Alteration Of Limestone Research Articles

Sr-ISOTOPIC COMPOSITION OF PALEOOCEAN WATER ON THE BORDER OF 960 Ma (DATA FOR NIZHNY TUNGUSKA FORMATION OF TURUKHAN UPLIFT OF SIBERIAN PLATFORM)

At the boundary of the Meso- and Neoproterozoic, Grenville collision events were widely manifested on the margins of ancient continental blocks. Most of the sedimentary sequences that had accumulated by that time had undergone significant thermal–metamorphic changes. In many ways, this is the main reason for the lack of isotopic data for carbonate deposits in the interval of 1200–900 Ma in world practice. Sr–isotopic composition in carbonate rocks with an age of 980–920 Ma was determined in the only section of the World – the Huainan Formation of the North China Platform with accumulation time determined by U–Pb dating of detrital zircon grains. In the upper part of the Nizhny Tunguska suite of the Turukhansk uplift, among carbonate deposits, we established the presence of altered volcanic rocks, as well as weathering crust products along them – poor bauxite and chamosite ores. Based on U–Pb zircon isotope dating, the age of the volcanic rocks is 964 Ma. For the least altered limestones of the upper part of the Nizhny Tungusska Formation, received 87Sr/86Sr values are 0.70532–0.70578, which are close enough to those found in the rocks of the Huainan Formation. Geochronological age of the studied limestones gives us more correct data. These data can be used to refine the previously proposed configuration of the Sr–isotopic composition variation curve in the Early Neoproterozoic. The Nizhny Tunguska Formation is the only carbonate section of the Early Neoproterozoic in the world with correctly geochronologically substantiated dating (based on zircon from subsynchronous volcanic rocks). This compares favorably with the Huainan formation.

Sr-Isotopic Composition of Paleoocean Water 960 Ma Ago (according to the Data for the Nizhny Tunguska Formation of the Turukhansk Uplift, Siberian Platform)

The Sr-isotopic composition in carbonate rocks aged 980–920 Ma has been defined in only one place in the world, the Huainan Formation of the North China Platform, with the accumulation time limited by U–Pb data of detrital zircon grains. The upper part of the Nizhny Tunguska Formation in the Turukhansk uplift is found to contain altered volcanic rocks, as well as weathering crust products along them, such as poor bauxite and chamosite ores, among carbonate deposits. Based on U–Pb zircon isotope dating, the age of the volcanic rocks is 963 Ma. For the least altered limestones in the upper part of the Nizhny Tunguska Formation, the 87Sr/86Sr values were obtained equal to 0.70532–0.70578, which are close to those found in the rocks of the Huainan Formation. These data about the geochronological age of the studied limestones make it possible to obtain more accurate data, which allows refining the previously proposed configuration of the Sr-isotopic composition variation curve in the Early Neoproterozoic. The Nizhny Tunguska Formation compares favorably with the Huainan Formation in being the world’s only Early Neoproterozoic carbonate sequence that is correctly geochronologically substantiated (based on zircon from subsynchronous volcanic rocks).

Petrographic and Geochemical Inferences for Genesis of Terra Rossa: A Case Study from the Apulian Karst (Southern Italy)

Terra rossa is a reddish clay soil which is often present on the surface of limestone in regions with a Mediterranean-type climate. Its genesis is a controversial subject in terms of the origin of the parent material, from the residuum of underlying (carbonate/dolomite) bedrock in the absence/presence of an external silicate contribution (e.g., aeolian dust). Within this context the main goal of the present work was the understanding of the geochemical processes leading to the formation of the terra rossa starting from the carbonate bedrock. We report the results of a multi-method analysis on a terra rossa deposit occupying the bottom of a Quaternary karst depression on Mesozoic limestones exposed in the Murge area (Apulia Foreland, southern Italy). Geological, petrographic, textural, and chemical data were collected on karst products (reddish calcite incrustations and nodules, and fine-grained portion of terra rossa) by a detailed field mapping, optical microscopy, XRF and fusion ICP/MS analyses and by scanning electron microscope. New collected data show that the mineralogical composition of reddish incrustations and nodules is comparable, consisting of fibrous and impure calcite, detrital fragments of quartz, K-feldspar, zircon and authigenic minerals as (Mn, Ba, Ca) phases, (Al, Si, Mn, Fe, Mg, Ba, Ca) minerals, Fe-kaolinite and anatase. The prevailing minerals, instead, in the fine-grained portion of terra rossa are hematite, kaolinite, and goethite. Based on the chemical composition, and especially on REE patterns, a progressive interaction of silicate aqueous solutions (with Al, Si, Fe), containing pelite material, with the calcareous bedrock, as a source of carbonic acid, was the process driving the formation of terra rossa. Obtained results add new elements to the definition of the long-lasting question about the genetic processes responsible for the formation of terra rossa, corroborating their polygenetic origin, as result of limestone alteration in conjunction with the chemical interaction with allochthonous siliciclastic material.

