Use of in situ laser sulphate-sulphur isotope analyses to establish temperature of precipitation of pyrite-anhydrite in the Cameros Basin, NE Spain (original) (raw)
Related papers
Geochimica Et Cosmochimica Acta, 1999
We describe a system for the in situ sulfur isotope analysis of small (Ͼ100 m) anhydrite crystals and investigate its application to anhydrite-pyrite geothermometry. Anhydrite inclusions (Ͻ1 mm) have been analyzed in spectacular, museum quality pyrite crystals from the Mesozoic Cameros Basin in NE Spain. Some of the data yield isotopic equilibrium temperatures (367 Ϯ 6°C) consistent with other geothermometric estimates of metamorphic temperature. This suggests that isotopic equilibrium was established between the host pyrite and anhydrite inclusions and was not affected by re-equilibration. However, other data points yield anhydrite compositions consistently too 34 S-depleted, resulting in a geologically unrealistic temperature (610 Ϯ 20°C). Experiments show that where pyrite becomes overheated by the laser during anhydrite decomposition, solid phase reaction can incorporate pyrite-sulfur into the sampled gas in a stoichiometric fashion, therefore, the consistency of the erroneous temperature estimates from this group. Successful analyses are only obtained when overheating of the pyrite is avoided during laser decomposition of anhydrite inclusions. The laser system allows isotopic measurement of anhydrite inclusions too small to be analyzed conventionally.
Mineralium Deposita, 1998
The sulphide deposits of the Iberian Pyrite Belt (IPB) represent an ore province of global importance. Our study presents 113 new sulphur isotope analyses from deposits selected to represent the textural spectrum of ores. Measured d 34 S values range from À26 to 10& mostly for massive and stockwork ores, in agreement with data previously published. In situ laser d 34 S analyses reveals a close correlation of d 34 S with texture. Primary diagenetic textures are dominated by relatively low d 34 S (À8& to À2&), whereas stockwork feeder textures are dominated by higher d 34 S ($+3& to 5&). Intermediate textures (mainly coarse textures in stratiform zones) have intermediate d 34 S, although they are mostly dominated by the high d 34 S component. Rare barite has a homogeneous d 34 S around 18&, which is consistent with direct derivation from Lower Carboniferous seawater sulphate. A dual source of sulphide sulphur in the IPB deposits has been considered. A hydrothermal source, derived from reduction of coeval seawater sulphate in the convective systems, is represented by sulphide in the feeder zones. Here variations in d 34 S are caused by variations in the extent of the sulphate reduction, which governs the SO 4 :H 2 S ratio. The second end-member was derived from the bacterial reduction of coeval seawater sulphate at or near the surface, as re¯ected in the primary textures. A distinct geographical variation in d 34 S and texture from SW (more bacteriogenic and primary textures) to NE (more hydrothermal textures and d 34 S) which re¯ects a variation in the relative input of each source was likely controlled by local geological environments. Given that the sulphur isotope characteristics of the IPB deposits are unlike most VMS and Kuroko deposits, and noting the dominance of a mixed reduced sedimentary and volcanic environment, we suggest that the IPB could represent an ore style which is intermediate between volcanic and sedimentary hosted massive sulphide types.
2001
The low-grade metasediments of1the Cameros Basin, northem Spain, host a number of deposits of spectacular quality pyrite mineralization. These formed dunng regional metamorphism and the pyrite crystals exhibit a wide range of morphologies. On the basis of pyrite crystal habit, the deposits can be classified into two groups: Group 1 comprises deposits with cubic, elongated or platy crystals; Group II comprises deposits characterized by pyritohedra and cubo-pyritohedra with striated faces, along with blocky crystals and fine-grained aggregates. Group 1 deposits are formed in sequences dorninated by meandriform fluviatile sediments, while Group 11 is hosted by deltaic plain and lacustrine metasediments.
