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Lead isotopic compositions of 61 samples (55 galena, one cerussite [PbCO3] and five whole ore samples) from 16 Volcanic Hosted Massive Sulphide (VHMS) deposits in the Urals Orogeny show an isotopic range between 17.437 and 18.111 for 206Pb/204Pb; 15.484 and 15.630 for 207Pb/204Pb and 37.201 and 38.027 for 208Pb/204Pb. Lead isotopic data from VHMS deposits display a systematic increase in ratios across the Urals paleo-island arc zone, with the fore-arc having the least radiogenic lead compositions and the back-arc having the most radiogenic lead. The back arc lead model ages according to Stacey–Kramers model are close to the biostratigraphic ages of the ore-hosting volcano-sedimentary rocks (ca. 400 Ma). In contrast, less radiogenic lead from the fore-arc gives Neoproterozoic (~ 700 Ma) to Cambrian (480 Ma) lead model ages with low two-stage model μ values of 8.8 (parameter μ = 238U/204Pb reflects the averaged U/Pb ratio in the lead source), progressively increasing stratigraphically upwards to 9.4 in the cross-section of the ore-hosting Baymak–Buribai Formation. The range of age-corrected uranogenic lead isotopic ratios of the volcanic and sedimentary host rocks is also quite large: 206Pb/204Pb = 17.25–17.96; 207Pb/204Pb = 15.48–15.56, and generally matches the ores, with the exception of felsic volcanics and plagiogranite from the Karamalytash Formation being less radiogenic compare to the basaltic part of the cross-section, which would potentially imply a different source for the generation of felsic volcanics. This may be represented by older Neoproterozoic oceanic crust, as indicated by multiple Neoproterozoic ages of mafic–ultramafic massifs across the Urals. The relics of these massifs have been attributed by some workers to belong to the earlier Neoproterozoic stage of pre-Uralian ocean development. Alternative sources of lead may be Archean continental crust fragments/sediments sourced from the adjacent East-European continent, or Proterozoic sediments accumulated near the adjacent continent and presently outcropping near the western edge of Urals (Bashkirian anticlinorium). The contribution of Archean rocks/sediments to the Urals volcanic rock formation is estimated to be less than 0.1% based on Pb–Nd mixing models. The most radiogenic lead found in VHMS deposits and volcanics in the Main Uralian Fault suture zone, rifted-arc and back-arc settings, show similar isotopic compositions to those of the local Ordovician MORBs, derived from highly depleted mantle metasomatized during dehydrational partial melting of subducted slab and oceanic sediments. The metasomatism is expressed as high Δ 207Pb/204Pb values relative to the average for depleted mantle in the Northern hemisphere, and occurred during the subduction of oceanic crust and sediments under the depleted mantle wedge. A seemingly much younger episode of lead deposition with Permian lead model ages (ca. 260–280 Ma) was recorded in the hanging wall of two massive sulphide deposits.
55 galena samples from 18 deposits. Volcanite-hosted deposits have a less radiogenic lead isotope composition than sediment-hosted deposits, while galena in quartz segregations and crack fillings shows the most pronounced radiogenic lead isotope composition. Application of the plumbotectonic model to the lead isotope data for the stratiform deposits suggests that the volcanite-hosted deposits show the most significant influence of a mantle-related source.--Modified journal abstract.
"Mineralogical and lead isotopic control of polymetallic quartz veins, southwestern Sweden." Geologiska Föreningen i Stockholm Förhandlingar, 114(4), pp. 450–451
Gold-quartz veins occurring in the Mjosa-Vanern ore district, southeast Norway and southwest Sweden, represent early Neoproterozoic members of the orogenic gold type of deposit. The Harnas gold-quartz veins, in the central part of the ore district, are steeply dipping veins hosted in a local, west-northwest–east-southeast–trending brittle shear zone, which transects the north-south–trending deformational fabric in the surrounding greenschist grade orthogneisses. This deformation and subsequent vein formation occurred at around 1.0 Ga in a late phase of the Sveconorwegian (Grenvillian) orogeny. Fluid inclusions show that the ore-bearing vein system at Harnas developed essentially in three successive stages: a quartz stage at a depth of ≈ 4 km, a pyrite-gold stage at a shallower crustal level (≈1.5 km) after rapid exhumation of the area, and finally a galena stage. All stages involved fracturing subparallel to the strike of the host shear zone. During the first two stages, the ore fluid was an aqueous H 2 O-CO 2 fluid with a salinity of 4 to 10 wt percent NaCl equiv and a temperature of ≈200°C, whereas in the galena stage it was a purely aqueous fluid with a similar salinity and a temperature of ≈150°C. Oxygen and sulfur isotope results imply a predominantly metamorphic origin for the ore fluid and suggest that important ore constituents, such as lead and sulfur, were derived from the regional orthogneisses. Other gold-anomalous quartz veins in the Harnas area, as well as the Brustad gold-quartz vein in the northernmost part of the Mjosa-Vanern ore district, show some variation in fluid composition. However, aqueous fluid inclusions containing CO 2 and calcite were identified in all veins. This, and other similarities, strongly suggests that the veins throughout the district were formed contemporaneously and were controlled by deformation that, at least in part, affected the entire Mjosa-Vanern region. It is inferred from geologic evidence and pressure estimates that veins began to form during the final phase of Sveconorwegian continent-continent collision and were completed during incipient rapid exhumation of the thickened crust. A set of barren quartz-calcite veinlets, which crosscut the ore-bearing veins at Harnas, is unrelated to the ore formation. These veinlets were deposited from a surface-derived, low-temperature, saline aqueous fluid during some significantly later, but regionally extensive, hydrothermal event.
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