Biogenetic peloidal micrites within coral skeletons cause geochemical anomalies in reef limestones, Zhongsha Atoll, South China Sea

High-magnesian calcite (HMC) peloidal micrite is a ubiquitous synsedimentary cement observed in abundance in the Holocene cryptic niches of coral intraskeletal pores and borings at Zhongsha Atoll in the South China Sea. In this study, we assessed the petrographic and geochemical effects of these intraclasts during early diagenesis. Peloidal micrites with an average diameter of 20–40 μm were composed of radially arranged rhombic micro-crystals of HMC (∼20 mol% MgCO3) and massive micritic nuclei. Some living bacterial/algal clumps in peloidal micrites are associated with bioluminescence and manifest an organic-rich micro-environments. Fragmented calcified cyanobacterial sheaths, which are components of the peloidal nuclei, further enhance biogenetic carbonate precipitation throughout peloidal micrite formation. The stable isotope characteristics of the peloidal microfabrics (average δ13C of 2.55 ± 0.15‰ and δ18O of −1.64 ± 0.25‰) suggest that photosynthesis and heterotrophic decomposition promoted peloidal micrite precipitation by increasing the carbonate saturation. Plentiful peloidal micrites within the intraskeletal pores also influenced the geochemical signatures of the bulk reef limestone. Owing to the presence of mixed HMC peloidal micrites, the δ13C and δ18O values of the bulk limestone (0.26 ± 1.04 and-3.82 ± 1.02‰, respectively) were elevated and fluctuated widely compared with pristine coral skeletons (−3.43 ± 0.48‰ and − 5.75 ± 0.278‰, respectively). Enriched Mn and depleted Sr contents led to an increase in the Mn/Sr diagenetic proxy. The co-occurrence of HMC peloidal micrites and elevated Mg/Ca, Mn/Sr, δ13C, and δ18O ratios indicate that the HMC peloidal micrites played an essential role in these geochemical variations. We, therefore, propose performing a comprehensive diagenetic evaluation before geochemical analyses of reef limestone. In such cases, selective micro-analysis of pristine areas in altered limestone would yield more robust reconstructions based on corals.

Integrated static and dynamic geophysical and geomechanical data for characterization of transport properties

The observatory of transfers in the vadose zone (OZNS-France) offers a unique support for characterizing the Beauce limestone aquifer at various spatial and temporal scales. This observatory consists of a large well associated with external boreholes dedicated to imaging, monitoring, and understanding mass and heat transfers through the vadose zone. An initial geological characterization of core samples gave valuable information on the facies encountered within the vadose zone. It is composed of highly incoherent limestones down to 7 m-depth and a massive and altered limestone rock layer from 7 to 20 m-depth. The latter presents fracturation and karstification increasing with depth. This study focuses on the hard limestone rock units and aims to quantify the level of heterogeneity through static and dynamic laboratory tests performed on core samples. Along 20 m of three boreholes, representative samples are tested through uniaxial or triaxial conditions. Elastic wave velocities are also measured. The obtained Young’s modulus, compression strength, and P- and S-wave velocities highlight strong variations with depth. For example, at 14 m-depth, uniaxial maximal strength ranges from 13 to 123 MPa. In addition, at 16 m-depth, P-wave velocities are distributed from 3,650 to 5,700 m.s −1 . These large scatterings confirm the strong heterogeneity of the geological formations. It is interpreted in terms of a high porosity, degree of fracturation and/or initiate active karstification. The double porosity is discussed from the difference between the static and dynamic Young’s modulus. Their variations are interpreted in terms of connected porosity and important crack density. This last parameter is estimated from the P- and S- wave velocities, and ranges from 0 to 0.43, with an extremum value of 1.21 around 19 m-depth. These values and variations are in good correlation with log imaging and independent permeability laboratory measurements made on the hard limestone rock units. • Coupled methods allow to characterize transport property of heterogeneous limestones. • Comparison of static and dynamic data shows a strong variation in double porosity. • High heterogeneities are in good correlation with permeability measurements.

