Low-grade Metamorphic Rocks Research Articles

An overview of copper ore mineralization in the Nepal Himalaya

Exploration of copper ore in the Nepal Himalayas has been carried out by different researchers at different times. The author conducted detailed geological studies and mineral exploration in some of the copper and polymetallic prospects and also reviewed the literature related to copper prospects in Nepal. The present research is able to delineate two distinct copper ore mineralization belts extending from west to east and identify the promising sites for copper ore and polymetallic mineralization that could be feasible for mining. The northern mineralization belt lies close to the Main Central Thrust where the mineralization occurs mostly in metamorphic rocks of garnet amphibolite facies in Darchula,Bajhang, Gorkha, Dolakha, Solukhumbu and Ilam districts. The southern mineralization belt in the Lesser Himalaya occurs in low-grade metamorphic rocks of greenschist facies in Dadeldhura, Rukum, Baglung, Myagdi, Tanahun, Dhadhing, Makwanpur, Dolakha, Udaypur, Bhojpur and Ilam districts. Most of these prospects were locally exploited on a small scale by traditional mining before 1951. Almost all mineralization in both belts is related to hydrothermal dissemination, irregular multiple vein type, replacement and cavity fillings and only a few polymetallic mineralizations at the exo-contact zone of granite bodies. The ore minerals were concentrated during the processes of regional metamorphism at the time of Himalayan upheaval. In general, the copper content in the northern belt appears to be higher in grade and tonnage as compared to the southern belt. The frequent association of tungsten, molybdenum, bismuth, nickel, tin, lead, zinc ores and traces of gold suggests the urgent need for further detailed exploration and evaluation of selected prospects which could be economic deposits and feasible for mining.

Petrology, age, and tectonic setting of the Park Spur pluton, Aspy terrane, central Cape Breton Highlands, Nova Scotia, Canada

The Park Spur pluton was emplaced in the Aspy terrane of the central Cape Breton Highlands at 374.2 ± 2.9 Ma (U–Pb zircon). It consists mainly of muscovite-biotite monzogranite with small areas of garnet-bearing muscovite monzogranite and abundant pegmatite and aplite dykes. On its northern margin the pluton intruded metamorphic rocks of the Ordovician–Silurian Cape North Group and associated orthogneiss and on the south it intruded low-grade metamorphic rocks of the Silurian Calumruadh Brook Formation. Deformation along the southern margin of the Park Spur pluton is consistent with emplacement during dextral transpression between the Bras d’Or and Aspy terranes. The Late Devonian age combined with S-type petrological characteristics show that the Park Spur pluton and nearby Canal pluton are related to the ca. 375 Ma Black Brook Granitic Suite, all interpreted to have been emplaced during later stages of the Acadian orogeny in Ganderia as a result of delamination.

Formation and distribution of landslides controlled by thrust-strike-slip fault zones and fluvial erosion in the Western Qinling Mountains, China

In active orogenic belts, fault zones and fluvial erosion are the two principal factors that control the development of landslides. The Langma-Baiya fault zone (LBFZ) and the Bailongjiang fault zone (BFZ) in the Western Qinling Mountains (WQM) in China are part of an intracontinental orogenic zone, where landslides are the principal geomorphological process. It is also one of the regions with the most severe landslide disasters in China. We established a historical landslide database for the LBFZ and BFZ, based on high-precision remote sensing image interpretation and field investigation. The results indicated a total of 2764 landslides, including 1190 large landslides and 79 giant landslides. The quality of the landslide database was evaluated using the frequency-area curve distribution characteristics, and the results showed that the small-medium size landslides are underrepresented in the database, likely due to the influence of factors such as surface erosion, vegetation coverage, and human activities. Therefore, we focused on the spatial distribution and formation mechanism of large and giant landslides. Landslides are unevenly distributed in space; specifically, our field investigation and statistical analysis of swath profiles revealed that landslides are aggregated within tectonic zones, in areas of weak rock masses, and on the banks of rivers. Finally, based on the combination of the various factors controlling landslides, we divided the study area into three subregions, for which conceptual models were established. These models highlight the importance of the distance of the fault zone to a river, together with the degree of fluvial erosion, in controlling the development of landslides. The combined effects of fluvial erosion, geological structures, and the low-grade metamorphic rocks exposed along fault zones are major controls on the development of large landslides, and the local structural characteristics of thrust-strike-slip fault zones control the type of landslides.

