Dolomitic Marbles Research Articles

Influence of carbonate rocks on soil properties in the humid tropical climate of atlantic forest, Rio de Janeiro –Brazil

Soil carbonate accumulation plays an important role in C sequestration, accounting for approximately 40% of the C stored in soils worldwide. Under humid tropical climate these carbonates are rapidly dissolved, intensifying CO2 emissions. Despite this, little is known about the weathering of carbonates in areas with complex geology. This study aimed to evaluate the influence of the parent material on soil attributes and weathering processes under humid tropical climate conditions along toposequences formed by dolomitic marbles and calc-silicate rocks interspersed with garnet-biotite gneiss. Samples and soils profiles were collected on sites of both parental materials and at different points of a toposequence, namely: upper third, middle third and lower third of a slope. The profiles were analyzed for their morphological, physical, chemical and mineralogical attributes. The attributes of soils formed from metamorphic sedimentary rocks were more influenced by geological processes than pedological ones. It was found that the nature of the parent material was the main responsible for the attributes of the profiles, especially the strongly alkaline pH, being greater than 8.0 in most soil horizons, and the high contents of exchangeable Ca2+ and CaCO3 equivalent, with maximum values of 40.6 cmolc kg−1 and 763 g kg−1, respectively. As for mineralogical composition, it was verified the occurrence of minerals such as talc, vermiculite and Fe and Ti oxides, with metamorphic processes and the addition of hydrothermal fluids being the main responsible for the presence of these minerals. The supply of high contents of basic cations, with maximum values of 61.4 cmolc kg−1, high Si/Al ratio (reaching values close to 5.0) and the high contents of CaCO3 conditioning the strongly alkaline pH hinder the dissolution of silicates, even those with low resistance to weathering, despite the hot and humid climate conditions. The action of weathering processes in humid tropical climate is more marked in acidic materials, being responsible for the in situ formation of kaolinite from the alteration of mica and feldspar.

Effect of marble slurry on the microstructure of cement mortars subjected to salt crystallization and alternate wetting and drying cycles

The scarcity of river sand has triggered the utilization of industrial waste to produce concrete and mortars. One such waste is marble slurry, which is being produced in huge quantities during marble processing, was evaluated as a partial substitute to sand for the production of mortars. This study aims to assess the properties of mortar mixes of proportions 1:3 and 1:6 made with dried marble slurry of dolomitic origin by substituting 20 and 30% of fine aggregate. It was observed that mortars could be produced with lesser water content on the incorporation of this waste. This has helped improve compressive strength, while properties like tensile and flexural strengths, water absorption are comparable to conventional mortars. On examining the microstructure using scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction, the augmented formation of tobermorite and gismondine like minerals on cement hydration were detected. Traces of dedolomitization of the dolomitic marble were also identified along with the formation of calcium carbo-aluminates, enhancing the quality of the microstructure and mechanical performance. While the phenomenon of salt crystallization restricts the usage of dried marble slurry for the production of mortars with a rich mix proportion of 1:3 only, alternating cycles of wetting and drying do not significantly affect the performance of both 1:3 and 1:6 proportions on the usage of dried marble slurry as fine marble slurry.

Igneous genesis of the Bayan Obo REE–Nb–Fe deposit: New petrographical and structural evidence from the H1–H9 cross-section and deep-drilling exploration

Rare earth elements (REEs) are a focus of global attention because of their unique properties and high-tech applications. Bayan Obo is the world's largest REE deposit and is hosted in the Paleoproterozoic Bayan Obo Group, Inner Mongolia. The REE resources are concentrated in unit H8, “dolomite marble.” However, whether unit H8 has an igneous or sedimentary origin is debated, and relationships among units H1–H9 are unclear, hindering our understanding of the formation of such a giant REE deposit. It was previously assumed that the unit H8 orebody is located in both limbs of the Bayan syncline and unit H9 in the core, which also needs to be confirmed because it is a guide for ore exploration. Field observations and petrographic studies of units H1–H9 were undertaken to re-evaluate the petrology and structure of the Bayan Obo Group. It was found that some units (e.g., H5–H7) are missing from the studied cross-section, with the two sides of unit H9, across the hinge of the so-called syncline, not being directly comparable. Unit H8 displays petrological, mineralogical, and whole-rock REE geochemical characteristics distinct from those of the other units investigated, suggesting they are of different origin. Moreover, deep-drilling exploration indicates major differences between the two limbs of the orebodies in terms of thickness, occurrence, and spatial distribution. The East orebody has a steep dip angle of 70°–80° and continuously extends to depth without reduction of angle, suggesting the syncline is probably nonexistent. These results, combined with the reported intrusion relationship of unit H8 with surrounding rock and in situ isotopic studies previously reported, indicate that the orebody is a carbonatitic igneous conduit rather than being of sedimentary origin. SHRIMP U–Pb dating of detrital zircons from the units H2–H9 yielded ages of 2574 ± 25 to 132 ± 2 Ma, overlapping the ages of units H1–H9 and the main REE mineralization periods reported previously. These protracted and complicated ages of the orebody likely represent fluid-aided resetting ages.

