Horizontal Veins Research Articles

The impact of the suction process on the blood flow in the peristaltic horizontal vein of Henle (Gastro-colic venous trunk GCT)

The impact of the suction process on the blood flow in the peristaltic horizontal vein of Henle (Gastro-colic venous trunk GCT)

Study on the Color Genesis of South Red Agate and the Geological Characteristics of the Siliceous Gravel Layer

The aim of the study was to explore the gem mineralogical characteristics of southern red agate and then analyze its structural characteristics and color causes. The mineral composition, structure, and chromogenic minerals of Nanhong agate were tested by a conventional gemmological test and observed under the microscope, infrared spectrum, and microlaser Raman spectrum. It is found that the color of southern red agate samples is mainly dark red or orange red, and colorless, white or yellow bands can be seen in some samples; the relative density is in the range of 2.59∼2.65. The matrix structure of the sample is mainly long fiber and short fiber, and some of them are fine-grained and crystalline plasmid. There are concentric ring-banded structures, and some horizontal banded and vein structures can also be seen. The analysis and test results show that the south red agate has a typical radial fiber structure, and the main chemical component is SiO2; Under the orthogonal polarizing microscope, a large number of round reddish brown mineral inclusions with banded distribution can be seen, the spot-colored mineral is iron oxide hematite, and calcite of the late hydrothermal origin can be seen in the cracks. In addition, the study shows that the formation of south red agate is multistage and the formation of hematite inclusions is also multistage.

Skempton’s poroelastic relaxation: The mechanism that accounts for the distribution of pore pressure and exhumation-related fractures in black shale of the Appalachian Basin

Middle and Upper Devonian gas shales of the Appalachian Basin host a sequence of mode I cracks from early veins to late unmineralized joints. These fractures develop during a cycle of burial and exhumation starting with horizontal veins, then vertical veins, then vertical joints, and ending with pancake joints. Veins are most common in the cross-fold orientation in gas shale of the deep hinterland, whereas joints are common in the shallow foreland fringe. Of particular interest for the recovery of natural gas are joints striking northeast to east-northeast in shallow core and outcrops of Appalachian gas shales, particularly in the underpressured, southwestern part of the basin. Northeast to east-northeast fractures are sparse in the deeper, northeastern part of the basin where the Marcellus gas shale is highly overpressured. The common occurrence of unmineralized joints in shallow gas shales is best explained with a stress–pressure–depth model for the evolution of pore pressure in the Appalachian Basin. Our model shows that by Skempton’s poroelastic relaxation, gas-charged porosity can drive natural hydraulic fractures during exhumation. In the shallow foreland fringe of the Appalachian Basin, exhumation was sufficient to favor the propagation of east-northeast–striking neotectonic joints in the contemporary tectonic stress field. These joints appear mainly at shallow depths where they facilitate leakage to form an underpressured gas column trapped by groundwater (i.e., Big Sandy gas field, Kentucky). Exhumation was insufficient for Skempton’s poroelastic relaxation to favor propagation of east-northeast joints in the deep, central part of the basin where capillary forces maintain overpressured gas shale.

Structural control and Sn-Ta-Nb mineralization potential of pegmatitic bodies in Numbi, South Kivu Eastern D.R Congo