Lower Paleozoic low-grade metamorphic units from the Central Balkan Zone, Bulgaria: tectonic relationships, framework and geodynamic significance

The Central Balkan Zone belongs to the north-vergent fold-and-thrust belt of the Alpine Balkan orogen. In this zone, pre-Permian low-grade rocks, metamorphosed during the Variscan orogeny, were later reworked at two main stages of the Alpine compressional tectonics. Several tectono-stratigraphic subdivisions of these metamorphic rocks were previously presented, based on a purely stratigraphic approach in the absence of detailed structural studies and, most importantly, of reliable paleontological and geochronological records. In this study, we propose a new framework of the low-grade metamorphic rocks from the Central Balkan Zone, applying a critical analysis of the existing data combined with new geochronological data and detailed lithological and structural observations. Based on the structural relationships and geochronological constraints, several new entities, such as the Korduna, Bilo and Zvezdets units, were established together with a reassessment of the previously recognized Diabase-Phyllitoid Complex (DPC). Both DPC and the Bilo Unit are now defined as Cambrian–Lower Ordovician (?) mélange complexes part of an accretionary wedge and/or forearc basin formed along the north Gondwanan margin. In the Bilo, Murgash and Etropole mountains, a tectonically uninterrupted Cambrian–Upper Ordovician section, characterized by a normal metamorphic gradient from low-grade to nonmetamorphosed terrigenous rocks, is recognized. The deposition of these sediments is related to the subduction of the Prototethys and the early evolution of the Rheic Ocean. An inverted metamorphic gradient recognized along the southern slopes of the Etropole and Zlatitsa-Teteven mountains through the Korduna and Zvezdets units is related to Variscan syn-metamorphic deformation in the Stargel-Bulovanya Tectonic Zone.

Progress of Deep Resources Exploration and Mining (DREAM) program in China: Critical minerals

China launched the Deep Resources Exploration and Mining (DREAM) program in 2016. Since then, the program has made significant progress in the exploration of critical minerals, such as rare earth, rare, and rare scattered metals. A “five-in-one” model, based on climate, landform, parent rocks, carrier minerals, and pH values of weathering crust, has been established for rare earth prospecting in South China. It has led to a major breakthrough in the discovery of a new type of ion-adsorption rare earth deposit in the weathered crust of low-grade metamorphic rocks in southern Jiangxi, South China. A pegmatite beryllium (Be), skarn beryllium-tungsten (Be-W), cassiterite sulfide tin-tungsten-beryllium (Sn-W-Be), independent fluorite, and lead-zinc (Pb-Zn) vein five-in-one model was developed for the prospecting of rare metals such as Be and W and polymetals in the Zhaxikang-Cuonadong ore-concentrated area of the eastern Himalayas. A series of metallogenic models has been proposed for the investigation of lithium (Li) resources, leading to important breakthroughs in the prospecting of large to superlarge Li deposits in the Jiajika (western Sichuan), Dahongliutan (southwestern Xinjiang), and Xiaoshiqiao (central Yunnan) ore fields. Meanwhile, the DREAM program has achieved significant advancements in its knowledge of the ultranormal enrichment of indium, germanium, gallium, niobium, and rare earth elements in the western Yangtze block, Southwest China.

Geochronology and skarn mineralogy of the Akesayi Fe deposit in the Western Kunlun, Xinjiang: Implications for mineralization process