ANALYSIS OF THE CAUSES OF CIRCULATION LOSS IN THE LOWER CRETACEOUS DEPOSITS DURING THE HORIZONTAL DRILLING UNDERCROSSING THE DANUBE RIVER (GIURGIU-RUSE SECTION)

The geotechnical, electrometric and magnetometric investigation data, correlated with the technological data from the horizontal directional drillings made for the undercrossing of the Danube River with gas pipelines, showed that in the Giurgiu-Ruse section, under a clastic sequence of 14-19 m thickness, consisting of layers of sandy clays, sands and white-grey, alluvial gravels, are developed Cretaceous grey altered limestones, yellowish-brown lumachelle limestones and grey-pink, compact limestones. The altered, eroded, and karstified limestones seems to be the explanation of drastic circulation loss of mud in the technological process of digging through horizontal directional drillings. In the studied studied, on the Giurgiu-Ruse alignment in the southern area of the Moesian Platform (Wallachian sector), there are several relevant papers occasioned by the construction of the Saint Gheorghe harbour channel related to the Giurgiu harbour, at the time of the 19th century (1888-1890), as well as the papers related to the investigations occasioned by the construction of the Giurgiu-Ruse Bridge in 1959. Older or newer papers are added, relating to the carbonate deposits of different ages of the Cretaceous in the outcrops of the Bulgarian bank of the Danube, as well as descriptions from drillings adjacent to the studied section.

Constraints on the preservation of proxy data in carbonate archives – lessons from a marine limestone to marble transect, Latemar, Italy

AbstractThis work evaluates an exceptionally complex natural laboratory, the Middle Triassic Latemar isolated platform in the northern Italian Dolomite Mountains and explores spatial and temporal gradients in processes and products related to contact metamorphism, dolomitization and dedolomitization of marine limestones. The relation between petrographic change and re‐equilibration of geochemical proxy data is evaluated from the perspective of carbonate‐archive research. Hydrothermal dolomitization of the limestone units is triggered by dykes and associated hydrothermal fluids radiating from the nearby Predazzo Intrusion. Detailed petrography, fluid inclusion analysis, δ13C and δ18O data and 87Sr/86Sr isotope ratios shed light on the extreme textural and geochemical complexity. Metamorphic and diagenetic patterns include: (i) peak‐metamorphic and retrograde‐metamorphic phases including three dolomite marbles, two dedolomite marbles, brucite, magnesium silicates and late‐stage meteoric/vadose cement at the contact aureole; (ii) four spatially defined episodes of dolomitization, authigenic quartz, low magnesium calcite and late‐stage meteoric cement at the Latemar isolated platform; and (iii) kilometre‐scale gradients in δ13C values from the contact aureole towards the platform interior. Results shown here are relevant for two reasons: first, the spatial analysis of alteration products ranging from high‐grade metamorphic overprint of marbles at temperatures of 700°C in the contact aureole to moderately altered limestones in the platform interior at temperatures <100°C, allows the observation of processes that commonly occur along vertical (prograde) gradients from shallow burial to metamorphism at depths >20 km. Second, under rock‐buffered conditions, and irrespective of metamorphic to diagenetic fluid−rock interactions, both marbles, and low‐temperature hydrothermal dolomites have conservative marine δ13C and δ18O values. The fact that metamorphism and hydrothermal dolomitization of precursor limestones and early diagenetic dolostones did not per se reset environmental proxy data is of interest for those concerned with carbonate archive research in Earth’s deep time.

Mississippian chat and tripolite zones in Osage County, Oklahoma: Paleokarst interpretation based on 3D seismic and well logs

