Mineralogy and Geochemistry (HFSE and REE) of the Present-Day Acid-Sulfate Types Alteration from the Active Hydrothermal System of Furnas Volcano, São Miguel Island, The Azores Archipelago (original) (raw)
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Journal of Geochemical Exploration, 2011
The mineralogy and the trace element compositions of hydrothermally-altered volcanic materials collected from ash fall deposits and in four debris-avalanche deposits (DADs) at La Soufrière volcano in Guadeloupe have been determined. Phreatic explosions of the 1976 eruption and flank collapse events have sampled various parts of the active and ancient hydrothermal systems of the volcano. Hydrothermal mineral assemblages (smectite + silica polymorphs ± pyrite/jarosite ± gypsum) are typical of rock alteration by lowtemperature acid-sulphate fluids. High-temperature mineral assemblages are rare, indicating that phreatic explosions and flank collapse events have sampled mainly the upper parts of the volcanic edifice. Andesitic eruptive products affected by shallow hydrothermal alteration are complex assemblages of volcanic materials (glass, phenocrysts and xenocrysts with complex magmatic histories) of different ages and compositions. The use of incompatible element ratios and REE compositions normalised to an unaltered reference material overcomes the interpretation difficulties related to mass balance effects of alteration processes and the petrologic heterogeneity of the initial material. REE and other incompatible elements (Th, U, Hf, Zr) are mainly concentrated in the glassy matrix of unaltered andesitic rocks. Secondary S-bearing mineral phases (e.g., gypsum, jarosite) that have precipitated from acidsulphate fluids do not contain substantial incompatible elements (REE, U, Th, Hf, Zr). Compositional variations of incompatible elements in hydrothermally-altered andesitic materials reflect mainly volcanic glass-smectite transformation, which is characterised by (i) strong depletion of alkalis and alkaline earths (Ba, Sr) and first transition series elements (Zn, Cu, Cr, Co, Ni), (ii) immobility of highly incompatible elements (Th, Zr, Hf, LREE) and (iii) strong depletion of MREE and HREE. The sigmoid shape of normalised REE pattern is characteristic of glass-smectite transformation by low-temperature acid-sulphate fluids. This transformation also produces significant variations in U/Th values, which offer the opportunity to date the cessation of hydrothermal alteration and to reconstruct the evolution in space and time of hydrothermal activity in a volcanic edifice.
Geochemistry of Volcanic Rocks from Faial Island (Azores)
2008
Whole rock geochemistry of nine rocks from Faial Island (Azores) was used to characterize Faial Island volcanism. Studied rocks are lavas and were classified based on chemical data as basalts, hawaiites and trachyte. These rocks represent five stratigraphic units of island, from the base to the top: Ribeirinha Volcanic Complex, Cedros Volcanic Complex, Almoxarife and Capelo formations. The rocks belong to the sodic alkaline series, with the exception of the trachytic rock, that exhibits potassic affinity. Faial Island volcanism is characterized by low SiO2 contents (45 to 49 %), high TiO 2 (2 to 3 %) and P 2 O 5 (0.29 to 0.74 %). MgO shows values varying from medium to high (2 to 15 %). The trachyte has 62 % of SiO 2 , low MgO (0.42 %), TiO 2 (0.53 %) and P 2 O 5 (0.13 %) contents. Al 2 O 3 varies from low to high (11 to 18 %) and reflects the plagioclase abundance variation. Na 2 O+K 2 O values increase with fractionation and K 2 O/Na 2 O ratio is less than 0.54, with the exception of the trachyte (0.73). Ni content decreases with magmatic evolution, from 356 to 5 ppm. The presence of the cumulate phases in basalts of Almoxarife and Capelo formations explains the MgO and Ni high values. Almost all the samples show Rb, Ba, Ta, Nb, Zr enrichment and Th, Sm impoverishment. The trachyte shows Ba enrichment and K depletion. Zr positive anomaly and U, Sr, P, Ti negative anomalies are observed in the trachyte. The REE patterns are typical of oceanic island lavas, showing LREE enrichment relative to MREE and HREE. The REE patterns suggest a continuous fractionation from a common melt generated by low partial melting rates. The rocks of Faial Island are the result of fractional crystallization and derived from a magma, which has mixed characteristics, mainly reflecting PREMA and EM II reservoirs, with minor contribution of HIMU.
