Pb-Pb and U-Pb Dating of Cassiterite by In Situ LA-ICPMS: Examples Spanning ~1.85 Ga to ~100 Ma in Russia and Implications for Dating Proterozoic to Phanerozoic Tin Deposits

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

Age constraints on fluid inclusions in calcite at Yucca Mountain

The {sup 207}Pb/{sup 235}U ages for 14 subsamples of opal or chalcedony layers younger than calcite formed at elevated temperature range between 1.88 {+-} 0.05 and 9.7 {+-} 1.5 Ma with most values older than 6-8 Ma. These data indicate that fluids with elevated temperatures have not been present in the unsaturated zone at Yucca Mountain since about 1.9 Ma and most likely since 6-8 Ma. Discordant U-Pb isotope data for chalcedony subsamples representing the massive silica stage in the formation of the coatings are interpreted using a model of the diffusive loss of U decay products. The model gives an age estimate for the time of chalcedony formation around 10-11 Ma, which overlaps ages of clay minerals formed in tuffs below the water table at Yucca Mountain during the Timber Mountain thermal event

Impact of sedimentary-exhalative hydrothermal systems on marine chemistry and mass extinctions : applications for ore genesis research and mineral exploration

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