Metallogenetic Mn-model of the Rhyacian-aged Buritirama Formation, Carajás domain (Amazon Craton)

The Mn-bearing Buritirama Formation is composed of clastic and chemical metasedimentary rocks representing a Rhyacian platformal basin (ca. 2.18–2.06 Ga) that limits the northern border of the Archean-Paleoproterozoic Carajás domain with the Paleoproterozoic Bacajá domain in the southern Amazon Craton, northern Brazil. The Buritirama Formation is divided into three main stratigraphic units. The Lower and Upper units consist of a basal quartzite followed by carbonate–silicate rocks (marls), dolomitic marbles (dolostone) and minor biotite schist while the Intermediate Unit consists of Mn-carbonate rocks topped by a supergene Mn-oxide deposit (Cenozoic age). These rocks were deformed and metamorphosed during the Transamazonian event (ca. 2.06 Ga).Geochemical data from dolomitic marbles (Lower Unit) exhibit negative Ce anomalies, high Y/Ho ratios (ca. 48) and seawater-like REE + Y patterns that reinforce an open platform setting for the Buritirama basin. Positive δ13C values (+3.2‰ to +5.0‰ V-PDB) for dolomitic marbles are considered to be an expression of the Lomagundi Carbon Isotopic Excursion. The metallogenesis of Mn-carbonate rocks is evaluated based on redox-sensitive trace elements (e.g. Zn and Ni), PAAS-normalized REE + Y patterns and δ13C values (−2.56‰ to +0.15‰ V-PDB) and point out to a classical multi-stage redox-controlled Mn-model involving: i) a primary hydrothermal fluid source related to ocean vents and/or normal faults; ii) microbially-mediated reduction of Mn-oxides particles at the ocean bottom; and iii) reactions between Mn2+ and CO32− (organic matter and seawater related), giving rise to diagenetic Mn-carbonates. The Rhyacian age, Mn deposits, and presence of Lomagundi-like δ13C allows to correlate the Buritirama Formation with the Águas Claras Formation within the Carajás domain further south, thus establishing an important metalliferous event that was up to now largely overlooked within the southern Amazon Craton.

Adakites and associated granitoids from the Serra da Prata Arc: Evidence for a Tonian subduction setting within the Araçuaí-Ribeira orogenic system (AROS), SE Brazil

Within deeply eroded orogenic belts, the geochemical signatures of metamorphosed igneous rocks can be used to constrain modern-like tectonic models operating in Precambrian times. One important aspect of this discussion is the presence or not of pre-collisional arc-related rocks that would represent the magmatic product of subduction processes imprinted on the upper plate of the convergent setting. New detailed geological and geochemical data of the northern segment of the Tonian to Cryogenian juvenile Outer Arc of the Araçuaí-Ribeira Orogenic System (AROS) are presented. Results were combined with recently published U–Pb and Hf data to contribute to a comprehensive tectonic model. The arc-related rocks (Serra da Prata Complex), transformed in foliated orthogneisses, are interlayered with dolomitic marbles, amphibolites, and psammo-pelitic paragneisses (Italva Group). This association is similar to the one described in the southern part of the orogenic system, pointing to the extension of the Outer Arc system until the Rio Doce River, in the transition zone between the Araçuaí and Ribeira segments of the AROS. New geochemical analyses together with reinterpreted published data indicate the presence of typical adakites that, together with normal calc-alkaline expanded series, island arc tholeiitic series, and A-type granitoids, corroborate with a modern style plate tectonic model. The envisaged model started in the Tonian, with E-verging subduction that resulted in the production of very juvenile arc-related rocks and associated fore-to back-arc basins filled with carbonatic rocks, meta-volcanic rocks, and MORB-like basaltic rocks. The typical adakites are considered the result of partial melting of young and hot oceanic lithosphere under moderate to high pressures in which garnet was an important stable residue. Potassic C-type adakitic rocks formed by the partial melting of thickened lower crust occur associated with the typical adakites. The occurrence of both adakites, together with A-type granitoids, characteristic of the investigated terminal sector of the Serra da Prata arc, is interpreted to result from a slab window gap, either associated with slab tearing/ridge subduction or slab roll-back, hampering an intracontinental model for the Ribeira orogen.