Geochemistry and geothermometry of volcanic rocks from Serra Branca, Iberian Pyrite Belt, Portugal
Gondwana Research, 2006
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.
Geochemistry of Volcanic Rocks, Albernoa Area, Iberian Pyrite Belt, Portugal
International Geology Review, 2004
Volcanic rocks from the Albernoa area essentially consist of calcalkaline quartz-feldspar-phyric coherent and hyaloclastic rhyodacites, and alkaline and tholeiitic basaltic rocks. Binary plots show that high-field-strength elements behaved as immobile elements, and allow for the identification of two felsic rock suites. Silica and alkali mobility, however, is reflected by compositional scatter on major-element diagrams: felsic rocks display rhyolitic to apparent andesitic compositions, and the mafic rocks display basaltic to apparent dacitic compositions. Silica and alkali mobility was focused along fracture networks and within the matrices of hyaloclastic breccias. Problematic classification of geotectonic setting for the felsic rocks is a reflection of anomalous high-field-strength element systematics; this probably results from a low temperature of crustal fusion, causing decreased solubility of the refractory phases in which these elements reside. The mafic rocks, however, evidently were generated in an extensional setting without involvement of subduction; the existence of apparent arc signatures was caused by crustal assimilation. This is compatible with volcanism in an attenuated continental lithosphere setting, due to strike-slip tectonics during oblique continental collision.
Water
This study focused on 13 water samples collected from two of the main active volcanoes (Furnas and Fogo) at São Miguel, Azores. Based on the major element composition, the waters are classified into Na-HCO3 and Na-Cl types. While the concentrations of chloride seem to reflect the contribution of sea salt aerosols, the behavior of the main cationic species and Sr in the analyzed waters appear to have been largely controlled by the interaction between meteoric waters and the underlying bedrock. The temperature and input of CO2 from the secondary volcanic activity are enhancing the silicate leaching. The stable isotopic data show that these waters have a meteoric origin (δ18O = −2.03 to −4.29‰; δ2H = −7.6 to −17.4‰) and are influenced by a deep hydrothermal/volcanic carbon source (δ13C = −4.36 to −7.04‰). The values of δ34S (0.13 to 12.76‰) reflects a juvenile sulfur source derived from the leaching of volcanic rocks. The Sr isotopic ratios show a slight difference between the values f...
Geological Magazine, 2013
Magmas in Faial Island, Azores (Portugal), were mostly erupted from two fissure zones and the Caldeira central volcano during overlapping periods. The fissure zones follow extensional trends oriented WNW and ESE and erupted nepheline- to hypersthene-normative basalts and hawaiites. The Caldeira central volcano builds the central part of the island, which is cut by the fissure zones. Ne-normative basalts show similar high-field-strength element (HFSE) concentrations but higher large ion lithophile element (LILE) concentrations than hy-normative equivalents. Primitive melts were generated by small (3–5%) degrees of partial melting of garnet-bearing peridotite, variably enriched in incompatible elements. Overall, basalts from Faial show relatively higher LILE abundances and LILE/HFSE ratios than those of the other islands of the Azores and of many other volcanoes in the Atlantic area. This feature indicates the existence of chemical heterogeneities in the mantle sources characterized b...
