Metasedimentary Research Articles

Geochemistry, U-Pb Zircon Ages and Hf Isotopes of Basement Rocks Beneath the Northeastern Margin of the Ordos Basin: Constraints on the Paleoproterozoic Evolution of the Western North China Craton

SHRIMP zircon ages, Hf-in-zircon isotopic compositions and whole rock geochemistry were analyzed on basement metamorphic rocks from drill cores collected from the northeastern margin of the Ordos Basin. Geochemical data from four metasedimentary rocks show large variations in major element compositions, but have similar REE patterns and trace element compositions, with ΣREE = 161.80 × 10−6~341.82 × 10−6, δEu = 0.26~0.63 and LaN/YbN = 3.44~25.38. SHRIMP zircon U-Pb dating of granitic gneiss yielded the magmatic zircon ages between 1856 ± 14 Ma and 2188 ± 11 Ma, with the upper intercept age of 2229 ± 88 Ma. The cores have εHf(t) values of −5.46 to +6.57, and Hf model ages vary from 2869 Ma to 2300 Ma. The analyses of metamorphic overgrowths on zircon grains yield an average metamorphic age of 1865 ± 17 Ma (MSWD = 3.5). The zircon cores have εHf(t) values of −3.98 to 1.95, and Hf model ages vary from 2782 to 2416 Ma. Combined with data from earlier studies, we draw the conclusion that the metamorphic rocks in the borehole were formed during middle Paleoproterozoic time and were involved in a major late Paleoproterozoic tectono-thermal event which might be attributable to the collision between the Western and Eastern Block along the TNCO (the Trans-North China Orogen). The basement rocks are petrologically, geochronologically and geochemically comparable with rocks from the Khondalite series. The fact that the material properties of basement rocks beneath the northeastern Ordos Basin are different from those of the Western Block indicates that the extent and exact boundaries of the Ordos Block need to be refined.

Mesozoic thermal events and related gold mineralization in the еastern Mongol-Okhotsk Orogenic Belt: constraints from regional geology and 40Ar/39Ar dating

ABSTRACT The Mongol–Okhotsk Orogenic Belt is a major structural element in East Asia. Several aspects of its evolution remain unknown or controversial. In this study, we report the results obtained using 40Ar/39Ar geochronological studies performed on the metasedimentary rocks from various terranes and formations as well as host rocks and hydrothermally altered rocks of gold deposits in the еastern Mongol–Okhotsk Belt. Our new 40Ar/39Ar data suggest that four thermal events occurred in the eastern Mongol–Okhotsk Belt at 172–166, 157–149, 141–135, and 133–128 Ma. The ages of these events do not correlate with the geological age of the formations and not depend on the location of the terranes. None of these events were accompanied by magmatic activity in the east of the belt. Therefore, we believe that these events are related to tectonic. The 172–166 Ma event corresponds to regional metamorphism associated with the collision of the Siberian Craton and Amurian Superterrane. The remaining three younger events of 157–149, 141–135, and 133–128 Ma occurred in the post-collisional setting. The Malomyr, Albyn, Unglichikan, and Elga gold deposits formed during two thermal stages at 141–135 and 133–128 Ma, after the collision. 40Ar/39Ar isotopic data suggest a gap (~30 Ma) between orogenic events caused by the collision of the Siberian Craton and Amurian Superterrane and gold mineralization. Our data are consistent with the idea that the formation of the Mongol–Okhotsk Orogenic Belt and gold ore deposits in this belt occurred in the relation to strike–slip (or transform displacement) along the regional E–W Mongol–Okhotsk fault/thrust system, but without relation of these deposits to granites.

Tectonic control on the spatial distribution of Sn mineralization in the Gejiu Sn district, China

