Magnetite Ore Research Articles

The Geochemical Characteristics of Trace Elements in the Magnetite and Fe Isotope Geochemistry of the Makeng Iron Deposit in Southwest Fujian and Their Significance in Ore Genesis

The Makeng iron deposit in southwest Fujian is a significant iron polymetallic deposit containing various types of iron ore, including garnet magnetite, diopside magnetite, and quartz magnetite. The metallogenetic type of the deposit has been a subject of debate, particularly in relation to the genesis of magnetite and the source of iron. In situ microanalysis of trace elements in magnetite from different ores shows relatively low levels of V, Ti, Cu, and Zn, with higher concentrations of Ca and Si, indicating the characteristics of a skarn type deposit. The δ57Fe values of the magnetite range from −0.091‰ to 0.317‰. Combining these data, whole-rock iron isotope analyses, including Juzhou and Dayang granites, diabase, and the Lower Carboniferous Lindi Formation sandstone, suggest that Fe in the magnetite primarily originates from granitic pluton, with potential contributions from diabase and the Lower Carboniferous Lindi Formation sandstone. Combined with field work, these results indicate that Makeng iron deposit is a skarn-type magnetite deposit associated with Yanshanian granitic intrusions. Therefore, the initial ore-forming fluid is postulated to be a high-temperature magmatic hydrothermal fluid with high oxygen fugacity. This fluid infiltrates spaces such as interlayer fracture zones between the Upper Carboniferous Jingshe Formation–Middle Permian Qixia Formation carbonate rocks and the Lower Carboniferous Lindi Formation sandstone, resulting in diverse magnetite ores due to metasomatism. The mineralization process of the Makeng iron deposit is basically the same, as it is composed of typical skarn deposits. Magnetite was mainly formed during calcic skarn formation stage, and this process persisted until the initial phase of the retrograde alteration of skarns. In contrast, sulfide minerals, including molybdenite, sphalerite, and galena, precipitated during the quartz–sulfide stage.

Exploration of suspected magnetite ore minerals in Emure – Ekiti, Southwestern Nigeria using integrated geophysical methods of very low frequency electromagnetic (VLF-EM) and magnetics

Investigation on the occurrence of magnetite ore minerals in Emure – Ekiti, Southwestern Nigeria was carried out by integrating magnetic and very low frequency electromagnetic (VLF-EM) methods on the suspicion that the major part of the magnetite ore being sought lies in the subsurface. Eight traverses in total were established with five traverses occupied in the northeast–southwest direction and the other three in the northwest–southeast direction. Six traverses were established across the exposed part of the pegmatite (magnetite) veins to serve as a control. Inter-station spacing of 1 m was chosen, so as not to miss any significant ore occurrence. The VLF-EM profile is a plot of the real and filtered real components. The KH-filter geophysical software was used to invert real data into a two-dimensional subsurface map that revealed variations in conductivity. The results of the geophysical interpretation for both methods indicate anomalous signatures observed on the exposed section of the pegmatitic veins, which are indicative of magnetite ore. These anomalous signatures align with certain zones along the Tr4 and Tr8 transects. Some of the high magnetic anomalies observed on traverse 5, 6, and 7 coincide with low conductivity suggesting the concentration of minerals such as quartz, microcline, plagioclase, and biotite, which are commonly associated with magnetite. The study has not only enhanced the mapping of the subsurface characterisation of the deposit but also confirms the reliability and suitability of using magnetics and VLF-EM geophysical methods for mapping magnetite deposits.

