Anhydrite Inclusions Research Articles

Vanished evaporites, halokinetic structure, and Zn-Pb mineralization in the world-class Angouran deposit, northwestern Iran

The Angouran deposit (19.3 Mt at 23.4% Zn and 4% Pb) is the second-largest Zn-Pb deposit in Iran. The deposit is hosted in a Neoproterozoic−Cambrian marble-schist sequence within a breccia pipe in a domal structure, with sulfide mineralization under low-temperature hydrothermal conditions (<200 ºC). The features of the ore-hosting breccias are similar to known halokinetic diapir breccias in the world but evaporite minerals are subtle. The common types of breccia clasts in the Angouran breccia pipe include a matrix-supported angular clast (float breccia) with highly variable sizes and orientations and exotic volcanic clasts. The volcanic clasts were derived from the underlying Miocene volcanic rocks, evidenced by the consistent petrography and zircon U-Pb ages dated at 20−19 Ma. Abundant smithsonite pseudomorphs after anhydrite and anhydrite inclusions within sphalerite and pre-ore marcasite in the breccia matrix indicate that the breccia pipe contains abundant anhydrite prior to the Zn-Pb mineralization. The enrichment of evaporite is also supported by the occurrence of considerable double-terminated quartz crystals that contain spherical and tabular carbonate inclusions and anomalously high Li, Na, and K concentrations, relatively high B concentration, and high δ18O values (up to 28.3‰). These observations suggest the Angouran deposit formed in a former halokinetic diapir breccia pipe. The halokinetic diapirism was possibly triggered by thrust loading of the marble-schist sequence over the Miocene evaporite beds during the Arabia-Eurasia continental collision. Halokinetic structures elsewhere in the Angouran region warrant this consideration. Most of the evaporite minerals in the breccia pipe were dissolved and replaced before and/or during subsequent Zn-Pb sulfide and smithsonite mineralization events. This study provides a good example for the identification of vanished evaporites, halokinetic structure, and associated Mississippi Valley-type mineralization.

Black Quartz from the Burano Formation (Val Secchia, Italy): An Unusual Gem

The Burano Formation in Val Secchia in the province of Reggio Emilia is rich in black or very dark quartz. The crystals are often pitted by scars, rarely shiny, often opaque, rarely translucent, and about 3 cm long. However, they have a beautiful color and are generally euhedral and bi-terminate with simple habitus and well developed faces—for these characteristics, they are very sought after by collectors. Micro-Raman analyses showed the quartz contains abundant inclusions of anhydrite and graphite. The inclusions of anhydrite are responsible for the chromatic inhomogeneity, while the black color is linked to the presence of disordered graphite inclusions. LA-ICP-MS analyses did not show a significant presence of chromophore elements. Black quartz formed in the original evaporite deposits of gypsum, which, due to diagenesis and the increase in temperature during deep tectonic burial conditions, lost water of crystallization and subsequently transformed into anhydrite. After the formation of quartz crystals, the Burano Formation was exhumated and rehydrated, resulting in gypsification at conditions close to the surface. The black quartzes of the Burano Formation represent an appreciable gemological material for the development of local craftsmanship, even if the difficulties in reaching the deposit limit their marketing.

Origin of andradite in the Quaternary volcanic Andahua Group, Central Volcanic Zone, Peruvian Andes

Euhedral andradite crystals were found in trachyandesitic (latitic) lavas of the volcanic Andahua Group (AG) in the Central Andes. The AG comprises around 150 volcanic centers, most of wich are monogenetic. The studied andradite is complexly zoned (enriched in Ca and Al in its core and mantle, and in Fe in this compositionally homogenous rim). The core-mantle regions contain inclusions of anhydrite, halite, S- and Cl-bearing silicate glass, quartz, anorthite, wollastonite magnetite and clinopyroxene. The chemical compositions of the garnet and its inclusions suggest their contact metamorphic to pyrometamorphic origin. The observed zoning pattern and changes in the type and abundance of inclusions are indicative of an abrupt change in temperature and subsequent devolatilization of sulfates and halides during the garnet growth. This process is interpreted to have taken place entirely within a captured xenolith of evaporite-bearing wall rock in the host trachyandesitic magma. The devolitilization of sediments, especially sulfur-bearing phases, may have resulted in occasional but voluminous emissions of gases and may be regarded as a potential hazard associated with the AG volcanism.