The Akesayi deposit, located in the eastern part of the Tianshuihai terrane of the Western Kunlun Orogenic Belt, Xinjiang, China, is a recently discovered medium-sized magnetite deposit containing approximately 51 million tons of ore with an average grade of 41 wt%. This deposit is hosted by skarns at the contact between monzonitic granite, diorite porphyry, and medium to low-grade metamorphic clastic rocks of the Permian Huangyangling Group. The ore types include banded, disseminated, and massive ores. The metallogenic process can be divided into prograde skarn, retrograde skarn, oxide, quartz-sulfide, and carbonate stages. The metallic minerals include magnetite and pyrite, and the gangue minerals include olivine, garnet, hedenbergite, hornblende, actinolite, biotite, cummingtonite, chlorite, quartz, and calcite. The zircon U–Pb dating of biotite adamellite intrusion and schist together constrains the formation age of the host rocks in the Akesayi Fe deposit to 217–272 Ma. Rb–Sr dating of eight garnet samples yields an isochron age of 12.41 ± 0.31 Ma, which is almost identical to the 12.56 ± 0.50 Ma age obtained via Sm–Nd dating of eight garnet samples. Furthermore, zircon U–Pb dating yields ages of 12.76 ± 0.12 Ma and 12.37 ± 0.18 Ma for diorite porphyry and monzonitic granite near the Fe orebodies, respectively. The initial Sr isotopes of garnet ((87Sr/87Sr)i = 0.710901) prove that the diorite porphyrite ((87Sr/87Sr)i = 0.709283) is related to skarn mineralization. Thus, the Akesayi deposit is a skarn-type iron ore related to the middle Miocene diorite porphyry. The mineral assemblage indicates the extensive development of manganese-skarn in the Akesayi mining area. The manganese content of host rocks of the deposit is low, and the source of manganese in the skarn may mainly come from the hydrothermal fluids associated with Miocene diorite porphyry emplacement. The compositional variations between the cores and rims of single garnet grains and different stages of tephroite indicate the concentration characteristics of the hydrothermal system, particularly manganese content, which evidently changed during hydrothermal activity. The endmembers of garnet and pyroxene suggest that the ore-forming environment transformed from a relatively reduced acidic environment to an alkali-rich, high-oxygen fugacity environment. The widely developed chlorine-rich amphiboles and biotite indicate that the hydrothermal fluids were rich in chlorine and may have been related to the diorite porphyrite intrusion near the ore bodies. The iron mineralization of the Akesayi deposit is analogous to the Gangdese Miocene porphyry–skarn polymetallic mineralization, which occurred during the Miocene in a post-collisional tectonic setting. Considering the series of Miocene intrusives discovered in West Kunlun, we propose that Miocene skarn Fe mineralization events occurred in West Kunlun, and that this area has great prospecting potential.

Weathering Processes and Mechanisms of Low-Grade Metamorphic Rock following Freeze-Thaw Processes

A remarkable risk in the field of rock engineering is the natural exposure of low-grade metamorphic rock to the freeze-thaw process. Examples of such materials are slate and phyllite, which are employed approximately mountainous permafrost zones. In order to study the weathering processes and mechanisms in low-grade metamorphic rock following freeze-thaw weathering processes, the freeze-thaw cycles were applied. Accordingly to this process, low-grade metamorphic rock partially or completely deteriorates, and mechanical properties, microstructure, and mineral content changes are commonly associated with freeze-thaw processes. This study shows that, following numerous freeze-thaw cycles processes, the quality, wave velocity, and compressive strength of rock reduce; calcite, mica, feldspar, and pyrite contents decrease; and clay minerals and gypsum content increase. Furthermore, the study also demonstrated that the changes in quality, wave velocity, and compressive strength caused by weathering of the rock followed the sequence slate > phyllite. However, the mineral contents of slate and phyllite alter in the same way. Because of this increase in volume and subsequent F-T cycles, new cracks form on the surfaces of pores due to outward pressure, and a lot of water is absorbed by clay minerals. The amount of water increases once again due to freezing as the temperature falls below zero degrees Celsius. This course duplicates with cyclic temperature changes and causes deterioration of low-grade metamorphic rock materials. By analysis of an examination of the damage variables based on wave velocity, it is possible to conclude that mineral changes during the freeze-thaw effect may make for the formation of voids and the deterioration of metamorphic rock. This research contributes to a better understanding of weathering processes and mechanisms of low-grade metamorphic rock following freeze-thaw weathering processes.

Structural record of polyorogenic pre-Alpine and Alpine deformations within a major thrust nappe close to a suture zone (Internal-External Zones Boundary of the central Betic Cordillera, S Spain)