Relating physical properties to temperature-induced damage in carbonate rocks

Carbonate rocks have a widespread diffusion in the Earth crust and are extensively used in cultural heritage and buildings. These rocks can be naturally or anthropically exposed to high temperatures. Consequently, relating physical properties to temperature-induced damage is extremely important. Six sets of compositionally and texturally different carbonate rocks, spanning from limestones and marbles to dolomitic marbles, were analysed in this study. Different physical properties, such as porosity, seismic wave velocities and electrical resistivity, were measured before and after thermal treatments with heating/cooling ranges between 105 and 600°C. Microstructural observations and optical analyses were used to investigate how temperature-induced damage affects the physical measured properties of the different microstructures. This integrated approach allowed to define a generalised relationship between physical properties and thermal-induced damage, by way of an induced damage index valid for a broad suite of carbonate rocks.

Diagenetic Modifications and Reservoir Heterogeneity Associated with Magmatic Intrusions in the Devonian Khyber Limestone, Peshawar Basin, NW Pakistan

In the present study, an attempt has been made to establish the relationship between diagenetic alterations resulting from magmatic intrusions and their impact on the reservoir properties of the Devonian Khyber Limestone (NW Pakistan). Field observations, petrographic studies, mineralogical analyses, porosity-permeability data, and computed tomography were used to better understand the diagenetic history and petrophysical property evolution. Numerous dolerite intrusions are present in the studied carbonate successions, where the host limestone was altered to dolomite and marble, and fractures and faults developed due to the upwelling of the magmatic/hydrothermal fluids along pathways. Petrographic studies show an early phase of coarse crystalline saddle dolomite (Dol. I), which resulted from Mg-rich hydrothermal fluids originated from the dolerite dykes. Coarse crystalline marble formed due to contact metamorphism at the time of dolerite emplacement. The second phase of dolomitisation (Dol. II) postdates the igneous intrusions and was followed by dedolomitisation, dissolution, and cementation by meteoric calcite. Stable isotope studies likewise confirm two distinct dolomite phases. Dol. I exhibits more depleted δ18O (-15.8 to -9.1‰ V-PDB) and nondepleted δ13C (-2.05 to +1.85‰ V-PDB), whereas Dol. II shows a relatively narrow range of depleted δ18O (-13.9 to -13.8‰) signatures and nondepleted δ13C (+1.58 to +1.89‰ V-PDB). Dolomitic marble shows a marked depletion in δ18O and δ13C (-13.7 to -8.5‰ and -2.3 to 1.95‰, respectively). The initial phase of dolomitisation (Dol. I) did not alter porosity (5.4-6.6%) and permeability (0.0-0.1 mD) with respect to the unaltered limestone (5.6-6.9%; 0.1-0.2 mD). Contact metamorphism resulted in a decrease in porosity and permeability (3.3-4.7%; 0.1 mD). In contrast, an increase in porosity and permeability in Dol. II (7.7-10.5%; 0.8-2.5 mD) and dolomitic marble (6.6-14.7%; 8.2-13.3 mD) is linked to intercrystalline porosity and retainment of fracture porosity in dolomitic marble. Late-stage dissolution and dedolomitization also positively affected the reservoir properties of the studied successions. In conclusion, the aforementioned results reveal the impact of various diagenetic processes resulting from magmatic emplacement and their consequent reservoir heterogeneity.

Intriguing minerals: corundum in the world of rubies and sapphires with special attention to Macedonian rubies

This lecture text presents features of the fascinating mineral corundum and its gem varieties, ruby and sapphire. The geologic occurrences of its varieties and the best-known world localities are presented in detail. The mineralogy and crystallography of corundum, as well as its occurrence forms, are discussed. The origin of their colour varieties and their usage are described, in addition to their important physical characteristics. Applications in the watch and laser industries and as an abrasive material, among others, are included. The ruby variety from dolomite marble in North Macedonia exhibits the unique optical phenomenon known as diasporescence–the inclusion of diaspore crystals in the corundum matrix. This phenomenon affects the parting of corundum crystals and influences their colour, density and hardness. Therefore, special attention is paid here to morphological and physical characteristics of the Macedonian gem corundum.

Sr- and C-Chemostratigraphy Potential of the Paleoproterozoic Sedimentary Carbonates under Medium-Temperature Metamorphism: the Ruskeala Marble, Karelia