The Numbi pegmatites of South Kivu Province (D.R.C) in the Kibaran belt which intruded the Mesoproterozoic Formation (1.6-0.9 Ga) was investigated to elucidate the structural imprints and geochemical characteristics related to the pegmatitic rocks associated with Sn - Nb - Ta mineralization, source(s) of the mineralisation, processes responsible for its formation and the economic viability of Tin-Niobium-Tantalum mineralisation. The structural features include: the pegmatitic veins with orientation from N150°E to N160°E characterized with direction and dip values averages of N84°E/13°SSE for horizontal veins and N138°E/73°WSW for subvertical veins. From the different stresses, the pegmatite and quartz veins occurred during the same tectonic phase. The characteristic high SiO2, Al2O3, Fe2O3 and Na2O, indicates peraluminous and S-type pegmatite, ferroan and medium to high K, characteristics of a collisional tectonic environment. The pegmatite showed an enrichment of Rb (114.70 – 1000 ppm), Ta (27.50 – 370.90 ppm), Cs (28.90 – 682.80 ppm), Sn (238 – 2451 ppm) and Nb (31.80 – 139.20 ppm) trace element, consistent with indices for mineralisation. The La/Yb vs. La and Sm/Yb vs. La/Sm diagrams established magmatic compositional heterogeneities and/or mixing in the lava suits that formed the pegmatites. Variation plots of Ta vs Ga, Ta vs Cs, Ta vs Cs + Rb and Ta vs K/Cs showed consistent trend of mineralization using their position relative to the Beus and Gordiyenkos line. These ratios - K/Ba, K/Rb, Ba/Rb, Rb/Sr and geochemical models- Y vs. Nb, (Y+Nb) vs. Rb, Zr vs. Ta/Zr, B (=Fe+Mg+Ti) vs A(=Al-(K+Na+2Ca), R1(=4Si+11(Na+K)-2(Fe+Ti)) vs R2(=6Ca+2Mg+Al) with observed tectonic markers like lineation/foliation, joints and veins suggests a structurally controlled mineralisation. The Numbi pegmatites are of partial melting of lithospheric mantle sources, relatively rich in Sn, Ta and Nb of economic value that are structurally controlled.

Experimental Study of the Effect of Water Spray in the Air on Thermal Transfers Along a Plate of an Exchanger

Abstract This study is an experimental investigation of the oceanic environment effect on a plate heat exchanger performance. Indeed, an experiment was carried out on a single plate of the exchanger to generate a turbulent airflow in which fine water droplets were injected into a horizontal vein where a heated plate was placed. The experimental tests were conducted for different air velocities and various water concentrations of freshwater and saltwater. In fact, two plate forms were considered: The first one is flat while the second is corrugated. Three main facts were observed in this work: (i) the correlations linking the heat transfer rate to the operating conditions, (ii) the effect of fog addition and the plate form on convective heat transfers, and (iii) the impact of the formation of a salt layer on the surface of the plate in the case of salt water.

Direct evidence for fluid overpressure during hydrocarbon generation and expulsion from organic-rich shales

Abstract Fluid overpressures are widely expected during hydrocarbon generation and expulsion from source rocks, yet direct evidence for this phenomenon is lacking in the case of organic-rich shales. Here we show that formation of bed-parallel fibrous calcite veins in mature laminated organic-rich shales in the Eocene Dongying depression, Bohai Bay Basin, east China, occurred in direct response to fluid overpressure due to hydrocarbon generation. The evidence for overpressure is recorded by coexisting primary aqueous and petroleum inclusions in the calcite fibers. Our results show that all analyzed fluid-inclusion assemblages record variable degrees of overpressure during vein dilation, ranging from only modestly in excess of hydrostatic, to approaching and perhaps exceeding lithostatic. Thus, our results indicate that fluid pressures during dilation of horizontal veins are not necessarily equal to the opposing force of overburden throughout the history of opening. This suggests that at least some of the vein dilation is accommodated by concomitant narrowing of the adjacent wall-rock laminae, likely by scavenging (dissolution and reprecipitation) of CaCO3 from the adjacent wall rock.

Regional‐scale development of opening‐mode calcite veins due to silica diagenesis

AbstractThe formation and distribution of natural fractures in Cretaceous‐Paleogene strata in Jordan are strongly tied to diagenetic processes, which in turn reflect the lithology of the host material. Observations collected from subsurface cores show that widespread fracturing began before compaction of the host sediment was complete, based on ptygmatic folding of one set of mineral‐filled fractures (veins). Nonfolded veins are preferentially developed within heavily cemented layers. Calcium carbonate is the greatest volumetric component of the host sediment, and most fractures are at least partially filled by calcite. Dolomite‐bearing and silica‐bearing fractures are present in dolomitized and silicified host beds, respectively. Horizontal veins are filled by cone‐in‐cone calcite or, rarely, silica or dolomite. The stratigraphic arrangement and degree of compaction around ptygmatically folded calcite veins and chert nodules suggest that silica diagenesis was an important driver of early fractures. Nevertheless, those fractures were filled with carbonate cements as they opened, based on crack‐seal texture of the vein fill. The volume loss associated with silica diagenesis created fracture porosity, which was filled coevally by carbonate cements. The distribution of later veins reflects embrittlement of host layers by cementation and is consistent with crustal deformation as the primary fracture driver.