Late Cretaceous granitic rocks occur in the Gejiu ore district to the east and west of the N-S striking Gejiu Fault, whereas major Sn deposits are only known to occur to the east of the Gejiu Fault. Comparison of the whole-rock chemistry, the apatite trace-element chemistry, and zircon Hf and O isotope data of the various granites demonstrates that fertile granites occur to both sides of the Gejiu fault. The results demonstrate that the c. 83 Ma old granitic intrusions (i) have similar magma sources, which are dominated by metasedimentary rocks that had experienced intense chemical weathering, resulting in reduced melts, (ii) had similar melting conditions, i.e., high temperature biotite dehydration melting, and (iii) in part had experienced large extents of fractional crystallization. The most evolved granites to both sides of the Gejiu Fault have the characteristics typical of tin granites. Therefore, the absence of major deposits to the west of the Gejiu Fault is not due to the absence of fertile granites. The areas to the east and the west of the Gejiu Fault, however, have fundamentally different fault pattern, which indicates different orientation of the stress field to both sides of the Gejiu Fault at the time of the emplacement of the Cretaceous granites. Late Cretaceous dextral movement along the Ailaoshan Fault Zone resulted in a (trans)tensional setting in areas to the east of the Gejiu Fault and in a (trans)pressional setting to the west of that fault. We speculate that the tectonic setting influences the potential for mineralization because the Sn bearing fluids need efficient pathways to transporting metals from the roof zone of the batholith into the wall rocks. In contrast to regional compression, hydraulic fracturing in an overall extensional setting has the potential to develop efficient fluid pathways and, thus, may lead to major ore deposits. To the west of the Gejiu Fault, however, granite intrusions in an overall compressional setting are likely to develop no or only small mineralization.

Geochemistry and petrogenetic evolution of metasedimentary rocks in Bunu Area, part of Kabba-Lokoja-Igarra schist belt, SW Nigeria

Geochemistry and petrogenetic evolution of metasedimentary rocks in Bunu Area, part of Kabba-Lokoja-Igarra schist belt, SW Nigeria

Detrital zircon analysis of metasedimentary rocks of the Staré Město Belt, Sudetes: implications for the provenance and evolution of the eastern margin of the Saxothuringian terrane, NE Bohemian Massif

The Staré Město Belt (SMB) in the Central Sudetes forms a Variscan tectonic boundary zone that is located between the Saxothuringian and Brunovistulian terranes of the Bohemian Massif. The three thrust-bounded upper, middle and lower lithotectonic units of the SMB are composed of metasedimentary and Late Cambrian metavolcanic rocks. A new LA-ICPMS zircon geochronology supported by zircon typology studies of the mica schists of the upper unit and the migmatitic paragneisses of the middle unit provides new insights into the provenance and evolution of the SMB. Our new data were obtained from metasedimentary rocks and compared to the previously published zircon ages of the SMB metavolcanic rocks. The results indicate that the metasedimentary and bimodal metavolcanic rocks in the separate lithotectonic units of the SMB originally formed Late Cambrian volcano-sedimentary successions. The source areas of the sedimentary basins studied were dominated by Neoproterozoic and Paleoproterozoic crystalline rocks that were presumably located near the West African Craton of Gondwana. A comparison of the detrital age spectra obtained with those previously published from the region indicates a strong association of the entire SMB with the Saxothuringian terrane of the Bohemian Massif. During partial melting of the metasedimentary rocks of the middle unit of the SMB, Cambrian and older zircon grains were affected by solid-state transformations that caused partial resetting of the U-Pb dates, changes in internal zircon textures and reductions in Th/U ratios.

Implications of Ore Textures for Gold Recovery at Esaase Deposit, Asanko Gold Mine

The Esaase Deposit of the Asanko Gold Mine Ltd (AGM) contains a system of epigenetic gold-related quartz veins hosted by tightly folded and foliated metasedimentary rocks. The gold mineralisation is associated with disseminated sulphide minerals in the quartz-carbonate veins and within the alteration zones of the host metasedimentary rocks. AGM employs a combination of Gravity Concentration and Carbon-In-Leach (CIL) methods to recover its gold and this has resulted in a recovery rate that ranges from 81.3% to 94.1%. The high recovery rate (>90%) suggests that the gold is free and therefore not locked up in the sulphides that are associated with the mineralisation. However, the recovery rate drops from 94.1 to 81.3% even when the samples contain high amounts of sulphides. Since the presence of sulphides in altered rock is one major indicator of gold mineralisation, this research was conducted to determine the ore textural relationship between gold and sulphides and the implication of the ore textures on the current gold recovery methods employed on the deposit. Polished block sections were prepared from samples from the ore zones: Transition and Cobra. Ore petrographic studies were conducted on the samples using the Leica DM2700P petrographic microscope at the University of Mines and Technology, Tarkwa, Ghana. These studies revealed that the low gold recoveries may be attributed to the gold occurrence as inclusions in the sulphides, which is typical for refractory ores, thus, making it difficult to be extracted by the current method employed at Asanko. Therefore, AGM may have to consider the economics of employing a combination of other methods to liberate the refractory gold and possibly, result in optimum recovery.