Mineralium Deposita, 2002
A fluid inclusion and stable isotopic study has been undertaken on some massive sulphide deposits (Aguas Ten˜idas Este, Concepcio´n, San Miguel, San Telmo and Cueva de la Mora) located in the northern Iberian Pyrite Belt. The isotopic analyses were mainly performed on quartz, chlorite, carbonate and whole rock samples from the stockworks and altered footwall zones of the deposits, and also on some fluid inclusion waters. Homogenization temperatures of fluid inclusions in quartz mostly range from 120 to 280°C. Salinity of most fluid inclusions ranges from 2 to 14 wt% NaCl equiv. A few cases with T h =80-110°C and salinity of 16-24 wt% NaCl equiv., have been also recognized. In addition, fluid inclusions from the Soloviejo Mn-Fejaspers (160-190°C and %6 wt% NaCl equiv.) and some Late to Post-Hercynian quartz veins (130-270°C and %4 wt% NaCl equiv.) were also studied. Isotopic results indicate that fluids in equilibrium with measured quartz (d 18 O fluid %-2 to 4&), chlorites (d 18 O fluid %8-14&, dD fluid %-45 to -27&), whole rocks (d 18 O fluid %4-7&, dD fluid %-15 to -10&), and carbonates (d 18 O ankerite %14.5-16&, d 13 C fluid =-11 to -5&) evolved isotopically during the lifetime of the hydrothermal systems, following a waxing/waning cycle at different temperatures and water/rock ratios. The results (fluid inclusions, d 18 O, dD and d 13 C values) point to a highly evolved seawater, along with a variable (but significant) contribution of other fluid reservoirs such as magmatic and/or deep metamorphic waters, as the most probable sources for the ore-forming fluids. These fluids interacted with the underlying volcanic and sedimentary rocks during convective circulation through the upper crust.
Chemical Geology, 2006
Sulfate δ 34 S and δ 18 O and Sr isotope compositions are presented for Berriasian (Upper Jurassic) gypsum evaporites from the Cameros Basin, a continental basin in the Iberian ranges of northern Spain. Solute sources to the ephemeral lakes in which the evaporites formed are dominated by weathering of sediments containing older Keuper marine evaporites and granitic/metamorphic Variscan basement. Strontium isotopic ratios of the continental evaporites (87 Sr/ 86 Sr = 0.707882 to 0.707933) are elevated compared to the Keuper source (87 Sr/ 86 Sr = 0.707605 to 0.707799), most likely reflecting a radiogenic component from basement lithologies. Sulfate δ 34 S in the continental evaporites (at around 18.2 ‰ V-CDT) is higher than any possible mixture of solutes to the basin. The degree of 34 S enrichment is small (≈ 3.7‰) and is most likely the result of partial bacterial reduction of lake-water sulfate. The same process would also enrich sulfate in 18 O, but by only ≈1‰. Evaporite sulfate δ 18 O (at around 21.7‰ V-SMOW) is, in fact, enriched by ≈ 10‰ relative to sulfate sources to the basin. This large effect is most likely the result of re-equilibration of oxygen isotopes during low grade metamorphism of the basinal sequence. Sulfur and strontium isotopes in the evaporites remained internally buffered and thus unchanged during metamorphism while oxygen isotopes were open to exchange with aqueous fluid and/or interbedded carbonates that constituted a large exchangeable oxygen reservoir. The potential of sulfate oxygen to undergo isotopic exchange at elevated temperatures must be taken into account when interpreting the significance of evaporite sulfate oxygen data from units that have undergone deep burial or low grade metamorphism.
Applied Geochemistry, 2000
Massive sulphide deposits of the northern Iberian Pyrite Belt (IPB) are mainly hosted by felsic volcanic rocks of rhyolitic to dacitic composition. Beneath most of the massive ores of this area (e.g., Concepcio n, San Miguel, Aguas TenÄ idas Este or San Telmo deposits) there is usually a wide hydrothermal alteration halo associated with stockwork-type mineralization. Within these alteration envelopes there are two principal rock types: (1) chlorite-rich rocks, linked to the inner and more intensely altered zones and dominantly comprising chlorite+pyrite+quartz+sericite (+carbonate+rutile+zircon+chalcopyrite), and (2) sericite-rich rocks, more common in the peripheral zones and showing a dominant paragenesis of sericite+quartz+pyrite+chlorite (+carbonate+rutile+zircon+sphalerite). Mass-balance calculations comparing altered and least-altered felsic volcanic rocks suggest that sericitization was accompanied by moderate enrichment in Mg, Fe and H 2 O, with depletion in Si, Na and K, and a slight net mass loss of about 3%. Chloritization shows an overall pattern which is similar to that of the sericitic alteration, but with large gains in Fe, Mg and H 2 O (and minor enrichment in Si, S and Mn), and a signi®cant loss of Na and K and a minor loss of Ca and Rb. However, chloritization has involved a much larger net mass change (mass gain of about 28%). Only a few elements such as Nb, Y, Zr, Ti, P and LREE appear to have remained inert during hydrothermal alteration, whilst Ti and Al have undergone very minor mobilization. The results point to the severity of the physico-chemical conditions that prevailed during the waxing stage of the ore-forming hydrothermal systems. Further, mineralogical and geochemical studies of the altered footwall rocks in the studied deposits indicate that hydrothermal ore-bearing¯uids reacted with host rocks in a multi-stage process which produced a succession of mineralogical and chemical changes as the temperature increased.
