Volcanoclastic Rocks Research Articles

Intraplate volcanism in the Danube Basin of NW Hungary: 3D geophysical modelling of the Late Miocene Pásztori volcano

Three-dimensional geophysical modelling of the early Late Miocene Pasztori volcano (ca. 11–10 Ma) and adjacent area in the Little Hungarian Plain Volcanic Field of the Danube Basin was carried out to get an insight into the most prominent intra-crustal structures here. We have used gridded gravity and magnetic data, interpreted seismic reflection sections and borehole data combined with re-evaluated geological constraints. Based on petrological analysis of core samples from available six exploration boreholes, the volcanic rocks consist of a series of alkaline trachytic and trachyandesitic volcanoclastic and effusive rocks. The measured magnetic susceptibilities of these samples are generally very low suggesting a deeper magnetic source. The age of the modelled Pasztori volcano, buried beneath a 2 km-thick Late Miocene-to-Quaternary sedimentary sequence, is 10.4 +/− 0.3 Ma belonging to the dominantly normal C5 chron. Our model includes crustal domains with different effective induced magnetizations and densities: uppermost 0.3–1.8 km thick layer of volcanoclastics underlain by a trachytic-trachyandesitic coherent and volcanoclastic rock units of a maximum 2 km thickness, with a top situated at minimal depth of 2.3 km, and a deeper magmatic pluton in a depth range of 5–15 km. The 3D model of the Danube Basin is consistent with observed high ΔZ magnetic anomalies above the volcano, while the observed Bouguer gravity anomalies correlate better with the crystalline basement depth. Our analysis contributes to deeper understanding of the crustal architecture and the evolution of the basin accompanied by alkaline intraplate volcanism.

Palaeozoic Magmatism Associated with Gold-Antimony-Tin-Tungsten-Lead-Zinc and Silver Mineralization in the Neighbouring of Porto, Northern Portugal

In the studied area there are evidences of Palaeozoic magmatism along the stratigraphic succession from Cambrian to Carboniferous. Igneous, volcanic and sub-volcanic rocks occur along and around a significant Variscan structure, the Valongo Anticline, an asymmetrical antiform anticline, trending NW-SE, located in Central Iberian Zone in the neighbourhood of Porto (Northern Portugal). Mineralizations of gold-antimony-tin-tungsten-lead-zinc and silver occur in the area. Magmatism related to acid and basic subvolcanic and volcanic rocks occurs interbedded in Montalto Formation metasediments (conglomerates, quartzite, wakes and slates) of Cambrian age. In the transition Cambrian-Ordovician (Tremadocian?) underlying the Lower Ordovician massive quartzite of Santa Justa Formation (Floian age), volcanic rocks show bimodal composition and occur interbedded in a lithologic association mainly composed of conglomerate and quartzite with minor slate and wake intercalations. The acid volcanism consists of interbedded volcanoclastic rocks of rhyolitic affinities and black and green cherts in thick layers represent the basic volcanism. To the top of this basal Early Ordovician volcano-sedimentary succession, overlaying Floian massive quartzites, a succession of interbedded quartzite’s, wackes and slates, enriched in ironstones in the normal limb, and bearing prints of volcanic origin occur. Sub-concordant quartz veins (exhalative-origin) interbedded in the ironstones were interpreted as volcanic layers recrystallized during the circulation of hydrothermal mineralizing fluids. The presence of igneous rocks in Palaeozoic Carboniferous metasediments around Porto is known since middle of nineteen century. Granodioritic porphyry intrusion occurs interbedded in carboniferous metasediments. Metasedimentary country rocks are surrounded by pre- to post-orogenic Variscan granites with no direct spatial relationship with mineralization but hidden granitic apexes suggest an indirect genetic relationship with Sb-Au veins. Chemical analyses were performed on some of the rocks evidencing their origin. Some of them are related to mineralizations. Earlier the main Variscan Au-Sb metallogenic hydrothermal event, metallic preconcentrations (Au, Sb, Pb) occurred during the Palaeozoic, often related to magmatic events.

Spatial pattern analysis of Ni concentration in topsoils in the Campania Region (Italy)