Strength and deformation behaviors of veined marble specimens after vacuum heat treatment under conventional triaxial compression

The mechanical behaviors of rocks affected by high temperature and stress are generally believed to be significant for the stability of certain projects involving rocks, such as nuclear waste storage and geothermal resource exploitation. In this paper, veined marble specimens were treated to high temperature treatment and then used in conventional triaxial compression tests to investigate the effect of temperature, confining pressure, and vein angle on strength and deformation behaviors. The results show that the strength and deformation parameters of the veined marble specimens changed with the temperature, presenting a critical temperature of 600 $$^{\circ }\hbox {C}$$ . The triaxial compression strength of a horizontal vein ( $$\beta = 90^{\circ }$$ ) is obviously larger than that of a vertical vein ( $$\beta = 0^{\circ }$$ ). The triaxial compression strength, elasticity modulus, and secant modulus have an approximately linear relation to the confining pressure. Finally, Mohr–Coulomb and Hoek–Brown criteria were respectively used to analyze the effect of confining pressure on triaxial compression strength.

Gypsum veins in Triassic Moenkopi mudrocks of southern Utah: Analogs to calcium sulfate veins on Mars

AbstractWell‐exposed gypsum veins in the Triassic Moenkopi formation in southern Utah, USA, are similar to veins at Endeavour and Gale Craters on Mars. Both Moenkopi and Mars veins are hydrated calcium sulfate, have fibrous textures, and crosscut other diagenetic features. Moenkopi veins are stratigraphically localized with strontium and sulfur isotope ratios similar to primary Moenkopi sulfate beds and are thus interpreted to be sourced from within the unit. Endeavour veins seem to be distributed by lithology and may have a local source. Gale veins cut across multiple lithologies and appear to be sourced from another stratigraphic interval. Evaluation of vein network geometries indicates that horizontal Moenkopi veins are longer and thicker than vertical veins. Moenkopi veins are also generally oriented with the modern stress field, so are interpreted to have formed in the latest stages of exhumation. Endeavour veins appear to be generally vertical and oriented parallel to the margins of Cape York and are interpreted to have formed in response to topographic collapse of the crater rim. Gale horizontal veins appear to be slightly more continuous than vertical veins and may have formed during exhumation. Abrupt changes in orientation, complex crosscutting relationships, and fibrous (antitaxial) texture in Moenkopi and Mars veins suggest emplacement via hydraulic fracture at low temperatures. Moenkopi and Mars veins are interpreted as late‐stage diagenetic features that have experienced little alteration since emplacement. Moenkopi veins are useful terrestrial analogs for Mars veins because vein geometry, texture, and chemistry record information about crustal deformation and vein emplacement.

Strata-bound vein array in the basal Pierre Shale, Lake Francis Case, South Dakota, U.S.A.

A distinctive strata-bound vein array occurs in the basal Pierre Shale exposed along the shores of Lake Francis Case, a reservoir on the Missouri River in south-central South Dakota. Typically 2–4 meters in thickness, the array consistently outcrops over a >50-km distance, a significant areal footprint. Ash layers define the upper and lower bounds of the vein array. Two, suborthogonal, preferred directions of vertical veins (northeast and southeast strike) define a regional pattern. By volume, vertical veins comprise 1–2% of the rock. Thinner, more discontinuous, and irregular horizontal veins also occur. Comparisons between array orientations and the joint/vein pattern in the immediately underlying marls of the top of the Niobrara Chalk identify distinct differences. Traverse data suggest that the vein arrays are characterized by uniform horizontal extension. Vertical veins in the array are typically 1–2 centimeters thick and contain massive jarosite, selenite, and fibrous gypsum. The abundance of jarosite and fibrous gypsum distinctly correlates with position in the weathering profile, and these phases are interpreted as due to replacement of original selenite during modern weathering. However, for initial vein array formation, the following suggests that they are not related to modern weathering and formed at depth: (1) a lack of correlation of vein width/frequency with position in the weathering profile; (2) the regional extent; (3) the consistent preferred orientations; (4) the uniform horizontal extension; and (5) the coarse-grained character of the selenite. The consistent strike pattern suggests influence of a regional stress field. The mechanism/timing of vein array formation is unclear. Formation due to diagenetic processes, which are especially significant in mud rocks, would explain the strata-bound character and isotropic horizontal strain and is considered most likely. Formation during glacial loading is one intriguing possibility. Localization of the vein array may be due to the organic-rich character of the host Burning Brule Member of the Sharon Springs Formation.