Paleoproterozoic hydrothermal overprinting over Neoarchean banded iron formation produced high-grade iron ores in the giant Gongchangling deposit of North China: Evidence from O–S–B isotopes

The Gongchangling iron deposit in North China hosts abundant high-grade (>50 wt% total Fe) magnetite ores and represents one of the largest BIF-hosted magnetite deposits worldwide. These high-grade magnetite ores are generally accepted to be formed by replacement of banded iron formation (BIF) (∼30 wt% total Fe), but the mechanism of iron enrichment remains controversial. Here we report new O–S–B isotope data of BIFs, high-grade iron ores, and related altered wall rocks, from the Gongchangling iron deposit, together with the geological and geochronological data to constrain the formation of the high-grade iron ores, and proposed new genetic model. The high-grade iron ores have positive δ56Fe and negative δ30Si values, similar to the BIFs, whereas the δ18O values of the high-grade iron ores and altered wall rocks are significantly lower than those of the BIFs. The δ34S values of pyrite from the high-grade iron ores are much higher than those of the BIFs. The δ11B values of tourmaline in the altered wall rocks of the high-grade iron ores are anomalously high, which is distinct from those of the BIFs and wall rocks, but similar to those of the evaporites from the Liaohe Group. At ca. 1.85 Ga, intense hydrothermal activity was initiated by post-orogenic extension and uplift with 18O-poor meteoric water circulated along crustal structures. Compositions of Na+, K+, Mg2+, Cl–, CO32–, 34S-rich SO42–, 11B-rich borate, and other soluble salts minerals were leached from the evaporitic rocks of the Liaohe Group, which driving the ore-forming hydrothermal fluids toward slightly alkaline and weakly oxidized as system matures. As the alkaline fluids migrated along the structures and fractures in the BIFs and reacted with the wall rocks, SiO2 was mobilized from the low-grade iron ores and consequently formed chlorite, garnet, and tremolite in the wall rocks due to reactions with clay minerals. These Si-rich hydrothermal fluids was also related to vein type quartz and massive quartz-bearing rocks. Most Fe was retained in situ and resulted in the formation of magnetite-rich iron ores. The SO42– in the hydrothermal fluids was reduced to form pyrite while B in the hydrothermal fluids reacted with the wall rocks to form 11B-rich tourmaline.

Mining wastes as CO oxidation mesoporous catalysts from the Fe-skarns of Serifos Island, Cyclades, Greece

The Fe-skarns from the Greek Island of Serifos in Cyclades has been utilized only for the exploitation of magnetite ores, while significantly vast amounts of red and yellow hematite ores remained unexploited as mining wastes. This study explores the effective utilization of such mining wastes per se, as active mesoporous catalysts for energy applications, taking CO oxidation as a model reaction. The results of our multi-analytical study have illustrated that the physicochemical characteristics of the hematite materials can be positively influenced by the implementation of a simple calcination step, while the characteristics of magnetite remained unaffected. The calcined yellow and red mineral hematite from Serifos Fe-skarns mining wastes per se, act beneficial in the catalytic activity with respect to calcined magnetite catalyst. The calcinated counterparts of hematite, and especially yellow hematite, can efficiently be used as a support for depositing Au nanoparticles, and thus achieving even higher activity for CO oxidation (90% CO conversion), at much lower temperatures (T ∼62.0 °C). The mining wastes per se of Serifos, mainly of mesoporous hematite, are particularly active for CO oxidation, without any complex pretreatments or activation or functionalization in the context of circular economy.

Reduction characteristics of cold-agglomerated pellets in blast furnace conditions

There is a significant and necessary drive towards reducing CO2 emissions produced by iron and steelmaking industries. Binding Solutions Ltd has developed a technology to contribute to the reduction of CO2 in the short and long term, by introducing a cold-bonded iron ore pellet to the furnace burden. Binding Solutions’ cold-agglomerated pellets, manufactured from magnetite and hematite iron ore, were tested in accordance with ISO standard tests alongside a standard market pellet. ISO standards were developed to assess the quality of lump ore, sinter and hot-bonded or alternatively known as indurated pellets, and there can be anomalies when testing cold-agglomerated pellets by the same methods. Additional test methodology, outside the scope of international standards may be utilised to give a fuller picture of pellet characteristics. The HOSIM test method was used to assess the properties of cold-bonded pellets of different minerologies. Results found that the time needed to reach a set point of reduction for a cold-agglomerated pellet is shorter than for an indurated control pellet. The data also showed that for the same iron ore source, alternate cold agglomeration methods can have a significant impact on the balance of pellet properties.