Primary fluid exsolution in porphyry copper systems: evidence from magmatic apatite and anhydrite inclusions in zircon

Primary fluid exsolution in porphyry copper systems: evidence from magmatic apatite and anhydrite inclusions in zircon

Diagenetic Origin of Bipyramidal Quartz and Hydrothermal Aragonites within the Upper Triassic Saline Succession of the Iberian Basin: Implications for Interpreting the Burial–Thermal Evolution of the Basin

Within the Upper Triassic successions in the Iberian Basin (Spain), the occurrence of both idiomorphic bipyramidal quartz crystals as well as pseudohexagonal aragonite crystals are related to mudstone and evaporite bearing sequences. Bipyramidal-euhedral quartz crystals occur commonly at widespread locations and similar idiomorphic crystals have been described in other formations and ages from Europe, America, Pakistan, and Africa. Similarly, pseudohexagonal aragonite crystals are located at three main sites in the Iberian Range and are common constituents of deposits of this age in France, Italy, and Morocco. This study presents a detailed description of the geochemical and mineralogical characteristics of the bipyramidal quartz crystals to decipher their time of formation in relation to the diagenetic evolution of the sedimentary succession in which they formed. Petrographic and scanning electron microscopy (SEM) analyses permit the separation of an inner part of quartz crystals with abundant anhydrite and organic-rich inclusions. This inner part resulted from near-surface recrystallization (silicification) of an anhydrite nodule, at temperatures that were <40 °C. Raman spectra reveal the existence of moganite and polyhalite, which reinforces the evaporitic character of the original depositional environment. The external zone of the quartz contains no anhydrite or organic inclusions and no signs of evaporites in the Raman spectra, being interpreted as quartz overgrowths formed during burial, at temperatures between 80 to 90 °C. Meanwhile, the aragonite that appears in the same Keuper deposits was precipitated during the Callovian, resulting from the mixing of hydrothermal fluids with infiltrated waters of marine origin, at temperatures ranging between 160 and 260 °C based on fluids inclusion analyses. Although both pseudohexagonal aragonite crystals and bipyramidal quartz appear within the same succession, they formed at different phases of the diagenetic and tectonic evolution of the basin: bipyramidal quartz crystals formed in eo-to mesodiagenetic environments during a rifting period at Upper Triassic times, while aragonite formed 40 Ma later as a result of hydrothermal fluids circulating through normal faults.

The sapphirine-bearing rocks in contact with the Lherz peridotite body: New mineralogical data, age and interpretation

Sapphirine-bearing rocks are described in the Aulus Basin (Ariège, France) in a contact zone between the Lherz peridotitic body and Mesozoic metasediments which underwent the Pyrenean Cretaceous high-temperature, low-pressure metamorphic event (Monchoux, 1970, 1972a, 1972b). Sapphirine crystals occur in layered clastic deposits characterized by an uncommon suite of Al-Mg-rich minerals. A detailed petrographic study of sixteen samples representative of the diversity of the Lherz sapphirine-bearing rocks is presented. These rocks include breccias and microbreccias with various compositions. Some samples are composed of polymineralic clasts and isolated minerals that derive from regionally well-known protoliths such as ultramafic rocks, meta-ophites, “micaceous hornfels”, and very scarce Paleozoic basement rocks. Nevertheless, a large portion of the sapphirine-bearing clastic suite is composed of mono- and polymineralic debris that derive from unknown protolith(s). We define a "sapphirine-bearing mineral suite” (SBMS) composed of monomineralic debris including: sapphirine + enstatite + aluminous spinel + Mg-amphiboles + Ca-amphiboles + kornerupine + accessory minerals (apatite, diopside, rutile, serpentine, smectite, tourmaline, vermiculite and a white mica). We highlight the dominance of metamorphic Keuper clastic materials in the studied rocks and the presence of inclusions of anhydrite and F-, Cl-, Sr-rich apatite in minerals of the Al-Mg-rich suite. The brecciated texture and the presence of unequivocal sedimentary features suggest that the sapphirine-bearing rocks were mechanically disaggregated and then experienced winnowing in underwater conditions with poor mixing between the different sources. We measured U-Pb rutile age data in order to provide constraints on the age of (one of) the protolith(s) of those clastic deposits. The obtained age (98.6 + 1.2 Ma) is interpreted as the age of metamorphism of this protolith of the SBMS. Previous works interpreted the Lherz sapphirine-bearing rocks as crustal protoliths modified at depth along the contact with the ultramafic rocks of the Lherz body during their ascent towards shallower depths. These new data imply: (i) an Upper Triassic to Lower Jurassic origin for the main protolith of the sapphirine-bearing rocks; (ii) the metamorphism of this protolith along an active hot crust–mantle detachment during Cenomanian times with the involvement of metasomatic, brine-type fluids; and (iii) its brecciation during the exhumation of the material due to the evolution of the detachment, followed by subsequent sedimentary reworking of the metamorphic material.