Prolonged high thermal gradient metamorphism in the Curnamona Province, south-central Australia, during the latter stages of Nuna assembly

Granulite-facies metasedimentary rocks from the Broken Hill Block of the Curnamona Province, south-central Australia, record a prolonged period (c. 1630–1550 Ma) of high thermal gradient metamorphism and anatexis. We present monazite and zircon U–Pb geochronological and REE geochemical data from garnet-bearing paragneisses of the Willyama Supergroup, coupled with garnet trace element mapping and mineral equilibria modelling, to develop a petrochronological model for the Palaeo–Mesoproterozoic metamorphic evolution of this region. Our results indicate that the Curnamona Province underwent an early phase of high-temperature–low-pressure metamorphism between c. 1630–1610 Ma. Peak metamorphism involving wholesale anatexis subsequently occurred under an apparent thermal gradient of 150–170 °C/kbar at c. 1600–1580 Ma during regional compression. Monazite U–Pb dates reveal that suprasolidus conditions were maintained for at least 30 Myr, with final melt crystallisation potentially occurring as late as c. 1550 Ma, indicating that the system remained hot for an extended duration. We suggest that high thermal gradient metamorphism in the Willyama Supergroup was driven by elevated levels of radiogenic heat production hosted within the Willyama succession. Temperatures may have been initially boosted by low-to-moderate degrees of crustal extension, although there is a dearth of coeval magmatism and structural evidence to support this claim. Peak metamorphism coincides with the timing of compressional orogenesis and related metamorphism in other Proterozoic terranes that are inferred to have occupied the eastern margin of proto-Australia. Many of these terranes also exhibit prolonged metamorphic histories and elevated levels of radiogenic heat production, suggesting that radiogenic heating may have had a major influence in pre-disposing these terranes to be the locus of metamorphism and deformation during the final assembly of Nuna.

Paleoweathering, provenance and tectonic setting of metasedimentary rocks at Ayanfuri area in the Paleoproterozoic Kumasi basin in Ghana: evidence from petrography and geochemistry

Paleoweathering, provenance and tectonic setting of metasedimentary rocks at Ayanfuri area in the Paleoproterozoic Kumasi basin in Ghana: evidence from petrography and geochemistry

Dolomitic Marble in a Context of UHT Metamorphism: Possible Signs of Melting, Lenadora Quarry, Sri Lanka

Abstract The Lenadora quarry, located 25 km north of Matale, in central Sri Lanka, exploits dolomitic marble on an artisanal scale. It is located in the Highland Complex but very close to its thrust contact with the Wanni Complex. The rocks have undergone metamorphism to the granulite facies; by analogy with metasedimentary rocks in a similar position south of Kandy, conditions of metamorphism may even have surpassed 900°C and 0.9 GPa. How did dolomitic marble fare in such an ultrahigh-temperature environment? High-resolution large-area image mosaics of imaged polished thin sections were acquired by using both light microscopy and scanning electron microscopy. We focus on fifty areas of interest. As expected, diopside and forsterite porphyroblasts appeared during prograde metamorphism. However, we did not anticipate to see subhedral porphyroblasts of forsterite mimetically replaced by diopside + dolomite. The conversion could have involved a fluid phase or a carbonate melt that had become locally enriched in silica. The presence of a melt accounts for composite calcite + dolomite globules trapped in forsterite. There is no evidence of breakdown products of dolomite, like periclase or brucite. The evidence that a carbonate melt was present is circumstantial; carbonate systems are notorious for erasing telltale evidence of melting. Other minerals encountered in the dolomitic matrix are phlogopite, pargasite, rutile, hydroxylapatite, zirconolite, lakargiite, a magnesiobeltrandoite-like phase, pyrrhotite, and calcite as a minor phase, some of which is of a second generation. Unreplaced relics of forsterite were partially serpentinized at a retrograde stage.