Economic Geology, 2022
The Iberian Pyrite Belt is a world-class metallogenic district developed at the Devonian-Carboniferous boundary in the Iberian Variscides that currently has seven active mines: Neves Corvo (Cu-Zn-Sn) and Aljustrel (Cu-Zn) in Portugal, and Riotinto (Cu), Las Cruces (Cu), Aguas Teñidas (Cu-Zn-Pb), Sotiel-Coronada (Cu-Zn-Pb), and La Magdalena (Cu-Zn-Pb) in Spain. The Iberian Pyrite Belt massive sulfide ores are usually hosted in the lower sections of the volcano-sedimentary complex (late Famennian to late Visean), but they also occur in the uppermost levels of the phyllite-quartzite group at the Neves Corvo deposit, stratigraphically below the volcano-sedimentary complex. A Pb-Nd-Sr isotope dataset was obtained for 98 Iberian Pyrite Belt metapelite samples (from Givetian to upper Visean), representing several phyllite-quartzite group and volcano-sedimentary complex sections that include the footwall and hanging-wall domains of ore horizons at the Neves Corvo, Aljustrel, and Lousal mine...
Equilibrium multiple sulfur isotope fractionation factors (33 S/ 32 S and 34 S/ 32 S) between aqueous SO 4 , H 2 S, and coexisting pyrite under hydrothermal conditions were determined experimentally at 300–350 °C and 500 bars. Two different experimental techniques were used to determine the fractionation factors and the rate of S isotope exchange between pyrite and constituent aqueous species, H 2 S and SO 4 ; (1) closed system gold capsule pyriteÀH 2 S exchange experiments and (2) complimentary time-series experiments at 300 and 350 °C, 500 bars using flexible gold cell hydrothermal equipment, which allowed monitoring the multiple S isotope composition of dissolved S species during pyrite precipitation and subsequent recrystallization. The three isotope technique was applied to the multiple S isotope data to demonstrate equilibrium S isotope fractionation between pyrite and H 2 S. Results at 350 °C indicate ln 34 a Pyrite=H 2 S = À1.9& and ln 33 a Pyrite=H 2 S = À1.0&. The ln 34 a Pyrite=H 2 S is not only different in magnitude but also in sign from the commonly used value of 1& from Ohmoto and Rye (1979). This experimental study also demonstrated initial S isotope disequilibrium amongst the aqueous S-species and pyrite during rapid precipitation, despite aqueous speciation indicating pyrite saturation at all stages. Textural, crystallographic , and S isotope interpretations suggest that pyrite formed by means of the FeS pathway. The initial S isotope disequi-librium between formed pyrite and dissolved S-species was effectively erased and approached isotopic equilibrium upon recrystallization during the course of 4297 h. Interpretation of seafloor hydrothermal vent sulfides using the revised equilibrium 34 S/ 32 S fractionation between pyrite and H 2 S suggests that pyrite is close to S isotope equilibrium with vent H 2 S, contrary to previous conclusions. The experimental data reported here broaden the range of pyrite formation mechanisms at seafloor hydrothermal vents, in that mineral formation pathway and equilibration rates need to be considered to account for the well-recognized S isotope variability that often characterizes these systems. Published by Elsevier Ltd.
Contributions to Mineralogy and Petrology, 1988
Smectite, illite, celadonite and chlorite are the major products of alteration of rhyolites and andesites, in the upper part of the Los Azufres geothermal system. Changes in mineral assemblages and composition of phases are observed as a function of depth and host rock lithology. Two different sequences characterize the rhyolites and the andesites from the surface to a depth of about 1500 m: kaolinite-, smectite (4-_ interlayered illite/smectite) ~ illite ~ muscovite (rhyolites), and kaolinite~smectite (+_interlayered illite/smectite) ~illite + celadonite-*illite + chlorite-, chlorite (andesites). Illite, and chlorite at depth, are largely dominant. Similar substitutions and correlations among chemical constituents characterize illites from rhyolites or andesites, but their compositions in the two host rock lithologies exhibit slight but significant differences, especially in their Fe and Mg contents which are the highest in illites from andesites. Illite exhibits progressive changes in composition with depth: a strong increase in the K content in the interlayer, together with an increase of the Fe content in the octahedral site. These changes correspond to a slight increase in the molar fraction of Fe-(Mg) celadonite end-members, and mostly to a dramatic decrease of pyrophyllite with increasing temperature. Temperature of the mineralogical and compositional changes was estimated from fluid inclusions studies, combined with other geothermometric approaches (chemical geothermometers and direct measurements). Variation of X-pyrophyllite with temperature is proposed as a geothermometer for different host rock lithologies. Transitions between the stability fields of illite 4-interstratified illite-smectire and illite + chlorite is around 200_ 30 ~ C, and between illite + chlorite and chlorite around 290 • 20 ~ C.