Iron isotopes in acid mine waters and iron-rich solids from the Tinto–Odiel Basin (Iberian Pyrite Belt, Southwest Spain)

The isotopic composition of Fe was determined in water, Fe-oxides and sulfides from the Tinto and Odiel Basins (South West Spain). As a consequence of sulfide oxidation in mine tailings both rivers are acidic (1.45 < pH < 3.85) and display high concentrations of dissolved Fe (up to 420 mmol l^− 1) and sulphates (up to 1190 mmol l^− 1).The δ⁵⁶Fe of pyrite-rich samples from the Rio Tinto and from the Tharsis mine ranged from − 0.56 ± 0.08‰ to + 0.25 ± 0.1‰. δ⁵⁶Fe values for Fe-oxides precipitates that currently form in the riverbed varied from − 1.98 ± 0.10‰ to 1.57 ± 0.08‰. Comparatively narrower ranges of values (− 0.18 ± 0.08‰ and + 0.21 ± 0.14‰) were observed in their fossil analogues from the Pliocene–Pleistocene and in samples from the Gossan (the oxidized layer that formed through exposure to oxygen of the massive sulfide deposits) (− 0.36 ± 0.12‰ to 0.82 ± 0.07‰). In water, δ⁵⁶Fe values ranged from − 1.76 ± 0.10‰ to + 0.43 ± 0.05‰.At the source of the Tinto River, fractionation between aqueous Fe(III) and pyrite from the tailings was less than would be expected from a simple pyrite oxidation process. Similarly, the isotopic composition of Gossan oxides and that of pyrite was different from what would be expected from pyrite oxidation. In rivers, the precipitation of Fe-oxides (mainly jarosite and schwertmannite and lesser amounts of goethite) from water containing mainly (more than 99%) Fe(III) with concentrations up to 372 mmol l^− 1 causes variable fractionation between the solid and the aqueous phase (− 0.98‰ < Δ⁵⁶Fe_{solid–water} < 2.25‰). The significant magnitude of the positive fractionation factor observed in several Fe(III) dominated water may be related to the precipitation of Fe(III) sulphates containing phases.     

Trace elements contamination of agricultural soils affected by sulphide exploitation (Iberian Pyrite Belt,Sw Spain)

Agricultural soils of the Riotinto mining area (Iberian Pyrite Belt) have been studied to assess the degree of pollution by trace elements as a consequence of the extraction and treatment of sulphides. Fifteen soil samples were collected and analysed by ICP-OES and INAA for 51 elements. Chemical analyses showed an As–Cu–Pb–Zn association related with the mineralisation of the Iberian Pyrite Belt. Concentrations were 19–994 mg kg⁻¹ for As, 41–4,890 mg kg⁻¹ for Pb, 95–897 mg kg⁻¹ for Zn and of 27–1,160 mg kg⁻¹ for Cu. Most of the samples displayed concentrations of these elements higher than the 90th percentile of the corresponding geological dominium, which suggests an anthropogenic input besides the bedrock influence. Samples collected from sediments were more contaminated than leptosols because they were polluted by leachates or by mining spills coming from the waste rock piles. The weathering of the bedrock is responsible for high concentrations in Co, Cr and Ni, but an anthropogenic input, such as wind-blown dust, seems to be indicative of the high content of As, Cu, Pb and Zn in leptosols. The metal partitioning patterns show that most trace elements are associated with Fe amorphous oxy-hydroxides, or take part of the residual fraction. According to the results obtained, the following mobility sequence is proposed for major and minor elements: Mn, Pb, Cd, > Zn, Cu > Ni > As > Fe > Cr. The high mobility of Pb, Cu and Zn involve an environmental risk in this area, even in soils where the concentrations are not so high.     

Acid mine drainage and acid rock drainage processes in the environment of Herrerías Mine (Iberian Pyrite Belt, Huelva-Spain) and impact on the Andevalo Dam

The present work describes the process of acid water discharge into the Andévalo Dam (Iberian Pyrite Belt, Huelva-Spain) starting from the interpretation of rainfall data and chemical analyses regarding pH, conductivity, metal and sulphate content in water, from a time series corresponding to the sampling of two confluent channels that discharge water into the referred dam. Statistical data treatment allows us to conclude the existence of acid mine drainage processes in the Chorrito Stream, which are translated into very low pH values and high sulphate and metal concentrations in the water coming from Herrerías Mine. On the other hand, the Higuereta Stream shows, for the same parameters, much lower values that can be interpreted as the channel response to acid rock drainage processes in its drainage basin induced by the rocky outcrops of the Iberian Pyrite Belt.     