The objective of this study is to demonstrate the combined use of point pattern, multifractal and digital image processing analysis in order to recognise and characterise spatial patterns of Ni concentration in topsoil in the Campania Region (Italy). After an exploring univariate statistical analysis, spatial clusters and outliers were revealed by Moran-I autocorrelation among the original measurement points. The linear and accurate Triangular Irregular Network (TIN) interpolation of Ni concentrations fitted at 3663 sample points was used. Successive moving average smoothing was applied to generalise the TIN model and to suppress small irregularities. The resulting raster map was the subject of multifractal analysis to classify the TIN-based Ni raster concentration map and to define Ni-baseline values. Digital Image Processing including lineament density and spatial variability index calculation was performed not only on the TIN-based total Ni raster map but also on its baseline map to recognise spatial patterns. The Ni-baseline map was given particular attention because the spatial distribution and variance in this map are assumed to be related to different geogenic processes.The highest spatial variability zones of total Ni concentrations are located in Campi Flegrei and around Napoli where low Ni values and spatial outliers are clustered. The highest variability zones can be found in topsoils over volcanoclastic rocks and they are related to urbanised and agricultural land use areas. The lowest variability zones coincide with the Eocene-Miocene argillaceous sediments of the Apennine Mts where the highest Ni values were measured.The highest spatial variability in the Ni-baseline map is in a NW-SE zone following the carbonate outcrops of the Apennine Mts (e.g. Mt. Alburni, Mt. Picentini). Another interesting NW-SE trending moderate to high variability zone was also identified following the alluvial plains of the Apennine's rivers. The Apennine's rivers seem to have played a crucial role in the redistribution of Ni through sediment transport processes resulting in higher spatial variability of Ni in the studied soils. The lowest variability zone is located south of Campi Flegrei and Napoli and it occupies the vast area of the Campanian Plain.

Dehydration-hydration reactivity of laumontite: analyses and tests for easy detection

AbstractHydration reactions are known to affect rock or aggregate stability in construction; laumontite is not usually considered to be a ‘problem-mineral’ though drill cores from the very low-grade metamorphic altered andesites and volcanoclastic rocks from Central Chile showed detachments of shotcrete in a tunnel exposed to periodic water flow, with expandable clay phases presumed to be responsible for the observed failure. Abundant laumontite detected in the cores may also be responsible for the detachment, however, resulting from the structural expansion and contraction in response to hydration and drying. Clay reactivity in construction projects is often tested on site by 30 days of ethylene glycol exposure, but adequate monitoring options for laumontite are not deployed. Options for laumontite characterization involving a combination of water immersion and slaking and modified oedometer-based expansibility tests were used here to observe the response to laumontite expansion pressure. All tests were formulated considering minimal implementation efforts for building sites or the easy availability of analytical and testing facilities.Laumontite was identified by optical microscopy, semiquantitative X-ray diffraction, and automated mineralogical analysis. A combination of the latter two methods provided reliable information about the presence of sub-microscopic laumontite and a visual impression of the textural arrangement of the zeolite in the rock.A slaking test based on four cycles of immersion followed by drying and final application of weight (simulated overburden) is best suited to indirect detection and for demonstrating rock reactivity due to the presence of laumontite. Rocks with laumontite show expansion when crushed, recompacted and fitted into an oedometer, but mineralogical information is required for adequate interpretation of the results.

Tonian emplacement of ophiolites in the southern Brasiliano Orogen delimited by U-Pb-Hf isotopes of zircon from metasomatites

Key parameters in the evolution of the oceanic crust in the little-known Tonian period of the Brasiliano Orogen are presently established through the finding and characterization of zircon in chloritite and tourmalinite enclosed in serpentinite. The Ibaré and Palma ophiolites were selected for the study and both are from the juvenile São Gabriel terrane, Rio Grande do Sul state, Brazil. U-Pb and Lu-Hf isotopic analyses of zircon from metasomatites and a volcanoclastic rock with laser ablation indicate similar ages in both ophiolites, respectively 726±2Ma and 722±3Ma (metasomatites) and 758±4Ma (volcanoclastic rock). Other three representative ages recorded in the metasomatites are 880±12, 836±6 and 780±5Ma, interpreted as successive serpentinization events. Mantle Hf isotope compositions of zircon in the metasomatites range from εHf=+12.1 to +1.4 and in the volcanoclastic rock the sample displays εHf=+11.7 to +6.7. Zircon geochemistry is close to the mantle array. This direct dating of ophiolites sets new limits to the evolution of the Brasiliano Orogen during formation of Gondwana. The youngest possible age for ophiolite accretion is 722Ma.

Ore petrology, hydrothermal alteration, fluid inclusions, and sulfur stable isotopes of the Milin Kamak intermediate sulfidation epithermal Au-Ag deposit in Western Srednogorie, Bulgaria

