Occurrence Of Magnetite Research Articles

Genesis of the Devonian Habaqin ultramafic arc complex and associated low-grade Fe–Ti oxide mineralization in the northern North China Craton

In the northern North China Craton (NCC), abundant non-layered ultramafic complexes and associated low-grade Fe–Ti oxide ores (over 1000 million tons) were uncovered. However, their petrogenesis and metallogeny have remained unclear. The Habaqin complex is one of the larger intrusions of the region and hosts the largest low-grade magnetite occurrence. Here we present petrography, zircon U–Pb geochronology, mineral and whole-rock major and trace element compositions, and whole-rock Sr–Nd and zircon Lu–Hf isotopes of the complex to constrain its genesis. The complex includes early pyroxenite (∼75% clinopyroxene, ∼15% amphibole, ∼10% magnetite) and late hornblendite (∼80% amphibole, 15–20% magnetite, 0–5% apatite). Zircon grains from the hornblendite yielded a weighted mean 206Pb/238U age of 395 ± 2 Ma. The increasing whole-rock FeOtot and TiO2 contents from pyroxenite (FeOtot = 18.9 wt%, TiO2 = 1.31 wt%) to hornblendite (FeOtot = 19.1–30.4 wt%, TiO2 = 2.63–3.12 wt%) correlate with higher modal proportions of amphibole and magnetite. The pyroxenite and hornblendite display similar arc-like trace element characteristics of enrichment in LILEs and LREEs, and depletion in HFSEs. The complex formed by fractional crystallization of clinopyroxene, amphibole, magnetite, and apatite from a hydrous parent magma. Early removal of clinopyroxene caused H2O enrichment in evolved residual magmas and crystallization of magnetite and amphibole, forming low-grade mineralization with Ti-V-poor magnetite in hornblendite. The complex displays high initial 87Sr/86Sr ratios of 0.706413–0.707341, negative εNd(t) values of −16.0 to −14.2 and negative εHf(t) value of −30.6 to −12.0, suggesting a parent magma derived from an enriched lithospheric mantle, with possible existence of amphibole-bearing pyroxenite veins within the source. During the Early Devonian, the retreat of the subducted Paleo-Asian oceanic slab as well as arc-continental collision along the northern NCC induced partial melting of the veins and eventually formation of the complex.

Process Mineralogy Assisted Iso-Floatability of Copper-Zinc-Lead Minerals from Iron Tailings at Marcona Iron Ore Mine

ABSTRACT Process mineralogy was applied to design the process circuit of magnetite tailings containing copper, lead and zinc from Marcona Iron Ore Mine. Mineralogy parameters, such as mineral composition, occurrence, and grain size distribution of the main minerals, were investigated in detail. The results showed that the main valuable elements including 41.40 wt.% iron, 0.064 wt.% copper, 0.38 wt.% lead, and 0.96 wt.% zinc, were primarily in the occurrence of magnetite, chalcopyrite, galena, and sphalerite, respectively. In addition, complex interlocking relationships were observed among the copper-zinc-lead minerals. Therefore, a fine grinding process was required for mineral liberation, followed by magnetic separation for magnetite collection. Iso-floatability flotation process was further applied to the magnetic tailings, achieving a copper grade of 28.37 wt.% and a recovery of 25.11% in the copper concentrates. Subsequently, zinc and lead grades of 49.04% wt.% and 8.06% wt.% were obtained in the zinc-lead concentrates, with a recovery of 51.72% and 25.21%, respectively. Therefore, this study provides an available separation of copper-zinc-lead minerals from iron tailings at Marcona Iron Ore Mine based on process mineralogy, thereby giving a prior diagnosis strategy to increase the utilization efficiency of iron tailings.

TARAGAI PERIDOTITE MASSIF AS AN EXPLOSIVE PIPE IN THE WESTERN BUREYA TERRANE (SOUTHERN RUSSIAN FAR EAST)

Taragai peridotite massif is interpreted as an explosive ultramafic pipe emplaced through the Neoproterozoic carbonate skarnoids and Early Paleozoic granites of the western Bureya terrane. Peridotites at the surface are represented by strongly eroded bowl-shaped structure filled with disintegrated explosive material containing abundant peridotite boulders and host rock xenoliths. Explosive character of the Taragai massif is further emphasized by the occurrence of magnetite, iron-carbonate-silicate (with magnetite) and Cu-Ag-Au microspherules characteristic of explosive eruptions. Formation of the Taragai ultramafic pipe is related to the assimilation of mantle wedge or lithospheric mantle material over the stagnant Izanagi slab by the ultra-hot and reduced upwelling fluid flow within the transform-type continental margin tectonic setting.

