with mineralization at the Coles Hill deposit, the largest unmined uranium deposit known in the United States. The deposit is hosted in metasomatized rocks of the 450- to 430-Ma-old Martinsville Intrusive Complex in south-central Virginia. Direct dating of metamict uranium-ore minerals, mostly coffinite, is not possible due to open-system radon loss. Instead, U-Pb isotopes in cogenetic apatite were investigated as a means of evaluating the age of mineralization. Here we report in situ electron probe microanalyses (EPMA) of coffinite, isotope-dilution thermal-ionization mass spectrometry (ID-TIMS) U-Pb data for mineralized whole rock samples, and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb isotope data for apatite in both unmineralized and U-mineralized host rocks. Massive deficits in radiogenic Pb preclude reliable U-Pb "chemical ages" calculated from EPMA data obtained from coffinite. In contrast, LA-ICPMS data for secondary apatite in unmineralized rocks indicate low-U concentrations (10(6)-10(2) ppm), "normal" (consistent with models of terrestrial Pb isotopic evolution) initial Pb isotope compositions, and U-Pb age estimates of similar to 330 Ma, which is consistent with dates previously proposed for the regional Paleozoic shear zone that hosts the deposit. Ore-stage apatite associated with coffinite has high-U concentrations (typically 10(2)-10(3) ppm but up to 2.4 wt% U) and large excesses of Pb-206 (Pb-207/Pb-206 < 0.01) unsupported by in situ U decay. Data show that initial Pb had variable isotopic compositions including both "normal" Pb derived from host rocks and Pb-206-enriched Pb introduced by secondary metasomatic fluids. Evaluation of the complex evolution and mixing of Pb sources has broader implications for UPb dating of hydrothermal apatite. Excess Pb-206 in apatite is derived from decay products of Rn-222 lost from coffinite and mobilized by Na-, P-, and U-enriched metasomatic fluids during the main mineralizing event at similar to 230 Ma. Ore-stage alteration did not uniformly reset the U-Pb systematics in host rocks precluding a well-constrained whole-rock isochron age. However, whole-rock isotope data imply U mobility at similar to 200-220 Ma and support a Triassic age for the final stages of mineralization. Results also indicate that apatite with up to several weight percent uranium is able to retain U and its decay products for hundreds of millions of years; an important consideration when assessing this mineral as a potential matrix for long-term storage of radioactive waste.