Interaction of acid mine drainage with waters and sediments at the Corona stream, Lousal mine (Iberian Pyrite Belt, Southern Portugal)

This study investigates the geochemical characteristics of the acid mine drainage discharged from the abandoned mine adits and tailing piles in the vicinity of the Lousal mine and evaluates the extent of pollution on water and on the stream sediments of the Corona stream. Atmospheric precipitation interacting with sulphide minerals in exposed tailings produces runoff water with pH values as low as 1.9–2.9 and high concentrations of (9,249–20,700 mg l⁻¹), Fe (959–4,830 mg l⁻¹) and Al (136–624 mg l⁻¹). The acidic effluents and mixed stream water carry elevated Cu, Pb, Zn, Cd and As concentrations that exceed the water quality standards. However, the severity of contamination generally decreases 4 km downstream of the source due to mixing with fresh waters, which causes the dilution of dissolved toxic metals and neutralization of acidity. Some natural attenuation of the contaminants also occurs due to the general reduced solubility of most trace metals, which may be removed from solution, by either co-precipitation or adsorption to the iron and aluminium precipitates.     

Stratigraphy of the tectonically imbricated lithological succession of the Neves Corvo mine area,Iberian Pyrite Belt,Portugal

Biostratigraphic research, based on palynomorphs and ammonoids, of the tectonically imbricated lithological succession of the Neves Corvo mine, in the Portuguese part of the Iberian Pyrite Belt, has yielded ages for all formerly recognised lithostratigraphic units. These can be assembled in three main lithological sequences: (1) detrital sandy/shale substrate (Phyllite-Quartzite Formation) of late Famennian age; (2) Volcano-Sedimentary Complex, divided into a lower and an upper suite, in which one basic, three dolerite sills and four felsic volcanic units and a mineralised package of massive sulphides are identified with ages which range from the late Famennian to the late Visean; (3) flysch succession (Mértola Formation) composed of shale and greywacke dated as late Visean to early Serpukhovian. Precise biostratigraphic dating of the sedimentrary host rocks of massive sulphide mineralisation constrains the age of the latter to the late Strunian (354.8–354.0 Ma). Three stratigraphic hiatuses, corresponding to early/middle Strunian, Tournaisian and early Visean respectively and a south-westward progressive unconformity were also recognised. Sequences 1 and 2 are related to extensional episodes while sequence 3 marks the beginning of compressive tectonic inversion which gave rise to south-westward flysch progradation in close relation to a foreland basin development. Our results lead to the reinterpretation of the tectonic structure of the Neves Corvo mine, with implications for the interpretation of the regional basin dynamics and metal exploration.Editorial handling: F. Tornos     

Chemical characteristics of groundwater around two massive sulphide deposits in an area of previous mining contamination, Iberian Pyrite Belt, Spain

A detailed chemical study of groundwater was carried out to elucidate the processes controlling the oxidation and dissolution of sulphide minerals at two massive sulphide deposits in the Iberian Pyrite Belt (IPB), i.e. the mined La Zarza deposit and the unmined Masa Valverde deposit. It was found that major-element compositions varied according to the hydrological regime, La Zarza being in a relatively high area with groundwater recharge (and disturbance due to the human factor) and Masa Valverde being in a relatively low area with groundwater discharge. The variations mainly concern pH, Eh, SO₄ and Na concentrations. Metal concentrations were determined (a) by ICP-MS after filtration, and (b) in some cases by voltammetric measurement of Cu, Pb, Zn, Cd and Mn using the Voltammetric In situ Profiling (VIP) System, which allows detection of only the mobile fractions of trace elements (i.e., free metal ions and small labile complexes a few nanometers in size). If one compares the results obtained by each of the two methods, it would appear that the groundwater shows significant enhancement of metal solubility through complexing with organic matter and/or adsorption onto colloids and/or small particles. In areas of sulphide oxidation, however, this solubility enhancement decreases according to Cu>Zn>Cd>Pb. Under very low redox conditions, the attained metal concentrations can be several orders of magnitude (up to 10⁸–10⁹ for Cu and 10²–10³ for Pb) larger than those expected from equilibrium with respect to sulphide minerals as calculated with the EQ3NR geochemical code; Zn concentrations, however, are close to equilibrium with respect to sphalerite. The implication of these results is discussed with respect both to mineral exploration and to environmental issues.     