ABSTRACT The Malaguide Complex occupies the highest position in a thick-skinned Alpine thrust stack. It includes polydeformed, sedimentary and low-grade metamorphic rocks of Palaeozoic basements and their Meso-Cenozoic covers. NE of Granada the complex results severely sheared and stretched along the Betic suture zone in the metamorphic hinterland (orogenic backstop). The suture developed after the Burdigalian, due to oblique collision of the Alboran Domain (including the Malaguide Complex) with the South-Eastern Iberian Paleomargin. The orogenic evolution in the backstop region started with late Palaeogene to Early Miocene N-directed pre-collisional shortening and early thrust stacking, associated with low-grade metamorphism in the lowest Malaguide units increasing downwards into the underlying Alpujarride Complex. An accretionary prism developed at the expense of deep-marine (Flysch) sediments and the sedimentary covers of the Iberian and Alboran conjugate margins (External Zones and Frontal units, respectively). Later orogenic stages were characterized by SSE-directed back-folding and -thrusting in the Middle Miocene affecting both sides of the Betic suture related to dextral wrenching causing plurikilometric displacements. In this way, the suture evolved towards a several kilometres wide megashear zone (Cogollos-Diezma Corridor), prior to a final phase of SW-directed post-orogenic extension in the Late Miocene. Despite this protracted Alpine evolution, the Malaguide basements also preserve a record of orogenic activity during Late Carboniferous stages of the Variscan Orogeny in the form of a set of NE-SW trending folds with SE vergence and associated slaty cleavage, which are not found in the post-Carboniferous cover. The temperature conditions of this event (>300°C) in the less metamorphic Malaguide units are determined by colour alteration index (CAI) data from conodonts that have also allowed biostratigraphic dating of the Devonian-Carboniferous successions.

Paleoenvironmental conditions over the Caribbean Large Igneous Province during the Late Cretaceous in NW of South American Margin: A sedimentological and ichnological approach

The Late Cretaceous in northwest South America was a period marked by synchronous regional geological processes (e.g., accretion of oceanic terranes, arc magmatism, sea-level fluctuations, oceanographic dynamics), related to the interaction of the Caribbean Large Igneous Province (CLIP) with the NW margin of South America. This synchroneity makes it difficult to reconstruct the complex geological evolution of this region. In Colombia, Upper Cretaceous marine sedimentary and low-grade metamorphic rocks are commonly exposed in both the Western Cordillera and the Sinú-San Jacinto Basin in the Pacific and Caribbean regions respectively. Significant deformation and structural complexity are the main limiting factors for sedimentological and paleontological studies of these deposits.Detailed sedimentological and ichnological analyses were carried out in eight sections, two in the Caribbean region and six in the Pacific region. The Caribbean sections are composed of limestones, mudrocks, siliceous and calcareous mudrocks, and cherts, associated with pelagic and hemipelagic deposits (E2.2 and G2.1 facies). The Pacific sections are characterized by two facies associations, one composed of mudrocks, limestones, as well as chert associated with pelagic and hemipelagic deposits (E2.2 and G2.1 facies) and the other of interlayered siliceous mudrocks and sandstones associated with turbiditic systems (C2 facies). Upper Cretaceous deposits in both regions are characterized by relatively low to moderate abundant and diverse trace fossils assemblages, consisting of Chondrites, Planolites, and Zoophycos in the Caribbean region and Chondrites, ?Nereites, Phycodes, Phycosiphon, Planolites, Thalassinoides, and Zoophycos in the Pacific region, assigned to the Zoophycos ichnofacies. Sedimentological and ichnological features suggest deposition in deep-marine environments characterized by pelagic, hemipelagic, and turbiditic sedimentation. Integrated with the biostratigraphical framework, this allows us to establish a regional correlation between both regions. High nutrient supply, poor oxygenation, and low hydrodynamic energy and sedimentation rates interrupted by episodic increases associated with turbiditic deposits are main paleoenvironmental factors controlling the sedimentation over the CLIP during Late Cretaceous.

Mineral chemistry of chlorite in different geologic environments and its implications for porphyry Cu ± Au ± Mo deposits