Textural, geochronological, and geochemical data are presented here for cassiterite from the giant (149.7 million tonnes [Mt]) Mesoproterozoic Sullivan Pb-Zn-Ag deposit, which has been subjected to several tectonothermal events. These data provide constraints on the age and origin of the tin concentrations and new insights into related base metal mineralization. Sullivan is rare among sediment-hosted, stratiform Pb-Zn-Ag deposits in having high tin contents in ore (up to 2.5 wt %; avg 310 ppm Sn). Cassiterite occurs in all facies of this deformed and metamorphosed deposit, including (1) high-grade veins with arsenopyrite and pyrrhotite, (2) bedded Pb-Zn-Ag ores, (3) massive pyrrhotite, (4) footwall and hanging-wall tourmalinites, and (5) other altered wall rocks. New in situ U-Pb dates for Sullivan cassiterite obtained by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) are modeled by a multicomponent-based algorithm that yields three age peaks: 1475 +/- 4 Ma (51% of the data), 1366 +/- 10 Ma (25%), and 1074 +/- 7 Ma (24%). These dates are attributed, respectively, to primary tin mineralization at ca. 1475 Ma, the East Kootenay orogeny at ca. 1370 to 1300 Ma, and the Grenvillian orogeny at ca. 1100 to 980 Ma. Based on the presence and local abundance of cassiterite in all ore and ore-related rocks at Sullivan, the U-Pb date of 1475 +/- 4 Ma reported here represents the first direct age for ore mineralization in the deposit. Occurrence of texturally discordant rims on Sullivan cassiterite grains having U-Pb dates coeval with the East Kootenay and Grenvillian orogenies suggests that these young dates reflect dissolution-reprecipitation processes associated with channelized metamorphic fluid flow. LA-ICP-MS U-Pb dates obtained on low-U (<10 ppm) cassiterite also indicate that U-Pb dates for cassiterite from other metamorphosed deposits should be viewed with caution and not assumed to record an age of primary tin mineralization. Aqueous transport conditions for tin are evaluated to gain insights into the cassiterite mineralization at Sullivan. Based on fo(2)-pH topology of aqueous tin species at 250 degrees C, tin transport was dominated by an SnCl3- complex at fo(2) of about -40 and pH of <4.0, conditions that were constrained, respectively, by widespread occurrence of pyrrhotite in deep footwall siliciclastic metasedimentary rocks of the host Aldridge Formation and by release of CO2 from shallow mafic sills and resulting formation of carbonic acid in condensed brine. The low fo(2) value also reflects inferred production of CH4 from heating of organic matter in the sediments during emplacement of these sills. Based on a fluid pH restriction of <4.0 and a requirement for sparse or no K-feldspar in the source, the tin likely derives from previously altered Lower Aldridge strata. This model relies on the early diagenetic dissolution of K-feldspar from these sediments by basinal brines, followed by interaction with a later, more acidic hydrothermal fluid generated during the emplacement of large mafic sills in the shallow subsurface that leached tin from accessory minerals such as titanite in siliciclastic sediments of the Lower Aldridge Formation. Mass balance calculations suggest that derivation of the tin from this sedimentary source (avg 2.0 ppm Sn) required similar to 40 km(3) and a cylinder diameter of 3.2 km (height 5.0 km) in order to supply the 0.1 Mt of tin contained in the deposit. The presence of mafic sills in the footwall of several other tin-bearing, sediment-hosted, stratiform Pb-Zn-Ag deposits and in modern, tin-rich, sediment-hosted sulfide deposits in the northeast Pacific Ocean suggests that siliciclastic marine basins that contain mafic sills-with or without stratiform sulfide deposits-should be evaluated for possible tin mineralization.

The Abu Dabbab rare-metal granite in the Eastern Desert of Egypt is a highly-evolved alkali-feldspar granite with transitional magmatic-hydrothermal features. Extreme geochemical fractionation and the associated significant Ta-Sn resource make the Abu Dabbab intrusion an important feature in the metallogenic evolution of the Arabian-Nubian Shield. U-Pb dating by laser ablation sector field (SF)-ICPMS analysis of igneous monazite yields a Concordia age of 644.7 +/- 2.3 Ma, identical within uncertainty to a lower intercept Tera-Wasserburg isochron age of 644.2 +/- 2.3 Ma obtained from hydrothermal cassiterite. Both ages place tight constraints on the timing of magmatic-hydrothermal processes in the Abu Dabbab granite which represents the oldest highly-evolved granite recognized so far in the Pan-African Arabian-Nubian Shield. Thus, the new ages also date the start of a period of late-orogenic metalliferous granite magmatism, when the basement of the Eastern Desert underwent a geodynamic transition from a compressive subduction-collision regime towards orogenic collapse in the late Cryogenian. (C) 2020 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