Clausthalite (PbSe) and tiemannite (HgSe) from the type locality: New observations and implications for metallogenesis in the Harz Mountains, Germany

Clausthalite and tiemannite from the type locality, Clausthal, in the Harz Mountains, Germany, have virtually gone undocumented since their discovery in the nineteenth century. The minerals and their selenide assemblages are here documented in historical samples from the Königin Charlotte mine in the former Clausthal Pb–Zn–Ag mining district. Clausthalite and tiemannite are the main selenide components; naumannite is generally subordinate, whereas klockmannite and eskebornite are accessory minerals. The absence of inversion lamellae in naumannite constrains the formation temperature to <130 °C, a temperature that is compatible with salbands of tiemannite and clausthalite that occur in the wall rock of bleached and reddened greywacke along a calcite–quartz veinlet. The veinlet calcite and quartz trapped highly saline, Ca-rich brines (26–33 wt% NaCl equivalent), at temperatures between 96 and 212 °C. Tiemannite and clausthalite also form massive to semi-massive pockets. Tiemannite hosts inclusions of celestine, anhydrite and carrollite. This inclusion assemblage indicates that tiemannite precipitated from sulfate-bearing brines that likely originated from the overlying Zechstein evaporitic sediments. Such an origin is reflected in the less radiogenic 87Sr/86Sr values of 0.70914 and 0.71133 for two samples of tiemannite aggregates containing celestine–anhydrite inclusions. The Clausthal selenide assemblages postdated the main sulfide-bearing, polymetallic vein-style mineralisation of the Harz Mountains.

Implication of Apatite and Anhydrite for Formation of an Iron‐Oxide‐Apatite (IOA) Rare Earth Element Prospect, Benjamin River, Canada

AbstractThe Benjamin River apatite prospect in northern New Brunswick, Canada, is hosted by the Late Silurian Dickie Brook plutonic complex, which is made up of intrusive units represented by monzogranite, diorite and gabbro. The IOA ores, composed mainly of apatite, augite, and magnetite at Benjamin River form pegmatitic pods and lenses in the host igneous rocks, the largest of which is 100 m long and 10–20 m wide in the diorite and gabbro units. In this study, 28 IOA ore and rock samples were collected from the diorite and gabbro units. Mineralogical observations show that the apatite–augite–magnetite ores are variable in the amounts of apatite, augite, and magnetite and are associated with minor amounts of epidote‐group minerals (allanite, REE‐rich epidote and epidte) and trace amounts of albite, titanite, ilmenite, titanomagnetite, pyrite, chlorite, calcite, and quartz. Apatite and augite grains contain small anhydrite inclusions. This suggests that the magma that crystallized apatite and augite had high oxygen fugacity. In back scattered electron (BSE) images, apatite grains in the ores have two zones of different appearance: (i) primary REE‐rich zone; and (ii) porous REE‐poor zone. The porous REE‐poor zones mainly appear in rims and/or inside of the apatite grains, in addition to the presence of apatite grains which totally consist of a porous REE‐poor apatite. This porous REE‐poor apatite is characterized by low REE (&lt;0.84 wt%), Si (&lt;0.28 wt%), and Cl (&lt;0.17 wt%) contents. Epidote‐group minerals mainly occur in grain boundary between the porous REE‐poor apatite and augite. These indicate that REE leached from primary REE‐rich apatite crystallized as allanite and REE‐rich epidote. Magnetite in the ores often occurs as veinlets that cut apatite grains or as anhedral grains that replace a part of augite. These textures suggest that magnetite crystallized in the late stage. Pyrite veins occur in the ores, including a large amount of quartz and calcite veins. Pyrite veins mainly occur with quartz veins in augite. These textures indicate pyrite veins are the latest phase.Apatite–augite–magnetite ore, gabbro–quartz diorite and feldspar dike collected from the Benjamin River prospect contain dirty pure albite (Ab98Or2–Ab100) under the microscope. The feldspar dikes mainly consist of dirty pure albite. Occurrences of the dirty pure albite suggest remarkable albitization (sodic alteration) of original plagioclase (An25.3–An60 in Pilote et al., 2012) associating with intrusion of monzogranite into gabbro and diorite.SO42− bearing magma crystallized primary REE‐rich apatite, augite and anhydrite reacted with Fe in the sodic fluids, which result in oxidation of Fe2+ and release of S2− into the sodic fluids. REE, Ca and Fe from primary REE‐rich apatite, augite and plagioclase altered by the sodic fluids were released into the fluids. Then Fe3+ in the sodic fluids precipitated as Fe oxides and epidote‐group minerals in apatite–augite–magnetite ores. Finally, residual S2− in sodic fluids crystallized as latest pyrite veins.In conclusion, mineralization in Benjamin River IOA prospect are divided into four stages: (1) oxidized magmatic stage that crystallized apatite, augite and anhydrite; (2) sodic metasomatic stage accompanying alteration of magmatic minerals; (3) oxidized fluid stage (magnetite–epidote group minerals mineralization); and (4) reduced fluid stage (pyrite mineralization).

