The significance of galena Pb model ages and the formation of large Pb-Zn sedimentary deposits
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Abstract:
In an attempt to clarify the significance of Pb model ages in Pb-Zn sedimentary deposits, we report high-precision Pb isotopic compositions for 64 galenas and 52 K-feldspars, the former from ores and the latter separated from granites. All samples are from Spain and the French Pyrenees. Lead from galena ores is of unequivocal continental origin. With few exceptions, Pb model ages systematically exceed emplacement ages by up to 400 Ma, a gap which is well outside the uncertainties of ~ 30 Ma assigned to the model. The histogram of the new high-precision Pb isotope data shows prominent peaks of galena Pb model ages at 94 ± 38 Ma and 392 ± 39 Ma. When the data are consolidated with literature data and examined in 3-dimensional Pb isotope space, cluster analysis identifies five groups. The model ages of the peaks occur, in order of decreasing peak intensity, at 395 ± 40 (Middle Devonian), 90 ± 34 Ma (Middle Cretaceous), and 613 ± 42 Ma (Neoproterozoic), with two minor peaks at 185 + 26 Ma (Jurassic) and 313 ± 41 (Upper Carboniferous). To a large extent, the model ages centered around these peaks correspond to distinct localities. The ages of the peaks do not coincide with any of the Betic, Variscan, or Pan-African tectonic events, which are the main tectonic episodes that shaped Iberian geology, but instead match well-known global oceanic anoxic events. It is argued that surges of metals weathered from continental surfaces scorched during anoxic events accumulated and combined in anoxic water masses with unoxidized marine sulfide released by submarine hydrothermal activity to precipitate the primary Pb-Zn stock. Frozen Pb isotope compositions require that galenas from black shales are the source of the final ores. The sulfides were later remobilized by large-scale convective circulation of basinal and hydrothermal fluids. The peaks of K-feldspar Pb model ages are distinct from those of galenas and do not correlate with magmatic emplacement ages. It is suggested that they instead reflect local circulation in Paleozoic sediments surrounding individual plutons. While Pb isotopes can be used as a regional provenance tool, such an approach requires that the data are considered in a fully 3-dimensional space.Keywords:
Radiogenic nuclide
Devonian
Abstract Galena in drill core samples from sandstone-lead deposits in the Dorotea district in the Caledonian Front was analysed for its lead isotope composition. Six samples were taken from Bellvikserg and three from the minor deposit of Ormsjö. The composition of all samples was highly radiogenic with 206Pb/204Pb-ratios between 22 and 23, and 207Pb/204Pb-ratios in the range 16.0 to 16.2. Plotted on a207Pb/204Pb versus 206Pb/204Pb diagram, the Dorotea lead follows a linear trend together with lead from Laisvall and Vassbo, suggesting their radiogenic component to be derived from source rocks of Svecokarelian age.
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Sulfide Minerals
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Galenas occurring in vugs and veins in the dolomites of the Eramosa, Goat Island, and Gasport members of the Middle Silurian Lockport Formation in the Niagara Escarpment have a small but significant range in lead isotopic composition. These differences are recorded in specimens from exposures in the Niagara Gorge in the east to the Guelph quarry, 100 km to the northwest. The galena isotopic data fit linear trends on 207 Pb/ 204 Pb– 206 Pb/ 204 Pb and 208 Pb/ 204 Pb– 206 Pb/ 204 Pb plots within experimental error. The most radiogenic ratios are anomalous, giving model ages (based on the growth curves of Zartman and Doe) up to 200 Ma younger than the most likely age of primary deposition of ~ 300 Ma. Sulphides currently being deposited from springs at the floor of the Vinemount quarry have a lead isotopic composition similar to that of some of the galena in the overlying dolomites. Rocks from several of the formations exposed at this site contain lead only slightly more radiogenic than the most radiogenic galenas. These results are consistent with the hypothesis of Haynes and Mostaghel that the modern sulphides result from local solution and redeposition of older sulphides. Previously published isotopic ratios for lead in the potassium feldspar constituents of ~ 300 Ma plutonic intrusives in the Appalachians cluster around the least radiogenic galena ratios and suggest that the lead in the Niagara Peninsula galenas has been remobilized from rock units in the Appalachian Basin. Grenville crystalline basement rocks do not appear to have contributed significantly to this lead. A single sample of galena from the Eramosa Member, exposed in the Ebel quarry in the Michigan Basin at the northwestern extension of the escarpment, has 207 Pb/ 204 Pb and 206 Pb/ 204 Pb ratios that are substantially more radiogenic than those in any of the galenas from the Niagara region of the Appalachian Basin and a 208 Pb/ 204 Pb ratio that lies significantly below the linear distribution defined by the Niagara region data set. We tentatively interpret this difference as reflecting derivation of the lead in the two basins from different crustal blocks.
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Escarpment
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Oxidizing agent
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