Mineralogy and environmental relevance of AMD-precipitates from the Tharsis mines,Iberian Pyrite Belt (SW,Spain)

This paper documents the solid phases associated to acid mine drainage (AMD) at the Tharsis mines (SW Spain). It provides an inventory of the AMD-precipitates, describing their main modes of occurrence and mineral assemblages. Results indicate that iron, aluminum and magnesium sulfates predominate in the assemblages. They occur as efflorescences composed of complex mixtures of metallic salts, and as ochres (jarosite combined with goethite). Also, their distributions illustrate two hydrochemical environments: the open pits, which reflect a proximal secondary paragenesis; and the downstream river banks (Meca River), which represent a more evolved paragenesis, resulting from the evolution of AMD produced throughout the system. These environments can be differentiated by composition and variety of minerals, which is considerably lower along the Meca River.The newly-formed minerals have monitoring significance and proved capable of participating in cycles of retention–liberation of hydrogen ions, sulfate, and metals. In a semi-arid climate, the importance of the AMD-precipitates as environmental indicators is stressed. They may help to understand the response of the system to the episodic rainfall events that occur after prolonged dry periods.     

Facies analyses and volcanic setting of the giant Neves Corvo massive sulfide deposit,Iberian Pyrite Belt,Portugal

In the Iberian Pyrite Belt, volcanic rocks are relatively scarce, accounting for approximately only 25% of the geologic record, with the remaining 75% consisting of sedimentary units. This association is very clear in the host succession to the Neves Corvo massive sulfide deposit in Portugal. The Neves Corvo host succession comprises the products of explosive and effusive rhyolitic eruptions intercalated with mudstone that records a submarine below-wave-base environment and provides precise biostratigraphic age constraints. The first and second volcanic events involved eruptions at local intrabasinal vents. The first event generated thick beds of fiamme breccia that are late Famennian in age. The fiamme were originally pumice clasts produced by explosive eruptions and were subsequently compacted. The second event was the late Strunian (latest Famennian) effusion of rhyolitic lava that was pervasively quench-fragmented. The third and final event is younger than the massive sulfide deposits poorly represented in the mine area and minor compared with the two other events. The integration of biostratigraphic data with the volcanic facies architecture indicates that the Neves Corvo ore deposits are similar in age to the late Strunian rhyolitic lava. Although regionally the Iberian Pyrite Belt is essentially a sedimentary succession, ore formation at Neves Corvo can be closely linked to discrete volcanic events that produced a relatively narrow range of volcanic facies.     

Geochemistry and geothermometry of volcanic rocks from Serra Branca, Iberian Pyrite Belt, Portugal

Volcanic rocks from Serra Branca, Iberian Pyrite Belt, Portugal, consist of calc-alkaline felsic and intermediate rocks. The latter are massive andesites, whereas the former include four dacitic to rhyolitic lithologies, distinguishable on spiderdiagrams and binary plots of immobile elements. Zircon thermometry indicates that two felsic suites may have formed from different magmas produced at distinct temperatures, with only limited fractionation within each suite. Alternatively, all the felsic rocks can be related through fractionation of a single magma if the lower zircon saturation temperature obtained for one suite merely results from Zr dilution, mostly reflecting silicification.

The relatively high magma temperatures at Serra Branca ease the classification of felsic rocks based on their HFSE contents and also indicate volcanogenic massive sulfide deposit favorability. This contrasts with other areas of the Belt that register lower magma temperatures and are subsequently barren. However, magma temperatures may have not been high enough to cause complete melting of refractory phases in which HFSE reside during crustal fusion of an amphibolite protolith, implying difficult discrimination of tectonic environments for the felsic rocks. The intermediate rocks were possibly formed by mixing between basaltic magmas and crustal material, compatible with volcanism in an attenuated continental lithosphere setting.     

Evaluation of heavy metal bio-availability from Almagrera pyrite-rich tailings dam (Iberian Pyrite Belt,SW Spain) based on a sequential extraction procedure

The beginning of polymetallic sulphide ore exploitation at Almagrera, Iberian Pyrite Belt, SW Spain, has been dated back to pre-Roman times; mining activity lasted until 2001 when it was definitely halted. The Almagrera sulphide tailings dam was built at the Sotiel–Coronada mine, where Cu, Zn and Pb concentrates were obtained by flotation processes, and sulphide (mainly pyrite) wastes formed. The dam was built as a water-reservoir, on a tectonic fault; its hazard is currently documented by the released wastes after a strong storm in the region, then, its stability is not monitored by the owner.     