The occurrence and origin of celestite in the Abolfares region, Iran: Implications for Sr-mineralization in Zagros fold belt (ZFB)

The major celestite deposits in Zagros Fold belt are associated with coastal marine carbonate and evaporate sediments of Oligo-Miocene Asmari and Lower Miocene Ghachsaran Formations. In the Abolfares region, celestite mineralization is extended in the western limb of Bangestan anticline in the carbonates of Early Miocene (middle part of Asmari Formation), underlying by dolomitic carbonates of Burdigalian. From bottom to top three main types of mineralization can be distinguished in the study area: (1) layer texture resulting from replacement of algal limestone by celestite minerals with some parts showing idiomorphic crystals (geodes) along the walls of the cavities, (2) celestite occurrence as irregular massive shape interconnected small crystals and nodules, and (3) celestite mineralization associated with steeply dipping veins and open space fracture fillings, resulting from late-stage epigenetic processes. Highlightly, the ore-hosting carbonate rocks were deposited in an intertidal – supratidal protected setting with hypersaline conditions, in accordance with other celestite deposits of the Zagros Fold belt. The abundance of diagenetic crystallization rhythmites, carbonate and anhydrite inclusions as confirmed by Laser Raman spectroscopy analysis, high Sr/Ba values (average; 8726.1) and strong negative correlations between SO3 vs CaO (R2 = 0.98), SrO vs CaO (R2 = 0.96) with positive correlations between Ba vs SrO (R2 = 0.54) and SO3 vs SrO (R2 = 0.98) highlight the role of high Sr late-diagenetic brines in replacement of carbonates with celestite minerals. It seems that the inception of compressional folding during or soon after the deposition of the Asmari Formation in the carbonate platform at the margin of NW-trending basin in the foreland of the Zagros orogenic belt lead to the upward refluxing of penetrated high-Sr diagenetic brines and celestite mineralization.

Large euhedral quartz crystals in the Triassic dolomites and evaporites of central Tunisia: implications for silica diagenesis in sulphate-rich and high-Mg environments

The Triassic salt diapirs in central Tunisia show splendid large multicoloured crystals of quartz. The study of these crystals and their host rocks sheds light on their origin and, more specifically, the relationships between silica authigenesis and its host rocks. Petrographic analyses of quartz crystals show strongly undulose euhedral to subeuhedral crystals (generally rhombohedral crystals) with concentration of many solid inclusions including sulphates and dolomite. Voids, shelter pores and negative crystals of gypsum, anhydrite and dolomite rhombs are found near the central parts of the crystals. Preserved anhydrite inclusions and dissolved evaporite are also found in these crystals. XRD analyses of white and black quartz crystals show crystalline alpha-quartz as the only silica phase present in most samples with a crystallinity index of 5. Accessory minerals include small relict patches of unreplaced calcite, gypsum, dolomite and anhydrite that escaped replacement. Fourier transform infrared (FTIR) spectra reveal the presence of abundant sulphate and organic compound included in the silica masses. The quartz crystals occurring in the allochthonous Triassic salt bodies are typically authigenic owing to the euhedral shape, the absence of any siliciclastic grains in the host rocks of quartz crystals, and also, to the absence of any sedimentary, wind or water-induced controls on the crystal distribution in their hosting rocks. Quartz growth in Triassic salt diapirs is a complex multistage and possibly continuous mechanism. During the progressive uplift of Triassic evaporites and dolomites, the dissolution of evaporites is enhanced and creates pores for the precipitation of the silica with development of larger quartz crystals. The colour variability in the quartz is ascribed to different ambient materials or fluids during crystallization. Remnant organic matter, before being altered and oxidised, could likely be included in the silica masses of the quartz and may serve not only to catalyse the precipitation and the growth but also to darken the quartz as well. Silica necessary for the formation of the quartz is mainly derived from two potential sources: (1) the presence of green and red clays in the Triassic salts and (2) silica-rich diagenetic fluids percolating from adjacent Lower Cretaceous sandy aquifers.

Melt and Fluid Inclusions in Hydrothermal Veins: The Magmatic to Hydrothermal Evolution of the Elatsite Porphyry Cu-Au Deposit, Bulgaria