Mississippian paleokarst chat and tripolitic chert (tripolite) zones associated with the Mississippian Lime have been hydrocarbon exploration targets in Osage County for many decades. Chat is residual chert, either in place or transported, weathered out of chert-bearing Mississippian Limestone that was eroded at the Mississippian-Pennsylvanian unconformity. Thus, the formation of chat is an epigene paleokarst process. Tripolite occurs as a highly porous, silica-rich interval within the Mississippi Lime. It is formed by in-place alteration of limestone by silica-rich surface waters or deep-seated hydrothermal fluids, making tripolite formation a mixed or hypogene paleokarst process. Here, we have studied chat and tripolite by seismic analysis calibrated by well control with full-wave sonic and density log data. We identify that chat and tripolite show clear separation from Mississippian Lime log-based acoustic impedance and [Formula: see text]/[Formula: see text], but there is no meaningful separation of chat from tripolite, and they both exhibit total porosities greater than 20% with evidence of fracture porosity. We find that the sonic-based normal incidence wedge model for chat bounded above by the Pennsylvanian Shale and below by the Mississippian Lime indicate that two seismic expressions are plausible: first, a strong negative amplitude when the chat thickness is above the tuning thickness (for this survey it is 56 ft) and, second, a weak positive or negative amplitude associated with the small impedance contrast between chat and overlying Pennsylvanian Shale. Our analysis suggests that the traditional chat “strong response” and a new “dim-out” exploration strategy may be usefully applied in Osage County. We show that the tripolite response is consistently a negative amplitude event that strengthens with increasing tripolite thickness. We provide an interpretive framework for characterizing chat and tripolite zones associated with the Mississippian Lime in the U.S. midcontinent, which may apply to regions around the world.

Sr Isotope Chemostratigraphy and Pb–Pb Age of the Riphean Carbonate Deposits of the Kharaulakh Uplift (Northeastern Margin of the Siberian Platform)

Abstract —The least altered limestones of the Ukta and Eselekh formations in the Precambrian section of the Kharaulakh uplift have a minimum 87Sr/86Sr ratio of 0.70673–0.70715. The lowest 87Sr/86Sr ratio of the overlying Neleger and Sietachan formations is 0.70791–0.70817. Based on these data, along with the earlier obtained positive δ13С values (up to 8‰) for the Kharaulakh section, we have estimated the age of the Ukta and Eselekh formations at 800–670 Ma and the age of the Neleger and Sietachan formations at ~640–580 Ma. The Pb–Pb isochron age of the least altered limestones of the Eselekh Formation calculated from eight samples is 720 ± 30 Ma. This age permits us to define the lower part of the Kharaulakh section of the Ukta and Eselekh formations to be the late Tonian of the International Chronostratigraphic Chart or to the Upper Riphean of the General Stratigraphic Scale of Russia. The presence of reliably dated Upper Riphean sediments in the Kharaulakh uplift indicates a more complex structure of the Precambrian sedimentary cover on the Arctic margin of the Siberian Platform than assumed earlier.

Reactivity of secondary phases in weathered limestone using isotopic tracers (D and 18O)

For a long time, limestone has been massively used in stone building and monuments because of its easy extraction and common presence in the landscape. On ancient monuments, mostly built in urban areas, it is exposed to urban-borne pollutants responsible for specific alteration mechanisms and weathering kinetics. Especially, the dissolution of calcite and the precipitation of new phases will affect the limestone pore network, modify the stones capillary properties, and influence the further alteration. In order to better understand these processes, an altered limestone sample from 'Tribunal Administratif' (TA) in Paris was studied. The main secondary phase was found to be syngenite, which can be explained by the location of the sample close to the soil, a potential source of K (fertilizers). This phase is more soluble than gypsum that is commonly found on altered limestone. In order to assess the reactivity of the system (limestone and new phases), oxygen and hydrogen isotopes were used to trace the transfer of water (D218O) and identify the location of the reactive areas (susceptible to alteration). For that, TA samples were exposed in a climatic chamber to relative humidity (RH) cycles (25% RH for 2.5 days and 85% RH for 4.5 days) for 2 months with a D218O vapor to simulate alteration occurring in conditions sheltered from the rain. Results have shown that the water vapor easily circulates deep in the sample and reacts preferentially with syngenite the most reactive phase (compared with calcite and quartz). This phase could evolve in gypsum when exposed to an environment different from the one resulting in its formation.

Influence of Nano Fluid on Interfacial Tension oil/water and Wettability Alteration of limestone

Nano materials have a great potential for enhanced oil recovery applications. The present research investigated the effect of nanoparticles on interfacial tension and wettability alteration. The silica oxide nanoparticles with different size 52, 65 and 5nm were used. The wettability alteration was evaluated by dipping the limestone rock in different concentrations of Nano silica (0.01-1wt. %) for a certain silica size suspended in salinity water. Measurements performed in three phase system (oil/water/solid) using pandel droplet and sessile drop method respectively by opitical-tensiometer. Results showed that Nano fluid can significantly reduce contact angle and alterated rock wettability with increasing Nano-silica concentration due to their high surface area in which make it more reactive, yielding the best results when using 0.01 wt.% Nano silica that decreased contact angle to (29°, 59°, 69°, 26°) for 10, 52, 65nm Nano silica size and modified Nano silica respectively for gas / liquid / solid system. in addition, for liquid / liquid/solid system, 0.01 wt.% Nano silica was decreased contact angle to (32°, 36°, 41°, 30°), interfacial tension to (2.26, 2.8, 3, 3.6Nm) for 10, 52, 65nm Nano silica size and modified Nano silica respectively, Interfacial tension decreased with increasing Nano silica concentration, but NPs size have slightly effect on interfacial tension.