Economic geology and the bulletin of the Society of Economic Geologists, 2006
The Neves-Corvo volcanic-hosted massive sulfide deposit is one of the largest (>300 Mt) and richest known deposits of the Iberian pyrite belt. The deposit is unique because of its extremely high Cu and Sn grades (45 Mt at 6% Cu plus 4.5 Mt at 12% Cu and 2.2% Sn). More than 300,000 metric tons (t) of tin metal is contained in several types of tin mineralization that grade from 500 ppm to 60 percent Sn. The deposit consists of five orebodies, including Corvo, the focus of the present study. The volcanic, volcaniclastic, and sedimentary lithofacies that directly host the Neves-Corvo mineralization consist of a rhyolitic dome-cryptodome-hyaloclastite complex. Facies interpretation indicates that volcanic activity occurred at moderate water depth (below stormwave base) in variably subsiding basins of different orders. Several syn-and postore modification events included gravity-driven mass transport processes and subsequent low-angle thrusting and asymmetric detachment folding, which shaped the present deposit. The ore geology and geochemistry, and the textural, mineralogical, and geochemical aspects of the ore-forming process in the Corvo orebody collectively suggest that ore formation resulted from a long-lived hydrothermal system, including two main mineralizing events: an early stage of stringer and massive cassiterite deposition and a subsequent episode of massive sulfide ore generation. Two spatially independent stockworks fed these genetically related mineralizing systems. The early deposition of stringer and massive cassiterite ores took place almost exclusively in the "tin corridor"-a structural alignment bounded by synvolcanic faults. Textural analysis and geochemical data both indicate minimal fluid-rock interaction during ascent of the tin-bearing fluid. The massive and semimassive cassiterite ores at Corvo most likely formed by direct venting into seawater and/or emplacement in water-laden (unlithified) sediments and early hydrothermal products that mantled the footwall volcanic sequence. Stringer and massive cassiterite precipitation is thought to have been a product of a short-lived episode in the overall ore-forming system. The vein-filling mineral assemblage in the cassiterite stockwork (quartz, cassiterite, pyrite, donbassite, kaolinite, chamosite), coupled with whole-rock and mineral geochemical data, indicate that the tinbearing fluid had a low pH and relatively high temperature (~380ºC). The massive sulfide-related hydrothermal alteration in Corvo is essentially strata bound. The permeability contrasts in the uppermost portion of the footwall succession played an important role in controlling the fluid flow. The hydrothermal alteration is zoned and consists of an inner chlorite/donbassite-quartz-sulfide-(sericite) core that grades outward into successive enveloping K-sericite-quartz-sulfide and Na-sericite-quartz-sulfide halos. Textural evidence, from the mine to the microscopic scale, and mass-balance considerations indicate that extensive silicate replacement in the coherent volcanic rocks of the footwall sequence and disseminated replacement mineralization in the uppermost volcaniclastic and/or sedimentary units were major mechanisms of massive sulfide deposition at Corvo. Alteration mineralogy and geochemistry indicate that the Neves-Corvo ore fluids were hotter and more acidic than typical ore-forming fluids of the Iberian pyrite belt. The geochemical characteristics of the tin and copper mineralization also differ from the ores of typical deposits of the Iberian pyrite belt and imply the involvement of additional metal sources, possibly magmatic, in the ore-forming system. Copper enrichment due to mechanical and fluid-assisted tectonometamorphic remobilization processes accounts for the formation of extremely high grade ore shoots in the deposit.
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.
Geochemical evidence concerning sources and petrologic evolution of Faial Island, Central Azores
Volcanic rocks that make up Faial Island, Central Azores, consist of four volcanostratigraphic units, with ages between 730 ka and the present. Lavas range from alkali basalts to trachyandesites and belong to the alkaline-sodic series. The oldest unit is the Ribeirinha Volcanic Complex, generally characterized by low MgO contents. The Cedros Volcanic Complex is composed of basalts to benmoreites with low MgO contents. The Almoxarife Formation represents fissure flows, containing MgO contents similar to to slightly higher than those of the underlying Cedros Volcanic Complex. The youngest unit, the Capelo Formation, consists of mafic rocks with MgO values higher than those of the other units. Bulk-rock major and trace element trends suggest that differentiation of the three earliest units were dominated by fractional crystallization of plagioclase ± clinopyroxene ± olivine ± titanomagnetite. Capelo bulk-rock compositions are the most primitive, and are related to a period when volcanic activity was fed by deep magmatic chambers, and melts ascended more rapidly. Comparison among geochemical patterns of the trace elements suggests a strong similarity between the lavas from Faial and Pico islands. Corvo Island volcanism contrasts with the geochemistry of Faial and Pico lavas, reflecting its strong K and Rb depletion, and Th, U, Ta, Nb, La, and Ce enrichment. Absence of the Daly gap in the Faial volcanics is attributed to early crystallization of Ti-Fe oxides. The probable source of the Faial magma coincides with the MORB-FOZO array, which implies the presence of ancient recycled oceanic crust in the mantle source. Ratios of incompatible trace elements suggest the similarity of Corvo volcanic rocks with magmas derived from HIMU sources, whereas the Faial and Pico volcanic rocks could have been produced from sources very close to EMII-type OIB.