J. C. Melgarejo

Methane-related carbonates formed at submarine hydrothermal springs: a new setting for microbially-derived carbonates?

Abstract Numerous small calcite mounds, up to 2.5 m in diameter and 0.75 m in height, accompanied by Ba, Hg and Tl mineralisation, occur in shallow submarine hydrothermal vents on the sea bottom, at 10 m depth, near Punta Mita, on the western coast of Mexico. The hydrothermal activity consists in water and gas (mainly nitrogen and methane) venting at 85°C, through a 100-m-long fissure hosted in basaltic rocks and partially covered by a thin layer of unconsolidated detrital sediments. The mounds consist of travertine-like calcite aggregates that develop around the main submarine hot springs amidst a hydrothermally altered basaltic host rock. Two main calcite generations are texturally recognisable: the first generation shows a radial-fibrous texture; the second is fine-grained calcite, which cements detrital grains and fills the pore spaces. The δ 13 C analyses of calcite reveal a strong depletion in 13 C, with values as low as −39.2‰ (Vienna PeeDee Belemnite), which suggest that microbial communities may have induced calcite precipitation through microbial methane oxidation. Barite, sulphides (mainly pyrite and cinnabar) and phosphates (carbonate–hydroxylapatite) are also present in the mounds in lower concentrations and form by direct precipitation from the hydrothermal fluid. The Punta Mita hydrothermal carbonate mounds represent a potentially novel environment for microbially induced carbonate mineralisation, which is characterised by high temperatures not encountered in areas of cold seep carbonate formation. Stable isotope results suggest that microorganisms responsible for the oxidation of methane may be present and active at temperatures near 85°C at the Punta Mita vents.

D, O and C isotopes in podiform chromitites as fluid tracers for hydrothermal alteration processes of the Mayari-Baracoa Ophiolitic Belt, eastern Cuba

The Mayari — Baracoa Ophiolitic Belt (MBOB, eastern Cuba) is composed of two large, chromite-rich massifs: Mayari —Cristaland Moa-Baracoa. The chromitites and hosting dunites were firstly affected by a regional serpentinization event, a subsequent episode of hydrothennal alteration (chloritization mainly) and, finally, these already altered bodies were crosscut by thin calcite-dominated veins. Analysed serpentines from serpentinized chromitites and dunites present very similar isotopic compositions (δ18O=+4.7 to +6.3 and δD= −67 to −60 , suggesting that the serpentinization process took place at moderate temperatures, in an oceanic environment. Serpentine formation by interaction with ocean water is also supported by the isotopic composition of chlorite and calcite. These results suggest that the serpentinization, chloritization and fracture filling processes of the Mayari — Baracoa Ophiolite Belt took place in a subocean floor scenario and, thus, that the Mayari — Baracoa serpentines represent a good example of serpentine formed during interaction with seawater. The oceanic origin of the serpentines from serpentinized chromitites and dunites from the MBOB indicate that the serpentinization of the mantle sequence occurred pre-thrusting (pre-emplacement in age).

Stratigraphy of Lower Cambrian and unconformable Lower Carboniferous beds from the Valls unit (Catalonian Coastal Ranges)

The Palaeozoic rocks outcropping around Valls are divided into two stratigraphic units. The boundary between both is an unconformity. The lower unit is composed by nearshore platform sediments and a Lower Cambrian age is indicated according to ichnotaxa content. The upper unit consists of pink nodular limestones and dark limestones, and it is followed by siliciclastic Culm Facies rocks. These limestones contain conodonts of the Uppermost Tournaisian at its base (anchoralis-latus Zone) and Lower Bashkirian (Namurian B) in the upper part. This condensed carbonate sequence was coeval with the thick siliciclastic Culm sedimentation in the surrounding areas.

Sulfur isotope geochemistry of the submarine hydrothermalcoastal vents of Punta Mita, Mexico

Abstract Coastal hydrothermal submarine vents occur near Punta Mita (Mexico) through a seafloor fissure hosted by basaltic rocks and partially covered by recent sediments. Hydrothermal venting produces calcareous tufa mounds, with minor sulfides, barite and apatite. Pyrite is the most abundant sulfide mineral that precipitates around the submarine hot springs. It occurs as framboids, euhedral crystals, thin botryoidal coatings interbedded with calcite, and as replacement of detrital magnetite grains. Vent fluids have temperature below 100 °C, and methane is abundant. Sulfur isotopic composition of pyrite predominantly has δ 34 S values from − 10.7 to +4.9 suggesting that the source of sulfur is microbially reduced sulfate seawater.