The magmatic to hydrothermal transition in the Late Cretaceous Elatsite porphyry Cu-Au-(Mo-platinum group element) deposit has been studied in a suite of samples with clear timing relations between porphyry dikes, magmatic-hydrothermal veins, silicate melt inclusions in quartz veins, fluid inclusion generations, and ore minerals. Ore mineralization occurs late, at temperatures ~200° to 300°C below those of the early, multistage interplay between magmatic and hydrothermal processes at near-magmatic temperature-pressure conditions. Crosscutting relations and petrography indicate that shortly after the intrusion of the earliest, monzodioritic dikes, fluids precipitated a first generation of granular quartz veins accompanied by potassic alteration. The second vein generation with crystalline quartz textures and K-feldspar alteration halos formed at the same time as the second, granodioritic pulse of porphyry intrusions. Cathodoluminescence imaging of quartz growth textures reveals that the earliest fluid inclusions in the crystalline quartz veins are of intermediate density and ~8 wt % NaCl equiv salinity, probably trapped at near-lithostatic pressures of ~1,200 to 1,300 bars at near-magmatic temperatures (≤730°C), and implies a depth of ~4 to 5 km. Depressurization led to phase separation, indicated by a first generation of coexisting brine and vapor inclusions trapped at temperatures of ≥640°C and suprahydrostatic pressures of ≥920 bars. A second quartz generation in crystalline quartz veins precipitated during progressive depressurization and hosts assemblages of coexisting brine, vapor, and silicate melt inclusions, trapped at temperatures in excess of 600°C and suprahydrostatic pressures of 630 to 880 bars. Some open-spaced quartz veins were filled with aplite during this stage. Field relations and geochemical evidence suggest that the aplites as well as the silicate melt inclusions in hydrothermal quartz veins represent volumetrically minor residual melts that evolved directly from granodiorite porphyries at the level of deposit formation, and do not represent aliquots of metal-supplying magma at depth. Fluid inclusions coexisting with the silicate melt inclusions are metal rich, but these fluids predate sulfide precipitation and are, therefore, not the dominant fluids responsible for the Cu-Au mineralization at Elatsite. Melt-fluid-metal separation processes recorded in these co-trapped silicate melt and fluid inclusions in vein quartz are small-scale, local phenomena and do not appear to be suited for understanding and quantifying metal segregation in the deeper source. Microthermometry data of fluid inclusions, trapped during the later stages of the formation of the second quartz generation, show further depressurization to ~260 to 325 bars and cooling to ~460°C, representing a hot hydrostatic regime. Bornite, chalcopyrite, and magnetite seem to be slightly later precipitated at temperatures ≤460°C in separate veins or opened spaces of preexisting veins. Dissolution textures of the second quartz generation indicate a subsequent local redissolution of vein quartz, most likely as a result of passing through a window of retrograde quartz solubility upon further cooling below 460°C. The next, economically most important chalcopyrite-pyrite stage is largely devoid of quartz precipitation and is mostly expressed as “paint veins.” Due to the lack of fluid inclusions, the temperature-pressure conditions of formation of these veins could not be constrained. The waning phase of hydrothermal activity is represented by a quartz-carbonate-zeolite stage, formed at low temperature (~145°C), as indicated by microthermometry of fluid inclusions trapped in quartz from this stage. Laser ablation-inductively coupled plasma-mass spectrometry analyses of fluid inclusions show very high Cu contents in early intermediate-density fluid inclusions and in the first vapor inclusion generation, followed by a drastic decrease in the second generation of vapor inclusions. The decrease correlates with the appearance of anhydrite inclusions in the second quartz generation and may indicate that a lack of sulfur in these later vapor inclusions led to Cu partitioning into the brine. Alternatively, it is also possible that, unlike early vapor inclusions, the later vapor inclusions were not susceptible to postentrapment copper enrichment, in accordance with recent experiments. A progressive Cu enrichment in the brine phase correlates well with depressurization prior to mineralization. Mass balance considerations based on analyzed fluid components and fluid phase relations indicate that brine was volumetrically minor and therefore likely stagnant, but this may have prepared ore precipitation by accumulating Cu stripped from ascending vapor.

The origin of celestine–quartz–calcite geodes associated with a basaltic dyke, Makhtesh Ramon, Israel

AbstractSpectacular celestine geodes occur in a Jurassic peri-evaporitic sequence (Ardon Formation) exposed in Makhtesh Ramon, southern Israel. The geodes are found only in one specific location: adjacent to an intrusive contact with a Lower Cretaceous basaltic dyke. Celestine, well known in sedimentary associations worldwide and considered as a low temperature mineral, may therefore be associated with magmatic-induced hydrothermal activity. Abundant fluid inclusions in celestine provide valuable information on its origin: gas-rich inclusions in celestine interiors homogenized at T≥200°C whereas smaller liquid-rich inclusions record the growth of celestine rims at T≤200°C. Near 0°C melting temperatures of some fluid inclusions and the occurrence of hydrous Ca-sulphate solid crystals in other inclusions indicate that celestine precipitated from variably concentrated Ca-sulphate aqueous solutions of meteoric origin. Celestine crystallized from meteoric water heated by the cooling basaltic dyke at shallow levels (c. 160 m) during a Lower Cretaceous thermal perturbation recorded by regional uplift and magmatism. The 87Sr/86Sr ratio of geode celestine, 0.7074, is similar to that measured in the dolostones of the host Jurassic sequence, but differs markedly from the non-radiogenic ratio of the dyke. Strontium in celestine was derived from dolostones preserving the 87Sr/86Sr of Lower Jurassic seawater, while sulphur (δ34S = 19.9‰) was provided by in situ dissolution of precursor marine gypsum (δ34S = 16.8‰) indicated by relict anhydrite inclusions in celestine. Low-temperature meteoric fluid flow during the Campanian caused alteration of the dyke into secondary clays and alteration of geodal celestine into quartz, calcite and iron oxides.