The Cornwall and Devon vein- and greisen-type copper and tin deposits of southwest England are spatially and genetically related to shallow-seated granitic intrusions. These late Variscan intrusions, collectively known as the Cornubian Batholith, extend over 200 km and form a continuous granitic spine from the Isles of Scilly Granite in the west to the Dartmoor Granite in the east. The granitic plutons of the Cornubian Batholith were intruded from ~ 295 to 270 Ma without a major hiatus. Twelve samples of cassiterite (SnO2) were obtained from tin deposits associated with seven different plutons within the Cornubian Batholith for in situ LA-ICPMS U–Pb dating. This study of cassiterite was undertaken to obtain the first results of direct dating of ore mineral to refine the geochronology of tin mineralization in this region. Of the cassiterite samples analyzed, the oldest ages were determined within the Kit Hill and Hingston–Gunnislake Granites in the central part of the Cornubian Batholith. The Hingston–Gunnislake cassiterite, from Drakewalls Mine, was the oldest sample dated at 291.8 ± 3.4 Ma. The next oldest dates, 290.5 ± 2.8 and 288.5 ± 2.9 Ma, were from two cassiterite samples extracted from the adjacent Kit Hill Consolidated Mines within the Kit Hill Granite. At the eastern end of the study area, two cassiterite samples within the Dartmoor Granite produced ages of 286.0 ± 1.8 and 284.1 ± 1.3 Ma. The youngest sample from this study, 275.4 ± 1.6 Ma, is from the Balleswidden Mine within the westernmost Land’s End Granite. The cassiterite dates do not reveal any readily observable relationship between ore ages and geographic relationship from west to east throughout the Cornubian Batholith. Incorporating the associated errors, the geochronology does indicate continuous mineralization within the granites for ~ 21 million years, from ca. 295 to 274 Ma. This span falls within the established period of granitic magmatism of ca. 295 to 270 Ma for the Cornubian Batholith and further confirms the reliability of in situ LA-ICPMS U–Pb dating of cassiterite.

Cassiterite (SnO 2 ) is increasingly used for U-Pb dating of tin mineralization. In situ analysis by LA-ICPMS is the preferred analytical method because (1) complete digestion of cassiterite needed for ID-TIMS dating is extremely difficult, and (2) cassiterite typically has low U contents making SIMS analysis problematic. The recently developed HBr-based digestion technique (Tapster and Bright, 2020; Carr et al., 2020) allows reliable ID-TIMS characterization of matrix-matched cassiterite reference materials useful for LA-ICPMS geochronology. By comparing in situ data with the ID-TIMS results, we estimate accuracy and precision of cassiterite U-Pb ages for our LA-ICPMS analyses at about 1.5% (2SD), based on long-term reproducibility of an ~155 Ma internal secondary cassiterite matrix-matched reference material. Cassiterite is found not only in tin and rare metal ore deposits, but also as an accessory phase in “tin granites”. Additionally, cassiterite may be the only suitable phase for direct U-Pb dating of mineralization in some VMS and SEDEX base-metal deposits. In this talk we discuss newly discovered issues with cassiterite dating that include the of

Cassiterite (SnO2), a main ore mineral in tin deposits, is suitable for U-Pb isotopic dating because of its relatively high U/Pb ratios and typically low common Pb. We report a LA-ICPMS analytical procedure for U-Pb dating of this mineral with no need for an independently dated matrix-matched cassiterite standard. LA-ICPMS U-Th-Pb data were acquired while using NIST 612 glass as a primary non-matrix-matched standard. Raw data are reduced using a combination of Iolite (TM) and other off-line data reduction methods. Cassiterite is extremely difficult to digest, so traditional approaches in LA-ICPMS U-Pb geochronology that utilize well-characterized matrix-matched reference materials (e.g., age values determined by ID-TIMS) cannot be easily implemented. We propose a new approach for in situ LA-ICPMS dating of cassiterite, which benefits from the unique chemistry of cassiterite with extremely low Th concentrations (Th/U ratio of 10(-4) or lower) in some cassiterite samples. Accordingly, it is assumed that Pb-208 measured in cassiterite is mostly of non-radiogenic originit was initially incorporated in cassiterite during mineral formation, and can be used as a proxy for common Pb. Using Pb-208 as a common Pb proxy instead of Pb-204 is preferred as Pb-204 is much less abundant and is also compromised by Hg-204 interference during the LA-ICPMS analyses. Our procedure relies on Pb-208/Pb-206 vs Pb-207/Pb-206 (Pb-Pb) and Tera-Wasserburg Pb-207/Pb-206 vs U-238/Pb-206 (U-Pb) isochron dates that are calculated for a similar to 1.54Ga low-Th cassiterite reference material with varying amounts of common Pb that we assume remained a closed U-Pb system. The difference between the NIST 612 glass normalized biased U-Pb date and the Pb-Pb age of the reference material is used to calculate a correction factor (F) for instrumental U-Pb fractionation. The correction factor (F) is then applied to measured U/Pb ratios and Tera-Wasserburg isochron dates are obtained for the unknown cassiterite analyzed in the same analytical session. This allows for U-Pb dating of cassiterite of any age with no need for an independently dated matrix-matched reference material, nor assumptions about the isotopic composition of common Pb. Results for cassiterite from tin deposits in Bolivia, Brazil, China, Russia, Saudi Arabia, South Africa, Spain, and the United Kingdom, with ages ranging from similar to 20 Ma to similar to 2060 Ma, demonstrate the applicability of this approach across a broad range of geologic time. These ages are in good agreement with published geochronology of the host rocks associated with the tin deposits and with previously published U-Pb ages of some cassiterites from the same deposits. Thus, our in situ LA-ICPMS methodology verifies the use of cassiterite as a reliable U-Pb mineral-geochronometer with the advantages of fast and relatively low cost in situ analyses with moderate spatial resolution.