In the porphyry Cu ± Au ± Mo deposits (PCDs), chlorite and its mineral assemblages are indicators for exploration. However, chlorite found in PCDs may have different origins because it is a common mineral in different geological environments. Therefore, it is necessary to identify the differences in the composition of PCDs-related chlorites (chloritePCDs) and chlorite formed in other geological environments (chloriteBarren). Here, based on occurrences, mineral assemblages, and compositions of the chlorites from thirty PCDs and barren areas (including geothermal systems, chloriteGSs; low-grade metamorphic rocks, chloriteLGMRs; sedimentary rocks, chloriteSRs), we used principal component analysis (PCA) and random forest (RF) methods to find discrimination approaches. Results show that chloritePCDs and chloriteBarren differ in the Fe, Mg, Al, Ti, and possibly Mn, K, and Ca elements. Based on these differences, wt.% ratios of (MgO + 100*TiO2)/(FeO + 100*CaO), TiO2/(CaO + K2O + Na2O), Al2O3/(100*K2O*CaO), MgO/(100*CaO*Na2O), SiO2/100(CaO + K2O), and FeO/100TiO2 and diagrams of Fe/(Fe + Mg) (a.p.f.u. ratio) vs TiO2/(Na2O + K2O + CaO) (wt.% ratio), Fe/(Fe + Mg) (a.p.f.u. ratio) vs MgO/(100*Na2O*CaO) (wt.% ratio), and Fe/(Fe + Mg) (a.p.f.u. ratio) vs SiO2/Al2O3 (wt.% ratio) are proposed to distinguish chloritePCDs from chloriteBarren. These discrimination approaches have better results in distinguishing chloritePCDs from chloriteLGMRs and chloriteSRs but are less effective for chloriteGSs. By comparing the physicochemical conditions, the highly oxidized conditions and high temperature of the ore-related intrusions and fluid properties result in the compositional differences between chloritePCDs and chloriteBarren. Although further researches remain to be done, this study provides potential approaches for identifying chloritePCDs based on major elements.

Timing and origin of orogenic gold mineralization in the Kanggurtag area, NW China

The Kanggur, Kangxi and Matoutan deposits are three important orogenic gold deposits located in the Kanggurtag gold mineralization belt of Eastern Tianshan, NW China. These deposits are hosted in low-grade metamorphic (greenschist-facies) volcaniclastic rocks of the Early Carboniferous Yamansu Formation. The orebodies are distributed in approximately E–W-striking shear zones and show evidence of hydrothermal alteration and four deformation events. The first deformation (D1) is associated with progressive regional N–S shortening without growth of gold mineralization. The second deformation (D2) is manifested by right-lateral shear zones under ductile conditions, which triggered the initial enrichment of gold in the stage I pyrite. The third deformation (D3) is characterized by a transpressional event under ductile–brittle conditions and formed stage II–Ⅳ pyrites with dominant Au mineralization; the native gold is mainly distributed in the interstices or interface of the pyrites. The fourth deformation (D4) is archived by post-collision extensional structures accompanied by magma emplacement and numerous limonite and comb-like quartz veins.Six pyrite samples (stage III) separated from the Kanggur deposit yield a Re–Os isochron age of 321 ± 6.7 Ma (MSWD = 3.0, initial 187Os/188Os = 0.35 ± 0.22). Three pyrite samples (stage III) separated from the Kangxi deposit yield a Re–Os isochron age of 327 ± 4.5 Ma (MSWD = 1.2, initial 187Os/188Os = 0.94 ± 0.12). The δ34S value significantly increases from stage I pyrite (−37.3‰ to −25.5‰, average = −32.2‰) to stage II−Ⅳ pyrite (−0.90‰ to 6.30‰, average = 1.96‰), indicating that sulfur in stage I pyrite migrated from sedimentary sources whereas in stage II–Ⅳ pyrites it was derived from the pyroclastic rocks in the country rocks. The H–O isotopic compositions of quartz show that the fluid in stage I (δ18Ow = 2.25‰ to 5.15‰, δ2D = −66.0‰ to −42.5‰) was metamorphic water and the fluid in stage Ⅴ was mixed with a large proportion of meteoric water. The fluids in stages II–Ⅳ maybe a “second hand” magmatic origin related to the andesite in the country rocks. Therefore, we propose that Au mineralization in the Kanggur, Kangxi and Matoutan deposits derived mainly from a metamorphic fluid source associated with the pyroclastic rocks in the country rocks. In addition, we conclude that these deposits are orogenic gold deposits that formed during the Late Carboniferous collisional tectonism (ca. 320 Ma) and the epithermal Au deposits (ca. 280 Ma) maybe associated with the post-collision extensional setting.