The Milin Kamak gold-silver deposit is located in Western Srednogorie zone, 50km west of Sofia, Bulgaria. This zone belongs to the Late Cretaceous Apuseni-Banat-Timok-Srednogorie magmatic and metallogenic belt. The deposit is hosted by altered trachybasalt to andesitic trachybasalt volcanic and volcanoclastic rocks with Upper Cretaceous age, which are considered to be products of the Breznik paleovolcano. Milin Kamak is the first gold-silver intermediate sulfidation type epithermal deposit recognized in Srednogorie zone in Bulgaria. It consists of eight ore zones with lengths ranging from 400 to 1000m, widths from several cm to 3–4m, rarely to 10–15m, an average of 80–90m depth (a maximum of 200m) and dip steeply to the south. The average content of gold is 5.04g/t and silver – 13.01g/t. The styles of alteration are propylitic, sericite, argillic, and advanced argillic. Ore mineralization consists of three stages. Quartz-pyrite stage I is dominated by quartz, euhedral to subhedral pyrite, trace pyrrhotite and hematite in the upper levels of the deposit. Quartz-polymetallic stage II is represented by major anhedral pyrite, galena, Fe-poor sphalerite; minor chalcopyrite, tennantite, bournonite, tellurides and electrum; and trace pyrrhotite, arsenopyrite, marcasite. Gangue minerals are quartz and carbonates. The carbonate-gold stage III is defined by deposition of carbonate minerals and barite with native gold and stibnite.Fluid inclusions in quartz are liquid H2O-rich with homogenization temperature (Th) ranging from 238 to 345°C as the majority of the measurements are in the range 238–273°C. Ice-melting temperatures (Tm) range from −2.2 to −4.1°C, salinity – from 3.7 to 6.6wt.% NaCl equiv. These measurements imply an epithermal environment and low- to moderate salinity of the ore-forming fluids.δ34S values of pyrite range from −0.49 to +2.44‰. The average calculated δ34S values are 1.35‰. The total range of δ34S values for pyrite are close to zero suggesting a magmatic source for the sulfur.

Geochemistry and isotopic signatures of Paleogene plutonic and detrital rocks of the Northern Andes of Colombia: A record of post-collisional arc magmatism

Between the Late Cretaceous and Paleogene, the Northern Andes experienced subduction and collision due to the convergence between the oceanic Caribbean Plate and the continental margin of Ecuador and Colombia. Subduction-related calc-alkaline plutonic rocks form stocks of limited areal expression or local batholiths that consist mostly of diorites and granodiorites. We investigated two stocks (Hatillo and Bosque) exposed in the Central Cordillera of Colombia that had U–Pb zircon crystallization ages between 60 and 53Ma. Relatively low radiogenic Sr, Nd and Pb isotopes from selected samples account for a heterogeneous crustal source, whereas negative anomalies of Nb and Ti, high LREE/HREE and Sr/Y>28 ratios indicate that the magmas were emplaced in a continental magmatic arc setting. ƐHf(i) values of the dated zircons were between −4 and +7 and suggest some contamination of the magmas during their ascent through the crust. The high Sr/Y ratios recorded both in the investigated plutons as well as in other Paleogene plutons in the Central Cordillera suggest that the magmas differentiate in high-pressure conditions (garnet stability field). This differentiation probably occurred at the base of a thickened crust through the Mesozoic subduction and accretion of oceanic arcs to the continental margin during the Lower Cretaceous and Paleocene. The existence of other Paleogene granitoids with evidence of shallower differentiation signatures may be also an inheritance of along strike variations in the Northern Andean continental crust due to Cretaceous to Paleogene oblique convergence. The Hf isotope results from Paleogene detrital zircons from volcanoclastic rocks of the eastern Colombian basins reinforce the possibility of a distal magmatic focus.

New U-Pb age constraints for the timing of gold mineralization at the Pampalo gold deposit, Archaean Hattu schist belt, eastern Finland, obtained from hydrothermally altered and recrystallised zircon

In this paper we present new U-Pb data on zircon and titanite from the host rocks of the Pampalo gold deposit located within the Neoarchaean Hattu schist belt in eastern Finland. We also present new U-Pb data on nearby plutonic rocks. Plagioclase porphyries at the mine site demonstrate inheritance from 2.82–2.83Ga sources while a suggestive intrusive age is at c. 2.76Ga. Zircon grains extracted from the altered felsic units hosting the gold ore show complex alteration and recrystallisation textures and demonstrate Zr mobility. This mobility is most probably related to the alteration event although direct link to gold mineralization remains to be shown. The preserved or recrystallized parts of the altered zircon grains, texturally homogeneous grain aggregates and some overgrowths yield heterogeneous ages which cluster between ca. 2.73 and 2.70Ga, with a mean age of 2.71Ga. This age is considered to place a new constraint on the timing of mobility of Au at the Pampalo. This event postdates the known crustal formation event at 2.76–2.73Ga in the area as recorded by the adjacent plutonic rocks and volcanoclastic rocks within the Hattu schist belt. The Naarva tourmaline granite, 10kmNW of Pampalo, is 2.69Ga and is thus temporally associated with known regional crustal anatexis and metamorphism in the Archaean of eastern Finland. Whether this crustal reworking event played a role in the genesis of the gold in eastern Finland needs to be further studied.