Source of ore-forming fluids in El Cobre VHMSdeposit (Cuba): evidence from fluid inclusions and sulfur isotopes

Abstract The El Cobre deposit, east of Cuba, lies in the intermediate volcanosedimentary sequence of the Sierra Maestra intraoceanic island arc. The structure of the deposit corresponds to that of a volcanogenic-hosted massive sulfide (VHMS) model. It comprises (a) thick stratiform bodies (baryte and anhydrite), (b) three stratabound bodies (formed by silicification and sulfidation of limestones or sulfate strata), (c) stockwork zones, an older anhydrite stockwork and a younger quartz-pyrite stockwork grading downwards to (d) simple veins (quartz with sulfide ores). Pyrite, chalcopyrite and sphalerite are the most abundant sulfides. Fluid inclusions from this deposit have a salinity between 2.3 and 5.7 wt.% NaCl eq., homogenization temperatures range between 177 and 300 °C. Sulfur exhibits a range of δ 14 S values from − 1.4 to +7.3 for sulfides and from + 16 to +21 for sulfates. Fluid inclusions and sulfur isotope data at El Cobre deposit indicate that the hydrothermal fluid from which the sulfide precipitated was seawater, modified by reaction with volcanic host rocks during hydrothermal circulation.

Stable isotope geochemistry of the Carboniferous Zn-Pb-Cu sediment-hosted sulfide deposits, Northeastern Spain

Carboniferous sedimentary rocks in the southwestern Catalonian Coastal Ranges contain stratiform (Zn, Pb, Cu) sulfide deposits. Stable isotope compositions of sulfur, oxygen, and hydrogen were studied in five localities in order to establish the source of hydrothermal fluids and the influence of metamorphism on these deposits. Sulfur isotopes were analyzed in pyrrhotite, pyrite, sphalerite, chalcopyrite, and galena. The δ34S(CDT) values lie mostly between -1.0 and +7.0‰ (average +4.4‰). Except for pyrrhotite-pyrite pairs, all the sulfide minerals show isotopic disequilibrium. δ34S values suggest that sulfur was derived by thermochemical reduction of seawater sulfate in a deep-circulating convective system. Fluid inclusions were studied in quartz crystals from the sediment-hosted stratiform occurrences. They are two-phase (L-V), with salinity between 1.1 and 18.0 wt% NaCl eq., and homogenization temperatures mainly in the range of 220-260°C. Oxygen and hydrogen isotopes were analyzed in chlorite crystals from four ore-bearing beds. δD(V-SMOW) values in chlorite range from -63 to -33‰, whereas δ18O values lie between +2.7 and +6.8‰. δ18O(V-SMOW) values in quartz range between +8.4 and +10.7‰. Most of the calculated oxygen and hydrogen isotope compositions of ore-forming fluids plot close to the isotopic composition of seawater. Therefore, the ore-forming fluid must have been mainly seawater that interacted with sedimentary rocks during convective circulation within a basin. This process took place in a subsiding Carboniferous basin divided into sedimentary highs and depressions by synsedimentary normal faults, which acted as permeable areas that favored fluid circulation.

Fluid evolution in the beryl-columbite-phosphate pegmatitesof Cap de Creus (Catalonia, Spain)

Abstract In the Cap de Creus peninsula (NE Spain), four granite pegmatite types are distinguished: microcline-rich; beryl- columbite;beryl- columbite-phosphate; and albite. Beryl -columbite-phosphate pegmatites are zoned, with border, wall, intermediate zones and a quartz core. In addition, albitic and quartz-muscovite replacement bodies occur. The intermediate zones are mainly constituted by quartz, microcline, albite, beryl, Nb Ta minerals and phosphates. In early stages, crystallization takes place from a volatile-saturated magma and a hypersaline (39–46 wt.% NaCl eq.), CO 2 -rich fluid exsolved from the magma. In the last stages of the crystallization of the intermediate zones, this fluid produced unmixing in two fluids: one CO 2 -rich, salts-poor (3–5 wt.% NaCl eq.) and the other saline (up to 34 wt.% NaCl eq.) and CO 2 -poor. The first fluid formed a quartz core at 430 °C and 2.7 kbar. The second fluid circulated through all the pegmatite due to the development of a fracture network produced by a hydraulic fracturation. This fluid replaced the previous zones. In addition, crystallization of fine-grained albitites (Ta P-rich) and late quartz-muscovite veins takes place in the fractures. The fluids that formed these veins are more saline than those from the albitites, with 36–38 wt.% NaCl eq. and with CO 2 and minor N 2 .