Complex 40Ar ∕ 39Ar age spectra from low-grade metamorphic rocks: resolving the input of detrital and metamorphic components in a case study from the Delamerian Orogen

Abstract. In this study, we provide 40Ar / 39Ar geochronology data from a suite of variably deformed rocks from a region of low-grade metamorphism within the Cambro–Ordovician Delamerian Orogen, South Australia. Low-grade metamorphic rocks such as these can contain both detrital minerals and minerals newly grown or partly recrystallised during diagenesis and metamorphism. Hence, they typically yield complex 40Ar / 39Ar age spectra that can be difficult to interpret. Therefore, we have undertaken furnace step heating 40Ar / 39Ar geochronology to obtain age spectra with many steps to allow for application of the method of asymptotes and limits and recognition of the effects of mixing. The samples analysed range from siltstone and shale to phyllite and contain muscovite or phengite with minor microcline as determined by hyperspectral mineralogical characterisation. Whole rock 40Ar / 39Ar analyses were undertaken in most samples due to their very fine-grained nature. All samples are dominated by radiogenic 40Ar, and contain minimal evidence for atmospheric Ca- or Cl-derived argon. Chloritisation may have resulted in limited recoil, causing 39Ar argon loss in some samples, which is especially evident within the first few percent of gas released. Most of the age data, however, appear to have some geological significance. Viewed with respect to the known depositional ages of the stratigraphic units, the age spectra from this study do appear to record both detrital mineral ages and ages related to the varying influence of either cooling or deformation-induced recrystallisation. The shape of the age spectra and the degree of deformation in the phyllites suggest the younger ages may record recrystallisation of detrital minerals and/or new mica growth during deformation. Given that the younger limit of deformation recorded in the high-metamorphic-grade regions of the Delamerian Orogen is ca. 490 Ma, the ca. 470 to ca. 458 Ma ages obtained in this study suggest deformation in low-grade shear zones within the Delamerian Orogen may have persisted until ca. 20–32 million years after high-temperature ductile deformation in the high-grade regions of the orogen. We suggest that these younger ages for deformation could reflect reactivation of older structures formed both during rift basin formation and during the main peak of the Delamerian orogeny itself. The younger ca. 470 to ca. 458 Ma deformation may have been facilitated by far-field tectonic processes occurring along the eastern paleo-Pacific margin of Gondwana.

The distribution and hydrological significance of intact rock glaciers in the north-west Himalaya

ABSTRACT Intact rock glaciers are common periglacial landforms that occur in cold alpine mountains and are often considered indicators of alpine permafrost. They can be both active and inactive, but both have ice in them and can be hydrologically significant. Little is known about the distribution of the rock glaciers in the Himalaya at basin-scale and content of water that can be hydrologically significant during the water-stressed months. We have found 967 intact rock glaciers covering an area of 306 km2 in the river basins of Beas, Chenab, Ravi, Sutlej, and Yamuna. Out of these, around 77% are moraine derived and the rest are talus derived. Most of them occur in the elevation between 4000 and 4500 m having a slope of 10° to 30° with a westerly and south-westerly aspect. They start occurring from 3220 to 6206 m, in places with −10°C to 10°C mean monthly temperature and within 50–100 cm of average monthly precipitation. Their alignment suggests that these rock glaciers occur in the area with low to medium incoming solar radiations. These rock glaciers are generally found where the general rock types are low-grade metamorphic rocks and feldspar-rich intrusive granites occurring beyond the Main Central Thrust in the Higher Himalaya. Finally, we calculated about 8.5 billion m3 volume of water (ice water equivalent) stored in them, considering 60% of the volume of rock glaciers is ice.

MINERALOGIA E ASPECTOS GEOQUÍMICOS DOS LATERITOS FOSFÁTICOS DA ILHA PEDRA GRANDE DO GURUPI (PA)

The Pedra Grande Island located at the mouth of the Gurupi River, is equivalent to a small plateau, which is currently partially drowned by the sea, where the impacts of the waves have already left longitudinal fractures and caves on the island. The island is entirely made up of lateritic rock, represented by the ferruginous lateritic crust, exhibiting a dominantly brownish red coloration. Texturally it consists of nodules, pisoliths and pseudo-fragments, which are surrounded by a whitish cream cement, giving the rock a general brechoid appearance, in addition to columnar and cavernous features. Two mineralogical associations are individualized in these laterites: a group formed by iron and calcium phosphates, widely developed in the ferruginous lateritic crust, together with iron oxy-hydroxides; and another formed by the aluminous phosphates, which constitute the whitish cream phosphate cement. The quantification of major, minor and trace elements revealed high contents of Fe2O3, P205 and A1203, and low of SiO2. In terms of traces, Mn and Zn display low contents, associated with ferruginous lateritic crust. On the other hand, rare earth elements are more concentrated in phosphate cement, mainly in Ce, Nd and La. The Ilha de Pedra Grande do Gurupi exhibits some mineralogical and chemical similarities with other occurrences of lateritic phosphates in the region, located on the continent. This correlation reveals that in the recent geological past, the ocean coastline presented a very different morphology from the current one. The geochemical and mineralogical analyzes of the Pedra Grande laterites allow us to suppose that this lateritic profile developed under a tropical climate and semi-flat topography, enriching itself in phosphorus from a phosfor-bearing parent rock, which could have the following characteristics: a clayey rock, with of shales, carbonates and phosphorites, or even a low-grade metamorphic rock, composed of schists and phyllonites containing phosphorite levels. Keywords: ferruginous crust; breccia like rocks; duphrenite; mitridatite; crandallite.