Listvenite-related gold deposits of the South Urals (Russia): A review

The Urals is a complex fold belt, which underwent long geological evolution. The formation of most gold deposits in the Urals is related to the collision stage. In this paper, we review some relatively small listvenite-related gold deposits, which are confined to the large Main Uralian fault zone and some smaller faults within the Magnitogorsk zone. The Mechnikovskoe, Altyn-Tash, and Ganeevskoe deposits are studied in detail in this contribution. They comprise the ore clusters along with other numerous small gold deposits, and constituted the sources for the gold placers exploited in historical time. The gold is hosted by metasomatites (listvenites, beresites) and quartz veins with economic gold grades (up to 20g/t Au). Listvenites are developed after serpentinites and composed of quartz, fuchsite, and carbonates (magnesite, dolomite)±albite. Volcanic and volcanoclastic rocks are altered to beresites, consisting of sericite, carbonates (dolomite, ankerite), quartz and albite. Pyrite and chalcopyrite are major ore minerals associated with gold; pyrrhotite, Ni sulfides, galena, sphalerite, arsenopyrite and Au-Ag tellurides are subordinate and rare. Gold in these deposits is mostly high-fineness (>900‰). The lower fineness (∼800‰) is typical of gold in assemblage with polymetallic sulfides and tellurides. The ores have been formed from the NaCl–CO2–H2O±CH4 fluids of low (∼2wt% NaCl-equiv.) to moderate (8–16wt% NaCl-equiv.) salinity at temperatures of 210–330°C. The oxygen isotopic composition of quartz (δ18O) varies from 14.7 to 15.4‰ (Mechnikovskoe deposit), 13.2 to 13.6‰ (Altyn-Tash deposit) and 12.0 to 12.7‰ (Ganeevskoe deposit). The oxygen isotopic composition of albite from altered rocks of the Ganeevskoe deposit is 10.1‰. The calculated δ18OH2O values of the fluid in equilibrium with quartz are in a range of 5.7–6.3, 4.2–4.6 and 6.3–6.7‰ respectively, and most likely indicate a magmatic fluid source.

Fluid evolution and mineralogy of Mn-Fe-barite-fluorite mineralizations at the contact of the Thuringian Basin, Thüringer Wald and Thüringer Schiefergebirge in Germany

Abstract Numerous small deposits and occurrences of Mn-Fe-fluorite-barite mineralization have developed at the contact of the Thuringian Basin, Thüringer Wald and Thüringer Schiefergebirge in central Germany. The studied mineralizations comprise the assemblages siderite+ankerite-calcite-fluorite-barite and hematite-Mn oxides-calcite-barite, with the precipitation sequence in that order within each assemblage. A structural geological analysis places the origin of the barite veins between the Middle Jurassic and Early Cretaceous. Primary fluid inclusions contain water vapour and an aqueous phase with NaCl and CaCl2 as the main solutes, with salinities mostly between 24–27 mass. % CaCl2 eq. Th measurements range between 85 °C and 160 °C in barite, between 139 °C and 163 °C in siderite, and between 80 °C and 130 °C in fluorite and calcite. Stable isotopes (S, O) point to the evaporitic source of sulphur in the observed mineralizations. The S,C,O isotopic compositions suggest that barite and calcite could not have precipitated from the same fluid. The isotopic composition of the fluid that precipitated barite is close to the sea water in the entire Permo–Mesozoic time span whereas calcite is isotopically distinctly heavier, as if the fluids were affected by evaporation. The fluid evolution in the siliciclastic/volcanic Rotliegend sediments (as determined by a number of earlier petrological and geochemical studies) can be correlated with the deposition sequence of the ore minerals. In particular, the bleaching of the sediments by reduced Rotliegend fluids (basinal brines) could be the event that mobilized Fe and Mn. These elements were deposited as siderite+ankerite within the Zechstein carbonate rocks and as hematite+Mn oxides within the oxidizing environment of the Permian volcanic and volcanoclastic rocks. A Middle-Jurassic illitization event delivered Ca, Na, Ba, and Pb from the feldspars into the basinal brines. Of these elements, Ba was deposited as massive barite veins.

Seismic stratigraphy, structure and morphology of Makarov Basin and surrounding regions: tectonic implications