Quaternary extension in the Rio Grande rift at elevated strain rates recorded in travertine deposits, central New Mexico

Calcite-filled extension veins and shear fractures are preserved in numerous travertine deposits along the western margin of the Albuquerque Basin of the Rio Grande rift. Calcite veins are banded and show geometries suggesting incremental cracking and calcite precipitation. U-series and 234U model ages from calcite infillings indicate that vein formation was active in the Quaternary, from ca. 2 Ma to ca. 250 ka. Vein orientations are systematic within each deposit and record a dominant extension direction that was horizontal and varied from E-W to NW-SE, consistent with both the regional finite extensional strain in the rift and with the global positioning system (GPS)–constrained deformation field. Three sites contain three orthogonal vein sets that crosscut one another nonsystematically, suggesting alternating times of: (1) regional E-W horizontal extension (dominant), (2) alternating N-S and E-W vertical veins that suggest vertical s1 and s2 » s3, and (3) horizontal veins that are interpreted to reflect times of highest pore fluid pressures and subequal principal stresses. One site contains conjugate normal faults that also record the dominant E-W extensional tectonic stress. Quaternary extensional strain rates calculated from vein opening for three locations range from 3.2 ± 1.4 × 10–16 s–1 to 3.2 × 10-15 ± 2.7 × 10–16 s–1, which are up to ∼40 times higher than the long-term (Oligocene–Holocene) finite strain rates calculated for different basins of the Rio Grande rift (8.5 × 10–17 to 4.5 × 10–16 s–1), and up to ∼100 times higher than modern strain rates measured by GPS data (3.9 × 10-17 ± 6.3 × 10–18 to 4.4 × 10-17 ± 6.3 × 10–18 s–1). These high Quaternary rates are comparable to modern strain rates measured in the Basin and Range Province and East African Rift. Thus, this paper documents persistent E-W regional extension through the Quaternary in the Rio Grande rift that bridges geologic, paleoseismic, and GPS rates. Anomalously high strain rates in the Quaternary were facilitated by ascent of travertine-depositing CO2-rich waters along rift-bounding normal faults, leading to locally very high stain accumulations. These sites also provide examples of natural leakage of deeply sourced CO2 interacting with regional tectonism, and they emphasize that rift maturation is a highly dynamic process, both spatially and temporally.

How travertine veins grow from top to bottom and lift the rocks above them: The effect of crystallization force

Travertine mounds form at the mouth of springs where CO 2 degassing drives carbonate precipitation from water flowing from depth. Building of such mounds commonly involves the successive stratigraphic deposition of carbonate layers that precipitate from waters rising from depth along vertical to horizontal open fissures that are episodically sealed by radiating crystals. Much more intriguing structures can also be observed, such as widespread horizontal white veins of carbonate with vertical aragonite fibers, parallel or oblique to the stratigraphic travertines, which extend laterally over distances of several tens of meters and could represent up to 50% of the total volume of the travertine mound. Using highly precise U-Th dating, the growth direction of these horizontal veins is shown to be from top to bottom, and this fact clearly indicates that they developed within the mound over a period of ∼1000 yr for the vein analyzed. A vein growth mechanism is proposed that is able to uplift the rock above the vein thanks to the force of crystallization. The possible development of such structures in other places and the consequences of reverse growth direction when interpreting travertine data are discussed.