Comprehensive petrological and isotope-geochemical study of marble was carried out in the upper part of the Sortavala Group of the northeastern lense at the Ruskeala deposit in the Northern Ladoga area. The petrological study showed that the carbonate rocks of the Sortavala Group underwent medium-temperature low-pressure amphibolite-facies metamorphism. The mineral assemblages of Ruskeala marbles were formed at temperature of 550–600°C and pressure of ~3–5 kbar in equilibrium with a mixed water-carbon dioxide fluid with $${{X}_{{{\text{C}}{{{\text{O}}}_{2}}}}}$$ ~ 0.5–0.8. Dolomite marbles contain admixture (up to 2%) of finely disseminated carbonaceous matter and about 8–15% of calcite. Dolomites contain small amounts of Mn (70–110 ppm) and Fe (1600–3600 ppm) and high content of Sr (122–256 ppm). The initial 87Sr/86Sr ratio in the dolomites is within 0.70465–0.70522, δ13C values vary between +0.6…+1.9 ‰, and δ18О within –13.2 … –10.2‰ (V-PDB). Calcite marbles are free of carbonaceous matter, have very low contents of Mg (0.2–0.8%), Mn (10–90 ppm) and Fe (160–640 ppm) and very high Sr concentration (850–2750 ppm). The initial 87Sr/86Sr ratios in the calcite marbles range from 0.70482 to 0.70489, δ13С from 1.5 to 2.1‰, and δ18О from –10.9 to ‒8.1 ‰ (V-PDB). Tremolite-bearing marbles have higher 87Sr/86Sr ratio (up to 0.70522), while their δ13С and δ18О values decrease to 0.1‰ and –12.2‰, respectively. The metamorphism of Ruskeala carbonate even under medium-temperature amphibolite-facies conditions was essentially isochemical process, which retained the unique Sr and C chemostratigraphic potential of calcite and dolomite marbles for the reconstruction of 87Sr/86Sr and δ13C in the Paleoproterozoic seawater. The 87Sr/86Sr ratio in the 1.9–2.0 Ga Svecofennian ocean was 0.70463–0.70492, and δ13С value +1.5 ± 1‰. New Sr-isotope data record an increase in radiogenic Sr input in the ocean about 2 billion years ago, which was probably related to the growth of continental crust and more intense weathering. The δ13С in Ludicovian carbonates mark the beginning of C-isotope stasis in ocean after the Lomagundi-Jatulian anomaly of 13Ccarb.

The Beni Bousera marbles, record of a Triassic-Early Jurassic hyperextended margin in the Alpujarrides-Sebtides units (Rif belt, Morocco)

The timing and process of exhumation of the subcontinental peridotites of the Gibraltar Arc (Ronda, Beni Bousera) have been discussed extensively over the last decades. In this work, we contribute to this debate through the first mapping, structural and petrological analyses, and SHRIMP U-Th-Pb dating of high-grade marbles that crop out around the Beni Bousera antiform of the Alpujarrides-Sebtides units of northern Rif (Morocco). These marbles, here termed the Beni Bousera marbles (BBMs), instead of being intercalations in the granulitic envelope (kinzigites) of the Beni Bousera peridotites, as previously described, form minor, dismembered units within a ∼30 to 300 m thick mylonitic contact between the kinzigites and the overlying gneisses of the Filali Unit (Filali–Beni Bousera Shear Zone, FBBSZ). They display silicate-rich dolomitic marbles, sandy-conglomeratic calcareous marbles and thinly bedded marble with interleaved biotite-rich schists. An unconformable contact, either of stratigraphic or tectonic origin, with the underlying kinzigites, is observed locally. Pebbles or detrital grains include K-feldspar, quartz, almandine garnet and zircon. Peak mineral assemblages consist of forsterite, Mg-Al-spinel, geikielite (MgTiO3), phlogopite and accessory zirconolite, baddeleyite and srilankite in dolomite marble, as well as K-feldspar, scapolite, diopside, titanite and accessory graphite and zircon in calcite marble. These assemblages characterize peak HT-LP metamorphic conditions close to 700–750 °C, ≤4.5 kbar. The FBBSZ includes minor ductile thrusts that determine kinzigite horses or slivers carried NW-ward over the marbles. Within the latter, NNE-trending folds are conspicuous. Brittle, northward-dipping normal faults crosscut the FBBSZ ductile structures. Detrital cores of zircon from the BBMs yield two U-Th-Pb age clusters of ∼270 Ma and ∼340 Ma, whereas their rims yield ∼21 Ma ages. Correlations with comparable settings in other West Mediterranean Alpine belts are discussed. The BBMs compare with the Triassic carbonates deposited over the crustal units of the Alpujarrides-Sebtides. The assumed Triassic protoliths may have been deposited onto the kinzigites or carried as extensional allochthons over a detachment in the Early Jurassic during the incipient formation of the Alboran Domain continental margin. Thus, it is concluded that the Beni Bousera mantle rocks were exhumed to a shallow depth during early rifting events responsible for the birth of the Maghrebian Tethys.

