Khanh Thien gold-ore-magmatic cluster (North Vietnam)

New data on the gold mineralization related of the Late Paleozoic alkaline complexes of North Vietnam are presented. Two types of gold mineralization associated with syenites and granosyenites have been identified: gold only and gold-polymetallic. Geochemical characteristics are given for both types and the first data on the sulfur isotopic composition of ore minerals.

Coarse-crystalline pyrite of the Central Timan

The results of mineralogical, geochemical, X-ray, isotopic and spectroscopic studies of coarse-crystalline pyrite (Kyvvozh Formation, Dimtem’el Creek, Central Timan) are presented. Pyrite is characterized by cubic habit and zonal Co distribution. It contains galena, thorite, zircon, monazite, rutile, chlorite, muscovite, quartz, albite, apatite and calcite inclusions. The unit cell parameter of pyrite vary from 5.4137 ± 0.0002 to 5.4187 ± 0.0010 A and correspond to an ideal pyrite. The sulfur isotopic composition of coarse-crystalline pyrite of 15.8‰ indicates its formation as a result of sulfate reduction during epigenesis.

The Carbon-Sulfur Link in the Remineralization of Organic Carbon in Surface Sediments

Here we present the carbon isotopic composition of dissolved inorganic carbon (DIC) and the sulfur isotopic composition of sulfate, along with changes in sulfate concentrations, of the pore fluid collected from a series of sediment cores located along a depth transect on the Iberian Margin. We use these data to explore the coupling of microbial sulfate reduction (MSR) to organic carbon oxidation in the uppermost (up to nine meters) sediment. We argue that the combined use of the carbon and sulfur isotopic composition, of DIC and sulfate respectively, in sedimentary pore fluids, viewed through a δ13CDIC vs. δ34SSO4 cross plot, reveals significant insight into the nature of carbon-sulfur coupling in marine sedimentary pore fluids on continental margins. Our data show systemic changes in the carbon and sulfur isotopic composition of DIC and sulfate (respectively) where, at all sites, the carbon isotopic composition of the DIC decreases before the sulfur isotopic composition of sulfate increases. We compare our results to global data and show that this behavior persists over a range of sediment types, locations and water depths. We use a reactive-transport model to show how changes in the amount of DIC in seawater, the carbon isotopic composition of organic matter, the amount of organic carbon oxidation by early diagenetic reactions, and the presence and source of methane influence the carbon and sulfur isotopic composition of sedimentary pore fluids and the shape of the δ13CDIC vs. δ34SSO4 cross plot. The δ13C of the DIC released during sulfate reduction and sulfate-driven anaerobic oxidation of methane is a major control on the minimum δ13CDIC value in the δ13CDIC vs. δ34SSO4 cross plot, with the δ13C of the organic carbon being important during both MSR and combined sulfate reduction, sulfate-driven AOM and methanogenesis.

Microbial sulfur cycling during the formation of Primary Lower Gypsum in Mediterranean marginals basins

<p>During the first phase of the Messinian Salinity Crisis, massive amounts of sulfate (SO<sub>4</sub><sup>2-</sup>) have been sequestred in the form of up to 200m thick gypsum deposits (Primary Lower Gypsum) in Mediterranean marginal basins. The sulfur isotopic composition of the sulfate ion of this unit (δ<sup>34</sup>S<sub>SO4</sub>) (on average 22.3 ‰) strongly suggests that gypsum was formed by concentration of marine sulfate. Interestingly, the preservation of sulfide globules within the gypsum and marls interbeds suggests that the basin sulfate was not only involved in gypsum formation but a fraction was also reduced through microbial sulfate reduction. Moreover, filamentous fossils interpreted to be the remnants of sulfide oxidizing bacterias are entrapped in this gypsum and indicate, together with the occurrence of sulfide globules and dolomite, that an active biogeochemical sulfur cycling was active at the time of Primary Lower Gypsum deposition. To investigate the role of this active sulfur cycling in Mediterranean marginal basins, we analyzed the multiple sulfur isotopic composition of sulfate and sulfide minerals (δ<sup>34</sup>S andΔ<sup>33</sup>S)<sub></sub>from Primary Lower Gypsum of the Vena del Gesso basin (Italy). Whereas the isotopic composition of gypsum (δ<sup>34</sup>S<sub>SO4 </sub>from 21 to 24‰ and Δ<sup>33</sup>S<sub>SO4 </sub>from -0.001 to 0.049‰) display very homogenous values that are close to those of the Messinian ocean (δ<sup>34</sup>S<sub>MSC </sub>~22±0.2‰ and Δ<sup>33</sup>S<sub>MSC</sub>~0.039±0.015), the analyzed reduced sulfur compounds display a wide range of variability  with -36 to +9‰ in δ<sup>34</sup>S and -0.017 to 0.125‰ in Δ<sup>33</sup>S. This suggests huge hydrologically-driven redox variations during Primary Lower Gypsum deposition in the Vena del Gesso basin, possibly involving intermittent stratification of the water column and an active microbial cycling of sulfur.</p>