Schist Research Articles

The inherent link between ore formation and geometallurgy as documented by complex tin mineralization at the Hämmerlein deposit (Erzgebirge, Germany)

A comprehensive quantitative mineralogical study on the Hammerlein tin deposit in the Erzgebirge, Germany, not only yields insights into the genesis of Sn mineralization but also provides also important clues for beneficiation. The lithological units of the skarn and greisen deposit show significant differences in modal mineralogy and Sn deportment. These systematic differences are attributed to several stages of ore formation. Of greatest significance is a paragenetically late cassiterite-chlorite-fluorite-sulfide assemblage. This assemblage replaces pre-existing skarn lithologies and also forms stockwork mineralization in greisen-type ores developed at the expense of mica schist that surrounds the skarn. The co-genetic formation of the cassiterite-chlorite-fluorite-sulfide assemblage is captured by the mineral association parameter—a parameter that can be easily quantified from data acquired during automated mineralogy studies. To document the preferred mineral association, a ratio is introduced that illustrates how closely cassiterite—the only Sn mineral of economic relevance—is associated with chlorite, fluorite, and sulfides. This so-called MAMA ratio illustrates the strongly preferred association between cassiterite and chlorite. The results also illustrate that the abundance of rock-forming chlorite may be used as a proxy for the abundance of the much less common cassiterite. This proxy is well-suited to sort ore from poorly mineralized/unmineralized rock fragments early during the beneficiation process. Such separation may well be achieved by using a short wave infrared detector that is already deployed in commercially available sorting equipment. The case study illustrates the inherent link between the processes responsible for ore genesis, the definition of geometallurgical domains, and the selection of suitable beneficiation strategies.

Early Precambrian tectono-thermal events: coupled U–Pb–Hf of detrital zircons from Jiao–Liao–Ji Belt, North China Craton

The Dashiqiao Formation on the Liaodong Peninsula constitutes an important component within the Jiao–Liao–Ji Belt, North China Craton. It is composed dominantly of dolomitic marbles intercalated with minor carbonaceous slates and mica schists, hosting one of the largest magnesite deposits on Earth. This study presents zircon cathodoluminescence (CL) images and U–Pb–Hf isotope data, as well as single-mineral geochemical data for the staurolite–garnet–mica schist from the Dashiqiao Formation, in order to constrain its protolith age and provenance, and further to discuss the early Precambrian tectono-thermal events of the North China Craton. U–Pb isotopic dating using the LA–ICP–MS method on detrital zircons from the schist preserves at least three age populations ranging in age from 2.99 to 2.02 Ga, and grains as old as ca 4087 Ma. The dominant Neoarchean detrital zircons were most probably sourced from the basement within the Longgang and Nangrim blocks, while the minor Mesoarchean zircons were only sourced from the Longgang Block. The subordinate middle Paleoproterozoic zircons are consistent with ages of the regionally distributed coeval Liaoji granites and volcanics within the Jiao–Liao–Ji Belt. Zircon U–Pb dating yields a metamorphic age of 1930 Ma for the sample, interpreted to represent the peak stage of epidote amphibolite facies metamorphism. Thus, the depositional age for the protolith of the schist was proposed in the period between 2.01 and 1.93 Ga. LA–MC–ICP–MS Lu–Hf isotopic data show that all Archean (2.45–2.55) detrital zircons possess positive eHf(t) values from + 0.7 to + 7.5 with juvenile depleted mantle model ages, suggesting a significant crustal growth event during the Neoarchean in the North China Craton. The Paleoproterozoic detrital zircons possess variable eHf(t) values (− 5.5–+ 8.3) and depleted mantle model ages from Mesoarchean to Paleoproterozoic. The zircons with negative eHf(t) values implies the Mesoarchean to Neoarchean crust undergoing a recycling event in the period 2.40–2.01 Ga, while those with positive eHf(t) value suggest some indication of juvenile addition to the crust during the Paleoproterozoic. Using regional geological and new detrital zricon U–Pb–Hf isotopic data, the early Precambrian tectono-thermal events can be subdivided into the following episodes: Mesoarchean, late Neoarchean, middle Paleoproterozoic, and late Paleoproterozoic times.

Not all moissanites are created equal: New constraints on moissanite from metamorphic rocks of Bulgaria