Formation of clinohumite ± spinel in dolomitic marbles from the Makrohar Granulite Belt, Central India: Evidence for Ti mobility during regional metamorphism

Abstract The mobility of Ti, a member of high field strength elements, in metamorphic fluids is crucial to understand the recycling of commonly perceived nominally soluble elements and for mass-flux calculations during crustal processes. In this study, we present evidence for large-scale Ti mobility from a suite of clinohumite±spinel-bearing dolomitic marbles from the Makrohar area in central India. The studied rocks mostly contain dolomite and calcite (in subequal proportions) and a subordinate amount of forsterite. It commonly develops 1–5 cm thick, laterally continuous, mostly parallel, sometimes anastomosing, brown-colored clinohumite rich bands with variable spinel. Clinohumite has moderate Ti and F (TiO2 = 0.55–2.88 wt%; F = 0.94–1.88 wt%; n = 32). Textural and phase equilibria modeling indicate that clinohumite grew at the expense of forsterite + dolomite under static conditions due to infiltration of F- and Ti-bearing extremely H2O-rich fluids (XCO2 < 0.03), at ~5–6 kbar pressure and ~650–700 °C temperature. The Ti and F were most likely supplied by highly channelized aqueous fluids restricted within the centimeter-thick bands. The negative volume change of the reactions further facilitated fluid ingress. The lateral continuity of the bands over several meters across multiple out-crops indicates that Ti was mobile at the meter to kilometer scale. The results are in accordance with experimental studies that solubility of Ti increases in the presence of halides and imply that Ti may be much more mobile in metamorphic fluids during regional metamorphism than previously anticipated.

Multiple Episodes of Fluid Infiltration Along a Single Metasomatic Channel in Metacarbonates (Mogok Metamorphic Belt, Myanmar) and Implications for CO2 Release in Orogenic Belts

AbstractFluid infiltration into metacarbonates is a key mechanism to induce orogenic decarbonation, which influences the global carbon cycle and long‐term climate evolution. Little is known regarding the fluid pathways during episodic infiltration events and how flow patterns control time‐integrated CO2 outflux. We investigate the “vein‐like” polycrystalline mineral reaction zones (PMRZs) in dolomite marbles (Mogok metamorphic belt, Myanmar), which are formed by metasomatism via the infiltration of Si–Al–K–Ti–Zr‐bearing fluids. The petrographic textures and mineral U–Pb chronology reveal three episodes of fluid influx in a single PMRZ: (1) the initial episode (Stage‐I) transformed most dolomite into Mg‐rich silicates/oxides and calcite at ∼35–36 Ma indicated by baddeleyite cores; (2) baddeleyite rims gave ages of ∼23–24 Ma, representing a subsequent infiltration episode (Stage‐II) that modified Stage‐I minerals via a dissolution–precipitation mechanism; (3) the final episode (Stage‐III) is recorded by zircon replacing baddeleyite, which yielded ages of ∼17 Ma. Stage‐III fluid has a higher SiO2 activity and [CO2/(CO2 + H2O)] than Stage‐I/Stage‐II fluids. Thermodynamic and mass‐balance analyses indicate that Stage‐I infiltration causes >62–67% loss of CO2 by both dolomite‐consuming reactions and calcite dissolution, whereas the latter two infiltration episodes induce <12–18% loss of CO2 via calcite dissolution. Our results provide compelling evidence that repeated episodes of infiltration (each separated in time by 7–13 Ma) occurred along a single channel in marbles. The initial infiltration episode may create high‐permeability regions, offering favorable channels for later‐stage fluids that transfer obviously less CO2 than the initial metasomatism. This considerably complicates a quantitative assessment of CO2 liberation from metacarbonates during orogenesis.

X-RAY FLUORESCENCE AND STABLE ISOTOPES ANALYSES OF IKPESHI MARBLE, SOUTH SOUTHERN NIGERIA

Integration of X-ray fluor escence and stable isotope spectrometric techniques for quality assessment and provenance study of exposed marble deposit at Fakunle Quarry, Ikpeshi, South Western Nigeria constitute the fundamental aims of this research. Fourteen fresh (14) marble samples obtained at different localities within the quarry were subjected to geochemical and isotopic analyses to ascertain the quantitative abundance of major oxides and stable isotopes using X-Ray Fluorescence and Thermo Fisher mass spectrometer respectively. The major oxides revealed by XRF analysis of the marble samples are CaO, MgO, SiO2, Al2O3, Fe2O3 and Na2O with percentage composition ranging between 11.66 – 13.25, 7.75 – 9.65, 41.36 – 47.55, 12.36 – 15.23, 7.79 – 10.55 and 1.44 – 1.75respectively. Na2O + K2O value ranges between 1.48 and 1.78.The classification of marble in relation to percentage of calcite-dolomite indicate a percentage range of -5 to 4% and 93-103% for Calcite and Dolomite respectively. Chemical Index of Alteration (CIA) ranges from 45.16 to 51.59 % and Chemical Index of Weathering (CIW) ranges from 46.19 to 52.30 %. Stable isotope ( ? –180) of marble ranges from -10.50 to -7.00 with a corresponding value from 25.50 to 55.33.Interpretation of the overall results indicates an impure quartz-rich dolomitic marble; metamorphosed from a low carbonate sedimentary/meta sedimentary protolith which shallowly precipitated within a passive marginal marine environment under humid condition. The high silica impurity can however be attributed to the inordinate influx of terrigenous sediments during the precipitation process. Weathering effect is minimal on the marble deposit. Conclusively, strong correlation is apparent between the obtained geochemical result and the basement geology of the study area.