Raman study of the laser-induced decomposition of ZnFe2O4 nanoparticles

ZnFe2O4 (ZFO) nanostructures are widely used systems thanks to their intriguing functional properties, which can lead to several crucial applications like magnetic hyperthermia, Magnetic Resonance Imaging (MRI) and water splitting. All of these, rely on the peculiar physico-chemical response of ZFO when exposed to different atmospheres and/or to specific ambient conditions. In this frame, it is important to monitor and control the ZFO response to different external stimuli and to understand the relation with their chemical, structural and defect-related properties to avoid performance worsening. In this work, we study the laser-induced transformations of ZnFe2O4 pure and Mg-, Ga-doped nanoparticles through microRaman spectroscopy thanks to its in-situ real-time monitoring of structural and phase changes in micrometer areas of the compounds which were subjected to laser irradiation in high (3 ×10-6 mbar) and low (1.5 mbar) vacuum environments. We report the occurrence of magnetite following the in-vacuum irradiation interpreted based on the sublimation of Zn cations and the subsequent reorganization of the Fe and O ions. We also report that the presence of Mg and Ga co-doping blocks the depletion of Zn-ions therefore maintaining the spinel ferrite structure. This work provides indications for the design and the synthesis of more robust Zinc Ferrite nanostructures with low or no photoinduced degradation. Moreover, it confirms the mechanism at the basis of water splitting experiments and it extends the knowledge on the fundamental instability triggers that hinder ferrite nanostructures' applications.

Petrophysical Characterization of Non-Magnetic Granites; Density and Magnetic Susceptibility Relationships

In this work we establish reliable correlations between density and magnetic susceptibility in three paramagnetic granites from the Pyrenees. In total, 128 sites (310 density measurements and >2600 susceptibility ones) were studied in the Mont Louis-Andorra, Maladeta and Marimanha granitic plutons covering the main range of variability of magnetic susceptibility. Regressions were calculated for every granitic body and an integrated linear function was obtained for the entire dataset: ρ (kg/m3) = 2566 (kg/m3) + 0.541κ (10−6 S.I.) (R:0.97). This relationship is only valid in the paramagnetic domain, where iron is mostly fractioned in iron-bearing phyllosilicates and the occurrence of magnetite is negligible (or at least its contribution to the bulk susceptibility). This relationship, likely different in other bodies, allows for transforming magnetic susceptibility data into density data, helping to constrain gravity modelling when density data from rock samples are scarce. Given the large amount of AMS studies worldwide, together with the quickness and cost-effectiveness of susceptibility measurements with portable devices, this methodology allows for densifying and homogenizing the petrophysical data when modelling granite rock volumes based on both magnetic and gravimetric signals.

Micro‐Raman spectroscopy and complementary techniques for the study of iron weapons from Motya and Lilybaeum (Sicily, Italy): Corrosion patterns in lagoon‐like and calcarenitic hypogea environments

AbstractMicro‐Raman spectroscopy (μ‐RS) has been used to characterize mineralogical phases of corroded iron materials, buried in lagoon‐like and calcarenitic hypogea environments. A set of samples from the Phoenician site of Motya (8th–6th centuries BC, Sicily) and from the Punic Necropolis of Lilybaeum (4th century BC, Sicily) were analyzed combining μ‐RS with scanning electron microscopy‐energy‐dispersive X‐ray spectroscopy (SEM‐EDS) and high‐resolution field emission scanning electron microscopy (HR‐FESEM). Micro‐Raman results revealed the presence of magnetite, goethite, lepidocrocite, and hematite as the main corrosion products and soil minerals as quartz, calcite, barite, actinolite, microcline, zircon, and Ti‐oxides. SEM and HR‐FESEM allowed exploring micro‐ and sub‐micrometer structures of iron oxy‐hydroxides though sections from rim to core. The different corrosion models suggest polymorphic inter‐conversions among iron oxy‐hydroxides and dissolution‐re‐precipitation reactions. In addition, the occurrence of magnetite and metal core that survived in armors buried in the hypogea site of Lilybaeum indicates more stable environmental conditions than those of Motya.

