Isabel Fanlo

The evolution of the Lorraine evaporite basin: implications for the chemical and isotope composition of the Triassic ocean

Abstract During the Upper Triassic times about 75 m of halite with minor anhydrite was deposited in the Lorraine basin. Samples of halite and sulfates were systematically collected from a borehole intersecting the sequence. The chemical evolution of the original brine was investigated by X-ray microanalysis of primary inclusions trapped in halite. The original brines were of the Na–Mg–K–Ca–Cl type, being consistent with the mineral association of halite and minor anhydrite. Their solute content can be reproduced by evaporation of present day ocean composition in an open basin. However, some modifications are required. The inflow of a small excess of CaCl 2 into the basin (1.2 to 1.9% of the total recharge) is required to explain the low SO 4 values analyzed in fluid inclusions. The same inflow helps to explain more satisfactorily the low Na and high Cl contents. The back-reaction of the brine with previous Ca-sulfate that forms polyhalite could explain the low and variable K values analyzed. The excess of Ca with respect to present day seawater, and the formation of SO 4 -poor evaporite sequences is the rule rather than the exception in the geological record. We have not been able to assess the ultimate origin of the Ca excess in the basin. Nevertheless, the great variation of the SO 4 content within short vertical distances (centimeter scale), suggests that these variations are due to processes taking place within the evaporite basin, rather than to changes in the composition of the ocean at a global scale. The isotopic composition of sulfur and oxygen was determined using samples of sulfates from the same borehole. The δ 34 S values of 21 samples analyzed show a narrow variation: δ 34 S=+15.5±0.4‰. The δ 18 O values of eight samples analyzed show a wider range of variation: δ 18 O=+13.0±1.1‰. None of the processes which modify the δ values, such as sulfate reduction or additional sulfate sources (continental, recycling of sulfates from the margins), were clearly identified. The values of δ 34 S SW =+15.7 to +14.5‰ and δ 18 O SW =+13.7 to +12.2‰ for the sulfate dissolved in the Triassic ocean explain the δ values of the sulfates interbedded in the halite of the Lorraine basin, and most analyses reported in the literature.

A model for the diagenetic formation of sandstone-hosted copper deposits in Tertiary sedimentary rocks, Aragón (NE Spain): S/C ratios and sulphur isotope systematics

Abstract Numerous small, noneconomic copper deposits are associated with syntectonic sandstones of Tertiary age (Campodarbe and Uncastillo Formations) in the South Pyrenean foreland basin (SPFB). Sulphides, native metal, and their oxidation products occur within elongate lenses in fluvial sandstones and conglomerates. In all cases, the geometry of the ore is highly consistent with the primary depositional and biogenic structures of the host sandstones. Copper ores replace wood debris and diagenetic pyrite or, more frequently, appear as intergranular cement replacing previous diagenetic calcite cement. Sulphide sulphur and organic carbon distribution shows that pyrite-rich sandstones are the copper ore precursor, and that mineralizing processes provoked the depletion of both reduced S and organic C as a consequence of interaction with an oxidized Cu-bearing fluid. On the other hand, low δ34S values are consistent with bacteriogenic derivation of sulphur after Pyrenean evaporites. Petrographic studies, sulphide sulphur and organic carbon concentrations, and δ34S values suggest that Cu-rich fluids were introduced into reduced sediments during diagenesis. Mineralizing processes took place in two different periods: the first one related to the filling of the Jaca Basin during erosion of the Pyrenean basement and the second one related to the emplacement of a thrust ramp which initiated the erosion of the previously formed cupriferous sandstones.

Compositional effects on the solubility of minor and trace elements in oxide spinel minerals: Insights from crystal-crystal partition coefficients in chromite exsolution

Abstract Chromite from Los Congos and Los Guanacos in the Eastern Pampean Ranges of Córdoba (Argentinian Central Andes) shows homogenous and exsolution textures. The composition of the exsolved phases in chromite approaches the end-members of spinel (MgAl2O4; Spl) and magnetite (Fe2+Fe23+

X-ray photoelectron spectroscopy characterization of natural chromite from Mercedita Mine (Eastern Cuba): quantification of the Fe3+/Fe2+ ratio

{\text{Fe}}_2^{3 + }

Polymetallic VMS deposits of the Andes Fueguinos (southernmost Argentina): Preliminary report

O4; Mag) that define the corners of the spinel prism at relatively constant Cr3+/R3+ ratio (where R3+ is Cr+Al+Fe3+). The exsolution of these phases from the original chromite is estimated to have accounted at ≥600 °C on the basis of the major element compositions of chromite with homogenous and exsolution textures that are in equilibrium with forsterite-rich olivine (Fo95). The relatively large size of the exsolved phases in chromite (up to ca. 200 μm) provided, for the first time, the ability to conduct in situ analysis with laser ablation-inductively coupled plasma-mass spectrometry for a suite of minor and trace elements to constrain their crystal-crystal partition coefficient between the spinel-rich and magnetite-rich phases (DiSpl/Mag

Geochemical, and stable and radiogenic isotope records in Devonian and Early Carboniferous carbonates from Valle de Tena, central Pyrenees (Spain): evidence for their diagenetic environments

D_{\text{i}}^{{\text{Spl/Mag}}}

Simple deposition versus replacement and re-equilibration at the Crescencia Ni-(Co-U) deposit (Central Pyrenees, Spain)

). Minor and trace elements listed in increasing order of compatibility with the spinel-rich phase are Ti, Sc, Ni, V, Ge, Mn, Cu, Sn, Co, Ga, and Zn. DiSpl/Mag

Unusual PGE concentration in early disulfides of a low-temperature hydrothermal Cu-Ni-Co-Au deposit at Villamanin (Leon, northern Spain)

D_{\text{i}}^{{\text{Spl/Mag}}}

Formation of ferrian chromite in podiform chromitites from the Golyamo Kamenyane serpentinite, Eastern Rhodopes, SE Bulgaria: a two-stage process

values span more than an order of magnitude, from DTiSpl/Mag

Fingerprints of metamorphism in chromite: New insights from minor and trace elements

D_{{\text{Ti}}}^{{\text{Spl/Mag}}}