Formation of chocolate-tablet boudins in a foreland fold and thrust belt: A case study from the external Variscides (Almograve, Portugal)

Chocolate-tablet boudins of quartzite are restricted to steep limbs of D1-folds along the SW coast of Portugal. The chocolate-tablet geometry results from older vertical and younger horizontal quartz veins. Both sets of veins developed during similar conditions by extension fracture in pre-existing necked domains: (1) both veins show stretched crystal fibers; (2) the boudin aspect ratio is the same in vertical (3.0 ± 1.4) and in horizontal sections (2.9 ± 1.2); (3) temperatures obtained from fluid inclusions are similar (200 ± 20° for vertical and 230 ± 22 °C for horizontal veins) and are compatible with temperatures obtained from illite crystallinity (ca. 200–ca. 250 °C). Given thermal equilibrium between the host rock and the precipitating fluid, the chocolate-tablet boudins formed close to the metamorphic peak. We interpret that the vertical veins developed after the limbs of the D1-folds had attained their steep attitude and the orientation of the greatest and intermediate principal strain axes had interchanged. Subsequently, the initial strain field was restored and opening of horizontal veins led to the chocolate-tablet geometry. The direction of the main shortening direction was constant from the initial buckling stage via isoclinal folding and during all boudinage stages.

An overview of hydrodynamic studies of mineralization

Fluid flow is an integral part of hydrothermal mineralization, and its analysis and characterization constitute an important part of a mineralization model. The hydrodynamic study of mineralization deals with analyzing the driving forces, fluid pressure regimes, fluid flow rate and direction, and their relationships with localization of mineralization. This paper reviews the principles and methods of hydrodynamic studies of mineralization, and discusses their significance and limitations for ore deposit studies and mineral exploration. The driving forces of fluid flow may be related to fluid overpressure, topographic relief, tectonic deformation, and fluid density change due to heating or salinity variation, depending on specific geologic environments and mineralization processes. The study methods may be classified into three types, megascopic (field) observations, microscopic analyses, and numerical modeling. Megascopic features indicative of significantly overpressured (especially lithostatic or supralithostatic) fluid systems include horizontal veins, sand injection dikes, and hydraulic breccias. Microscopic studies, especially microthermometry of fluid inclusions and combined stress analysis and microthermometry of fluid inclusion planes (FIPs) can provide important information about fluid temperature, pressure, and fluid-structural relationships, thus constraining fluid flow models. Numerical modeling can be carried out to solve partial differential equations governing fluid flow, heat transfer, rock deformation and chemical reactions, in order to simulate the distribution of fluid pressure, temperature, fluid flow rate and direction, and mineral precipitation or dissolution in 2D or 3D space and through time. The results of hydrodynamic studies of mineralization can enhance our understanding of the formation processes of hydrothermal deposits, and can be used directly or indirectly in mineral exploration.

Melt Segregation in the Lower Part of the Partially Molten Mantle Zone beneath an Oceanic Spreading Centre: Numerical Modelling of the Combined Effects of Shear Segregation and Compaction

The physical processes capable of concentrating melt within the lower part of the partially molten mantle horizon beneath an oceanic spreading centre have been considered using one-dimensional numerical models that take into account the effects of shear segregation during ductile deformation, compaction and liquid–solid surface tension. These models show that when the viscosity of the solid fraction of the mantle is constant, shear segregation is too slow to be a geologically relevant process. However, if, as suggested experimentally, the ductility of the solid fraction of the mantle increases in the presence of interstitial melt, shear segregation is found to develop on time-scales compatible with mantle upwelling. The critical melt fraction at which shear segregation occurs is predicted to be positively correlated with the melt-free viscosity of the mantle, a value of ∼5% being typical for simulations run with a melt-free viscosity of 1018 Pa s. No characteristic length scale is found for this segregation, suggesting that melt will tend to concentrate at the length scale at which it is produced (i.e. that of mantle olivine grains). At face value this would imply that shear is not an efficient mechanism to concentrate liquid. However, segregation will occur parallel to the direction of compression, such that towards the bottom of the zone of partial melting beneath an oceanic spreading centre, horizontal veins are expected. Because of the local extraction of the interstitial melt, these horizontal veins will drastically reduce vertical permeability, a fact that will trigger compaction in that direction. In this case, rapid formation of melt-rich horizons approximately one compaction length in height is predicted. High permeability and high melt-free viscosity both act to efficiently drain liquid from throughout the lower region of the melting column, leading to the formation of thick widely spaced compaction waves. On the other hand, low values of these parameters lead to formation of thin closely spaced horizons derived from locally produced liquids. Liquid contained in these melt-rich horizons will be transported through the uppermost mantle, either in horizontal structures if the viscosity of the mantle is high (>1019 Pa s), or rapidly drained towards the surface if the direction of compression rotates vertically, a situation expected if the viscosity of the melt-free mantle is ∼1018 Pa s. This complex interplay of shear segregation, compaction and regional stress fields may potentially help to explain certain geochemical and petrological features of rocks from mid-ocean ridges and ophiolite complexes, such as the apparent lack of interaction of mid-ocean ridge basalt-like liquids with the uppermost mantle and the planar geometry of dissolution channels observed in ophiolite complexes.