Terrestrial moissanite (SiC) is widely reported as an ultra-high pressure mineral occurring in kimberlites, diamonds and ultramafic/mafic rocks of mantle origin. However, the conditions of crystallization remain largely unknown. Moreover, dozens of SiC occurrences have been reported from continental crust sources such as granitoids, andesite–dacite volcanic rocks and their breccia, metasomatic and metamorphic rocks, and even limestones. The validity of many of these reports is still debated primarily due to possible contaminations from the widespread use of synthetic SiC abrasives in samples preparation. Indeed, reports of well-documented in-situ occurrences of moissanite in association with co-existing minerals are still scarce. The only condition of moissanite formation that is agreed upon is that extremely reducing media are required (e.g. 4.5–6 log units below the iron-wustite buffer). Here, we report the new occurrence of moissanite that was found in-situ within the garnet–staurolite–mica schists of Topolovgrad metamorphic group of Triassic age in Southern Bulgaria. The 10–300 μm moissanite crystals are situated within 0.1–1.2 mm isolated clusters, filled with amorphous carbon and nanocrystalline graphite. Most of moissanite crystals are 15R (rhombohedral) and 6H (hexagonal) polytypes, and one prismatic crystal, found within them, exhibits unusual concentric polytypical zoning with core (15R), intermediate zone (6H) and rim (3C-cubic). Experimental data show that this type of polytypical zonation is likely due to a decrease in temperature (or/and pressure?) and changes in Si/C ratio. Indeed, amphibolite facies metamorphism (500–580 °C – garnet–staurolite zone) followed by a subsequent cooling during the retrograde stage of green schist facies metamorphism (∼400–500 °C) could have provided a change in temperature. The SiC containing clusters exhibit evidence that they are pre-metamorphic, and we hypothesize that their protolith was a “black shale” material likely rich in carbon, hydrocarbon and terrigenous silica. The latter served as a source of isolated chemically-reduced media, which is required for SiC formation. Other concepts to explain moissanite occurrences in metasedimentary rocks are also discussed. Importantly, our findings show that the formation conditions of moissanite are likely more variable than previously recognized.

Structural setting of the Yalaxiangbo dome, SE Tibet (China)

The Yalaxiangbo gneiss dome (SE Tibet) in eastern Himalaya is one of the major metamorphic culminations in the Tethyan Himalayan Sequence, referred to as North Himalayan Gneiss Domes (NHGD). It comprises three main tectonic units separated by an upper ductile/brittle and a lower ductile detachment. The upper tectonic unit, above the upper ductile/brittle detachment, includes unmetamorphosed to low-grade metamorphic Triassic-Lower Cretaceous slate andmetapsammite of the Tethyan Himalayan Sequence. The middle tectonic unit, sandwiched between the upper and lower detachments, consists of mylonitic granite, staurolite-garnet-two mica schist and biotite-plagioclase gneiss affected by the ductile top-to-the north extensional shear of the lower detachment. The lower tectonic unit consists of mylonitic gneiss, leucogranite plutons, dikes and sills.By integrating meso-/micro-structural analyses, petrographic and Laser Ablation Inductively Coupled Plasma-Mass Spectrometer (LA-ICP-MS) in situ U-(Th)-Pb monazite geochronological data from selected samples affected by ductile shear, we constrain the activity of the lower detachment at ~ 18 Ma and the shearing along the upper detachment later than ~ 15 Ma. The kinematic, geochronology and petrologic features of the Yalaxiangbo detachments are similar to the South Tibetan Detachment system, which crops out in southern Tibet.

High‐temperature, decompressional equilibration of the eclogite facies orogenic root (Western Gneiss Region, Norway)

AbstractGarnet–clinopyroxene mafic rocks have been investigated in the outer coastal area of the northwestern‐most part of the Western Gneiss Region (WGR), South Norway. The garnet–clinopyroxene rocks occur as lenses with amphibolitized and deformed margins ranging in size from 1 m2 up to 2–3 km2. They are regionally widespread and included in migmatitic gneisses, mica schists and amphibolites. The mafic lenses vary from fine‐ to coarse‐grained with a strain variation from massive to coaxial S > L tectonite fabric. Garnet is Alm42‐53Prp17‐35Grs20‐33Sps0‐3. Clinopyroxene is a Na–Al diopside (En34‐43Fs8‐17Wo48‐52) with Al up to 0.50 a.p.f.u. and Na content up to Jd24. Garnet and clinopyroxene occur in an assemblage with edenitic‐pargasitic amphibole (Ti < 0.32 a.p.f.u.), plagioclase (An16‐43Ab57‐71), quartz, locally biotite (Mg# = 0.0.46–0.56; Ti = 0.51–0.59 a.p.f.u.), calcite, epidote and accessory rutile, ilmenite, zircon and apatite. Garnet porphyroblasts occur commonly as euhedral crystals, and locally with corroded rims surrounded by a corona of plagioclase or amphibole–plagioclase. Growth of secondary garnet is locally observed in S > L tectonite rock. Clinopyroxene occurs as elongated subhedral crystals forming a strong fabric, or as a coarse symplectite with plagioclase. Amphibole is present as matrix grains in the garnet–clinopyroxene assemblage, but occurs also in coronas on garnet as symplectite with plagioclase, or as replacement textures on clinopyroxene. Secondary titanite is produced on rutile, and spinel+plagioclase on ilmenite. The P–T evolution is modelled by P–T pseudosections (TheriakDomino software), thermobarometry and by mapping of garnet chemistry. Garnet porphyroblasts show a decrease in CaO and MnO, an increase in MgO and variable FeO with resulting increasing Mg# from core to rim, indicating growth under increasing temperatures and decompression. Calculation of garnet+clinopyroxene+plagioclase+quartz+rutile stability combined with garnet and clinopyroxene isopleths of grossular and Mg# composition yields a maximum temperature metamorphism of 1.4–1.8 GPa and >900°C. The P–T modelling supports high‐P granulite facies conditions for the equilibration of the garnet–clinopyroxene‐dominated mafic lenses. The maximum temperature metamorphism is associated with partial melting. In addition, an outermost small Mn increase, and local reversal of the Mg# ratio and CaO‐flattening in garnet of the mafic lenses are interpreted as retrogression into amphibolite facies. This is in accordance of mineral replacement of clinopyroxene to amphibole and titanite growth on rutile. The data support an evolution where the eclogite facies crust in the northwestern‐most coastal part of WGR underwent decompression during heating into high‐P granulite facies conditions, followed by cooling and amphibolitization. Our investigation gives a regional petrological documentation and illustrates an extensive high‐T equilibration in the Caledonian root zone subsequent to the deep crustal burial.