Structural study of a semiductile strike-slip system in the Central Iberian Zone (Variscan Fold Belt,Spain): structural controls on gold deposits

The Villalcampo shear system is a regional dextral strike-slip fault zone that affects Late Variscan granites and their metamorphic country rocks over an area of about 150 km². The detailed geometry of this subvertical north-west—south-east shear zone is outlined. The system forms an extensional fan to the northwest and extends to the south-east as a broad extensional duplex. Particular attention is focused on the distribution of fault rocks and associated veins in its north-west splay. A structural study of the shear bands (encompassing both geometric and kinematic criteria) and a microscopic study of the fault rocks has led to the interpretation of the system as a brittle—ductile shear zone. Calculations give a shear strain value of = 1.5 and a minimum displacement of s = 3700 m. The localization of gold mineralization in mylonite-filled subvertical extensional veins is a product of the formation of the Villalcampo shear system. The subvertical faults and veins underwent a process of cyclical sealing and reopening. As such they acted as valves controlled by fluid pressure regulating fluid—rock interactions and gold deposition. Conditions favouring these processes occur near the base of the seismogenic zone in the vicinity of the frictional—quasi-plastic transition at mid-greenschist metamorphic conditions (T = 350°C and 10–15 km depth).     

The Removal of Dissolved Metals by Hydroxysulphate Precipitates during Oxidation and Neutralization of Acid Mine Waters, Iberian Pyrite Belt

This study examines the removal of dissolved metals during the oxidation and neutralization of five acid mine drainage (AMD) waters from La Zarza, Lomero, Esperanza, Corta Atalaya and Poderosa mines (Iberian Pyrite Belt, Huelva, Spain). These waters were selected to cover the spectrum of pH (2.2–3.5) and chemical composition (e.g., 319–2,103 mg/L Fe; 2.85–33.3 g/L SO₄⁼) of the IPB mine waters. The experiments were conducted in the laboratory to simulate the geochemical evolution previously recognized in the field. This evolution includes two stages: (1) oxidation of dissolved Fe(II) followed by hydrolysis and precipitation of Fe(III), and (2) progressive pH increase during mixing with fresh waters. Fe(III) precipitates at pH < 3.5 (stages 1 and 2) in the form of schwertmannite, whereas Al precipitates during stage 2 at pH 5.0 in the form of several hydroxysulphates of variable composition (hydrobasaluminite, basaluminite, aluminite). During these stages, trace elements are totally or partially sorbed and/or coprecipitated at different rates depending basically on pH, as well as on the activity of the SO₄⁼ anion (which determines the speciation of metals). The general trend for the metals which are chiefly present as aqueous free cations (Pb²⁺, Zn²⁺, Cu²⁺, Cd²⁺, Mn²⁺, Co²⁺, Ni²⁺) is a progressive sorption at increasing pH. On the other hand, As and V (mainly present as anionic species) are completely scavenged during the oxidation stage at pH < 3.5. In waters with high activities (> 10⁻¹) of the SO ₄⁼ ion, some elements like Al, Zn, Cd, Pb and U can also form anionic bisulphate complexes and be significantly sorbed at pH < 5. The removal rates at pH 7.0 range from around 100% for As, V, Cu and U, and 60–80% for Pb, to less than 20% for Zn, Co, Ni and Mn. These processes of metal removal represent a significant mechanism of natural attenuation in the IPB.     

Synchronous deposition of massive sulphide deposits in the Iberian Pyrite Belt: New data from Las Herrerías and La Torerera ore-bodies

Two small to medium sized massive sulphide deposits, Las Herrerías and La Torerera, located in the Iberian Pyrite Belt (IPB) are examined from a geological and palynostratigraphic perspective. The palynological assemblages are assignable to the Retispora lepidophyta–Verrucosisporites nitidus (LN) miospore Biozone (Latest Devonian: Latest Famennian/Strunian) of Western Europe. This age permits correlation with some of the main massive sulphide deposits dated so far in the region (viz., Tharsis, Aznalcóllar, Sotiel-Coronada or Neves-Corvo), and validates once again the hypothesis that a single mineralizing event was responsible for the genesis of most of the IPB’s massive sulphide deposits. The present study confirms that palynostratigraphy is an invaluable high-resolution biostratigraphic tool in the IPB, applicable to dating, correlation and ore-exploration.     