Boracite Mg3[B7O13Cl] from the Zechstein salt deposits

AbstractAmong the borates in the Middle European Zechstein Salt Succession boracite MgBoracite is to be found in two varieties: individual crystals in cubic, tetrahedral or dodecahedral habit on the one hand and fibrous crystals so-called “stassfurtite” on the other hand.The formation conditions such widely spread borates in the salt succession are ambiguous in two respects. First of all the synthetic formation of boracites is to be made by hydrothermal or melt conditions. Both processes can be suspended for the salt succession. Furthermore the cubic modification is stable above 265 ºC for the Mg-boracite. The cubic, tetrahedral or dodecahedral habit could be used as a geothermometer, but such conditions can be exclude by the paragenetic minerals, esp. carnallite (MgKClThe chemical composition of orthorhombic, pseudo-cubic boracite depends on the location.Pure Mg-boracite in hexahedral habit and in fibrous habit, so-called “stassfurtite”, occurs in the North Harz region, whereas the Fe-, Mn-, Mg-boracite appears in the South Harz region.Until now the source of boron, the time of formation of crystals, but also the reasons for the differences in habit of the single hexahedral crystals are still unclear. The formation during a diagenetic/metamorphic process is evident. However, the preferred formation in Stassfurt seam could be an indication for the boron enrichment in an early diagenetic process.Furthermore permit the determination of the thermal stability and the volatile content of crystals conclusions to the chemical composition of the fluid.The observed variation suggests that the condition of crystal growth as well as the chemical composition of fluid repeatedly changed over the time. Randomly occuring xenomorpheous anhydrite and magnesite inclusions within single boracite crystals have been interpreted as an indication to factors of chemical milieu during the formation of crystals.The reversible phase transition temperature of the boracite is a linearly function of the iron and manganese content and varies from 265 ºC for Mg-boracite to 330 ºC for Fe(Mn)-boracite.The thermal decomposition of boracite is determined by two processes. The decomposition started with a boronchlorine release (BOCl?), having a maximum rate at 1050 ºC. Additionally to this release one observes a simultaneous emission of HThe results give evidence for the aged approach of a secondary formation of boracite within the complete Stassfurt seam, possibly in connection with the formation of salt diapirs in the Jura and Cretaceous period.The wider environmental distribution of borates is an indication of chemical transport processes within the salt succession. This should be a more important issue in the discussion about the utilisation of salt diapirs for the storage of nuclear waste.

Mineralogy, geochemistry and petrogenesis of igneous inclusions within three inactive diapirs, Zagros belt, Shahre-kord, Iran

AbstractThe Kaj-Rostam Abad, Dashtak and Doab diapirs are part of the Precambrian–Cambrian Hormuz series that are rich in igneous inclusions concentrated by dissolution of diapiric salt. They are situated in the Iran–Pakistan salt range and commonly associated with inclusions of basalt, trachyte, andesite, micro-gabbro, gypsum and anhydrite, with lesser amounts of carbonate rocks. The mineral assemblage in these inclusions developed in three stages: (I) magmatic stage (diopside, Ti-augite, kaersutite, plagioclase, apatite, biotite and opaque minerals), (II) late magmatic stage (biotite, quartz, chlorite, albite, calcite, titanite, epidote, actinolite and opaque minerals) and (III) vein mineralization (quartz, chlorite, albite, calcite, garnet, epidote, opaque minerals and actinolite). Clinopyroxene is diopside to Ti-augite. Actinolite, kaersutite, albite and pycnochlorite are constituents of the metasomatic rocks of the area. Chlorite geothermometry yielded a temperature of 330–500 °C for chlorite formation. Clinopyroxene thermobarometry ranges from 960 ≤T≤ 1440 °C and 1 ≤P≤ 10 kbar. The presence of halite-bearing fluid inclusions in hydrothermal quartz veins with homogenization temperatures between 320 and 350 °C points to strong evidence of hydrothermal events. The salinity of these fluids is 39.8–42.7 wt% NaCl. δ18O data on hydrothermal quartz veins range from 14.89 to 22.09 ‰ (SMOW), indicating that the studied samples were affected by fluids originated from sedimentary-evaporitic rocks. Meteoric water that penetrated the evaporitic rocks likely mixed with late magmatic fluids while subjected to magmatic heat, when buried to depths of several kilometres by the Phanerozoic cover sequence. Whole-rock geochemistry data for the studied rocks emphasize their alkaline to sub-alkaline affinities, in a transitional magmatic series.

Cl-bearing lizardite in metamorphosed kimberlites from the Udachnaya Vostochnaya Pipe, Yakutia

Anhydrite-bearing dolomites and kimberlites from the contact zone of the Udachnaya Vostochnaya pipe, northern part of the Eastern Siberian Platform, were affected by low-grade metamorphism to the zeolite facies. The kimberlites are serpentinized and carbonatized and contain metasomes of anhydrite and saponite pockets. Twenty lizardite pseudomorphs after olivine (Fo91–82) in the kimberlites were examined on an electron microprobe. The lizardite is rimmed by saponite and contains dolomite, calcite, magnetite, and anhydrite inclusions. Lizardite in the central parts of the pseudomorphs contains 1.5–1.9 wt % Cl and 6–9 wt % Fe2O3, and this mineral in the outer portions of the pseudomorphs bears 0.7–1.0 wt % Cl and 2–6 wt % Fe2O3 (the paper presents nine microprobe analyses and images showing the distribution of Cl, Mg, Al, Si, S, Ca, Ti, and Fe obtained in characteristic X-ray radiation). The amounts of Fe3+ in octahedrally and tetrahedrally coordinated sites of the Cl-bearing lizardite are roughly equal. Cl was borrowed in the course of serpentinization from the host Early Paleozoic evaporites and brines contained in them. The Cl concentration in our lizardite from the metamorphosed kimberlites from the Eastern Siberian Platform (continental lizardite) is much higher than the Cl concentration in oceanic lizardite from serpentine replacing peridotites (0.03–0.2 wt % Cl). This is likely explained by differences in the Cl concentrations in the metamorphic fluids, their salinity (3% for oceanic water and 65% for brines in the platform cover).