Unveiling the polyphasic evolution of the Neoarchean IOCG Salobo deposit, Carajás Mineral Province, Brazil: Insights from magnetite trace elements and sulfur isotopes

The Salobo iron oxide copper-gold (IOCG) deposit host an important Cu-mineralization of the Carajás Mineral Province. The ore is characterized by an association of (bornite-chalcocite-digenite) hosted by magnetite and biotite-garnet schists. These rocks record distinct stages of superimposed hydrothermal alteration and dynamic metamorphism. Three different occurrences of magnetite constitute the magnetite bodies: a pristine to inclusion-poor Mgt I; inclusion-rich magnetite-bearing breccia Mgt II, and the granoblastic-foliated Mgt III. In general, the magnetite types present a similar trace element composition with some variations in Si, Al, Mg and K indicating that the magnetite types share the same magmatic-hydrothermal origin with post-formation processes (dissolution/reprecipitation) recorded in Mgt III. In addition, sulfur isotope signatures for chalcopyrite (1.3–3.35‰), pyrrhotite (0.88–1.98‰), and pyrite (1.7–5.04‰) associated with magnetite I and II, are consistent with a magmatic-hydrothermal sulfur source at Salobo without/or minimal external sulfur contributions. Our data indicate that ore-forming fluids for Salobo ore could have undergone sulfur disproportionation processes during sulfide formation due to magnetite precipitation, which results in the classic IOCG mineral association of chalcopyrite + magnetite described for the first time in this contribution. Therefore, the main mineralization event in the Salobo deposit could be coeval with similar IOCG deposits in the Carajás Mineral Province related to the Neoarchean magmatism at ca. 2.7 Ga. Subsequent deformation and low-grade dynamic metamorphism related to reactivation of the Cinzento Shear Zone (ca. 2.5 Ga) and A-type granite emplacement probably Paleoproterozoic in age resulted in remobilization-recrystallization of magnetite and Cu-sulfides.

Mineralogical-geochemical features of magnetite occurrences in serpentinites of the Kalkan area (Main Uralian fault zone, South Urals)

Chemical and mineralogical composition of magnetite occurrences from serpentinites of the Kalkan melange area of the Main Uralian Fault zone (South Urals) are studied. Abundant Cu minerals including native copper, Cu sulfdes (covellite, bornite, minerals of the chalcocite-digenite series), chlorides (nantokite), Cu carbonates and Cu silicates are found in magnetite ores. Magnetite also contains abundant inclusions of cobaltpentlandite, rare Ag and Bi sulfdes, heazlewoodite and awaruite. All magnetite occurrences host relicts of chromites, the composition of which (moderate to high Cr / (Cr + Al) values) indicates a strong depletion of a peridotite protolith, which formed in the upper mantle below a forearc basin. A two-stage formation model of magnetite occurrences is suggested. At an early stage, the disseminated magnetite with inclusions of Cu-Fe-Co sulfdes formed under the infuence of metalliferous fuids in serpentinized ultramafc rocks at the bottom of the oceanic basin. During collision, serpentinites were tectonized with the formation of small bodies of massive magnetite ores and Cu, Ag and Bi sulfdes and native minerals.

Origin of the Xiaohekou skarn copper deposit and related granitoids in the Zha-Shan ore cluster area, South Qinling, China

The Xiaohekou copper deposit is one of the few typical skarn-type, commercial deposits in the Zha-Shan ore cluster area in the eastern part of South Qinling. The ore-related granitoids in the Xiaohekou mining district include a number of small plutons of granodiorite porphyry and granite porphyry, as well as numerous dykes. They are characterized by relatively low SiO2 (65.9–73.3 wt%) and MgO (0.26–1.72 wt%) contents, variable Mg# (29.4–64.0) values, high K2O (3.11–5.75 wt%) content and negligible δEu (1.00–1.15) values. The samples also exhibit depletion in Cr, Ni, Nb, Ta, and Ti content, and high Sr/Y (46.6–64.7) and La/Yb (17.0–25.0) ratios. These features are similar to those of high-K calc-alkaline adakite-like magma formed by partial melting of thickened lower crust in continental collision zones or intracontinental settings. The majority of the samples have TDM2(Hf) of 1.5–1.2 Ga, corresponding to weakly negative εHf(t) values (−4.75 to −0.13) typical of crustal material, but more than one quarter have TDM1(Hf) of 0.80–0.71 Ga, corresponding to positive εHf(t) values of +0.07 to +2.45, indicating that the granitoids resulted from partial melting of Mesoproterozoic lower crustal rocks with significant input from enriched lithospheric mantle-derived magmas. Zircons from the Xiaohekou granitoids have ΔFMQ values concentrated around −0.62 to +5.54, most of which show the magma fO2 of FMQ to HM, in accordance with the widespread occurrence of magnetite, hematite and specularite in the Xiaohekou skarn system, which is an indicator of relatively high fO2 for a magmatic–hydrothermal system. The granitoids exhibit similar or even higher Ce4+/Ce3+ (average of 452) and Eu/Eu* (average of 0.72) values in comparison with those of fertile Cu deposits worldwide, indicating that the Xiaohekou granitoids may possess the potential to form a large magmatic–hydrothermal Cu deposit. Zircons of the granitoids show obvious positive correlations between oxidation indices (Ce and Eu anomalies, Ce4+/Ce3+ ratios, and ΔFMQ values) and εHf(t) values, implying the involvement of an oxidized mantle component which might have caused the fO2 elevation and played an important role in the formation of Cu deposits. The zircon U–Pb ages (141.3 ± 1.3 and 138.1 ± 2.0 Ma) of the Xiaohekou ore-related granitoids obtained in this study are similar to the zircon U–Pb and molybdenite Re–Os ages yielded from porphyry Mo mineralization at the Chigou and Lengshuigou deposits in the Zha-Shan ore district, indicating that they are possibly part of a single tectonic, magmatic, and metallogenic event in the Yanshan orogeny. The ore-related granitoids in the Xiaohekou area were formed in a post-collisional compression–extension transition regime during the Early Cretaceous Qinling intracontinental orogeny. The orogeny can be attributed to the far-field effect of a switch from pre-Mesozoic Tethyan tectonics to Late Mesozoic Pacific tectonics. This triggered a stress field transition from N–S to E–W trending and from intracontinental subduction thickening to large-scale lithospheric delamination and thinning.