Beneficiation Processing of Magnetite ore from Lampung as Dense Media for Dense Medium Separator in Coal Washing Plant

Until recently, coal washing plants in Indonesia are still using imported magnetite from Australia as dense media for dense medium separator units. To be effectively utilized as a dense media, magnetite ore needs to be concentrated to remove gangue minerals so that the final product will have more than 95% magnetic content, 95% weight passing 53 microns, and relative density ranging from 4.9 – 5.2 g/cm3. Experimental studies have been performed at the Department of Metallurgical Engineering ITB to concentrate fine magnetite ores from Lampung Province. Two process routes were chosen: grinding-magnetic separation and magnetic separation-grinding. Products from the two routes were sieve size analyzed, assayed and characterized. Magnetism characteristic was analyzed with VSM and relative density was measured with pycnometer. The first process route products have maximum magnetic content of 99.1% and particle weight passing 53 microns of 95.7%, while the second route have magnetic content of 95.2% and particle passing 53 microns of 97.1%. Concentrates from both routes have the same relative density of 4.5 g/cm3. Characterization by XRF and AAS gives Fe content of 46.6% and 48.8% for the first route product, and 52.1% and 52.9% for the second route. Lampung magnetite ore gives lower magnetism characteristic compare to Australian magnetite ore. Finer particle size gave lower magnetic saturation value, hence lower magnetism.

Mineralogical, geochemical characteristics and formation mechanism of skarns in Banlun magnetite ore cluster area, southeastern Yunnan

Mineralogical, geochemical characteristics and formation mechanism of skarns in Banlun magnetite ore cluster area, southeastern Yunnan

Comparative study on grinding kinetics behavior of vanadium-titanium magnetite ore before and after microwave pretreatment

Vanadium-titanium magnetite underwent closed circuit crushing before and after microwave pretreatment. Subsequently, batch grinding tests were conducted on the crushed products, with varying grinding times. Ultimately, the m-order grinding kinetics model of the ore was established both before and after microwave pretreatment. The results show that microwave pretreatment causes a significant increase in the number of micro-cracks on the ore's surface. Moreover, the specific surface area and pore volume of the ore increase by 17.29% and 40.98% respectively. These changes result in a reduction in the mechanical properties of the ore. During the grinding tests, the number of micro-cracks is found to be the primary factor influencing the grinding speed in the initial stage. However, as the grinding progresses, the grinding probability becomes the main factor affecting the grinding speed. The coarse particle size of the microwave-treated ore exhibits a significantly higher grinding speed than the untreated ore. Conversely, the grinding speed of fine particle size ore change slightly. The application of microwave-assisted grinding for vanadium-titanium magnetite proves advantageous in improving the particle size composition of the grinding product. Consequently, this can lead to enhanced separation efficiency and concentrate quality.

Iron ore to lodestone: With lightning assist

A lodestone is a piece of magnetite or titanomagnetite iron ore having an extremely intense remanent magnetization; it is nature's only permanent magnet. This magnetization intensity is larger than could be acquired in any iron ore by any known natural mechanism through exposure to just the Earth's magnetic field. This is revealed by the large ratio (REM) of the measured (NRM) - natural remanence, to the (SIRM) - saturation remanence.We describe new details of the mechanism by which part of an iron ore body becomes a lodestone: Exposure to lightning bolt related electric discharge's magnetic fields is required.While the lightning bolt related magnetic fields are necessary, they are not sufficient for creation of a lodestone. We show that the iron ore must be previously magnetically “hardened” into what we term a “latent lodestone iron ore” condition before the transformation to lodestone can be achieved.A magnetic hardness continuum (characterized with magnetic hysteresis loop parameters) for a range of magnetite and titanomagnetite iron ores demonstrates how the hardening is created by alteration; in magnetite through oxidation scenarios and in titanomagnetite by solid state exsolution and oxidation.Large REM (evidence of lightning), the *Polarity signature (which we identify with iron powder patterns) reflecting the large REM, and the location on the magnetic hardness continuum (latent lodestone state resulting in largest possible magnetization), provides important details to the lodestone story.