Low-grade metamorphic rocks of the Tethys subduction–collision zone in the Medvednica Mt. (NW Croatia)

Low-grade metamorphic rocks of the Tethys subduction–collision zone in the Medvednica Mt. (NW Croatia)

Quantitative texture analysis of slates: An insight into slate’s elastic properties and their contribution to the understanding of seismic wave reflections in anisotropic materials

Slates are low-grade metamorphic rocks characterized by an alignment of the phyllosilicates, resulting in the development of a marked foliation. A consequence of this is a strong directional dependence of their elastic properties, which are tightly bound to their texture. The Rietveld analysis applied to synchrotron x-ray diffraction images of slates from the Truchas Syncline (NW Iberian Peninsula) will provide us with the orientation distribution function (ODF) of the principal minerals that constitute the rock. This textural analysis shows an exceptional alignment of phyllosilicates (muscovite and chlorite) in contrast with a poor orientation of other minerals such as quartz. In a later treatment of the data, the elastic properties of the slates, as well the variations of the seismic wave velocities, have been calculated from the ODF and the single crystal stiffness tensors. The results of both the elastic properties and the wave velocities can be used for the construction of models that allow to understand the reflections of seismic waves in markedly anisotropic materials, which is a matter of a great interest in the studies of the Earth’s lithosphere with seismic methods.

Formation of the Banxi Sb deposit in Eastern Yangtze Block: Evidence from individual fluid inclusion analyses, trace element chemistry, and He-Ar-S isotopes

The Banxi Sb deposit (>100,000 t Sb metal with a grade of 0.02%–64.5% Sb (typically 15.3%–25.9% Sb) in the Eastern Yangtze Block is hosted in Neoproterozoic low-grade metamorphic clastic rocks of the Banxi Group. The deposit is characterized by vein-type Sb-only mineralization. Analyses of individual quartz-hosted fluid inclusions obtained by LA-ICP-MS identified Na as the most abundant cation (thousands to tens of thousands of ppm) in the ore-forming fluids of the Banxi Sb deposit, together with variable contents of K, Ca, Al, As, Fe and Sb (tens to thousands of ppm). Multiple chemical criteria, including high Na+/K+ (typically 3–15) and low Mn2+/Na+ (0.003–0.2) and Sr2+/Na+ (0.001–0.01) ratios of individual fluid inclusions, suggest that the fluids responsible for quartz and stibnite precipitation in the Banxi Sb deposit were likely dominated by basinal brines within low-grade metasedimentary rocks. Moreover, the extremely low 3He/4He ratios (0.001–0.048Ra) but relatively high 3He/36Ar (0.5 × 10−5–5.8 × 10−4) and 40Ar/36Ar ratios (323–1111 with an average of 737) of fluid inclusions trapped in ore-stage quartz further support a sedimentary origin of ore-forming fluids in this deposit. The positive and homogeneous sulfur isotopic values (δ34S = 8.0–9.5‰) of stibnite and arsenopyrite are comparable to those of sulfides in Proterozoic metasedimentary rocks (δ34S = 5.6–11.5‰), indicating these metasedimentary rocks may have served as the primary source of S for the Banxi Sb deposit. Combined with geological features and new trace element chemistry data for quartz and stibnite, we envisaged a new formation model for the Banxi Sb deposit that assumes that circulation of weakly alkaline basinal brines through faults under an extensional tectonic setting during the Early Cretaceous (∼130 Ma), mobilization of Sb and S from the underlying and host metasedimentary rocks and ascent of ore- forming fluids along faults, and ultimately precipitation of Sb minerals in or around fault/fracture zones as a result of the decreasing pH of the ore-forming fluids as well as sulfidation during fluid-rock reaction.