Investigation of limestone and dolomite clast alteration in petrocalcic horizons, southern Nevada, USA

Petrocalcic horizons formed on Quaternary surfaces in southern Nevada contain altered limestone and dolomite clasts in the enveloping carbonate matrix. Three profiles from two geomorphic surfaces contain pebble to cobble clasts with distinct voids and alteration textures at their tops. Clast and alteration zone characteristics were analyzed with scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, electron microprobe analysis, and with an inductively-coupled plasma spectrophotometer. Limestone and dolomite clasts display considerable top-down alteration marked by vertical tubular voids and an increase in neoformed fibrous clay in the altered regions. Downward percolating soil waters moved through the cemented horizon and contacted the upper clast boundaries as evidenced by selective alteration and void development at the tops of clasts. The preservation of voids indicates that alteration and void development began no earlier than initial petrocalcic formation when soil matrix pore spaces were largely filled with pedogenic carbonate. Undersaturation of soil fluids with respect to primary clast components (Ca and/or Mg) and/or increased pCO2 under conditions enhanced during wetter (pluvial) climates is interpreted to drive dissolution and alteration of clasts in these horizons. Resulting morphological and mineralogical changes in these petrocalcic horizons may mark past climate and geomorphic influence on their development over time. Misinterpretation of parent material provenance as a result of selected removal of individual clast lithologies may occur.

Deep seated fault‐related volcanogenic H2S as the key agent of high sinkhole concentration areas

AbstractKarst collapse sinkholes (KCS) are a peculiar karst morphology resulting from the collapse of the topographic surface caused by subsurface karstification. In the Southern Apennines these phenomena are not randomly distributed but concentrate in several zones, named High Sinkhole Concentration Areas (HSCA), showing peculiar geological, structural and hydrogeological conditions. Gas vents and mineral springs occur widely in these HSCA, and are often stricty related to the KCS. Starting from four representative areas, the aim of this study is to ascertain if there is a genetic link between peculiar mineralogical vs geochemical features of densely fractured/altered rocky masses in KCS, gas vents/springs occurrences and diffuse hypogenic karstification. By means of a multidisciplinary approach (geological/geomorphological, mineralogical and geochemical) we selected and analysed four different kinds of samples related to KCS, gas vent and spring occurrences: (i) altered to deeply altered limestones sampled in the sinkholes; (ii) unaltered limestones sampled in close proximity to sinkhole areas; (iii) gypsum‐rich crusts/patinae, precipitated near to gas vents; (iv) gypsum‐ and calcite‐rich precipitates occurring at springs or inside caves. Among neoformed non‐carbonate minerals, gypsum is virtually ubiquitous, halides (fluorite, halite, sylvite) also occur in small to trace amounts. The mineralogical assemblages of the different samples show similarities and may also be compatible with hypogenic speleogenesis and with a process of alteration of the carbonate bedrock by means of uprising mineralizing fluids along structural discontinuities. Stable isotopic compositions (S, O) display strong variability in δ34S and δ18O for sulfate in the different areas, but a deep‐seated sulfur source can be hypothesized for many of the studied KCS‐related samples. This study has important implications for the relationships between areas of high concentration of sinkholes, regional fault systems, mechanical characteristics of rocks and the high seismicity typical of these areas of the Southern Apennines. © 2018 John Wiley & Sons, Ltd.