Geochemistry and mineralogy of surface pyritic tailings impoundments at two mining sites of the Iberian Pyrite Belt (SW Spain)

Two pyritic tailings impoundments located in two mining areas of the Iberian Pyrite Belt (Cueva de la Mora and Minas de Ríotinto-Zarandas) were selected to asses their potential environmental impact. Mineralogical (XRD diffraction study), physico-chemical characterization (colour, particle size, pH, acid–base account, total Fe, As, Cu, Pb and Zn) as well as a speciation study (by means of a seven-step sequential extraction procedure) were performed in superficial (0–20 cm) tailings samples. Arsenic and metal contents in soils around the tailings impoundments were also studied. Zarandas dam, a reclaimed impoundment, which has been limed, partially topsoiled and planted, has supported and allowed the growth of pine trees and other plants for many years. The surface of this impoundment can be considered very acid but nonacid forming. Although total As and metal concentrations were relatively high, it is not possible to conclude that the Zarandas tailings have polluted the surrounding soils. Tailings in Cueva de la Mora showed high total and easily mobilizable concentration of toxic elements. The net neutralization potential was strongly negative as a consequence of the acid generation caused by the sulphide oxidation, the presence of secondary acid-generating minerals and the absence of neutralizing materials. Coquimbite and rhomboclase efflorescences formed during the Mediterranean dry summers on the surface of this impoundment contained very high levels of soluble As, Cu and Zn that were easily dissolved and released to the running water in the first rains of autumn.     

A new 3D geological model and interpretation of structural evolution of the world-class Rio Tinto VMS deposit,Iberian Pyrite Belt (Spain)

A new 3D geological model and interpretation of structural evolution of the Rio Tinto world-class VMS deposit are presented in this work. The Rio Tinto volcanogenic massive sulfide (VMS) deposit is located in the Spanish segment of the Iberian Pyrite Belt and is hosted by felsic porphyritic volcanic rocks and tuffs. Computer generated 3D modeling of the different orebodies and host rocks has been carried out using data from around 3000 drill-core logs, allowing us to build 93 cross-sections and 6 plants (both 50 m spacing). This has enabled us to recognize the geometry and relationships between the mineralization and the earliest Carboniferous transtensional tectonics through the development of an extensional pull-apart basin with two sub-basins separated by the NW-SE trending Eduardo Fault. The sub-basins, Cerro Colorado and San Dionisio, were limited by two E-W strike-slip faults, the Northern and Southern faults, and bounded in the east and west by the NW-SE-trending Nerva and Western faults, respectively. The generated pull-apart basin was first filled by a basaltic magmatism of mantle origin and later, following the deposition of the intermediate complex sedimentary unit, by rhyodacitic volcanic rocks of crustal origin. The evolution of the subsiding basins caused the development of an E-W oriented rollover anticline that affected these filling rocks.As a result of a counterclockwise rotation of the stress axes, the primitive pull-apart basin evolved into a basin affected by E-W transtensional sinistral shearing. Its northern and southern limits were favorable areas for increased hydrothermal fluid flow, which gave way to the huge concentration of VMS mineralization located near the limits. The Northern and, to a lesser degree, the Southern extensional faults thus become channel areas for feeding and discharging of the VMS and stockwork ores. The main mineralizing period was related to this stage. Subsequently, during the Variscan transpressional phase, the E-W extensional faults were reactivated as inverse faults, affecting the volcanic sequence of mafic to felsic composition and the intermediate complex sedimentary unit. Fault propagation folds developed above these faults, affecting the massive sulfides, the transition series and the Culm flysch sediments, with buttressing playing a significant role in the geometry of tectonically inverted structures. The VMS mineralization and cupriferous stockworks were folded and dismembered from the original conduits in the volcanic series, and a dextral reactivation of the NW-SE trending faults also developed.Finally, it should be emphasized that this new 3D geological model is an approach to provide a better insight into the 3D structure of the world-class VMS Rio Tinto deposit and could be a key-point for further studies providing a new tool to increase knowledge of the VMS mineralizations and exploration guidelines elsewhere in the IPB.     