The tectonic history of Amerasia Basin, Arctic Ocean, is not well known because of a paucity of data and complexities introduced by the Alpha–Mendeleev Ridge large igneous province. Makarov Basin, at the northern limit of Amerasia Basin and adjacent to Lomonosov Ridge, may provide a window into understanding the larger tectonic framework. The objective of this study is to decipher the sedimentary and tectonic history of northern Amerasia Basin by analysing the seismic stratigraphy, structure and morphology of Makarov Basin and surrounding regions (Alpha and Lomonosov ridges) of the central Arctic Ocean. The principal data sources for this study are a 400km long multi-channel seismic line, extending from Alpha Ridge to the crest of Lomonosov Ridge via central Makarov Basin, and the Arctic bathymetric chart.The seismic record within Makarov Basin is divided into five seismic units. The first unit overlying basement hosts Late Cretaceous (minimum age) slope to base of slope sediments. Some of these sediments are interbedded with volcanic or volcanoclastic rocks with a minimum age of 89Ma. Makarov Basin becomes isolated from proximal sources of sediments after the onset of rifting that separated Lomonosov Ridge from the Barents Shelf, which may have occurred as early as the mid-Late Cretaceous, and led to the creation of Eurasia Basin. Sediments are largely pelagic to hemipelagic as a result of this isolation. This deposition style also applies to the draped sedimentary strata on Alpha and Lomonosov ridges. The uppermost seismic units within Makarov Basin arejump-correlatedto the stratigraphic record of the ACEX drill site on top of Lomonosov Ridge to provide age control. This correlation shows that the 44.4–18.2Ma hiatus documented in the drill core is not apparent in the basin. Inter-ridge correlations and the absence of an obvious planate surface on Alpha Ridge also suggest that sedimentation was uninterrupted on this ridge during the hiatus.Seismic data reveal a deep subbasin (~5km thick) within Makarov Basin. This subbasin is immediately adjacent to Lomonosov Ridge within major bends in the general strike orientation of the ridge. The rhomboid shape of the deep subbasin, the straight and steep morphology of the Amerasian flank of Lomonosov Ridge and the presence of numerous sub-parallel ridges (e.g. Geophysicists and Marvin spurs) created by splay faulting are evidence of strike-slip (transtensional) tectonics. This interpretation supports the “rotational” model of opening of Amerasia Basin with a transform to transtensional margin at Lomonosov Ridge. As spreading continued, however, the tectonics became increasingly extensional perpendicular to Lomonosov Ridge. There is no evidence of major tectonic deformation in Makarov Basin beyond the late Paleocene.

New evidence of effusive and explosive volcanism in the Lower Carboniferous formations of the Moroccan Central Hercynian Massif: Geochemical data and geodynamic significance

The Azrou-Khénifra basin, located in the SE sector of the Moroccan Central Hercynian Massif of the Western Meseta of Morocco comprises volcanic and volcanoclastic rocks where two magmatic sequences can be distinguished: i) the Dhar Lahmar Sequence, composed of Upper Visean basaltic lava flows and pyroclastic deposits; and ii) the Kef Al Asri Sequence, composed of Visean – Serpukhovian intermediate to acid rocks.A continuous spatial and temporal evolution between the two volcano-sedimentary sequences was observed during the detailed geological work performed in the studied area. Petrography and geochemical studies additionally suggest a continuous compositional evolution from the more basic magmatic rocks to the intermediate/acid rocks, which implies a cogenetic magmatic differentiation controlled by crystal fractionation (with minor crustal assimilation) of a calc-alkaline trend magmatic suite. The inferred magmatic evolution is consistent with a geodynamic environment of an orogenic zone within an active continental margin setting.This partly explosive Visean – Serpukhovian volcanism, identified for the first time in the Western Meseta of Morocco, displays very similar petrographic and geochemical characteristics to its Eastern Meseta analogues, which implies that the emplacement of these magmatic rocks must have occurred in similar collisional geodynamic settings for both major geological domains, further constraining the evolution of this major crustal segment within the Carboniferous events that shaped the Hercynian Orogeny.

Geochemistry and Petrogenesis of Pliocene Alkaline Volcanic Rocks of Dok Island, Korea

Dok island comprises Pliocene volcanic products such as a series of volcanoclastic rocks and lavas ranging in composition from alkali basalts, and trachyandesites to trachytes. Compositional variation of the basaltic rocks can be attributed to fractional crystallization of olivine, clinopyroxene, plagioclase, and magnetite. Chemical variations among the trachyandesites are caused by fractionation of clinopyroxene, plagioclase, and magnetite with minor amphibole, while trachytes are controlled mainly by feldspar fractionation. Incompatible element abundance ratios and chondrite normalized LREE/HREE ratios (e.g., (La/Yb)c: 24.8 to 32.8 for basalts, 15.6 to 31.2 for trachyandesites) suggest that the origins of the basalts and trachyandesites involve both different degrees of partial melting and subsequent fractional crystallization processes. Trace element ratios of the basalts from Dok island are characterized by high Ba/Nb, La/Nb, Ba/Th and Th/U and isotopic ratios (Tasumoto and Nakamura, 1991) that are similar to the EM 1 type of oceanic island basalts such as Gough and Tristan da Cunha basalts.