Metamorphic evolution and age constraints of the garnet-bearing mica schist from the Xindaduo area of the Sumdo (U)HP metamorphic belt, Tibet

AbstractAs one of the major components of the Himalayan–Tibetan Orogeny, the Lhasa terrane plays a key role in understanding the origin and evolution of this giant orogenic belt and the opening and closure of the Tethys oceans. The eclogite-bearing Sumdo Complex in the central Lhasa terrane was recognized as the main suture of the Palaeo-Tethys Ocean between the north and south Lhasa sub-terranes. Despite the eclogite having been studied for a long time, no attempts have been applied to studying the country rocks, causing confusion in understanding the relationship between the eclogite and the adjacent schist. Petrological investigations and phase equilibrium calculations on the garnet-bearing mica schist of the Sumdo Complex have been performed to constrain its metamorphic evolution. The P–T conditions for three metamorphic stages are constrained as P1 (480–500°C, 2.6–2.7 GPa), P2 (580–600°C, 1.3–1.4 GPa) and R (530°C, 0.9 GPa), which represent the prograde, temperature peak and retrograde stages. Two possible P–T paths were constructed, which experienced isothermal decompression (PT1) or heating with a decompression process (PT2), corresponding to the growth and extinction of garnet porphyroblasts in the matrix. The LA-MC-ICP-MS U–Pb dating method yielded a metamorphic age of 229.7±3.5 Ma, which was interpreted as the age of amphibolite-facies metamorphism at c. 600°C, 1.2–1.4 GPa during the closure of the Palaeo-Tethys Ocean, resulting in the aggregation of the north and south Lhasa sub-terranes. The close relationship between the eclogite and garnet-bearing mica schist, and their similar P–T–t paths indicate an in situ tectonic evolution rather than tectonic juxtaposition during exhumation.

Clay minerals in Arctic Kongsfjorden surface sediments and their implications on provenance and paleoenvironmental change

Kongsfjorden is a typical fjord on the edge of the ice cap of the Arctic Svalbard-Barents Sea. Its inner bay is connected with a modern glacier front along the direction of the fjord axis with a significant gradient change in the parameters of hydrology, sedimentation, and biology. In summer, ice and snow melt-water and floating ice collapse continuously and thus transport the weathering products on the surrounding land into the sea. Thus Kongsfjorden is regards as a natural laboratory for the study of unique sedimentation in polar fjords under modern glacial-sea water conditions. In this study, fifty-two surface sediments were collected in Kongsfjorden for clay mineral analysis to study the sediment source and sediment-transport process. Our results indicate that clay minerals in the surface sediments from Kongsfjorden are mainly composed of illite, chlorite, and kaolinite, and no smectite is found. Rocks from different periods exposed extensively in the surrounding areas of Kongsfjorden provide an important material basis for clay minerals in the Kongsfjorden. Kaolinite may be mainly derived from the fluvial deposits, weathered from reddish sandstones and conglomerates during the Carboniferous Period. Illite is mainly derived from Proterozoic low-grade and medium-grade metamorphic phyllite, mica schist, and gneiss. While chlorite is mainly from Proterozoic low-grade metamorphic phyllite and mica schist. In the direction from the fluvio-glacial estuary to the sea of the glacier front of Kongsfjorden, illite increase gradually, and the content of kaolinite declines gradually. However, the change pattern of chlorite is insignificant, which may be related to the provenance. Kongsfjorden detritus is mainly transported by the fluvio-glacial streams and icebergs into the sea and deposited in the inner bay. Coarse sediments are rapidly deposited in the glacier front, estuary, and near-shore areas. Clay fraction begins to deposit significantly by 200–400 m after flowing into the sea, which due to the crystal behavior of clay minerals, hydrodynamic condition and flocculation. Kaolinite and chlorite on the south of the bay near the Blomstrandhalvoya Island is mainly affected by ice-rafted detritus and thus can reveal the trajectory of transportation by the floating ice while entering the sea.