Ghost-rock karstification of Devonian limestone flooring the Athabasca Oil Sands in western Canada

This study interprets a new style of ghost-rock karstification at a site in northeast Alberta, western Canada. The karstic area of the Upper Devonian paleotopography was localized at the western end of the Bitumount Trough, a 50 km-long collapse structure that resulted from removal of a 120 m thick interval of salt beds of the Middle Devonian Prairie Evaporite only 200 m below. Ghost-rock karstification developed in massive light grey limestone beds that alternate with chlorite-rich argillaceous intervals, of the Moberly Member, Devonian Waterways Formation. These weathered rock profiles expanded latterly and were deepened by slowly moving hydraulic gradient-driven phreatic groundwater descending along clusters of closely spaced 2–10 cm-wide joints that cross-cut the fracture-shattered limestone beds. The uppermost beds of greyish green chlorite-rich argillaceous limestone were decalcified as the flows along the joint clusters carried insoluble residues consisting of quartz silt mixed with illite-chlorite clays into the substrate. These chlorite-rich insoluble residues accumulated as replacement fabrics within decalcified porous zones of the altered limestone in the substrate. As the joint opening plugged with insoluble residues within the deepened decalcification weathered rock intervals, the alterite zones expanded laterally to form vertical pod-shaped structures. These 5–20 m in diameter and 10–20 m-deep pseudo-sinkhole zones consisting of alterite retained ghost-rock traces of the original limestone strata. This area was subsequently covered by the Lower Cretaceous Athabasca Oil Sands.

Elemental geochemistry and strontium-isotope stratigraphy of Cenomanian to Santonian neritic carbonates in the Zagros Basin, Iran

A Neo-Tethyan upper Cenomanian–Santonian neritic carbonate ramp succession (Sarvak and Ilam formations), drilled in the Zagros Basin in southwest Iran, was investigated via detailed sedimentology, microfacies analysis, elemental geochemistry and Sr-isotope stratigraphy (SIS). The succession contains two exposure surfaces, which are known as the CT-ES and mT-ES (Cenomanian–Turonian and middle Turonian, respectively), and associated prominent negative carbon-isotope excursions that represent important regional stratigraphic marker horizons. Precise knowledge about the onset of platform exposure and the duration of the exposure-related hiatus, however, is currently lacking due to a rather low-resolved shallow-water biostratigraphic framework and a bulk carbonate carbon-isotope pattern that clearly differs from global Late Cretaceous reference curves. Therefore, the existing bio-chemostratigraphic framework was complemented by bulk carbonate strontium-isotope stratigraphy (SIS). As bulk carbonate material is in particular prone to diagenetic alteration, a careful selection of least altered samples has been carried out by means of elemental geochemistry and petrography. In contrast to what could be expected, the meteoric alteration of limestones beneath both exposure surfaces is not clearly expressed by increasing iron and manganese and coeval decreasing strontium contents. On the contrary, the impact of meteoric diagenesis is well illustrated via pronounced increases in Rb concentrations and concomitant prominent positive shifts to radiogenic strontium-isotope values, an observation that clearly reflects the decay of continentally derived 87Rb into 87Sr. Rubidium corrected strontium-isotope values place the CT-ES around the Cenomanian–Turonian boundary and point to an exposure duration of less than 0.4Myr. This rather short-term CT-ES related hiatus is supported by petrographic evidence, which indicates a youth karstification stage of strata beneath the CT-ES. Following SIS, the Ilam–Sarvak transition at the top of Nezzazatinella–Dicyclina interval zone coincides with the mT-ES. Carbonates placing above this transition (Ilam Formation) are ascribed to the earliest to latest early Santonian, while carbonates immediately beneath the mT-ES (Sarvak Formation) are dated as late Turonian. SIS thus indicates a long-lasting hiatus of 4.5Myr associated with the mT-ES. Emergence represented by the CT-ES is here proposed as harbinger of the mT-ES in the Zagros Basin, which most likely resulted from stepwise peripheral bulging due to ophiolite obduction in combination with a small-scale global sea level fall around 94Ma.

Stable Isotope and Fluid Inclusion Constraints on the Source and Evolution of Ore Fluids in the Hongniu-Hongshan Cu Skarn Deposit, Yunnan Province, China