Tapiaite, Ca5Al2(AsO4)4(OH)4·12H2O, a new mineral from the Jote mine, Tierra Amarilla, Chile

AbstractTapiaite (IMA2014-024), Ca5Al2(AsO4)4(OH)4·12H2O, is a new mineral from the Jote mine, Tierra Amarilla, Copiapó Province, Atacama, Chile. The mineral is a late-stage, low-temperature, secondary mineral occurring with conichalcite, joteite, mansfieldite, pharmacoalumite, pharmacosiderite and scorodite in narrow seams and vughs in the oxidized upper portion of a hydrothermal sulfide vein hosted by volcanoclastic rocks. Crystals occur as colourless blades, flattened on {101} and elongated and striated along [010], up to ∼0.5 mm long, and exhibiting the forms {101}, {101} and {111}. The blades are commonly intergrown in subparallel bundles and less commonly in sprays. The mineral is transparent and has a white streak and vitreous lustre. The Mohs hardness is estimated to be between 2 and 3, the tenacity is brittle, and the fracture is splintery. It has two perfect cleavages on {101} and {101}. The calculated density based on the empirical formula is 2.681 g cm–3. It is optically biaxial (+) with α = 1.579(1), β = 1.588(1), γ = 1.610(1) (white light), 2Vmeas = 66(2)° and 2Vcalc = 66°. The mineral exhibits no dispersion. The optical orientation is X ≈ [101]; Y = b, Z ≈ [101]. The electron-microprobe analyses (average of five) provided: Na2O 0.09, CaO 24.96, CuO 0.73, Al2O3 10.08, Fe2O3 0.19, As2O5 40.98, Sb2O5 0.09, H2 O 23.46 (structure), total 100.58 wt.%. In terms of the structure, the empirical formula (based on 32 O a.p.f.u.) is (Ca4.83Cu0.102+Na0.03)Σ4.96(Al2.14Fe0.033+)Σ2.17[(As3.875+Sb0.015+)Σ3.88O16][(OH)3.76(H2O)0.24]Σ4(H2O)10·2H2O. The mineral is easily soluble in RT dilute HCl. Tapiaite is monoclinic, P21/n, with unit-cell parameters a = 16.016(1), b = 5.7781(3), c = 16.341(1) Å, β = 116.704(8)°, V = 1350.9(2) Å3 and Z = 2. The eight strongest lines in the powder X-ray diffraction pattern are [dobs Å(I)(hkl)]: 13.91(100)(101), 7.23(17)(200,002), 5.39(22)(110,011), 4.64(33)(112,211,303), 3.952(42)(113,311,213), 3.290(35)(214,412,114,411), 2.823(39)(303,315) and 2.753(15)(513,115,121,511). The structure of tapiaite (R1 = 5.37% for 1733 Fo > 4σF) contains Al(AsO4)(OH)2 chains of octahedra and tetrahedra that are topologically identical to the chain in the structure of linarite. CaO8 polyhedra condense to the chains, forming columns, which are decorated with additional peripheral AsO4 tetrahedra. The CaO8 polyhedra in adjacent columns link to one another by corner-sharing to form thick layers parallel to {101} and the peripheral AsO4 tetrahedra link to CaO6 octahedra in the interlayer region, resulting in a framework structure.

Spectral reflectance characteristics of the Hamar Laghdad hydrothermal sequence, Morocco: Implications for the methane origin on Mars

We analyze and discuss reflectance spectra of carbonate rocks from the Hamar Laghdad area (Morocco), where evidences of interactions with hydrothermal, and, in some cases, methane enriched fluids derived from underlying volcanoclastic rocks, are reported in the literature. Deconvolution of the rock spectra into a sum of Gaussians, using MGM, resolved a number of both vibrational and electronic absorption features, mainly assigned to CO32−, Al–OH, and Fe2+, Fe3+, Cu, CF interaction processes, respectively. The associations of these absorption bands are exhaustive descriptions of the rock spectral properties. A spectral model of the Hamar Laghdad carbonate sequence was therefore delineated, based on the integration of these absorption band associations with XRF and Mössbauer analyses of the rock samples. The model involves: pure limestones and dolostones, limestone with iron carbonates, mixed carbonate and Al-silicate impure limestones, limestones with iron sulfides. The model points out the complexity of the spectral characteristics of rocks that underwent hydrothermal and partly methanogenic processes, but also suggests an alternative geologic scenario plausible for possible methanogenic activity in the Mars geologic past.