Geochronology and Zircon Hf Isotope of the Paleoproterozoic Gaixian Formation in the Southeastern Liaodong Peninsula: Implication for the Tectonic Evolution of the Jiao-Liao-Ji Belt

The Jiao-Liao-Ji belt (JLJB), in the Eastern Block of the North China Craton, is a major Paleoproterozoic orogen and underwent a complicated tectonic evolution during 2.2–1.8 Ga. The Liaohe Group, an important stratigraphic unit in the JLJB, is key to understanding the complex evolution of this belt. In this paper, we present new detrital zircon U–Pb ages and Hf isotope data for meta-sedimentary rocks from the Gaixian Formation in different areas of the JLJB, in addition to compiled data for other formations of the Liaohe Group, to establish the depositional age and source of detrital materials of the group. U–Pb age results show that the age ranges of zircons from the different samples are broadly similar. The youngest zircon group is ca. 2.06 Ga, and the youngest single-grain age is ca. 2.0 Ga, constraining the depositional age of the Gaixian Formation to between 2.0 Ga and the metamorphic age of ca. 1.9 Ga. The zircon age data indicate that the provenance was primarily Archaean basement of the Nangrim Block and Paleoproterozoic volcanic rocks of the Li’eryu Formation. On the basis of the new geochronological data and results from previous studies, it is inferred that the JLJB underwent a successive process of rifting–subduction–collision, with the different formations of the Liaohe Group being deposited in different stages from rift to passive continental margin and then to active continental margin. Zircon Hf isotope data from the JLJB and adjoining Longgang and Nangrim blocks indicate that a major crustal growth event occurred at 2.9–2.5 Ga, followed by crustal growth and intense recycling of ancient crust at ca. 2.2 Ga.

Geologic Evidence of Lithostatic Pore Fluid Pressures at the Base of the Subduction Seismogenic Zone

AbstractGeophysical and rheological work below the subduction seismogenic zone suggest that elevated pore fluid pressures (Pfs) may facilitate slow slip and tremor. However, quantification of Pf and its variation in this environment remain indirect and incomplete. We examine brittle‐frictional deformation within metasedimentary rocks from an exhumed subduction zone. Here, penetrative foliation formed under viscous deformation conditions by pressure‐solution creep at 0.9 GPa, 300–350°C, and differential stresses <35 MPa. Three sets of crack‐seal veins, precipitated at peak subduction temperatures, cross‐cut this foliation. Mechanical inversions of these veins indicate they formed within a single stress field orientation with foliation, but at different magnitudes of differential stress. These deformation structures reveal near‐lithostatic Pf that fluctuated in time by perhaps only ∼4 MPa, resulting in cyclic mode I and shear fracturing and viscous flow, and support slow slip and tremor mechanisms that require or are enhanced by high Pf and low differential stress.

New constraints for crustal sulfur contamination of gold source: Evidence from complex δ34S of pyrite in the northwestern Jiaodong gold province, China