ABSTRACT: Structural and Geochemical Analysis of the Little Grand and Salt Wash Faults to Investigate the Leakage of CO2 from a Natural Reservoir

The Little Grand and Salt Wash faults, located in southeastern Utah in the Colorado Plateau, discharge significant amounts of CO2 gas and cut a series of sandstones, shales, and siltstones. They have and are impacting the regional (basin-wide) flow evidenced from travertine deposits, calcite/aragonite veins, springs, seeps, and geysers erupting from abandoned exploration wells that are localized along the fault zones. The structure of the fault zones show that fault movement and cementation have been active contemporaneously; namely, the fault rocks are cross-cut by thick veins, and travertine veins are cut by faults. Water chemistry of samples taken from the CO2-charged geysers and springs suggest that the CO2 is sourced from Paleozoic rocks 1.5 - 2 km deep. Analysis of 13/12C and 18/16O isotopes of the travertine terraces and veins likewise suggest a deeper, sub salt, Paleozoic source of the CO2. Our hypothesis for these faults is that the CO2 flows upward along the faults from near the Pennsylvanian Paradox Formation. The CO2 charges a shallower water system, which then flows to the surface. The CO2-rich discharging water is supersaturated with respect to calcite and aragonite and deposits travertine. Horizontal travertine veins are found along the fault trace. We infer that when fluids reached a lower lithostatic load and/or higher permeability stratigraphic interval, horizontal veins were formed. At various points in the fault history, fluid pressures were high enough to cause flow to occur across the fault zones, despite the presumably low permeability of the clay-rich faults.

3-D analysis of vascular and capsular invasion in thyroid follicular carcinoma.

To clarify the mechanism of vascular invasion (VI) and capsular invasion (CI) in thyroid follicular carcinoma (FC), 3-D reconstruction of tumor tissues, vessels and capsules was performed from serial sections in five FC and three follicular adenomas (FA). Outflow veins in the lesions were well developed under the capsule, narrowed just beneath the capsule, and dilated within the capsule. Both VI and CI were observed at the poststenotic intracapsular dilated part of outflow veins. The tumor tissue extended along the intracapsular horizontal veins which flowed in parallel with the capsule in VI, or along the vertical veins which directly flowed outside the capsule in CI. There was tumor tissue in the subendothelial layer of the outflow veins within the capsule. Intravascular tumor nests and nodular lesions within the capsule continued from the main tumors. It can be concluded from these results that most VI and CI may not be true invasions but pre-invasive lesions.