Initial surface failure and wear of cemented carbides in sliding contact with different rock types

The initial wear, deformation and degradation of cemented carbide in contact with different rock types are studied using a crossed cylinder sliding test. The sliding distance is limited to centimetres at a time, interrupted by successive SEM analysis. This allows for careful studies of the gradually changing microstructure of the cemented carbide during the test. Five different rock types are included; granite, metal sulphide ore, mica schist, quartzite and marble. All rock types are very different in microstructure, composition and properties. The cemented carbide grade used for the evaluation contains 6 wt% Co and fine (~ 1 µm) WC grains, a grade commonly used in rock drilling. The results show that the cemented carbide microstructure becomes altered already during the very first contact with rock. The initial wear rate and wear character is highly influenced by the rock type. The initial wear of the cemented carbide is highest against quartzite and lowest against marble.

Effect of soil particle and pore orientations on sound velocity

The main purpose of compressing soil dams, highway constructions, bridge abutments and fillings behind retaining walls is to increase the bearing capacity, reduce consolidation settlement and permeability of clayey soils. However, the soil particles and pore orientation could be somehow anisotropic due to a compaction-induced anisotropic structure. Orientations of particles, pores and other constituents during compaction of a mixed mica schist clayey soil with fly ash were studied to investigate how soil structure, and in turn, sound velocity change during compaction of a cohesive soil at different moisture contents on all three sides at three are perpendicular to each other. Ultrasonic sound velocity measurements of the cube samples compressed in the optimum water content were investigated perpendicular to each other. The results showed that the ultrasonic sound velocity decreased in the compression direction of the soil samples, namely the overall degree of preferred orientation increases as opposed to the compression direction. On the other hand, the overall degree of preferred horizontal orientation increases and ultrasonic sound velocity decreases.

The Saxothuringian Terrane affinity of the metamorphic Stachów Complex (Strzelin Massif, Fore-Sudetic Block, Poland) inferred from zircon ages

The Saxothuringian Terrane defined in the western part of the Bohemian Massif is regarded to have easterly continuations in the Karkonosze–Izera Massif, the Kamieniec Ząbkowicki Belt and the Orlica–Śnieznik Dome. All these units comprise Early Ordovician (~500 Ma) metagranites associated with mica schists. Even more to the east, ~500 Ma metagranites and metasedimentary rocks occur also in the Strzelin Massif of the East Sudetes, where they are known as the pale and dark Stachow gneisses, respectively. Altogether, these rocks form the Stachow Complex which was thrust on the Strzelin Complex of the Brunovistulicum Terrane during the Variscan Orogeny. The contribution presents lines of evidence for a Saxothuringian affinity of the Stachow Complex rocks: (1) the new SHRIMP U-Pb age data of zircons from both the pale and dark Stachow gneisses; (2) the indication that the zircon age spectra from the ~500 Ma granitoids and their accompanying metasedimentary rocks are similar to those found in other parts of the Sudetes; (3) the “Armorican” age pattern of inherited zircons of the pale Stachow gneisses, as also observed in the Saxothuringian Terrane; (4) the similarity of trace elements and Sm-Nd isotope data of the Stachow gneisses and correlative rocks from the Karkonosze–Izera Massif and the Orlica–Śnieznik Dome.

N2-rich fluid in the vein-type Yangjingou scheelite deposit, Yanbian, NE China

Nearly pure N2 fluid inclusions (Th (L) = −151~−168 °C; Th (V) = ~150.3 °C) were identified in W-mineralized quartz veins from the Yangjingou scheelite deposit, in the eastern Yanbian area, NE China. Other fluid inclusion populations include N2-CO2, NaCl-H2O ± N2 and CO2 ± N2-NaCl-H2O, but no hydrocarbons were detected. The host rocks are part of the Wudaogou Group metamorphic series, which mainly consist of Ca-rich mica schist. Subhedral sulfide minerals occur in early disseminated W-mineralized quartz veins, or have partially replaced early scheelite. ThN2 and ThN2-H2O indicate N2 fluid-trapping from 315 °C to 410 °C and from 80 MPa to 350 MPa. Oxygen and hydrogen isotopic data (δD = −74.9‰~−77‰, δ18O = 9.6‰~12‰, V-SMOW) suggest that the mineralizing fluids were composed of mixed magmatic and metamorphic water, N2-rich inclusions (δ15N = −0.5‰ to 1.4‰) indicate fluid-rock interaction with metamorphic rocks. The N2-rich fluid was closely associated with scheelite precipitation. During thermal decomposition under high oxygen fugacity conditions, which occurred synchronously with metamorphism and magmatic activity, large amounts of N2 were liberated from NH4+-micas, which then accumulated in the parent fluid of the quartz scheelite veins.