Crustal structure and nature of emplacement of the 85°E Ridge in the Mahanadi offshore based on constrained potential field modeling: Implications for intraplate plume emplaced volcanism

An integrated interpretation of multi-channel seismic reflection, gravity and magnetic datasets belonging to northern most part of the 85°E Ridge in the Mahanadi offshore is carried out to study the crustal structure and mode of its emplacement. The basement structure map of the ridge reveals that it is 130–150km wide and is composed of an eastern high which appears as a continuous, broad and smooth topographyand the western high characterized by several steep isolated highs. The seismic velocities reported for the first time over the ridge indicate several sedimentary sequences ranging in velocities between 1.6 and 4.0km/s above the acoustic basement top. The salient aspects of the sedimentary velocities are; a low velocity layer (2.6–3.2km/s) within the Cretaceous sequence in the intervening depressions encompassing the flank region, and a regionally widespread higher velocity layer (3.5–3.8km/s) belonging to the Eocene–Oligocene section overlying the ridge. A layer having a velocity of 4.2–4.7km/s probably made of volcanoclastic rocks is observed immediately below the acoustic basement. The sediment isopach maps presented here for three major horizons are used to compute the 3-D sediment gravity effect to obtain a crustal Bouguer anomaly map of the region. Detailed analysis of the gravity and magnetic anomaly maps clearly demonstrates the continuity of ridge up to the Mahanadi coast at Chilka Lake. Seismically constrained gravity and magnetic models indicate that the ridge is composed of volcanic material that was emplaced on continental crust in the shelf-slope areas and over the oceanic crust in the deep offshore areas. The modeled crustal structure below the ridge further indicates volcanic emplacement of the ridge on a relatively younger lithosphere. We propose two alternative models for the emplacement of the ridge.

Comparison of the Mineral Trapping Capacity in three Reservoirs with Variable Mineral Compositions under CO2 Saturated Conditions

Abstract The mineral trapping capacity is considered the most secure long-term CO 2 storage mechanism as it permanently demobilizes injected carbon. Here we compare three different reservoir rock mineral compositions in terms of their mineral trapping capacity using a mass balance approach. The evolution of carbon fractions in the form of residual CO2, CO 2 in the formation water and carbon stored in minerals is predicted over time using reaction path modelling. The two studied siliciclastic reservoirs (Precipice Sandstone, Surat Basin, Australia; Pretty Hill Formation, Otway Basin Australia) have no or only a very small mineral trapping capacity, whereas the volcanoclastic reservoir (Takinoue Formation, central Hokkaido, Japan) has a very large mineral trapping capacity. This difference results from the abundance of reactive Ca-, Mg- and Fe-bearing silicate minerals. It is demonstrated that the mineral trapping capacity of volcanoclastic rocks can exceed residual and solubility trapping. The limitation of the chosen approach is a rather simplistic treatment of mineral dissolution and precipitation kinetics.

Produtos metamórficos de sistemas high-sulfidation oceânicos mesoproterozoicos, grupo Serra do Itaberaba, SP

O Grupo Serra do Itaberaba, no segmento central da Faixa de Dobramentos Ribeira (sudeste do Brasil), corresponde a uma sucessão meta-vulcanossedimentar do Mesoproterozoico. As rochas deste grupo foram afetadas por dois eventos metamórficos regionais na fácies anfibolito e um terceiro evento retrometamórfico na fácies dos xistos verdes. Esta sucessão depositou-se, inicialmente, em um ambiente oceânico contendo basaltos com assinaturas do tipo N-MORB, que, posteriormente, evoluiu para um ambiente de retro-arco. Neste grupo são conhecidas quatro ocorrências de rochas ricas em alumina (Guavirituba, Pedra Branca, Itaberaba e Pico Pelado), que afloram na forma de pequenas lentes intercaladas entre metabasitos, rochas metavulcanoclásticas, metatufos e metapelitos. A gênese destas rochas está associada à atividade magmática-hidrotermal oceânica em ambiente de retro arco, vinculada à colocação de pequenos corpos de rochas riolíticas e processos mineralizantes em ouro do tipo high-sulfidation. Análises por difração de raios X (DRX) de 15 amostras ricas em alumina, de granulação muito fina, permitiram identificar coríndon, topázio, margarita, rutilo e mica indiferenciada (possivelmente sericita). As associações minerais identificadas possibilitaram reconhecer a atuação de dois eventos de alteração argílica avançada e um terceiro evento de carbonatação ou retrometamórfico nas rochas do Grupo Serra do Itaberaba: o primeiro evento gerou o protolito 1, rico em óxidos de alumínio ± alunita, que, metamorfisadas, produziram litotipos azuis escuros formados por coríndon ± sericita; o segundo caracteriza um evento de silicificação que produziu o protolito 2, composto por topázio + zunyita + alunita ± rutilo ou por andalusita + alunita, correspondendo os seus produtos metamórficos a litotipos de cor marrom e esbranquiçada (ocorrência de Pico Pelado); no evento de carbonatação ou retrometamórfico cristalizou-se margarita a partir da andalusita, cianita ou sillimanita. A soma destes três eventos produziram, durante o metamorfismo ou retrometamorfismo, litotipos de cor marrom, esbranquiçada e heterogênea formados por margarita ou por margarita + sericita ± coríndon (ocorrências Guavirituba e Pedra Branca). A identificação desses litotipos em campo representa valiosa ferramenta em trabalhos de exploração mineral, uma vez que constituem rochas-guias para a localização de depósitos de ouro em sucessões meta-vulcanossedimentares metamofisadas em grau médio.