Magnetic susceptibility of Variscan granite-types of the Spanish Central System and the redox state of magma

Magnetic susceptibility (MS) has been measured in Variscan granites from central Spain. They yield values in the order of 15 to 180µSI units for S- and I-type granites, indicating that both types belong to the ilmenite series. Only samples from magnetite-bearing leucogranites from the I-type La Pedriza massif show high MS values, in the order of 500-1400µSI, reflecting the presence of this ferromagnetic mineral. Mineral chemistry of magmatic Fe-rich minerals (mainly biotite) suggests similar oxidation values for both granite types. MS values change in highly fractionated granites accordingly either with the presence of rare new Fe-oxide phases (some I-type leucogranites) or with the marked modal amount decrease of Fe-rich minerals (I- and S-type leucogranites). The redox state in highly fractionated granite melts is mostly controlled by magmatic processes that modify redox conditions inherited from the source region. Thus, the occurrence of magnetite or ilmenite in granites is primarily controlled by the oxidation state of the source material but also by the differentiation degree of the granite melt. The presence of magnetite in some Variscan I-type leucogranites might be a consequence of crystal fractionation processes in a more limited mafic mineral assemblage than in S-type granite melts.

Alkali Metasomatism and Th-REE Mineralization in the Choghart deposit, Bafq district, Central Iran

The Choghart iron oxide-apatite (IOA) deposit is located 124 km southeast of Yazd, in the Bafq district within the Central Iranian microcontinent. The Choghart deposit is hosted by the rhyolitic rocks of the Early Cambrian volcano-sedimentary sequence (the Esfordi formation). Both host rocks and the orebodies are crosscut by diabase dykes. Tectonically, the Choghart rhyolites represent the continental margin setting and the Choghart diabase dykes formed in the back-arcbasin environment, respectively, indicating that the evolution of the Bafq district is associated with subduction of Palaeotethys oceanic crust beneath the Central Iranian microcontinent followed by formation of continental arc related granitoids and rhyolites and then formation of back-arc basin diabase dykes. Similar to the other subduction-related rhyolites, the Choghart rhyolite is enriched in Th and LREE compared to Ia, Nb, and HREE.The main host minerals of Th and REE in the Th-REE mineralization zone are thorite and sphene. Albitization is the most important alteration aspect related to Th-REE mineralization (mainly Th, La, Ce, Nd, and Y). In addition to albite, Th-REE mineralization is associated with actinolite, augite, diopside, minor microcline and orthoclase, plus magnetite, calcite, pyrite, rutile, and minor amounts of chalcopyrite. The negative Eu anomaly in Th mineralization zone, as well as the paragenetic occurrence of magnetite, pyrite and chalcopyrite with thorite suggest that Th-REE mineralization formed in relatively reduced condition. The presence of paragenetic calcite accompanied by thorite and sphene in the Th-REE mineralization zone indicates that Th and REE were likely transported by the carbonate complexes in the mineralizing fluids. The similarity betweenthe chondrite-normalized REE patterns of the host rhyolite and the Th-REE mineralization zone suggests that post-magmatic driven fluids of continental margin rhyolitic magma played an important role in Th-REE mineralization.