The Williston Basin has produced oil and gas from conventional structural and stratigraphic traps for more than 60 years. The advent of horizontal drilling and hydraulic fracturing of shale in the Devonian-Mississippian Bakken Formation has increased the production of oil in this basin from areas that partially overlap the prairie pothole region of North America. Massive scale of oil production in the Williston Basin increases the risk of accidental releases of coproduced formation water (brine) into the environment. The prairie pothole region is named for a multitude of small lakes and wetlands that provide critical habitat for waterfowl and other wildlife. The increased risks raise the importance of developing robust tracers that can identify the source(s) and quantity of contamination of this premier wetland ecosystem. Radiogenic Sr-87/Sr-86 and U-234/U-238 isotope tracers are widely used in hydrological studies to detect potential sources of groundwater contamination. Here, we used paired Sr-87/Sr-86 and delta Sr-88 values in wetland water, which is essentially shallow groundwater at the Goose Lake and Fuller sites in Montana and North Dakota where produced water contamination was previously established. We also analyzed brines from the Bakken Formation and Mississippian Charles Formation to evaluate potential sources of contamination. This is the first attempt to combine these isotope tracers to estimate a magnitude of groundwater contamination by the oil-field produced water. Moreover, because U+6 is soluble in water under subaerial oxidizing conditions and the U+4 content is orders of magnitude lower (pg/g levels) in reducing brines, the U-234/U-238 isotope tracer is insensitive to brine contamination. However, it elucidates potential variability of uncontaminated end-members and, therefore, helps to improve the accuracy of estimated degrees of brine contamination based on binary mixing relationships. In addition, U isotopes in groundwater have a potential for detecting some U-rich anthropogenic contaminants (e.g., phosphate fertilizers). Using these isotopic systematics we confirm previous conclusions that surface water and shallow groundwater in two studied sites are variably contaminated by produced water and estimate the degree of the contamination at similar to 7% in a sample collected close to a brine tank and < 0.5% in most other samples. The U isotope analyses in combination with the Sr isotope results help outline variable groundwater pathways and uncontaminated endmembers. Also, U isotopes show no appreciable groundwater contamination from U-enriched P-fertilizers. Our study in the Williston Basin demonstrates the potential of using combined radiogenic (Sr-87/Sr-86, U-234/U-238) and non-traditional stable (delta Sr-88) isotopic systematics as tracers of anthropogenic contamination in surface and groundwater systems.