The formation processes of the complex δ34S variations for the Jiaodong-type gold deposits are still equivocal, which cannot give a robust identification for the sulfur sources, by inference, Au sources. The δ34S and trace elements of ore-related pyrite collected from the northwestern Jiaodong gold province (Jiaojia and Sanshandao-Xinli goldfields) are analyzed by in-situ methods, to reveal more clues for understanding the complex δ34S variations and ore material sources. Gold deposits in the northwestern Jiaodong contain quartz-Au-pyrite (Py-a) stage (III) and quartz-Au-pyrite (Py-b)-polymetallic stage (IV). In the Jiaojia goldfield, Py-a has a δ34S range of 9.9–24.4‰, whereas Py-b has a δ34S range of 7.3–9.3‰. In the Sanshandao-Xinli goldfield, Py-a has a δ34S range of 6.9–34.7‰, and δ34S of Py-b ranges in 0.2–19.2‰. Single-point and mapping analyses show that As-Au-Co-Ni contents have considerable variations in Py-a and Py-b. The δ34S and As-Au-Co-Ni contents in Py-a and Py-b show diverse correlations. Equilibrium and Rayleigh (or sulfidation) fractionations cannot account for the large δ34S variation due to the pyrite-chalcopyrite-pyrrhotite buffer. The rapid decompression with oxidization of ore-forming fluids by immiscibility can only result in the δ34S decrease, rather than increase to 14.0–34.7‰ from the concentrated δ34S range of 9.6–13.4‰ in Py-a and Py-b. Therefore, the analyzed δ34S values in the ore-related pyrites should represent those of ore-forming fluids. Considering the simple physicochemical processes cannot reconcile the complex matches between large δ34S variations and As-Au-Co-Ni contents, it is suggested the As-rich and Au-As-rich ore-forming fluids with large δ34S range and variable Co-Ni concentrations should flow into mineralization sites during stages III and IV in the Sanshandao-Xinli goldfield. The sulfur with large δ34S range and Au-As-Co-Ni elements with variable contents in the ore-related pyrites of the Jiaojia and Sanshandao-Xinli goldfields were most likely inherited from the sedimentary sulfides that were reduced from seawater sulfates, by comparisons with those in the typical sedimentary, metamorphic hydrothermal and magmatic hydrothermal pyrites. Combining with previous researches for the ore-forming materials sources of the Jiaodong-type gold deposits, the metamorphic Precambrian rocks, especially those metasedimentary rocks containing sulfides reduced from seawater sulfates, can be considered as the feasible ore-forming materials sources for the large δ34S range and variable trace elements in the Jiaojia and Sanshandao-Xinli goldfields. Through the δ34S comparison for the Jiaodong-type gold deposits, we propose an important process that the single-sourced ore-forming fluids, which were initially derived from the metasomatized lithospheric mantle, have extracted the ore materials (including sulfur-Au-As elements) from the metamorphic Precambrian rocks (possible Precambrian metamorphic basement in the Jiaodong gold province or the subducted Late Neoproterozoic rocks of the YC), and mixed heterogeneously to precipitate pyrites with complex δ34S and trace elements throughout the Jiaodong-type gold deposits.

Strain localisation and transcurrent reactivation in the granulite facies Kalinjala Shear Zone at Port Neill, South Australia

The central portion of the Kalinjala Shear Zone, in the Port Neill region, South Australia, is recognised as a northeast-trending anomalously magnetic corridor coinciding with a sub-vertical transpressional dextral shear zone. This shear zone is a primary lithospheric structure, separating the nucleus of the Archean Gawler Craton from a suite of Paleoproterozoic granites (ca 1850 Ma) and earlier paragneiss. At Port Neill, there is an apparent 17 km dextral offset of a pre-existing Bouguer gravity anomaly by the Kalinjala Shear Zone. This is attributed to Kimban-aged (ca 1740–1690 Ma) deformation that is accompanied by peak granulite facies metamorphic conditions reaching pressures of ∼1.0 GPa and temperatures of ∼800 °C. This is consistent with the presence of leucosomes, within the sheared fabric, in the axial surface of mylonitic folds and in mafic gneisses. Strain has been localised within and adjacent to mafic dykes, predominantly hosted in the granitic rocks, with the highest strained areas having the greatest evidence of leucosome extraction. The local orientation of stretching lineations in this shear zone do not necessarily correlate with the transport direction but may reflect along- and across-strike variations in finite shear strain into alternating zones dominated by either constrictional or a superimposed flattening strain. Overprinting the dominant dextral movement are sinistral shears, with horizontal strike-slip movements, which post-date the peak metamorphic assemblages and are generally confined to sites containing late K-feldspar-rich pegmatites. The major movement recorded in the Kalinjala Shear Zone was both horizontal and vertical, occurring in response to regional exhumation of the Cornian-aged (ca 1850 Ma) sequences in the east, during the Kimban Orogeny, and was basically controlled by the nature of lithospheric plate convergence during accretion of a continental backarc onto the Gawler Craton. KEY POINTS Dextral transpression with late-sinistral along-strike extension and east-up exhumation is associated with 17 km of horizontal offset along the central section of the Kalinjala Shear Zone. Deformation is dominated by three major Kimban-aged (ca 1730–1690 Ma) events, which overprint older ensialic backarc metasedimentary rocks, orthogneiss and granites of the Donington Suite. Granulite facies metamorphic conditions control the nature of deformation and strain distribution within the shear zone. The Kimban events record the collision of the Gawler Craton nucleus with the southern margin of the North Australian Craton.