The Hongniu-Hongshan Cu skarn deposit (77.8 Mt at 1.8% Cu) is located in the central part of the Zhongdian porphyry and skarn Cu belt in southwestern China. Skarn and orebodies occur mainly between the different units of the Upper Triassic Qugasi Formation or within altered limestone adjacent to Late Cretaceous intrusions (78–76 Ma). Three main paragenetic stages of skarn formation and ore deposition have been recognized on the basis of petrographic observations: (1) pre-ore-stage hornfels with diopside (Di87–72Hd12–7), small-scale endoskarn with reddish grossular (Adr22–57Gr78–43), diopside (Di83–92Hd7–15), vesuvianite, and abundant exoskarn with red-brown andradite (Adr75–98Gr2–22), sahlite (Di28–41Hd58–71), and wollastonite; (2) syn-ore-stage retrograde minerals, sulfides (pyrite, chalcopyrite, pyrrhotite, molybdenite, galena, and sphalerite), quartz, and calcite; and (3) post-ore-stage calcite veins. Sulfur isotope values of sulfides are relatively high, with an average δ 34S = 4.9‰ (n = 40), suggesting that the ore-forming fluid was magmatic and that the sulfides precipitated from a relatively reducing ore fluid. The coexistence of silicate melt and primary fluid inclusions in quartz phenocrysts of the mineralization-related quartz monzonite porphyry indicates the simultaneous entrapment of fluid and melt, and records the process of the aqueous fluid exsolving from the crystallizing melt. The initial single-phase fluid has a salinity of 8.8 to 12.7 wt % NaCl equiv and homogenization temperatures of 566° to 650°C, corresponding to pressures of 680 to 940 bar and lithostatic depth of 2.5 to 3.5 km. The primary fluid inclusions in the pre-ore-stage garnet and pyroxene composed of coeval vapor-rich (V type) and halite-bearing (S-I and S-II types containing sylvite) inclusions (32–>79 total wt % salts) share similar homogenization temperatures (450°–550°C), indicative of the occurrence of fluid unmixing under lithostatic pressures of ~550 to 780 bar (>2.0-km depth). Primary fluid inclusions trapped in syn-ore quartz, calcite, and epidote show the common development of S-type inclusions (~37.3 wt % NaCl equiv) with coexisting V-type, liquid-rich (L type), and CO2-bearing (C-I type) inclusions, all of which have homogenization temperatures of 300° to 400°C and trapping pressures of 100 to 400 bar (~1.5-km depth). Brine inclusions homogenized by halite dissolution after vapor disappearance in both the pre- and syn-ore stages are interpreted to have been trapped under overpressured conditions (>1,520 bar). Oxygen isotope analyses were conducted on garnet, wollastonite, epidote, quartz, and calcite. The pre-ore-stage garnet and wollastonite have δ 18Ofluid values of 5.6 to 8.1‰, whereas the syn-ore-stage epidote, quartz, and calcite have more variable δ 18Ofluid values in the range of 3.9 to 17.5‰. The δ 18Ofluid values of the post-ore-stage vein calcite (15.2–21.3‰) are much higher than both the pre- and syn-ore stages. The vapor phase of inclusions contains H2S, CH4, and C2H6 in the syn-ore stages. All these observations reveal that (1) the formation of the Cu skarn deposit was dominated by a magmatic hydrothermal system, (2) multiple fluid pulses contributed to the formation of the pre- and syn-ore-stage skarn minerals and sulfides, and (3) the increase in pH due to the neutralization of the acidic fluid could be the main factor controlling the large-scale ore deposition in Hongniu-Hongshan.

Laser cleaning and laser-induced breakdown spectroscopy applied in removing and characterizing black crusts from limestones of Castello Svevo, Bari, Italy: A case study

The laser cleaning process combined with laser-induced breakdown spectroscopy (LIBS) were applied to restore and characterize altered limestones of the ancient jamb of the historic entrance gate of Castello Svevo, Bari, Italy. This area of the masonry blocks of the limestone castle was chosen because of its evident degradation with an apparent deposit of black crusts. The combination of a Q-switched Nd:YAG pulsed laser with the diagnostics typical of the LIBS technique was shown to be very effective for monitoring, controlling and characterizing the laser cleaning process of limestone. The different elemental compositions of the black encrustations covering the stone surface and the underlying stone allowed to evaluate and avoid over-cleaning and/or under-cleaning. Further, coupling LIBS to the cleaning process provided important information about the optimal experimental conditions to be used for evaluating the conservation status and determining the most proper cleaning restoration procedure before operating the consolidation of the blocks. Thus a sufficient removal of unwanted layers could be achieved without modifying the surface underneath and ameliorating the effectiveness of traditional cleaning techniques. In this work, the elemental composition of the ablated black crust and the underlying stone were determined by the spectroscopic study of plasma emitted from either a single pulse (SP) or a double pulse (DP) LIBS configuration. With respect to SP LIBS, a marked enhancement of the signal emission was observed by DP-LIBS used after a previous stratigraphic DP-LIBS assessment of the cleaning depth.