Times of metal-rich brine discharge into ancient ocean basins, associated with the formation of sedimentary-exhalative (sedex) Zn-Pb-Ba ore deposits, coincided with short-duration positive excursions ("spikes") in the global marine Sr isotope record. While these spikes are unexplained by conventional oceanic models, chronostratigraphic correlations, combined with mass balance evidence and oceanographic modeling, suggest that the flux of radiogenic Sr from sedex brines during ore formation is sufficient to explain these previously enigmatic Sr-87/Sr-86 spikes. We review existing Sr-87/Sr-86 data and present new data as verification of these global Sr-87/Sr-86 spikes and their correlations with the formation of giant sedex ore deposits. Major events include an 1 x 10(-4) (similar to 0.7078-similar to 0.7079) excursion contemporaneous with formation of the Rammelsberg deposit at -389 Ma; spikes on the order of 1 to 3 x 10(-4), coeval with formation of the Meggen deposit at similar to 381 Ma, several ore deposits in the Macmillan Pass district at similar to 379 to 375 Ma, and the Silvermines deposits at similar to 352 Ma; and two >6 x 10(-4) spikes coincident with formation of the giant Navan deposit at similar to 346 Ma and Red Dog deposits at similar to 337 Ma. Moreover, the timing of peak 8(87)Sr/Sr-86 spikes correlates with global delta C-13 and delta O-18 spikes,deposition of metal-rich black shales and ironstones, metal-induced malformation (teratology) of marine organisms, and mass extinctions. The relationships among these features were poorly understood, but our new model explains how the flux of key biolimiting nutrients and metals contained in sedex brines, demonstrably equivalent to or exceeding that of the total modern riverine flux to the ocean, spurred ocean eutrophication, which, ultimately, through a series of positive feedback mechanisms, may have triggered global chemical and biological events. If, as we hypothesize, sedex hydrothermal systems are recorded in the global marine isotopic, geologic, and biological records, our findings define a new approach to the study of and exploration for sedex deposits. We demonstrate that fluid inclusion solute chemistry and isotopic and stratigraphic studies of sedex deposits, coupled with chronostratigraphic correlation and high-resolution Sr-87/Sr-86 isotope chemostratigraphy, can be used to answer long-standing questions about geologic processes responsible for formation of these extraordinary deposits. This approach provides evidence for the age, duration, and fluxes of fluids and metals vented into the ocean by these giant hydrothermal systems. Accordingly, the marine Sr-87/Sr-86 curve constitutes a global exploration tool that could be applied to assess the mineral potential of sedimentary basins.To illustrate the potential of this tool to identify favorable stratigraphic ages and basins with potential for undiscovered giant sedex deposits, we highlight several spikes, on par with those characteristic of the Red Dog and Navan deposits, which have not been correlated with known metal deposits. Given these strong temporal correlations, mass balance estimates, and results of ocean chemistry modeling, our study suggests that further work is warranted to determine the extent to which periodic venting of hydrothermal basinal brines into the ocean has influenced the evolution of marine chemistry. Ultimately, these global signatures can be applied to the study of and exploration for sedex deposits.

A method for the measurement of “common” lead isotope ratios in silicate glasses and minerals using LA-SC-ICPMS is described and evaluated.

Isotope dilution thermal ionization mass spectrometry (ID-TIMS) and laser ablation inductively coupled plasma mass spectrometry (LA–ICP-MS) are two popular analytical methods used to constrain the age of minerals, including cassiterite. Compared to ID-TIMS, the LA–ICP-MS analytical method is often favoured due to its simple analytical procedure and sample preparation, although the methodology constantly needs modification. Our recent paper on dating cassiterite from the Baiganhu W-Sn ore field in NW China based on LA–MC-ICP-MS and ID-TIMS methods has received a response from Dr. Leonid A. Neymark (2017). Definitely, Neymark is appreciated, and his suggestions are helpful to improve the methodology of cassiterite U–Pb geochronology. Based on a discussion on the cassiterite U–Pb dating method, we re-confirm the validity of the cassiterite U-Pb ages for the Baiganhu W-Sn ore field. The initial non-radiogenic 204Pb used in the U-Pb isochron approach can be replaced by the non-radiogenic 207Pb, because the radiogenic 207Pb decayed from 235U can be ignored in cassiterite with a high Pb content. Therefore, the cassiterite U-Pb isochron yields an age similar to that obtained from T-W concordia for the same sample, testifying the validity of the cassiterite 238U/207Pb–206Pb/207Pb isochron method.