Experimental deformation of a single-layer anhydrite in halite matrix under bulk constriction. Part 2: Deformation mechanisms and the role of fluids

Composite natural samples, consisting of a single layer of anhydrite, embedded in matrix of rock salt, were experimentally deformed at temperature, T = 345 °C and strain rate, ė = 10 −7 s −1. The geometry and kinematics of folds and boudins, which affect the stiff anhydrite layers, were described in Part 1 of the present study ( Zulauf et al., 2009). The microfabrics, deformation mechanisms, and the role of fluids are treated in the present paper. Anhydrite was deformed under brittle–viscous conditions as is indicated by fracturing, twinning, and local recrystallization, the latter based on strain-induced grain-boundary migration. Viscous deformation of halite was accommodated by slip on {110}〈110〉 which led to formation and rotation of subgrains and a striking 001-maximum parallel to the long axis, X, of the strain ellipsoid. Differential stress obtained from subgrain size of halite is largely consistent with stresses recorded by the load cells of the machine (<5 MPa). Despite of the high deformation temperature and the low water content of the starting samples, NaCl brine and hydrocarbons, expelled from fluid inclusions of both halite and anhydrite, led to hydraulic fracturing and redistribution of matter. The fluids migrated along newly formed microfractures in anhydrite and along halite–anhydrite boundaries. Released hydrocarbons and NaCl brine were redeposited in open space of neck domains in the form of black organic coatings (condensate) and fine-grained halite, respectively. As these fluid-controlled phenomena are common in salt domes of northern Germany, and released fluids may led to contamination of bed rock and biosphere, the results of the present study should be important for workers who are dealing with radioactive waste deposits in rock salt.

Experimental deformation of a single-layer anhydrite in halite matrix under bulk constriction. Part 1: Geometric and kinematic aspects

A new deformation apparatus has been used to model the internal kinematics of salt diapirs. Composite natural samples consisting of a single layer of anhydrite, embedded in halite matrix, were constrictionally deformed at temperature, T = 345 °C, strain rate, ė = 10 −7 s −1, maximum viscosity, η = 2 × 10 13 Pa s, and maximum finite strain, e x = 122%. The anhydrite layer, oriented parallel to the major stretching axis, X, was deformed by fracturing, whereas halite behaved viscously. At advanced state of constriction ( e x > 90%) a strong increase in strain hardening of halite led to a transient tension fracture that healed up and was shortened by folding during the final phase of viscous deformation. Tiny prismatic anhydrite inclusions disseminated inside the halite matrix were reoriented during constriction resulting in a linear grain-shape fabric. 3D-images of the anhydrite layer, based on computer tomography, revealed rare kink folds with axes subparallel to X, and boudins which result from tension fracture. With increasing layer thickness, H i, the width of boudins, W a, increases linearly ( W a = −0.3 + 1.3 × H i). The normalized width of boudins ( W d = W a/ H i) is almost constant at 1.5 ± 1.0. These geometrical parameters can be used to reveal fracture boudinage under bulk constriction. The oblique orientation of most of the boudins, with respect to the principal strain axes, results from folding of the boudins by a second generation of folds, the latter with axes subperpendicular to the layer.

Primary igneous anhydrite: Progress since its recognition in the 1982 El Chichón trachyandesite

Primary igneous anhydrite was first identified in 1982 El Chichón pumices. Analysis of the sulfur budget for the eruption provided compelling evidence that the pre-eruptive magma contained a significant gas phase at ∼ 7 km depth in order to account for the “excess gas release” of ∼ 5–9 million tons of SO 2 to the stratosphere by the eruption. Primary igneous anhydrite and a larger “excess gas release” of ∼ 20 million tons of SO 2 were noted for the significantly larger eruption of Mount Pinatubo in 1991, for which a separate gas phase at ∼ 7–9 km depth was also required by the sulfur budget. Pumices from both eruptions have mineral assemblages dominated by plagioclase and hornblende, with minor biotite, and show evidence for co-nucleation and mutual inclusions of anhydrite and apatite. Both magmas were also very water-rich and highly oxidized, with oxygen fugacities $1 log unit above the synthetic Ni–NiO buffer. Furthering the similarities between these two eruptions, ion-microprobe analyses of sulfur isotopic compositions of anhydrites in pumices from El Chichón and Mount Pinatubo both showed that individual crystals are isotopically homogeneous, but inter-crystalline variations in δ 34S are well beyond analytical error. This study reviews the history of thought regarding primary igneous anhydrite. Ten volcanic and plutonic analogues to the El Chichón and Mount Pinatubo magmas are discussed. All were similarly water-rich and highly oxidized magmas. Hornblende is the dominant mafic phenocryst in all but one plutonic example; in that case so much early anhydrite precipitated that no calcium remained to form hornblende. Biotite is very abundant in that exceptional plutonic case, and is present in six of the other examples with primary igneous anhydrite. Sphene is known as an indicator mineral of high magmatic oxygen fugacity; it was present in the 1982 El Chichón pumices, and in 6 of the other anhydrite-bearing samples. Hornblende and biotite phenocrysts are also characteristic of the intermediate-to-felsic plutonic rocks that typically host porphyry–Cu deposits. Such magmas are also known to be water-rich and highly oxidized. A close spatial and petrogenetic connection exists between magmas with primary igneous anhydrite and magmatic–hydrothermal ore deposits.