Chapter 7 Scottish mineral Geological Conservation Review sites – Metamorphic minerals

Much of Scotland north of the Highland Boundary Fault has been affected by regional and, to a lesser extent, thermal metamorphism and minor metasomatism which, through variable temperatures and pressures, has produced a range of metamorphic minerals. This paper, the seventh of eight on Scottish Mineral Geological Conservation Review Sites, describes ten sites of the highest conservation value within the Lewisian, Moine and Dalradian terranes of the Scottish Highlands, together with thermally altered inclusions in Paleogene volcanic rocks. The minerals featured include the constituents of pelitic and semipelitic schist, such as kyanite and staurolite, unusually large garnet with orthopyroxene and clinopyroxene in ultramafic rocks, and calc silicate minerals, such as tremolite, actinolite, diopside and epidote, together with phlogopitic mica. One site is noted as having stratabound pyrite, chalcopyrite and sphalerite along with large concentrations of chromium mica. The presence of kyanite and tourmaline on the Ross of Mull is one of the few significant mineral sites within rocks of the Neoproterozoic-age Moine Supergroup.Six of the sites encompass extremely rare minerals, generated by the metamorphism of precursors with unusual rock compositions. Foremost amongst these are the barium minerals celsian, cymrite and hyalophane, and the first recorded occurrence in Great Britain of armenite. Although a range of amphibole species occur in both meta-igneous and metasedimentary rocks of the Scottish Highlands, the iron-magnesium clinoamphiboles of the grunerite-cummingtonite series are extremely rare. Indeed the site described in this paper is one of only two occurrences of cummingtonite within the Precambrian-age Dalradian Supergroup. In some cases, the regional mineral assemblages, including the cummingtonite-bearing rocks, have been overprinted by subsequent high temperature/low pressure thermal events associated with the emplacement of igneous intrusions. Pre-eminent amongst the thermal sites must be the occurrence of mullite and sapphire in aluminous shale xenoliths entrained within Paleogene-age basaltic lava flows on the Isle of Mull.

The utility of rapid throughput single-collector sector-field ICP-MS for soil Pb isotope studies

Lead (Pb) isotope tracing is widely used in environmental monitoring and public health studies, to determine the provenance of agricultural and archaeological materials, and in mineral exploration campaigns. Combined with elemental analyses, Pb isotope ratios are used to ‘fingerprint’ the sources and transport paths of Pb found in multiple targets including dust, metal filings, soil/regolith, plants, water and blood, to name a few. Low cost and high throughput are important factors in such studies - typically performed using solution-mode Inductively Coupled Plasma -Mass Spectrometers (ICP-MS) - but these undoubted advantages need to be balanced against data quality. Here we compare Pb isotopic compositions (including the low abundance 204Pb isotope) determined for unseparated soils measured by high-throughput single-collector sector-field inductively-coupled-plasma mass-spectrometry (SC-SF-ICP-MS) and quadrupole ICP-MS (Q-ICP-MS) with results from more traditional high-precision multi-collector-ICP-MS (MC-ICP-MS) analysis following Pb separation chemistry. The soils, from outcrops of young basalts near Melbourne (Australia), have Pb isotope compositions similar to those of the underlying lava flows, with minor offsets possibly related to dust import from Palaeozoic metasedimentary rocks which dominate the region. Except for one sample, contamination with industrial Pb appears to be absent to minor, a surprising finding given the proximity of the sample sites to a major metropolitan area. In these soils, comparison of data for fully digested and acid-leached soil, some from a range of sampling depths, allows small and variable contributions from anthropogenic Pb to be resolved. Despite their lower precision, isotopic compositions measured by SC-SF-ICP-MS and Q-ICP-MS largely reproduce the isotopic patterns established by MC-ICP-MS. However, analytical uncertainties are larger and performance is more variable for Q-ICP-MS. The results confirm SC-SF-ICP-MS as a viable high-throughput, intermediate precision tool for large-scale Pb isotope surveys.