Nb–Ta mineralization in Ti-oxide minerals from the Bagolyhegy Metarhyolite Formation (Bükk Mountains, NE Hungary)

Abstract The foliated low-grade metamorphic rocks of the Triassic Bagolyhegy Metarhyolite Formation, mainly of pyroclastic origin, host post-metamorphic quartz-albite veins containing abundant tourmaline and occasionally rutile/ilmenite. The study of the Ti-oxide-mineralized veins with SEM-EDX revealed an unusual mineral assemblage comprising fine-grained Nb–Ta-bearing oxides (columbite-tantalite series, fluorcalciomicrolite and other Nb–Ti–Y–Fe-REE-oxide minerals) intergrown with Nb-rich polymorphs of TiO2 (anatase, rutile), ilmenite and zircon enriched with hafnium. This high field strength elements (HFSE)-bearing paragenesis is unexpected in this lithology, and was not described from any formation in the Paleozoic-Mesozoic rock suite of the Bükk Mountains (NE Hungary) before. The host metavolcanics are significantly depleted in all HFSE compared to the typical concentrations in felsic volcanics and the mineralized quartz-albite veins have even lower Ti–Nb–Ta concentration than the host rock, so the mineralization does not mean any enrichment. From proximal outcrops of the Triassic Szentistvánhegy Metavolcanics, potassic metasomatized lenses with albite-quartz vein fillings containing rutile/ilmenite are known. We studied them for comparison, but they only contain REE mineralization (allanite-monazite-xenotime); the Nb–Ta-content of Ti-oxide minerals is undetectably low. LA-ICP-MS measurements for U–Pb dating of Hf-rich zircon of the Nb–Ta-rich mineral assemblage gave 71.5 ± 5.9 Ma as lower intercept age while dating of allanite of the REE mineralized quartz-albite veins gave 113 ± 11 Ma as lower intercept age. The REE-bearing vein fillings formed during a separate mineralization phase in the Early Cretaceous, while the Nb–Ta mineralization was formed by post-metamorphic alkaline fluids in the Late Cretaceous., controlled by fault zones and fractures.

Experimental Application of Cement-Stabilized Pavement Base with Low-Grade Metamorphic Rock Aggregates

Low-grade metamorphic rock (LMR) is a kind of stone that is widely distributed in China. The alkali activity strictly prevents its application in conventional concrete. This paper evaluates the possibility of using LMR aggregate in cement-stabilized pavement base (CSPB). The compressive strength of CSPB prepared with LMR and limestone aggregates at various curing conditions was measured. Expansion rates were determined via accelerated simulation tests to assess the alkali reactivity of LMR, followed by microscopic analysis. Finally, the possibility of using LMR in CSPB was evaluated from the economic viewpoint. Results indicate that CSPB specimens prepared with LMR have similar compressive strength at each content of cement, regardless of curing conditions. The expansion rates of all CSPB specimens with LMR were lower than 0.1%, indicating the absence of an AAR, which was further validated by the absence of the AAR product in microscopic observations. It is inferred from the economic analysis that 70.9% lower cost can be achieved by the replacement of limestone aggregate with LMR aggregate. This demonstrates that technical, economic and environmental benefits endow LMR with wide market potential as the aggregate of CSPB.

Geochemistry of the Middle Eocene Gercus Sandstone, Shaqlawa Area, Northern Iraq: Implications for Provenance, Tectonic Setting and Paleoweathering

The study of the geochemical characterization of sandstone rocks contributes to understand the nature of the source rocks, their tectonic setting and the effects of paleoclimate and paleo-weathering. Accordingly, twenty sandstone samples from the red sandstone units of the Gercus Formation were collected from the northeastern limb of Safin Anticline at Shaqlawa Area – Northern Iraq. The studied sandstone samples show depletion in major, trace elements contents, except for CaO, MgO and Ni, Co and Cr respectively, and general depletion in the rare earth elements (REE) contents. Major element geochemistry indicates that the red bed sandstones of the Gercus Formation are of an oceanic island arc (OIA) tectonic setting. On the other hand, the results revealed that the paleo-weathering of the source area of the Gercus Formation experienced intensive chemical weathering under humid and/or warm climate conditions. The maturity and paleoclimate conclusions of the studied samples suggest a low feldspar content and low values of K2O / Al2O3 ratio, which in turn resulted from the dominance of low grade metamorphic and sedimentary rock types in the source area.