Metasomatism and ore formation at contacts of dolerite with saliferous rocks in the sedimentary cover of the southern Siberian platform

The data on the mineral composition and crystallization conditions of magnesian skarn and magnetite ore at contacts of dolerite with rock salt and dolomite in ore-bearing volcanic—tectonic structures of the Angara—Ilim type have been integrated and systematized. Optical microscopy, scanning and transmission electron microscopy, electron microprobe analysis, electron paramagnetic resonance, Raman and IR spectroscopy, and methods of mineralogical thermometry were used for studying minerals and inclusions contained therein. The most diverse products of metasomatic reactions are found in the vicinity of a shallow-seated magma chamber that was formed in Lower Cambrian carbonate and saliferous rocks under a screen of terrigenous sequences. Conformable lodes of spinel-forsterite skarn and calciphyre impregnated with magnesian magnetite replaced dolomite near the central magma conduit and apical portions of igneous bodies. At the postmagmatic stage, the following mineral assemblages were formed at contacts of dolerite with dolomite: (1) spinel + fassaite + forsterite + magnetite (T = 820−740°C), (2) phlogopite + titanite + pargasite + magnetite (T = 600–500°C), And (3) clinochlore + serpentine + pyrrhotite (T = 450°C and lower). Rock salt is transformed at the contact into halitite as an analogue of calciphyre. The specific features of sedimentary, contact-metasomatic, and hydrothermal generations of halite have been established. The primary sedimentary halite contains solid inclusions of sylvite, carnallite, anhydrite, polyhalite, quartz, astrakhanite, and antarcticite; nitrogen, methane, and complex hydrocarbons have been detected in gas inclusions; and the liquid inclusions are largely aqueous, with local hydrocarbon films. The contact-metasomatic halite is distinguished by a fine-grained structure and the occurrence of anhydrous salt phases (CaCl2 · KCl, CaCl2, nMgCl2 · mCaCl2) and high-density gases (CO2, H2S, N2, CH4, etc.) as inclusions. The low-temperature hydrothermal halite, which occurs in skarnified and unaltered silicate rocks and in ore, is characterized by a low salinity of aqueous inclusions and the absence of solid inclusions. The composition and aggregative state of inclusions in halite and forsterite indicate that salt melt-solution as a product of melting and dissolution of salt was the main agent of high-temperature metasomatism. Its total salinity was not lower than 60%. The composition and microstructure of magnetite systematically change in different mineral assemblages. Magnetite is formed as a result of extraction of iron together with silicon and phosphorus from dolerite. The first generation of magnetite is represented by mixed crystals, products of exsolution in the Fe-Mg-Al-Ti-Mn-O system. The Ti content is higher at the contact of dolerite with rock salt, whereas, at the contact with dolomite, magnetite is enriched in Mg. The second generation of magnetite does not contain structural admixtures. The distribution of boron minerals and complex crystal hydrates shows that connate water of sedimentary rocks could have participated in hydrothermal metasomatic processes.

Role of delayed release of sulphates from clinker in DEF

Two clinkers rich in sulphate burned in the pilot plant rotary kiln and cements prepared from them were investigated. Clinker richer in sulphate (SO3=3.6%) contained independent anhydrite grains as well as inclusions of anhydrite in belite. The mortar from it expanded after heat treatment at 90 °C and the addition of Na2SO4 or NaOH accelerated and increased this expansion. The expansion occurred irrespective of the fact that the clinker contained only 3% of C3A, although the C4AF content was 13%. The second clinker with 2.6% SO3 contained mainly calcium langbeinite and expanded only when 2% of Na2SO4 was added. The SEM examination of the mortars revealed the presence of numerous bands of massive ettringite around sand grains. Agglomerates of cracked ettringite in cement gel were also present. In addition, microcracks were seen inside the darker C–S–H gel. The conclusion is that anhydrite forming inclusions in belite gives an expanding mortar after heat treatment at 90 °C independently of the tricalcium aluminate content. Such clinkers are not typical of industrial conditions. The expansion is caused by the bands of massive ettringite as well as its agglomerates present in the cement gel and nanometric ettringite in the C–S–H phase.