Concentration of manganese gondite by magnetic separation and flotation – a preliminary study / Concentração de gonita de manganês por separação magnética e flutuação - um estudo preliminar

Traditionally, manganese mining exploited rich deposits and produced rich granular concentrates with high commercial acceptance. But the scarcity of these rich ores results in searches for new alternatives to support the market demand. This work, for example, proposed the evaluation of a low-grade manganese ore concentration, aiming to produce a concentrate to be destined steelmaking. The liberation study, realized in the characterization step, showed the associations between grouped manganese oxides/hydroxides, spessartine, and quartz (minerals with predominantly proportions in gondite). The material studied was a gondite (metasedimentary rock with low manganese content), submitted to characterization and pre-concentration. Flotation results pointed to a metallurgical reasonable recovery of 88.76%, but a product with manganese content equal to 21.51%, indicating a low enrichment of 1.11. The greatest metallic recovery was 92.48% to a size range of –147 +38 μm applying the magnetic concentration tests with field strengths of 15,500 Gauss. The magnetic concentration product with the best metallic recovery (92.48%) produced manganese content equal to 22.84%, representing an enrichment of 1.18. The BSE images revealed particles of manganese oxides with the liberated surface, but also the permanence of spessartine particles involved by the other manganese oxides.

Testing the global significance of the sulfur isotope record of the ca. 2.0 Ga Zaonega Formation: A micro-scale S isotope investigation

The Paleoproterozoic metasedimentary rocks of the Zaonega Formation (Onega Basin, NW Russia) are important archives of inferred global environmental change following the initial oxygenation of the Earth’s atmosphere and oceans. However, the geochemical signals preserved in these exceptionally organic-and pyrite-rich metasedimentary rocks and their environmental meaning remain contested. In particular, the Zaonega Formation’s unusually high pyrite sulfur isotope ratios (δ34S) have been explained by either global or local forcings acting on sulfur cycling processes. We tested former interpretations of the Zaonega Formation’s sedimentary pyrite record by integrating bulk and micro-scale δ34S analysis to discriminate the isotopic signatures of different generations of pyrite and determine the underlying mechanisms contributing to δ34S variability. We show that the prolonged genesis of pyrite occurred via multiple stages and included precipitation from early diagenetic fluids, organic matter pyritization, and late-stage alteration fluids. Our results demonstrate that early-stage pyrite typically carries more variable and lower δ34S values than late-stage pyrite. Although the early pyrite captures pore water S isotope signatures least evolved from the seawater, their contribution to the bulk δ34S results can be dwarfed by the greater volume of late-stage coarse pyrite. Consequently, determining the sequence of pyrite precipitation and δ34S characteristics of individual generations in any given sample are fundamental to interpreting bulk δ34S records. Our micro-scale results suggest that previous estimates based on bulk pyrite data (ca. 6 to 18‰) should not be related to the original seawater sulfate’s isotopic composition. These results demonstrate that a thorough understanding of the geological context and mechanisms associated with S-cycling, and pyrite formation is necessary to interpret bulk δ34S records accurately.

Diverse P‐T‐t Paths Reveal High‐Grade Metamorphosed Forearc Complexes in NW China

AbstractSubduction zones provide important constraints on reconstruction of tectonic configurations and convergent geodynamics. The timing and evolution of the subduction zones at the interaction between the Paleo‐Asian and Tethyan Oceans remain ambiguous, casting significant uncertainty on the reconstructions of proto‐Asia and subduction dynamics. Here, we report new petrologic and geochronologic data of a high‐grade metamorphic complex in the Dunhuang area, NW China, together with thermodynamic modeling, geothermobarometry, and second ion mass spectrometry U‐Pb chronology to reveal the complex metamorphic structure and history of the Paleozoic subduction‐exhumation channel at the intersection of the Paleo‐Asian and Tethyan domains. The subduction zone contains a diverse collection of eclogite, high‐pressure (HP) granulites, amphibolites, and metasedimentary rocks with a broad spectrum of P‐T‐t paths, which were buried and exhumed at different depths at ∼463–411 Ma. Our first dating of the oldest (∼463 Ma) HP granulite extends the orogenic period earlier to the Middle Ordovician. The uneven change in subduction gradients from ∼18°C/km to ∼10°C/km illustrates the thermal evolution of the subduction zone from infancy to maturity. A dramatic drop of the gradient at ∼420 Ma suggests a potential short‐term switch of the subduction dynamics from “hot” to “cool,” possibly due to plate geometry reorganization in response to slab roll‐back. Our data demonstrate that the Dunhuang Complex was chaotically mixed and juxtaposed at different levels in the subduction‐exhumation channel. The southward younging and increase in depth of the HP metamorphism indicate that the Dunhuang Complex was formed by north‐dipping subduction of the Proto‐Tethyan Ocean from the Early Paleozoic.