Rüdlingerite, Mn2+2V5+As5+O7·2H2O, a New Species Isostructural with Fianelite

The new mineral species rüdlingerite, ideally Mn2+2V5+As5+O7·2H2O, occurs in the Fianel mine, in Val Ferrera, Grisons, Switzerland, a small Alpine metamorphic Mn deposit. It is associated with ansermetite and Fe oxyhydroxide in thin fractures in Triassic dolomitic marbles. Rüdlingerite was also found in specimens recovered from the dump of the Valletta mine, Canosio, Cuneo, Piedmont, Italy, where it occurs together with massive braccoite and several other As- and V-rich phases in richly mineralized veins crossing the quartz-hematite ore. The new mineral displays at both localities yellow to orange, flattened elongated prismatic, euhedral crystals measuring up to 300 μm in length. Electron-microprobe analysis of rüdlingerite from Fianel gave (in wt%): MnO 36.84, FeO 0.06, As2O5, 25.32, V2O5 28.05, SiO2 0.13, H2Ocalc 9.51, total 99.91. On the basis of 9 O anions per formula unit, the chemical formula of rüdlingerite is Mn1.97(V5+1.17 As0.83Si0.01)Σ2.01O7·2H2O. The main diffraction lines are [dobs in Å (Iobs) hkl]: 3.048 (100) 022, 5.34 (80) 120, 2.730 (60) 231, 2.206 (60) 16-1, 7.28 (50) 020, 2.344 (50) 250, 6.88 (40) 110, and 2.452 (40) 320. Study of the crystal structure showcases a monoclinic unit cell, space group P21/n, with a = 7.8289(2) Å, b = 14.5673(4) Å, c = 6.7011(2) Å, β = 93.773(2)°, V = 762.58(4) Å3, Z = 4. The crystal structure has been solved and refined to R1 = 0.041 on the basis of 3784 reflections with Fo > 4σ(F). It shows Mn2+ hosted in chains of octahedra that are subparallel to [-101] and bound together by pairs of tetrahedra hosted by V5+ and As5+, building up a framework. Additional linkage is provided by hydrogen-bonding through H2O coordinating Mn2+ at the octahedra. One tetrahedrally coordinated site is dominated by V5+, T(1)(V0.88As0.12), corresponding to an observed site scattering of 24.20 electrons per site (eps), whereas the second site is strongly dominated by As5+,T(2)(As0.74V0.26), with, accordingly, a higher observed site scattering of 30.40 eps. The new mineral has been approved by the IMA-CNMNC and named for Gottfried Rüdlinger (born 1919), a pioneer in the 1960–1980s, in the search and study of the small minerals from the Alpine manganese mineral deposits of Grisons.

Formation of the giant Aynak copper deposit, Afghanistan: evidence from mineralogy, lithogeochemistry and sulphur isotopes

ABSTRACT Aynak is the largest known copper deposit in Afghanistan, with indicated resources of 240 Mt grading 2.3% Cu placing it in the ‘giant’ category. Host rocks are Neoproterozoic metasediments comprising dolomitic marble, carbonaceous quartz schist and quartz-biotite-dolomite schist containing garnet, scapolite and apatite. Chalcopyrite and bornite dominate the hypogene ore with lesser pyrite, pyrrhotite, cobaltite and chalcocite, and rare sphalerite, molybdenite, uraninite and barite. Sulphides occur as bedding-parallel laminae, disseminations, metamorphic segregations and crosscutting veins. Sulphide δ34S ratios range –14.5 to +17.3‰ in bedded and disseminated sulphides (n = 34). This broad range favours biogenic reduction of seawater sulphate as a major source of sulphur, although thermochemical reduction processes are not precluded. The narrower δ34S range of –6 to +12.2‰ in vein and segregation sulphides (n = 21) suggests localized redistribution and partial homogenization during metamorphism. Geochemical associations suggest that Al, P, Ca, Ti and Fe were primary sedimentary constituents whereas Cu, Mg, S, Se, As, Co and Bi were introduced subsequently. We infer that Aynak originated as a shale- and carbonate-hosted stratabound replacement deposit, resembling orebodies of the Central African Copperbelt, although underlying red-beds are absent at Aynak and mafic volcanics were the probable copper source. These giant deposits formed worldwide in the Cryogenian probably due to marine enrichment in copper, magnesium and sulphate coincident with profuse basaltic volcanism and ocean oxidation.