The metamorphic evolution and tectonic significance of the Sumdo HP–UHP metamorphic terrane, central-south Lhasa Block, Tibet

Abstract The Lhasa terrane, forming one of the main tectonic components of the Himalayan–Tibetan orogen, has received a lot of attention as it records multiple episodes of plate spreading, subduction and collision within the realm of the Paleo-Tethys Ocean. A review of the mineralogical, petrological, geochemical and geochronological data of eclogites, associated blueschists and garnet-bearing mica schists from the Sumdo high- and ultrahigh-pressure metamorphic (HP/UHP) complex in the central/southern part of the Lhasa terrane, is present here so that the origin and tectono-metamorphic evolution of this important suture can be deduced. By re-evaluating the original published P – T conditions for the metamorphic rocks of the Sumdo Complex, we consider that the Sumdo Complex has experienced low temperature HP/UHP metamorphic conditions, characteristic of fast subduction (and exhumation) in a typical oceanic subduction zone setting. The original wide spread in the maximal peak P – T conditions could be reduced in size due to thus far unknown inconsistencies in the usage of applied geothermobarometric techniques. The remaining spread in the maximal P – T conditions ( c. 200°C/10 kbar) of the HP/UHP regions can be explained by a mechanism that the rocks from individual tectonic slices were subducted to different depths and followed by juxtaposition on their way back to the surface. A re-consideration of the isotopic ages of eclogites from the Sumdo Complex demonstrates that the opening of the Paleo-Tethys Ocean, located in between the two major components of the Lhasa terrane, was initiated prior to c. 280 Ma and the eclogite facies metamorphism is likely to be of late Permian ( c. 260 Ma) to early Triassic age (245–225 Ma), recording different ages of subduction from individual slices of the oceanic crust. The closure of the Paleo-Tethys Ocean resulted, no earlier than 210 Ma, in the final collision between the northern and southern Lhasa blocks. This final collision event may have been triggered by the initial subduction of the Bangong–Nujiang Tethys Ocean in the north.

The origin and geochemical characteristics of rutile in eluvial and fluvial-alluvial placers and quartz veins of the Menderes Massif from the Neoproterozoic Pan-African Belt, Western Turkey

Rutile occur in important amounts within Quaternary detrital sediments, and quartz veins which cut down the Paleozoic aged Eşme formation in the Menderes Massif on the Neoproterozoic Pan-Afrikan Belt in the west of Turkey. As a result of erosion, transport and accumulation of rutile-bearing quartz veins in the mica schists of the Eşme formation outcropped between Eşme (Uşak) and Kula (Manisa), the placer rutile occurrences occur in terraces, fluvial-alluvial and eluvial deposits. This study investigates the element content of rutile, oxygen isotope ratios of rutile and quartz, and formation temperature of rutile related to titanium source rock.Field observations show that rutiles are located in quartz veins that cut the schists. After these veins were fragmented and eroded, blocks of quartz with rutile and rutile grains are transported into clastic sediments in slope debris (eluvial), terraces and recent fluvial-alluvial deposits. Whereas quartz, rutile, ilmenite, and albite are found in the paragenesis of the rutile-bearing quartz vein fragments, the placers forming the terraces and current fluvial-alluvial deposits contain quartz, albite, muscovite, orthoclase, kaolinite, rutile, dravite, ilmenite, and zircon. The rutile grains in the examined placers and quartz vein fragments have grain sizes ranging from mm fractions to 5–6 cm.The major oxide compositions of the rutile grains have average values of 94 wt.% TiO2, 1.5 wt.% Fe2O3, 0.5 wt.% SiO2, and 0.3 wt.% Al2O3. The rutile samples have average values Nb of 1424 ppm, V of 980 ppm, W of 192 ppm, Ta of 94 ppm, and Zr of 73 ppm. The rutiles in the study area are defined as iron-rich rutile “nigrine” due to high iron content. Quartz veins are host rocks of rutile grains in the terrace and fluvial-aluvial sediments.The rutile-bearing quartz veins may be sourced from the Lower-Middle Miocene aged granites which are intruded the rocks in the Menderes Massif. Based on the analysis results of the methods of Zr thermometer from Zr values containing rutile and quartz-rutile geothermometer from oxygen isotope results (13.5‰ and 6‰) of quartz and rutile samples, the formation temperature of the rutile-bearing quartz veins were found to be at temperatures of 537 °C and 561 °C, respectively. These temperatures indicate that the rutile-bearing quartz veins may be in a temperature range between pegmatitic and/or hydrothermal stages.