Signature of Himalayan orogenic features in Brahmaputra River sediments, Bangladesh: Evidence from single-grain heavy mineral chemistry

The present study describes results obtained from the chemistry of detrital heavy minerals i.e. pyroxene, amphibole, biotite, garnet, epidote and Fe-Ti oxides in fluvial sediments of the northern Brahmaputra River (Bangladesh) with an aim to determine conditions of their petrogenesis and provenance. The primary and secondary genera of ferromagnesian minerals occurred in calc-alkaline and peraluminous subduction zone. In which, the garnets are Fe-rich, indicating mostly almandine component (Alm65–Pyp16–Grs8–Sps6 averagely), occurred in medium to high grade metasedimentary rocks in the Lesser Himalaya (LH), along the Main Central Thrust (MCT) and the eastern Himalayan syntaxis. Besides, the fingerprint of omphacite and actinolite owe to ascertain the co-existence of garnet developed in ultrahigh-pressure (UHP) eclogites that may also be drained from the Tso Morari massif. Augite to aegirine-augite pyroxenes emphasizes Fe enrichment in basaltic systems and high to ultrahigh grade metamorphic rocks, which are exposed in the LH, Shillong Plateau, Mikir Hills, South Tibetan Detachment System (STDS), eastern Himalayan syntaxis and Tso Morari massif. Geochemistry and thermobarometry of the primary magmatic amphiboles and biotites manifest the source of granitoid and granodiorite like bodies, and their windows are exposed in the Bomi–Chayu, Gangdese arcs and the western Arunachal Himalaya. Again, metamorphosed Fe-Ti oxide minerals are well-exposed along the NE Lesser Himalaya, where magmatic derivative of Fe-Ti oxide minerals were modified through the diffusional processes in low-grade metamorphism (534–562 °C with 10–22.1–10−21.5fo2). Integrating the aforementioned discussion with the thermochronology, it is evident that the eastern Himalayan syntaxis is the major source of sediment flux, which is carried mostly by the upper Himalayan tributaries i.e. Yigong, Parlung, Dibang and Lohit. Also, the lower Himalayan tributaries i.e. Subansiri and Manas drain the sequestered derivatives dominantly from the Arunachal Himalayan. Tso Morari eclogites (NW Himalaya) have also contribution somewhat of dense minerals to the Tsangpo-Brahmaputra River system. Thus, scrutinizing the fingerprint of single-grain detrital minerals provides key information regarding the source terrains and tectonics of the Himalayan sequences.

Deciphering the geochemistry of two key Paleoproterozoic siliciclastic sequences of the Piedra Alta Terrane (Río de la Plata Craton, Uruguay)

The geochemistry of two metavolcano-sedimentary sequences deposited in the Piedra Alta Terrane is compared, and their geotectonic evolution is discussed. The Ojosmín Unit (OU) comprises MORB-like basic rocks at the base and a fine-grained siliciclastic sequence interpreted as marine turbidites towards the top. The succession was later obducted during the Orosirian. Whole-rock geochemistry of the metasedimentary rocks of the OU indicates the lack of source rock alteration during deposition (Chemical Index of Alteration, CIA ca. 40-53), implying ice-house climatic conditions in correlation with the worldwide Rhyacian glaciations. Recycling of zircon fractionates the Rare Earth Elements (REE) increasing the amount of HREE, Y, and Hf. Variation ranges of Th/Sc (0.4-4.9), Zr/Sc (30-410), Th/U (2.5-4.3), and of the Eu/Eu * negative anomaly (0.4-0.7) approximate Upper Continental Crust (UCC) values. A new clastic metasedimentary, gently folded unit, the Cerro de la Figurita Formation (CFFm), is erected. The CFFm clastic sedimentation (3,000 m in thickness) represents a deepening upward sequence, starting with polymictic conglomerates deposited in an alluvial fan-dominated environment that evolves to marine turbidites. The CFFm is probably related to a foreland geotectonic setting developed during the Orosirian. The geochemistry of the CFFm reveals similarities to unrecycled UCC, and weathering of the source rocks increases up section (CIA 45-92). Low ratios of Th/Sc (0.3-1.5), Zr/Sc (6-20), Th/U (3-6), high Cr/V (1.1-12.2), and a less pronounced Eu/Eu* negative anomaly of certain samples (0.5-0.9) suggest a contribution from mafic source rocks (probably ophiolitic).