Miocene Gypcretes from the Calama Basin, northern Chile

Gypcretes of Miocene age are preserved beneath a 9·53 ± 0·36 Ma ignimbrite along the eastern margin of the Oligo‐Pleistocene Calama Basin, northern Chile. They are restricted to a single stratigraphic horizon developed within laterally extensive (>35 km) coalesced alluvial fan deposits, developed along the margin of an endorheic basin. Two types of gypcrete are recognized. Type 1 comprises almost completely gypsum‐cemented sandstones containing alabastrine nodules and columns, sub‐vertical and horizontal veins of fibrous gypsum and ‘v‐shaped’ cracks infilled by clastic material, and are interpreted as surface weathered gypsic crusts. Type 2 gypcretes are composed of massive, reddened poikilitic and mesocrystalline gypsum (up to 80% of the rock) with isolated bedding‐parallel, clast‐rich lenses (200 × 30 cm) and sub‐vertical veins of fibrous gypsum. The massive texture resembles that of well developed B horizons in Quaternary alluvial desert soils. The crystal forms suggest an origin as a subsurface gypsic crust formed by a combination of hydromorphic (poikilitic) and illuvial (mesocrystalline) processes with the fibrous gypsum veins suggestive of periodic surface exposure.Gypcrete horizons are up to 25 m thick and composed of both gypcrete types. They represent superimposed phases of surface and subsurface gypcrete development. Quaternary gypcretes are developed in arid climatic regimes, but are not considered to develop under hyper‐arid climates. An arid climate is considered to have prevailed in the study area up to 9·5 Ma after which a change to hyper‐aridity favoured gypcrete preservation.

From alkali basalt to phonolite in hand-size samples: vapor-differentiation effects in the Bouzentès lava flow (Cantal, France)

The Bouzentès lava flow is a 20-m-thick alkali basalt flow emplaced during the last stage of formation of the Cantal stratovolcano at 4.2 Ma. Its upper part has 1- to 20-cm-thick vesicle-rich segregation sheets which recur every 0.1–2 m. These horizontal veins are hawaiitic in composition. They are characterized by hypertrophic development of their minerals (‘pegmatoids’) and by glassy phonolitic segregation vesicles. Internal differentiation within the Bouzentès lava flow was triggered by an unusually high water content, as suggested by pre-emptive iddingsite alteration of olivine phenocrysts. The proposed model of formation of the segregation sheets includes the upward motion of diapirs of residual melt plus addition of vapor from the bottom of the central liquid lens to the base of the upper solidified crust of the cooling lava flow. Olivine settling appears to have been inhibited or at least retarded by upward migration of melt plus vesicles. Most of the features observed in Bouzentès recall the internal differentiation processes usually described within thick Hawaiian lava lakes. The segregation vesicles are believed to result from an increase of gas solubility in residual melt during the crystallization process.

Mesothermal-type mineralization in the Sabie-Pilgrim's Rest gold field, South Africa

We describe here a different class of mesothermal gold deposit at Sabie-Pilgrim's Rest which is probably associated with the Bushveld igneous event in South Africa. Pressure and temperature estimates indicate that the ore fluids of the Sable-Pilgrim's Rest gold field, which occurs within the Early Proterozoic Transvaal Supergroup, were similar to those of mesothermal gold deposits. However, a number of features distinguish the Sable-Pilgrim's Rest ores from typical mesothermal deposits: (1) the presence of ubiquitous Cu and Ri in addition to Au, Ag, hs, and Sb; (2) high salinities (mean approximate to 12 wt % NaCl equiv); (3) low mean bulk CO2 content (approximate to 5 mole %); (4) veins largely hosted within a platform carbonate sequence; and (5) mineralization along both vertical and horizontal (stratiform) veins, showing a clear genetic link. Characteristic features of the Sable-Pilgrim's Rest gold field that correspond to those within the above spectrum of mesothermal gold deposits include: (1) comparable light stable isotope (delta D, delta(18)O. delta(34)S) ranges. together with a lack of isotopic evidence for meteoric water involvement; (2) high Au/Ag ratios; (3) presence of ore shoot structures, which are highly characteristic of mesothermal gold deposits; (4) variably deformed structures within individual sheeted veins which point toward cyclic opening and quartz accretion events separated by episodes of deformation; and (5) fluid inclusion properties which indicate high fluid pressures during entrapment. Bushveld magmatism is thought to have been the main heat source responsible for generating the hydrothermal cell that resulted in the formation of the Sable-Pilgrim's Rest gold field. Mineralization involved an originally pristine fluid transporting metals that were probably derived from the same source, as well as a measure of connate waters from the granitic basement and the overlying sedimentary rocks. The fluid was relatively acidic and anoxic and equally efficient in transporting gold either as a bisulfide complex or a chloride complex. Gold precipitation took place as a result of (1) a decrease in sulfur activity within the ore-bearing fluid because of sulfide precipitation, and (2) changes in the fluid chemistry as a result of fluid immiscibility.