Chemical and mineralogical composition of fluvial sediments (Bistrita River, Romania): Geogenic vs. anthropogenic input into rivers on its way through mining areas

The upper reaches of the Bistrita drainage system were selected as a natural test site to determine the geogenic and anthropogenic input into fluvio-lacustrine systems in humid mid-latitude morphoclimatic zones. The reason for this selection lies in the complex geology and its metallogenic evolution leading to a great variety of Fe, Mn, U, and polymetallic sulfides ore deposits. It sparked an intense mining activity during the past centuries with a strong impact on the drainage system similar to many mineralized sites in the world which are still under exploitation. Sediment samples from Bistrita River were analyzed by means of X-ray fluorescence spectrometry (XRF), X-ray diffraction (XRD), near-infrared (NIR) and micro-Raman spectroscopy (μ-Raman).Our results revealed that the chemical and mineralogical built-up of the stream sediments is mainly geogenic, with most of the trace elements accumulated in the river sediments derived from the source rocks exposed in catchment area of the River Bistrita. A strong input by man has been detected in the drainage system near abandoned mining sites. The trace elements are mainly accommodated in the structure of detrital minerals representative of the clastic aureole around the source rocks, and to a lesser extent adsorbed onto the surface of clay minerals. The REE incorporated into muscovite furnish evidence of having derived from the source rocks, prevalently mica schists exposed by supergene processes in the provenance area and rule out a neoformation of clay minerals on transport and deposition.

Deeply subducted continental fragments – Part 1: Fracturing, dissolution–precipitation, and diffusion processes recorded by garnet textures of the central Sesia Zone (western Italian Alps)

Abstract. Contiguous continental high-pressure terranes in orogens offer insight into deep recycling and transformation processes that occur in subduction zones. These remain poorly understood, and currently debated ideas need testing. The approach we chose is to investigate, in detail, the record in suitable rock samples that preserve textures and robust mineral assemblages that withstood overprinting during exhumation. We document complex garnet zoning in eclogitic mica schists from the Sesia Zone (western Italian Alps). These retain evidence of two orogenic cycles and provide detailed insight into resorption, growth, and diffusion processes induced by fluid pulses in high-pressure conditions. We analysed local textures and garnet compositional patterns, which turned out remarkably complex. By combining these with thermodynamic modelling, we could unravel and quantify repeated fluid–rock interaction processes. Garnet shows low-Ca porphyroclastic cores that were stable under (Permian) granulite facies conditions. The series of rims that surround these cores provide insight into the subsequent evolution: the first garnet rim that surrounds the pre-Alpine granulite facies core in one sample indicates that pre-Alpine amphibolite facies metamorphism followed the granulite facies event. In all samples documented, cores show lobate edges and preserve inner fractures, which are sealed by high-Ca garnet that reflects high-pressure Alpine conditions. These observations suggest that during early stages of subduction, before hydration of the granulites, brittle failure of garnet occurred, indicating high strain rates that may be due to seismic failure. Several Alpine rims show conspicuous textures indicative of interaction with hydrous fluid: (a) resorption-dominated textures produced lobate edges, at the expense of the outer part of the granulite core; (b) peninsulas and atoll garnet are the result of replacement reactions; and (c) spatially limited resorption and enhanced transport of elements due to the fluid phase are evident along brittle fractures and in their immediate proximity. Thermodynamic modelling shows that all of these Alpine rims formed under eclogite facies conditions. Structurally controlled samples allow these fluid–garnet interaction phenomena to be traced across a portion of the Sesia Zone, with a general decrease in fluid–garnet interaction observed towards the external, structurally lower parts of the terrane. Replacement of the Permian HT assemblages by hydrate-rich Alpine assemblages can reach nearly 100 % of the rock volume. Since we found no clear relationship between discrete deformation structures (e.g. shear zones) observed in the field and the fluid pulses that triggered the transformation to eclogite facies assemblages, we conclude that disperse fluid flow was responsible for the hydration.

Formation age and geodynamic setting of the Neoproterozoic Shalong iron formation in the Central Tianshan, NW China: Constraints from zircon U–Pb dating, geochemistry, and Hf–Nd isotopes of the host rocks

The recently discovered Shalong iron deposit in the Central Tianshan with a reserve of 14 Mt of iron at 25–30 wt.% Fe shows the typical iron formation geological characteristics: The host rocks are mainly metamorphosed volcanic rocks including quartz hornblende schist, mica quartz schist, and plagioclase amphibolite; the iron orebodies are stratiform and bedded, indicative of strata‐controlled deposits; metallic minerals are dominated by magnetite with haematite and ilmenite in minor quantities, whereas gangue minerals are quartz, amphibole, chloritoid, and plagioclase; and ore textures are banded and laminated. Here, we present LA‐ICP‐MS zircon U–Pb ages, whole‐rock geochemistry, Sm–Nd isotope, and zircon Lu–Hf isotope analyses on the host rocks in order to constrain the timing and geodynamic setting of the deposit. The geochemical characteristics, Sm–Nd isotope, and zircon Lu–Hf isotope compositions of the host rocks indicate that their protoliths were likely basic and felsic volcanic rocks. The bimodal kind rock types, the juvenile Nd–Hf isotopic characters, and the within‐plate trace element geochemical features of the host rocks together indicate that these rocks formed in a rift‐like extensional setting. Zircon U–Pb dating of the host rock interlayer suggests that the Shalong iron formation was formed in the Neoproterozoic (ca. 760 Ma). Combined with geological characteristics, close association with meta‐volcanic rocks, absence of glacial deposits, and within‐plate geochemical and juvenile isotopic signatures of the host rocks, the Shalong iron deposit is interpreted to be an Algoma‐type Neoproterozoic iron formation. The formation of this deposit can be linked to rift‐like volcanic activities related to the breakup of the Rodinia supercontinent. Our results in conjunction with previous studies suggest that the Central Tianshan should be related to the breakup of Rodinia, as indicated by the Neoproterozoic rift‐related igneous events of the Central Tianshan.