Fluid Infiltration and Mass Transfer along a Lamprophyre Dyke–Marble Contact: An Example from the South-Western Korean Peninsula

In this contribution, we report the metasomatic characteristics of a lamprophyre dyke–marble contact zone from the Hongseong–Imjingang belt along the western Gyeonggi Massif, South Korea. The lamprophyre dyke intruded into the dolomitic marble, forming a serpentinized contact zone. The zone consists of olivine, serpentine, calcite, dolomite, biotite, spinel, and hematite. Minor F and Cl contents in the serpentine and biotite indicate the composition of the infiltrating H2O-CO2 fluid. SiO2 (12.42 wt %), FeO (1.83 wt %), K2O (0.03 wt %), Sr (89 ppm), U (0.7 ppm), Th (1.44 ppm), and rare earth elements (REEs) are highly mobile, while Zr, Cr, and Ba are moderately mobile in the fluid. Phase equilibria modelling suggests that the olivine, spinel, biotite, and calcite assemblage might be formed by the dissolution of dolomite at ~700 °C, 130 MPa. Such modelling requires stable diopside in the observed conditions in the presence of silica-saturated fluid. The lack of diopside in the metasomatized region is due to the high K activity of the fluid. Our log activity K2O (aK2O)–temperature pseudosection shows that at aK2O~−40, the olivine, spinel, biotite, and calcite assemblage is stable without diopside. Subsequently, at ~450 °C, 130 MPa, serpentine is formed due to the infiltration of H2O during the cooling of the lamprophyre dyke. This suggests that hot H2O-CO2 fluids with dissolved major and trace elements infiltrated through fractures, grain boundaries, and micron-scale porosity, which dissolved dolomite in the marble and precipitated the observed olivine-bearing peak metasomatic assemblage. During cooling, exsolved CO2 could increase the water activity to stabilize the serpentine. Our example implies that dissolution-reprecipitation is an important process, locally and regionally, that could impart important textural and geochemical variations in metasomatized rocks.

Structural and microtectonic analyses of Barpak of Gorkha district, west-central Nepal

Geological mapping was carried out in the Barpak-Bhachchek area of the Daraudi River valley, Gorkha district, West-Central Nepal for structural analysis. The area comprises rocks of the Higher Himalayan Crystalline and the Lesser Himalayan Sequence. Pelitic and psammitic schist, quartzite, calc-quartzite, dolomitic marble, graphitic schist, gneiss are the main rock types within the Lesser Himalayan Sequence, whereas banded gneiss and quartzite form a significant portion of the Higher Himalayan Crystalline in the study area. The area is affected by poly-phase deformation. Lesser Himalayan Sequence has suffered five deformational phases (DL1-DL2, D3-D5) whereas the Higher Himalayan Crystalline has suffered four deformational events (DH1, D3-D5). The Lesser Himalayan Sequence lying to the northern limb of the Gorkha-Kuncha Anticlinorium is contort into doubly plunging to dome-and-basin-like en echelon type of non-cylindrical folds as Baluwa Dome and Pokharatar Basin (DL2 and D4). The direction of shearing as indicated by shear sense indicators (C' Shear band and Mica fish) is top-to-south coinciding with regional sense of shear related to the MCT propagation. The dynamic recrystallization direction, obtained from rock dominant with phyllosilicate minerals is top-to-south and coincides with mineral lineation and indicate the mineral lineation is contemporary with dynamic recrystallization during the MCT propagation.

Boron isotopic investigation of the Bayan Obo carbonatite complex: Insights into the source of mantle carbon and hydrothermal alteration