Thermal waters in the Lower Yarmouk Gorge and their relation to surrounding aquifers

Sources and mineralization of groundwaters from Cretaceous A7/B2 and B4 limestone aquifers in the Golan and Ajloun Heights are derived from conjoint discussion of stable isotopes of water and sulphate, major ion chemistry, rare earth distribution patterns and the (hydro)geological setting. Isotope fractionation of precipitation was used to derive elevation and continental influence of recharge areas. Positive shift of δ18O and δ2H occur due to evaporation before replenishment and the interaction with basalts. Major infiltration areas for Golan Heights are the limy foothills of the Mt. Hermon and for the Ajloun Heights the Plateau itself. To a less degree, precipitation infiltrates also the basaltic catchments. Groundwaters are mineralized by water/rock (i) variably altered limestones by diagenesis, (ii) evaporates and seawater brines enclosed in limestone matrix and (iii) locally occurring basaltic intrusiva. In the Yarmouk Gorge, a deep-seated brine of the Ha’on type ascends and mixes with the fresh shallow groundwater.Contrasting the macro chemical composition, REY distribution patterns indicate the lithology of the recharge areas. Infiltration over basalts results in REY patterns distinctly different to those derived from limy catchments.REY and isotope fingerprints prove that water from the Syrian Hauran Plateau is recharging springs and wells in the lowermost Yamouk gorge. The basaltic cover of the Golan Heights is of minor importance for recharge of the underlying A7/B2 aquifer, which becomes recharged at the foothills of Mt. Hermon and gets confined southwards. Even the wells in the basaltic cover receive water from the underlying limestone aquifer.Anomalous heat flux near the Yarmouk gorge and locally in the western escarpment of the Ajloun may be produced by ascending fluids from greater depth and/or by basaltic intrusions.

Tamarugite-bearing paragenesis formed by sulphate acid alteration in Diana Cave, Romania

An exceptional new occurrence of the mineral tamarugite, NaAl(SO 4 ) 2 · 6H 2 O, from a short karst cavity (Diana Cave, Baile Herculane; SW Romania) is described. It was formed by corrosion of the bedrock (limestone and marls) by a SO 4 2− -rich steam condensate resulting from oxidized S 2− ions escaping from the thermo-mineral water emerging from depth in the cave. Tamarugite forms dull white earthy aggregates. Scanning-electron microscope (SEM) observations reveal tabular subhedral crystals never exceeding 15 μm across. The cell parameters refined from the powder data for the monoclinic space group P 2 1 / a are: a = 7.358(6), b = 25.23(2), c = 6.093(5) A, β = 95.16(5) °, V = 1126.98(1) A 3 . The δ 34 S values of the cave sulphates and the thermal water confirm marine evaporites as the source of sulphur. The sulphate-acid alteration of limestone with contribution of Al 3+ and Na + from the marls and the thermal water is responsible for the formation of tamarugite. The steam-condensate alteration paragenesis includes native sulphur, bassanite, anhydrite, epsomite, pickeringite, halotrichite, apjohnite and alunite, as well as quartz and halite, all primary and secondary speleogenetic by-products.

Weathering of carbonate materials in ancient Maya constructions (Río Bec and Dzibanché): Limestone and stucco deterioration patterns

Carbonate materials – limestone and lime stuccos – were widely used in ancient Maya construction. Deterioration of these materials presents a major problem for the conservation of Maya architectural heritage. However, the mechanisms of deterioration and their interaction with the diverse inner structures of these materials are still insufficiently understood. We performed micromorphological and mineralogical research of the limestone alteration in the buildings of Río Bec and stuccos in Dzibanché, two Late Classic Maya sites in the southern Yucatan peninsula. Dissolution and recrystallization of calcite were found to be universal and the most important weathering process in both materials. Dissolution is selective, affecting mostly micrite. Larger crystals, especially rhombohedral calcitic pseudomorphs developed after dolomite, are more stable. Selective dissolution of micritic cement of limestones and stuccoes enhances physical breakdown by producing numerous pores and loosening bounds between coarser inclusions. This process clearly controls the deterioration grade of different limestones: those with more intensive fragmentation and powdering have higher contents of micrite. Secondary carbonates are represented mostly by acicular calcite filling fissures and forming coatings on the outer surface. Clear spatial relation to fungi and roots and their similarity to well-known soil neoformations indicate biogenic origin of these neoformations. In most cases, they contribute to macroscopic powdering being loose. However, in the cracks of some limestones they form dense infillings, which could plug the fractures and prevent further advance of weathering to the core of the stone. Neoformed sulfates and iron oxides are fewer, but also participate in the deterioration.