I. Subías

Hydrothermal events in the Valle de Tena (Spanish Western Pyrenees) as evidenced by fluid inclusions and trace-element distribution from fluorite deposits

Abstract This paper provides further evidences from fluorite and calcite geochemistry and from fluid inclusion studies for the origin of the Valle de Tena fluorite deposits. The Valle de Tena, Spanish Western Pyrenees, contains two kinds of fluorite mineralizations: (1) vein-type deposits located in the vicinity of the Panticosa granite and enclosed by Devonian carbonate facies; and (2) carbonate-hosted mineralizations on highly silicified Lower Carboniferous limestones. Based on geological evidences the latter deposits can be referred to as Permo-Triassic. Microthermometrical measurements on two-phase, water-rich inclusions with high liquid to vapor ratios yield homogenization temperatures ranging from 100° to 200°C for the two deposit-types. Salinities range from 4.0 to 11.6 eq wt% NaCl for carbonate-hosted deposits and from 12.5 to 21.3 eq wt% NaCl for vein mineralizations. REE contents are variable and highest abundances are found in the fluorite from Lanuza and Tebarray (green flurite). These later fluorite mineralizations have primary REE patterns with positive Eu anomalies which possibly reflect the alteration of feldspar. The REE distribution patterns of fluorites and calcites from the carbonate-hosted mineralizations are different from those occurring as veins; the decrease of LREE in the former resulted in roof-shaped REE patterns. The negative Eu anomalies as well as the trace-element distribution allow us to deduce that the fluid from which these later Ca-minerals were formed was probably equilibrated with the sedimentary country rocks. The fluid inclusion, geochemical, geologic and preliminary stable isotope data support two temporally distinct hydrothermal systems accounting for the fluorite formation and suggest a sedimentary basin source for the ore fluids. High-salinity fluids (basement brines) are involved in the formation of the vein-type deposits, whereas the subsequent carbonate-hosted mineralizations were formed by mobilization of the fluorite veins by formation waters. The most likely fluorite deposition mechanism is cooling with minor contribution of fluid-rock interaction. In such context, magmatic activity is considered as heat source. The above mineralization events possibly span a time from Permian to Triassic which is in close agreement with the timing of hydrothermal processes documented throughout the Hercynian Europe.

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.

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

The polymetallic volcanic-hosted massive sulphide deposits of the Andes Fueguios occur in a highly deformed but coherent stratigraphic succession of Late Jurassic metavolcanic and metasedimentary rocks. Local stratigraphy consists of porphyries along with rhyolitic lava, acidic tuff, breccias, conglomerates, radiolarian lutites and basaltic rocks: Mineralization occurs as massive stratiform, massive replacement and sulphide stringer veins. Sulphides are typically massive, fine-grained, layered and locally brecciated andconsists of pyrite and sphalerite, with lesser pyrrhotite, chalcopyrite, galena, tetrahedrite-freibergite, bourmonite, cobaltite and arsenopyrite. Styles of hydrothermal alteration identified in the host rocks include proximal silicification and more distal chloritization, sericitization and, in places, carbonatization. Future research will be focused on identifying the salient physico-chemical controls on the mineralization process and their implications for volcanic-hosted massive sulphide exploration in the district.

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

Mineralogical, textural and geochemical investigations were made to determine the post-depositional evolution of Devonian and Early Carboniferous carbonates from Valle de Tena. The carbonate association is made up of low-Mg calcite, which occurs as micrite, spar cements, neomorphic patches and spar filling veinlets. Non-stoichiometric dolomite and ankerite occur as cements (dolomite also as replacements) in the Middle Devonian, post-dating calcite types. All these phases pre-date tectonic stylolites, indicating compaction after stabilization of the carbonate minerals. Strontium concentrations indicate that Early Devonian and Early Carboniferous micrites initially precipitated as aragonite; Middle and Late Devonian micrites precipitated as high-Mg calcites. Both precursors were diagenetically stabilized to low-Mg calcites through interaction with meteoric waters in phreatic environments. Trace elements in dolomite and ankerite indicate precipitation from Sr-enriched meteoric water. All studied carbonates, except Middle Devonian limestones, precipitated in reducing environments, which favoured incorporation of Fe and Mn. Late calcite generations precipitated from more saline waters than micrites. Light δ18O values in micrites suggest alteration mainly in meteoric-phreatic environments. The dolomites and ankerites precipitated from more 18O-depleted fluids than the calcites, suggesting a greater contribution from meteoric waters. Variations in δ13C of micrites represent primary secular trends, according to published δ13C variations. The δ13C oscillations within each succession probably relate to sea-level oscillations. Strontium isotopes also point to a meteoric origin of diagenetic fluids. Model calculations suggest that O and Sr isotopes equilibrated between calcites and fluid at relatively low water/rock ratios, whereas C isotopic signatures are inherited from limestones.

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

The mineral assemblage of the Crescencia mine contains Ni sulfarsenides, arsenides and diarsenides, uraninite and pyrite. Three stages of mineral deposition can be distinguished: Stage I: nickeline, gersdorffite I, and rammelsbergite; Stage II: gersdorffite II; Stage III: uraninite. Gersdorffite II has been subdivided into six groups on the basis of textural and compositional criteria. Some of these gersdorffite types are clearly pertained to disequilibrium processes. Gersdorffite B, C, E and F represent intermediate steps during the nickeline replacement and progressive re-equilibration with residual fluids, representing by gersdorffite D. Thus, compositional variations found in gersdorffite are the result of nickeline replacement and reequilibration processes rather than a direct precipitation under different conditions of the ore-forming fluids.

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

The early mineral assemblage of the Providencia mine in northern Spain includes metastable Cu-Ni pyrite-type disulfides with economic concentrations of Au and subeconomic concentrations of PGE in solid solution. High chondrite-normalized relations of Ir, Rh and Pt versus Pd, and the lack of Ru also characterize the ore. Our studies show that the ore-forming fluids were quasi-hydrostatic, low-temperature hydrothermal, and had a high salinity, and a high sulfur fugacity. The source of PGE remains unknown.