The origin of the sediment-hosted Kanmantoo Cu-Au deposit, South Australia: Mineralogical considerations

Multiply deformed sediments of the Cambrian Kanmantoo Group, which were metamorphosed to the amphibolite facies, host numerous Cu-Au, Fe-S, and Pb-Zn-Ag-(Cu-Au) deposits, of which the largest Cu-Au deposit is Kanmantoo (34.5 Mt @ 0.6% Cu and 0.1 g/t Au). Mineralization at Kanmantoo is characterized by discordant and pipe-like orebodies (Kavanagh and Emily Star) along with mineralization locally concordant to bedding (Nugent) that is spatially related to meta-exhalative rocks. Previous studies have suggested a syn-sedimentary origin for the Pb-Zn-Ag-(Cu-Au) and Fe-S deposits, whereas the Kanmantoo deposit remains controversial, with syn-sedimentary, metamorphogenic, and post-peak metamorphic models having being applied. The stratiform nature of some parts of the Nugent orebody, and disseminated sulfides locally concordant to bedding along with the recognition of a zone of chalcopyrite-magnetite-rich rocks at the syn-sedimentary Wheal Ellen Pb-Zn-Ag-(Cu-Au) deposit, which shows a metallic mineral assemblage almost identical to the most common assemblage at Kanmantoo, supports a genetic link between the Pb-Zn-Ag-(Cu-Au) deposits and Cu-Au mineralization, and is consistent with a metamorphosed syn-sedimentary model for Cu-Au mineralization. The discordant nature of most orebodies at Kanmantoo is the result of the mineralization having formed as stockwork zones in sub-seafloor pipes. Varying degrees of remobilization were subsequently associated with syn- to post-peak metamorphism. Major and trace element compositions, coupled with principal component analyses, show that the compositions of garnet, biotite, staurolite, chlorite, muscovite, and magnetite in metamorphosed altered rocks spatially associated with sulfide mineralization at Kanmantoo can be distinguished from those, where present, in metamorphosed country rocks (i.e., metapsammites and metapelites). Garnet, chlorite, biotite, and muscovite in quartz garnetite within quartz mica schist associated with the Nugent orebody are elevated in Mn (up to 19.5 wt% MnO – garnet, 2,825 ppm – chlorite, 3,206 ppm – biotite, and 108 ppm – muscovite) and Zn (up to 170 ppm – garnet, 1,602 ppm – chlorite, 1,592 ppm – biotite, and 108 ppm – muscovite) relative to samples in other orebodies and in unmineralized rocks elsewhere in the Kanmantoo Group. Such enrichments in these elements mimic similar enrichments in the same minerals in metamorphosed altered rocks associated with Pb-Zn-Ag-(Cu-Au) deposits in the Kanmantoo Group. Biotite in metamorphosed altered rocks at Kanmantoo contains elevated concentrations of Pb (up to 110 ppm), and, in general, Zn (up to 841 ppm), whereas muscovite is also elevated in Pb, Zn, and Cu (up to 272 ppm Pb, 78 ppm Zn, and 173 ppm Cu). Staurolite in these same rocks contains up to 1.6 wt% ZnO (with one outlier that contains 3.2 wt% ZnO) and is considerably more enriched in Zn than in unaltered country rocks (∼0.1 wt% Zn). The trace element enrichments in silicates studied here constitute a potential pathfinder to metamorphosed Cu-Au mineralization in the Kanmantoo Group and emphasize the geochemical and genetic links between the Pb-Zn-Ag-(Cu-Au) and Cu-Au deposits.

Natural and experimental fluorine substitution in biotite: Implications for fluid-rock thermochronometry and application to the Seridó Belt, northeastern Brazil