Joteite, Ca2CuAl[AsO4][AsO3(OH)]2(OH)2·5H2O, a new arsenate with a sheet structure and unconnected acid arsenate groups

AbstractJoteite (IMA2012-091), Ca2CuAl[AsO4][AsO3(OH)]2(OH)2·5H2O, is a new mineral from the Jote mine, Tierra Amarilla, Copiapó Province, Atacama, Chile. The mineral is a late-stage, low-temperature, secondary mineral occurring with conichalcite, mansfieldite, pharmacoalumite, pharmacosiderite and scorodite in narrow seams and vughs in the oxidized upper portion of a hydrothermal sulfide vein hosted by volcanoclastic rocks. Crystals occur as sky-blue to greenish-blue thin blades, flattened and twinned on {001}, up to ~300 μm in length, and exhibiting the forms {001}, {010}, {10}, {20} and {111}. The blades are commonly intergrown in wheat-sheaf-like bundles, less commonly in sprays, and sometimes aggregated as dense crusts and cavity linings. The mineral is transparent and has a very pale blue streak and vitreous lustre. The Mohs hardness is estimated at 2 to 3, the tenacity is brittle, and the fracture is curved. It has one perfect cleavage on {001}. The calculated density based on the empirical formula is 3.056 g/cm3. It is optically biaxial (–) with α = 1.634(1), β = 1.644(1), γ = 1.651(1) (white light), 2Vmeas= 78(2)° and 2Vcalc= 79.4°. The mineral exhibits weak dispersion,r < v. The optical orientation isX≈c*;Y≈b*. The pleochroism isZ(greenish blue) >Y(pale greenish blue) >X(colourless). The normalized electron-microprobe analyses (average of 5) provided: CaO 15.70, CuO 11.22, Al2O38.32, As2O546.62, H2O 18.14 (structure), total 100 wt.%. The empirical formula (based on 19 O a.p.f.u.) is: Ca1.98Cu1.00Al1.15As2.87H14.24O19. The mineral is slowly soluble in cold, concentrated HCl. Joteite is triclinic,P, with the cell parameters:a= 6.0530(2),b= 10.2329(3),c= 12.9112(4) Å,α= 87.572(2),β= 78.480(2), γ = 78.697(2)°,V= 768.40(4) Å3andZ= 2. The eight strongest lines in the X-ray powder diffraction pattern are [dobsÅ(I)(hkl)]: 12.76(100)(001), 5.009(23)(020), 4.206(26)(120,003,121), 3.92(24)(022,02,02), 3.40(25)(13), 3.233(19)(031,023,123,03), 2.97(132,201) and 2.91(15)(22,13). In the structure of joteite (R1= 7.72% for 6003Fo> 4σF), AsO4and AsO3(OH) tetrahedra, AlO6octahedra and Cu2+O5square pyramids share corners to form sheets parallel to {001}. In addition, 7- and 8-coordinate Ca polyhedra link to the periphery of the sheets yielding thick slabs. Between the slabs are unconnected AsO3(OH) tetrahedra, which link the slabs only via hydrogen bonding. The Raman spectrum shows features consistent with OH and/or H2O in multiple structural environments. The region between the slabs may host excess Al in place of some As.

U–Pb zircon dating and nature of metavolcanics and metarkoses from the Monte Grighini Unit: new insights on Late Ordovician magmatism in the Variscan belt in Sardinia, Italy

In the external units of the Sardinian Variscides Nappe Zone, volcanic and volcanoclastic successions of Middle Ordovician age follow Lower Paleozoic calc- alkaline magmatism developed at the northern Gondwana margin. We present geochemical and zircon U-Pb isotopic data for the Truzzulla Formation, a low-to-medium-grade metamorphic volcanic-volcanoclastic succession belong- ing to the Monte Grighini Unit, the deepest unit in the Nappe Zone. Geochemical and radiometric data allow us to define a Late Ordovician (Katian) magmatic (volcanic) event of calc-alkaline affinity. These new data, in con- junction with previously published data, indicate that in the Sardinian Variscides, the age of Lower Paleozoic Andean- type calc-alkaline magmatism spans from Middle to Late Ordovician. Moreover, the age distribution of calc-alkaline volcanics and volcanoclastic rocks in the Nappe Zone is consistent with a diachronous development of Middle-Late Ordovician Andean-type magmatic arc through the portion of the northern Gondwanian margin now represented by the Sardinian Variscides. This reconstruction of the Sardinian Variscides reflects the complex magmatic and tectonic evolution of the northern margin of Gondwana in the Lower Paleozoic.