Mössbauer and Raman spectroscopic study of oxidation and reduction of iron oxide nanoparticles promoted by various carboxylic acid layers

The effect of coating with nine different carboxylic acids (glycolic, propionic, lactic, malic, tartaric, citric, mandelic, caproic and caprylic) on nanostructured magnetite (D ~ 10 nm) was studied by Raman and photoacoustic, magnetic and 57Fe Mossbauer measurements. Mossbauer spectra of frozen suspensions showed dominantly magnetically split envelopes at lower temperatures, which were evaluated by hyperfine field distribution method. Mossbauer and Raman spectroscopy indicated similar variation of relative occurrence of magnetite and maghemite phases. These results are discussed on the basis of the hypothesis that different carboxylic acids can promote either the oxidation or reduction of iron oxide nanoparticles.

Secondary Mineralization of Ferrihydrite Affects Microbial Methanogenesis in Geobacter-Methanosarcina Cocultures.

The transformation of ferrihydrite to stable iron oxides over time has important consequences for biogeochemical cycling of many metals and nutrients. The response of methanogenic activity to the presence of iron oxides depends on the type of iron mineral, but the effects of changes in iron mineralogy on methanogenesis have not been characterized. To address these issues, we constructed methanogenic cocultures of Geobacter and Methanosarcina strains with different ferrihydrite mineralization pathways. In this system, secondary mineralization products from ferrihydrite are regulated by the presence or absence of phosphate. In cultures producing magnetite as the secondary mineralization product, the rates of methanogenesis from acetate and ethanol increased by 30.2% and 135.3%, respectively, compared with a control lacking ferrihydrite. Biogenic magnetite was proposed to promote direct interspecies electron transfer between Geobacter and Methanosarcina in a manner similar to that of c-type cytochrome and thus facilitate methanogenesis. Vivianite biomineralization from ferrihydrite in the presence of phosphate did not significantly influence the methanogenesis processes. The correlation between magnetite occurrence and facilitated methanogenesis was supported by increased rates of methane production from acetate and ethanol with magnetite supplementation in the defined cocultures. Our data provide a new perspective on the important role of iron biomineralization in biogeochemical cycling of carbon in diverse anaerobic environments. It has been found that microbial methanogenesis is affected by the presence of iron minerals, and their influences on methanogenesis are associated with the mineralogical properties of the iron minerals. However, how changes in iron mineralogy affect microbial methanogenesis has not been characterized. To address this issue, we constructed methanogenic cocultures of Geobacter and Methanosarcina strains with different ferrihydrite mineralization pathways. The experimental results led to two contributions, i.e., (i) the transformation of iron minerals might exert an important influence on methanogenesis under anaerobic conditions and (ii) both biogenic and chemical magnetite can accelerate syntrophic ethanol oxidization between Geobacter metallireducens and Methanosarcina barkeri This study sheds new light on the important role of iron biomineralization in the biogeochemical cycling of carbon in diverse anaerobic environments, particularly in iron-rich natural and agricultural wetland soils.

Constraints on iron sulfate and iron oxide mineralogy from ChemCam visible/near-infrared reflectance spectroscopy of Mt. Sharp basal units, Gale Crater, Mars

Relative reflectace point spectra (400–840 nm) were acquired by the Chemistry and Camera (ChemCam) instrument on the Mars Science Laboratory (MSL) rover Curiosity in passive mode (no laser) of drill tailings and broken rock fragments near the rover as it entered the lower reaches of Mt. Sharp and of landforms at distances of 2–8 km. Freshly disturbed surfaces are less subject to the spectral masking effects of dust, and revealed spectral features consistent with the presence of iron oxides and ferric sulfates. We present the first detection on Mars of a ~433 nm absorption band consistent with small abundances of ferric sulfates, corroborated by jarosite detections by the Chemistry and Mineralogy (CheMin) X-ray diffraction instrument in the Mojave, Telegraph Peak, and Confidence Hills drilled samples. Disturbed materials near the Bonanza King region also exhibited strong 433 nm bands and negative near-infrared spectral slopes consistent with jarosite. ChemCam passive spectra of the Confidence Hills and Mojave drill tailings showed features suggestive of the crystalline hematite identified by CheMin analyses. The Windjana drill sample tailings exhibited flat, low relative reflectance spectra, explained by the occurrence of magnetite detected by CheMin. Passive spectra of Bonanza King were similar, suggesting the presence of spectrally dark and neutral minerals such as magnetite. Long-distance spectra of the “Hematite Ridge” feature (3–5 km from the rover) exhibited features consistent with crystalline hematite. The Bagnold dune field north of the Hematite Ridge area exhibited low relative reflectance and near-infrared features indicative of basaltic materials (olivine, pyroxene). Light-toned layers south of Hematite Ridge lacked distinct spectral features in the 400–840 nm region, and may represent portions of nearby clay minerals and sulfates mapped with orbital near-infrared observations. The presence of ferric sulfates such as jarosite in the drill tailings suggests a relatively acidic environment, likely associated with flow of iron-bearing fluids, associated oxidation, and/or hydrothermal leaching of sedimentary rocks. Combined with other remote sensing data sets, mineralogical constraints from ChemCam passive spectra will continue to play an important role in interpreting the mineralogy and composition of materials encountered as Curiosity traverses further south within the basal layers of the Mt. Sharp complex.