Earth's largest rare earth element (REE) ore deposit at Bayan Obo, China, is hosted in a dolomitic marble of Mesoproterozoic age, the genesis of which remains controversial, whether it is of sedimentary or igneous origin. Evidence of Caledonian-age recrystallization further complicates the interpretation of geochemical data from this complex. This study provides new insights into the petrogenetic history of the Bayan Obo host dolomite by documenting major and trace element abundances, and the first-reported δ11B values combined with radiogenic (Sr, Nd, Pb) and stable (C, O) isotope results. Boron abundances of the carbonates investigated are typical for carbonatites worldwide (0.4–1.1 ppm) and overlap with those for fresh (unaltered) mid-ocean ridge basalts (MORBs). Boron isotope ratios do not exhibit any systematic correlations with their corresponding B contents, light rare earth element (LREE) concentrations, δ13CPDB or δ18OSMOW compositions. For seven dolomite samples characterized by pristine 87Sr/86Sr(i) values (<0.70340), the δ11B compositions range from −3.6 to −9.0‰, which vary between those for typical asthenospheric (MORB) mantle (−7.1 ± 0.9‰) and an enriched mantle source. The δ11B values for the remaining samples span a wide range (−14.5 to −1.0‰) and given their corresponding more radiogenic 87Sr/86Sr(i) compositions (>0.70340) suggest hydrothermal alteration. Additionally, εNdCHUR values (1.3 Ga) define two groups corresponding to pristine samples (−1.5 to 1.2) and those containing recrystallized REE-bearing phases (−3.0 to −12.6). Together, the major and trace element abundances, δ11B values, and radiogenic isotope data for the host dolomitic marble at Bayan Obo are consistent with an upper mantle origin, and are best explained through the involvement and mixing of at least three distinct mantle reservoirs. When combined with radiogenic and B isotope results from other carbonatite complexes within China (Maoniuping, HYC- Huayangchuan, QMC- Qiganbulake mafic–ultramafic‑carbonatite complex, Miaoya, TNCC- Trans-North China Carbonatites) of varying emplacement ages (between ~26 and ~2000 Ma), these all suggest derivation from enriched upper mantle source regions during the past ~2 billion years. Comparison of the initial Pb isotope ratios reported here for Bayan Obo carbonates to those from the 440 Ma Miaoya carbonatite complex may be attributed to either a significant increase in the upper mantle's 238U/204Pb ratio under present-day China over a ~900-million-year interval, or simply reflects derivation from distinct upper mantle sources.

Lithologically controlled behaviour of the Dorozsma metamorphic hydrocarbon reservoir (Pannonian Basin, SE Hungary)

Numerous fractured hydrocarbon reservoirs occur in the metamorphic basement of the Pannonian Basin in Hungary, and among them, the Dorozsma field is one of the most productive. Here 66 boreholes penetrated the metamorphic massif, making detailed petrographic and microstructural evaluation possible. Well-log data (gamma-ray, density, neutron porosity and compressional slowness) are available for each well, which can be used to extend the petrographic information spatially.The data suggest three basic rock types in the study area, which align along a consequent rock column. In the deepest structural positions, amphibolite is typical, while the top of the basement is dominated by kyanite gneiss. Thermobarometric calculation results prove that the gneiss-rich realm was metamorphosed at a significantly higher temperature than the lowermost amphibolite, suggesting a post-metamorphic structural contact between them. The contact zone, observed in numerous boreholes, contains intensively deformed dolomite marble of a few tens of metres in thickness. The peak metamorphic temperature of this rock type is much lower than those typically seen in the two neighbouring blocks. Localising the position of the dolomite marble horizon in each well was accomplished using petrographic and well-log information, and through this investigation, a flat, low-angle shear zone became visible, cut by a set of younger normal faults. Hydrocarbon production data clearly shows that the best, most productive intervals in each well are strongly correlated to the shear zone between the gneiss- and amphibolite-dominated realms, namely the sheared dolomite marble zone.

Characteristics and timing of hydrothermal fluid circulation in the fossil Pyrenean hyperextended rift system: new constraints from the Chaînons Béarnais (W Pyrenees)

The evolution of hyperextended rift systems is linked to complex tectonic processes in which fluid activity is much more important than previously thought. This study sheds light on the fluid-sediment interactions within the distalmost parts of the Mauleon-Arzacq hyperextended basin (Western Pyrenees) focusing on the post-depositional modifications of pre- to syn-hyperextension sediments due to hydrothermal fluids. Strong field- and petrography-based evidence demonstrates the presence of fluid-related products affecting the Jurassic to Cretaceous sediments exposed in the Chainons Bearnais (easternmost Mauleon basin). These are supported by new U–Pb dating of carbonates and microthermometry of fluid inclusions showing temperatures of up to 250 °C and highly depleted δ18O and strongly enriched 87Sr/86Sr values. Two main stages of fluid activity can be defined: (i) a carbonate-rich stage leading to fabric-destructive replacement dolomitization of the pre-rift carbonates, widespread hydro-fracturation giving rise to different types of hydraulic breccias cemented by multi-phase dolomite and calcite dated at ~ 96 Ma; and (ii) a Na-SiO2-rich stage associated with authigenic albite and quartz, mainly affecting syn-rift deposits. Finally, the occurrence of dolomitic marbles and mylonites provide evidence for strong recrystallization and ductile shearing affecting the sediments during the latest stages of hyperextension at ~ 94 Ma. The Chainons Bearnais represent a primary target to investigate fluid-rock interactions linked to extensional tectonics that can be used as an analogue to compare to other fossil rift systems (e.g. Adriatic paleo-rifted margin) or present-day magma-poor, hyperextended rifted margins (e.g. Iberia-Newfoundland).