Abstract We modeled non-steady-state, fluid-assisted diffusion in a sheet plane, infinite cylinder and finite thin cylinder using analytical solutions of the diffusion equation and the experimental diffusivity of F-in-biotite. Diffusion experiments on natural biotite in the presence of hydrofluoric acid at 0.4 GPa and 650, 700, and 750 °C produced diffusive influx perpendicular to the c-axis. EPMA mapping and profiling allow us to define the following Arrhenius expression of F diffusion in biotite: D = 7.04 m 2 s × 10 − 4 × exp − 221 kJ mol RT − 1 The results yield the discrimination of a high-T homogenization domain where cooling paths continuously reequilibrate F-in-biotite, an intermediate-T zonation domain where cooling paths record F-in-biotite diffusion profiles and a low-T closure domain where F-in-biotite remains unchanged. The zonation domain significantly expands with increasing cooling rates and decreasing diffusion time. We systematically analyzed F-in-biotite with electron microprobe in samples collected along representative cross-sections within the Serido Belt, northeastern Brazil. Its metasedimentary units reached upper amphibolite grade metamorphic conditions, with constant F-in-biotite (F ≈ 0.33 wt.%) in grains from mica schist and paragneiss. The Ti-in-biotite geothermometer yielded nearly constant temperatures around 623 °C. The F-in-biotite regional background values remain unchanged at the contact with the major Ediacaran F-rich Acari pluton. It indicates the absence of magmatic fluid influx into the host rocks and is evidence for a fluid-absent nature of melts. By contrast, the F-in-biotite of the metric mica schist enclaves within dykes and sills of Cambrian pegmatitic granites was reequilibrated by interaction with exsolved fluids. The Ti-in-biotite geothermometer indicates temperatures around 642 °C, and our model suggests 100 kyr as a minimal duration for fluid-rock interaction. The preservation of intra-grain F-in-biotite homogeneity requires fluid flux cessation before cooling or fluid-present cooling rates below 100 °C Myr−1. Higher cooling rates would generate F-in-biotite intra-grain zonation. At the contact with the pegmatitic granites, the metasedimentary rocks show meter-scale F-in-biotite gradients due to thermal profiles setting the fluid (fH2O/fHF) gradient. The preservation of such gradients is related to a relatively fast fluid flux episode compared to the overall duration of the elevated thermal profile. The absence of intra-grain zonation indicates that F-in-biotite re-equilibration was fast relative to that of the fluid flux duration. We found intra-grain F-in-biotite zonation within an orthogneiss sample from the Paleoproterozoic Caico complex basement nearby the metasedimentary belt and intruded plutons. The fit of the natural profile by modeled profiles for cooling rates below 10 °C Myr−1, obtained from available Serido Belt U-Pb and 40Ar/39Ar ages, indicates that the orthogneiss cooled from 475 °C. It suggests the existence of a thermal gradient as the inner part of the belt cooled from 623 °C.

Structural, metamorphic and geochronological insights on the Variscan evolution of the Alpine basement in the Belledonne Massif (France)

A structural and petrochronological study was carried out in the southern part of the Belledonne crystalline massif. A first tectonometamorphic event, Dx, corresponds to the eastward thrusting of the Chamrousse ophiolitic complex characterized by a low-temperature–moderate-pressure metamorphism reaching 0.535 ± 0.045 GPa and 427.5 ± 17.5 °C. A subsequent D1 deformation is defined by a penetrative S1 foliation that mostly dips toward the west and displays an E–W- to NE–SW-trending mineral and stretching lineation L1. D1 is associated with a top-to-the east shearing and is responsible for the crustal thickening accommodated by the eastward nappe stacking and the emplacement of the Chamrousse ophiolitic complex upon the Rioupéroux-Livet unit. This event is characterized by an amphibolite facies metamorphism (0.58 GPa ± 0.06; 608 ± 14 °C) that attains partial melting at the base of the nappe pile (0.78 ± 0.07 GPa; 680.5 ± 11.5 °C). LA-ICP-MS U-Pb dating of monazite grains from the mica schists of the Rioupéroux-Livet unit constrain the age of D1 to 337 ± 7 Ma. The D2 tectono-metamorphic event is characterized by NE–SW trending, upright to NE-verging synfolial folding. Folding associated with D2 is pervasively developed in all lithotectonic units with the development of a steeply-dipping S2 foliation. In particular, D2 involves the uppermost weakly metamorphosed Taillefer unit. LA-ICP-MS U-Pb dating performed on detrital zircon grains shows that the Taillefer conglomerates was deposited during the Visean. A zircon SIMS U-Pb age of 352 ± 1 Ma from a plagioglase-rich leucocratic sill of the Rioupéroux-Livet unit is interpreted as the age of magmatic emplacement. Our results suggest that the D2 event took place between 330 Ma and 310 Ma. We propose a new interpretation of the tectonometamorphic evolution of the southern part of the Belledonne massif, focusing on the Middle Carboniferous stages of the Variscan orogeny.

Comparison between Predicted Rock Mass Classes and Actual Rock Mass Classes in Headrace Tunnel of Upper Mai Hydroelectric Project, Nepal

This study involves comparison of predicted rock mass classes in design stage with actual rock mass classes of headrace tunnel of Upper Mai Hydroelectric Project. The total length of headrace tunnel is 2070.52 m. The lithology of the study area consists of gneiss, schistose gneiss and mica schist. The rock mass classifications, Q-system were used to predict rock mass classes during the design phase as well as for classify rock mass during excavation of headrace tunnel. The applicability and validity of proposed classification has been checked by comparing the prediction with actual observation after completion of excavation. It was found that the predicted classes does not exactly matches with the actual rock masses as a results effecting in construction time as well as in cost and economy of the project. HYDRO Nepal JournalJournal of Water Energy and EnvironmentIssue No: 22Page: 41-44Uploaded date: January 14, 2018