페루 남동부 쿠스코주 콘스탄시아 반암동-몰리브데늄 광산의 지질 및 광화작용

콘스탄시아 광산은 페루 남동부 안다우아이라스-야우리 Cu-Mo-Au 광화대 내에 발달하는 광상으로서, 구조적으로는 아방까이 변형대의 남서쪽에 위치하며 반암동광형태로 부존한다. 콘스탄시아 광산의 광화대는 상부로부터 용탈대, 2차부화대(약 1% Cu), 혼합대, 초생광화대(약 0.5%), 스카른대(약 1.5% Cu)로 구성되어 있다. 주요 모암은 몬조니 반암이다. 용탈대는 갈철석 침전이 특징적이며 노두상에서는 갈색으로 보인다. 스카른대는 자철석과 석류석의 산출이 특징적이다. 현재 콘스탄시아 광산은 캐나다의 Hudbay사가 100% 지분을 보유하고 있으며, 2015년 1월부터 상용생산을 시작하였다. Constancia mine is a deposit developed within Andahuaylas-Yauri Cu-Mo-Au metallogenic belt, southeastern Peru and is located in the southwestern part of Abancay deformation zone structurally as the porphyry copper deposit type. Mineralized zone in Constancia mine are composed of leached zone, secondary enrichment zone(ca. 1% Cu), mixed zone, primary mineralized zone(ca. 0.5%), skarn zone(ca. 1.5% Cu) from the upper part. Main country rock is monzonitic porphyry. Leached zone are characterized by the precipitation of limonite and looks brown in the outcrop. Oxidized zone have green due to the occurrence of copper oxide and secondary enrichment zone are characterized by the occurrence of chalcocite. Skarn zone are characterized by the occurrence of magnetite and garnet. Now, Hudbay, Canadian mining company, have 100% share about Constancia mine and started to produce commercially from January, 2015.

Late Pleistocene to Holocene palaeoenvironmental variability in the north-west Spanish mountains: insights from a source-to-sink environmental magnetic study of Lake Sanabria

We present a source-to-sink environmental magnetic study of a sediment core from Lake Sanabria (north-west Iberian Peninsula) and rocks of its catchment. The results indicate the occurrence of magnetite, and probably also pyrrhotite, in sediments accumulated between ca. 26 and 13 cal ka BP in a proglacial lake environment. These minerals also appear to dominate the magnetic assemblage of Palaeozoic rocks from the lake catchment. This indicates that sedimentation was then driven by the erosion of glacial flour, which suffered minimal chemical transformation due to a rapid and short routing to the lake. A sharp change in magnetic properties observed in the lake sediments between 13 and 12.6 cal ka BP reflects the rapid retreat of glaciers from the lake catchment. Sediments from the upper half of the studied sequence, accumulated after 12.6 cal ka BP in a lacustrine environment with strong fluvial influence, contain magnetite and smaller amounts of maghemite and greigite. We suggest that greigite grew authigenically under anoxic conditions caused by enhanced accumulation of organic matter into the lake. The occurrence of maghemite in these sediments suggests pedogenic activity in the then deglaciated lake catchment before the erosion and transportation of detrital material into the lake.