Early kaolinization of detrital Weald facies in the Galve Sub-basin (Central Iberian Chain,north-east Spain) and its relationship to palaeoclimate

A set of samples from the Camarillas Formation (Barremian, Weald facies) in the Galve Sub-basin (Central Iberian Chain, north-east Spain) was studied to determine the origin of the abundant kaolinitic clays and their relationship to the sedimentary environment, palaeoclimate and diagenetic processes. The samples were examined by X-ray diffraction and scanning and transmission electron microscopy, with special emphasis on clay-mineral characterization. The analysed materials are a mixture of detrital (quartz, micas, and K-feldspars) and authigenic phases (kaolinite, Fe-oxides, gibbsite, dickite, and calcite). Therefore, the mineralogy of the rocks reflects the source area, the sedimentary conditions, and the diagenetic evolution. The most abundant authigenic phases are kaolinites. The combination of XRD and electron microscopy shows that the kaolinites are well crystallized and have as high a degree of ordering as those formed by weathering in palaeosols; this clay formed the rock matrix, intergrowths with muscovite, and vermicular booklets that replaced detrital silicates as a consequence of intense dissolution processes. The diagenetic processes have recrystallized kaolinites in the sandstones, producing larger crystallinity indices and dickite. In contrast, kaolinites from the claystones and siltstones probably reflect formation by weathering. The kaolinitization process described, associated with the crystallization of gibbsite and iron oxides, is in agreement with the relatively warm and humid conditions described for the Iberian Range basin in the early Barremian.     

Ordinary kriging and indicator kriging in the cartography of trace elements contamination in São Domingos mining site (Alentejo, Portugal)

The abandoned pyrite mine of São Domingos, in the southeast Portugal is still an acid mine drainage generator and a source for trace elements pollution. This study aims to evaluate and map the soils and sediments chemical pollution. With this purpose three test sites located in the vicinities of the mine were sampled for soils, sediments and mining waste materials. The samples were analysed for pH, organic carbon, iron oxides and total content of As, Cu, Cr, Hg, Pb, Sb, U, Zn, Mn, S, Fe Al, Ca, K, Mg and P. The hyperspectral image of the test sites was acquired using the aero transported sensor HymapTM. Chemical analysis and hyperspectral images data were processed to estimate the contamination maps by multivariate data analysis, ordinary kriging, cokriging and indicator kriging methodologies.The test sites located downstream the mine open pit, are highly contaminated mainly in As, Hg, Pb and Sb, but no contamination was found in the third test site, upstream the mining site. The acid mining drainage seems to be the main spreading agent of chemical contaminants mostly originating from the mine waste materials. Factorial analysis and the geostatistical methodologies allowed several approaches for the contamination cartography of mining areas.     

Temporal and spatial variations in tectonic subsidence in the Iberian Basin (eastern Spain): inferences from automated forward modelling of high-resolution stratigraphy (Permian–Mesozoic)

By subsidence analysis on eighteen surface sections and 6 wells, which cover large part of the Iberian Basin (E Spain) and which are marked by high-resolution stratigraphy of the Permian, Triassic, Jurassic and Cretaceous, we quantify the complex Permian and Mesozoic tectonic subsidence history of the basin. Backstripping analysis of the available high resolution and high surface density of the database allows to quantify spatial and temporal patterns of tectonically driven subsidence to a much higher degree than previous studies. The sections and wells have also been forward modelled with a new ‘automated' modelling technique, with unlimited number of stretching phases, in order to quantify variations in timing and magnitude of rifting. It is demonstrated that the tectonic subsidence history in the Iberian Basin is characterized by pulsating periods of stretching intermitted by periods of relative tectonic quiescence and thermal subsidence. The number of stretching phases appears to be much larger than found by earlier studies, showing a close match with stretching phases found in other parts of the Iberian Peninsula and allowing a clear correlation with discrete phases in the opening of the Tethys and Atlantic.     

Source and evolution of ore-forming hydrothermal fluids in the northern Iberian Pyrite Belt massive sulphide deposits (SW Spain): evidence from fluid inclusions and stable isotopes

A fluid inclusion and stable isotopic study has been undertaken on some massive sulphide deposits (Aguas Teñidas Este, Concepción, 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–Fe-jaspers (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 ¹⁸O _fluid ˜–2 to 4‰), chlorites (d ¹⁸O _fluid ˜8–14‰, dD _fluid ˜–45 to –27‰), whole rocks (d ¹⁸O _fluid ˜4–7‰, dD _fluid ˜–15 to –10‰), and carbonates (d ¹⁸O _ankerite ˜14.5–16‰, d ¹³C _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 ¹⁸O, dD and d ¹³C 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.