Composite carbonate and silicate multiphase solid inclusions in metamorphic garnet from ultrahigh‐P eclogite in the Dabie orogen

AbstractComposite multiphase solid (MS) inclusions composed of carbonate and silicate minerals have been found for the first time in metamorphic garnet from ultrahigh‐P eclogite from the Dabie orogen. These inclusions are morphologically euhedral to subhedral, and some show relatively regular shapes approaching the negative crystal shape of the host garnet. Radial fractures often occur in garnet hosting the inclusions. The inclusions are primarily composed of variable proportions of carbonate and silicate minerals such as calcite, quartz, K‐feldspar and plagioclase, with occasional occurrences of magnetite, zircon and barite. They are categorized into two groups based on the proportions of carbonate and silicate phases. Group I is carbonate‐dominated with variable proportions of silicate minerals, whereas Group II is silicate‐dominated with small proportions of carbonates. Trace element analysis by LA‐ICPMS for the two groups of MS inclusions yields remarkable differences. Group I inclusions exhibit remarkably lower REE contents than Group II inclusions, with significant LREE enrichment and large fractionation between LREE and HREE in the chondrite‐normalized REE diagram. In contrast, Group II inclusions show rather flat REE patterns with insignificant fractionation between LREE and HREE. In the primitive mantle‐normalized spidergram, Group I inclusions exhibit positive anomalies of Zr and Hf, whereas Group II inclusions show negative anomalies of Zr and Hf. Nevertheless, both groups exhibit positive anomalies of Ba, U, Pb and Sr, but negative anomalies of Nb and Ta, resembling the composition of common continental crust. Group I inclusions have higher Ba and U contents than Group II inclusions. Combined with petrological observations, the two groups of MS inclusions are interpreted as having crystallized from composite silicate and carbonate melts during continental subduction‐zone metamorphism. The differences in trace element composition between the two groups are primarily attributed to the proportions of carbonate and silicate phases in the MS inclusions. The silicate melts were derived from the breakdown of hydrous minerals such as paragonite and phengite, whereas the occurrence of carbonate melts indicates involvement of carbonate minerals in the partial melting and thus has great bearing on recycling of supracrustal carbon into the mantle. The coexistence of silicate and carbonate melts in the eclogitic garnet provides insights into the nature of hydrous melts in the subduction factory.

Hematite replacement of iron-bearing precursor sediments in the 3.46-b.y.-old Marble Bar Chert, Pilbara craton, Australia

The history of atmospheric oxygen prior to the Great Oxidation Event (2.45–2.2 Ga) is not well understood. Hematite in the Marble Bar Chert from a NASA-funded drill hole (ABDP1) in the Pilbara craton, Australia, has been cited as evidence for an oxygenated ocean 3.46 b.y. ago. However, isotopic data from the same drill hole have been used to argue for an anoxic ocean. It is generally agreed that the hematite is primary, representing either a direct hydrothermal precipitate or a dehydration product of iron oxyhydroxides that formed during anoxygenic photosynthesis. Here we present new petrographic evidence from the Marble Bar Chert (in drill hole ABDP1) that shows that hematite in jasper bands formed via mineral replacement reactions. The hematite mostly occurs as sub-micron–sized inclusions within chert (so-called “dusty” hematite) that are typically arranged into polygonal clusters surrounded by a rim of clear quartz, resembling shrinkage structures. The lateral transition from laminated chert enclosing minute inclusions of greenalite, siderite, and magnetite to chert dominated by dusty hematite provides evidence for in situ replacement of iron-bearing minerals. The presence of hematite-rich bands containing octahedral crystals with residual cores of magnetite indicates that some of the hematite was derived from the replacement of magnetite. This interpretation is supported by the widespread occurrence of magnetite in jasper displaying progressive stages of replacement, from unaltered octahedral inclusions in quartz to hematite pseudomorphs along quartz grain boundaries. The occurrence of dusty hematite in fractures, sedimentary laminae, and the outer margins of polygonal clusters containing greenalite is consistent with fluid-mediated oxidation of iron-rich precursor minerals. The presence of syn-sedimentary chert breccias comprising rotated fragments of laminated chert indicates that the precursor sediment was silicified shortly after deposition. The abundance of “dusty” greenalite inclusions, which are texturally the earliest components of the laminated chert, suggests that the precursor sediment contained an iron-rich clay mineral. Our results show that hematite has replaced ferrous-rich minerals after deposition and provide a mechanism to explain the origin of hematite in the Marble Bar Chert, which is consistent with the origin of hematite in adjacent basalts. A secondary origin for hematite invalidates arguments for an oxygen-bearing ocean ∼3.46 b.y. ago and provides a viable explanation for the formation of Archean jasper bands. Our findings show that misinterpretations about the origin of hematite in early Precambrian cherts could lead to false conclusions about the chemistry of the ancient ocean and atmosphere.

Abundant bacterial magnetite occurrence in oxic red clay

Magnetotactic bacteria (MTB) produce chains of intracellular magnetite and/or greigite crystals and respond to an ambient magnetic field. MTB are considered to be microaerophilic to anaerobic organisms that live at and below the oxic-anoxic transition zone of aquatic environments. On the basis of rock magnetic analyses, including first-order reversal curve diagrams and isothermal remanent magnetization component analyses, along with transmission electron microscopy, we demonstrate that bacterial magnetites (magnetofossils) dominate magnetic mineral assemblages throughout a 76 m thickness of red clay at Integrated Ocean Drilling Program Site U1365 in the South Pacific Gyre, as well as in subsurface red clay of the North Pacific Gyre, where the sediment column contains abundant dissolved oxygen and no oxic-anoxic transition zone exists. This implies that MTB inhabit red clay; this conflicts with widespread interpretations of MTB ecology, namely that they are microaerophilic, requiring low levels of oxygen to grow and produce magnetite, and that magnetotaxis is used to help them find optimal positions in a strong vertical chemical gradient. Most magnetofossils in the red clay have cubo-octahedral morphology. This supports the notion that magnetofossil morphology can be a paleoenvironmental indicator; the proportion of elongated magnetofossils increases in less oxic environments. Our results also have implications for red-clay paleomagnetism in that magnetofossils may cause much-delayed remanence acquisition if MTB can live at decimeter depths within red clay.

Geochemistry and origin of the early Mesoproterozoic mangerite–charnockite–rapakivi granite association of the Serra da Providência suite and associated gabbros, central–eastern Rondônia, SW Amazonian Craton, Brazil

The Serra da Providência suite (SPS) is constituted of quartz mangerite-charnockite-rapakivi granite and coeval mafic rocks, intruded during the 1.60–1.53 Ga interval into Paleoproterozoic metamorphic basement (Jamari Complex) in the Juruena-Rondônia geochronologic province, SW Amazonian Craton. In this region the Serra da Providência suite consists of two batholiths and several small plutons represented by syeno-monzogranites and igneous quartz mangerite and charnockites.The Serra da Providência granites are ferroan, calc-alkalic to alkali-calcic and characterized by high Na2O + K2O, Rb, Zr, Y, Nb, Ta, Ce, Zn, Ga, and REE (except for Eu), moderate Ba, and low Sr, MgO, and CaO. Ga/Al, Y/Nb, FeOT/(FeOT + MgO) and K2O/Na2O ratios are high and Sr/Ba and Rb/Ba are low and are geochemically similar to typical A2-subtype granites. The SPS charnockites and quartz mangerites display values of FeOT, MgO, CaO, Ba, Rb, Sr, Nb/Ta and Zr/Hf similar to the SPS A-type granites.The occurrence of magnetite as an accessory phase and the high FeOT/(FeOT + MgO) in granites, charnockites and quartz mangerites suggest crystallization from a relatively oxidized magma and also explain the occurrence of titanite as a primary phase in these granites, similar to those reported in several localities of the Amazonian Craton. The parental magma of the mafic rocks of the SPS probably had mainly enriched sources and resulted from melting of heterogeneous mantle reservoirs with coherent chemical characteristics. Trace elements data indicate subduction-modified mantle sources for the mafic rocks that are in agreement with the post-collisional character of this magmatism.A large set of whole-rock Sm–Nd isotope data (Bettencourt et al., 1999; Scandolara, 2006; Santos et al., 2008 and this work) demonstrate that granites, charnockites and quartz mangerites of the study area are the product of magmas derived from interaction between enriched mantle derived magmas (in very subordinate proportions) and recycled crust in larger proportion. The source region is dominated by Paleoproterozoic material, but several samples yield Archaean model ages that are the first evidence for such ancient source materials in the SW Amazonian Craton. The Serra da Providência suite was emplaced during the post-collisional stage of the Juruena-Jamari arc and Madeirinha orogeny (Scandolara et al., 2011), and post-dates the collision between the Tapajós (Tapajós-Parima geochronological province, Pará, Brazil) and Bolívia (palaeocontinent which precedes Sunsás geochronological province) blocks. Its geochemical and structural features are coincident with those recognized in post-collisional granitoids. Geochemical data suggest that complex processes which involved crustal melting, fractional crystallization, magma mixing and, in some extent, crustal assimilation were responsible for the magmatism of the Serra da Providência suite. The magmas that constitute the Serra da Providência suite resulted from varying degrees of partial melting of a compositionally heterogeneous source. The nature of the source and the degree of melting exerted a significant control over compositional variation in some of the parental magmas which were also affected by subsequent fractional crystallization and mingling processes. Internal variations in composition observed in several bodies can be the result of incremental amalgamation of different magma pulses that varied mainly in the degree of partial melting.