Geochemical features of sulfides of the Aktash magnetite-polymetallic deposit, Western Karamazar, Tajikistan (LA-ICP-MS data)

The distribution of trace elements in sulfdes from various ores of the Aktash magnetite-polymetallic deposit (Tajikistan) is studied using LA-ICP-MS method. Sulfdes partly replace banded magnetite ores. Each sulfde type exhibits specifc geochemical features. Galena from sulfde-magnetite ores contains the higher amount of elements of «high-temperature» association (Bi, Ag, Cu) than galena from pyroxene-sphalerite ores, which, in addition to Ag and Bi, also contains higher amount of elements of «medium-temperature» association (Se, Te, and Sb). Sphalerite from sulfde-magnetite and chlorite-pyrrhotite ores contains higher Fe, Cd and Mn amount than sphalerite from pyroxene-sphalerite ores, which is characterized by elevated Co and As contents. Pyrite is depleted in trace elements except for As, the content of which is maximum compared to other sulfdes. The Ag, Se, Bi, Zn, Cd and Ni contents of chalcopyrite from chlorite-pyrrhotite ores are higher than in chalcopyrite from sulfde-magnetite and pyroxene-sphalerite ores. Chalcopyrite from pyroxene-sphalerite ores is characterized by elevated Pb, As, Ge, Te and Sb contents. Relatively high Co, Ni and Se contents are determined in pyrrhotite. In addition to Fe, Cu, Zn, Pb and Au, which form minerals in all types of ores, we suggest by-product extraction of following elements from complex sulfde-magnetite, chlorite-pyrrhotite and pyroxene-sphalerite ores (the Bi, Ag, Se and Te contents of galena and Cd content of sphalerite are shown in brackets): Bi (19520–24650 ppm), Ag (7907–9650 ppm), Se (397–606 ppm) and Te (276– 436 ppm) from galena concentrate and Cd (8525–27670 ppm) from sphalerite concentrate.

Formation of magnetite-(apatite) systems by crystallizing ultrabasic iron-rich melts and slag separation

Magnetite-(apatite) ore deposits are interpreted as being formed by the crystallization of iron-rich ultrabasic melts, dominantly generated by the interaction of silicate melts with oxidized P-F-SO4-bearing sedimentary rocks. This hypothesis is supported by geologic evidence, experimental studies, numerical modeling, stable and radiogenic isotope geochemistry, mineralogy, and melt- and mineral-inclusion data. Assimilation of crustal rocks during ascent promotes separation from a silicate magma of Fe-rich, Si-Al-poor melts with low solidus temperatures and viscosities, allowing coalescence, migration, and emplacement at deep to subaerial crustal environments. When the iron-rich melt attains neutral buoyancy, fractional crystallization leads to melt immiscibility similar to that observed in industrial blast furnaces, which promotes separation of massive magnetite ore overlain by different types of “slag” containing actinolite or diopside ± phosphates ± magnetite ± feldspar ± anhydrite ± scapolite, commonly enriched in high field strength elements. The mineralogy and morphology of this iron-depleted cap strongly depend on the depth of emplacement and composition of the iron-rich magma. Most of these systems exhibit high oxygen fugacity, which inhibits the precipitation of significant sulfide mineralization. The initially high fO2 of these systems also promotes the formation of low-Ti (< 1 wt%) magnetite: Ti acts as an incompatible component and is enriched in the iron-poor caps and in the hydrothermal aureole. High fluid-phase pressures produced during massive crystallization of magnetite from the melt further facilitate the exsolution of magmatic-hydrothermal fluids responsible for the formation of aureoles of alkali-calcic-iron alteration with hydrothermal replacement-style iron mineralization. On the whole, these systems are dramatically different from the magmatic-hydrothermal systems related to intermediate to felsic igneous rocks; they are more akin to carbonatite and other ultramafic rocks.

Effective treatment of high-arsenic smelting wastewater: Immobilization arsenic and synthesis well-crystallized scorodite

As-containing acidic wastewater from the nonferrous heavy smelting industry is a hazardous As pollution source due to its complex composition, high acidity, and strong toxicity. In this study, an environmentally friendly strategy was proposed to simultaneously remove and immobilize As from acidic wastewater through the synthesis of highly stable scorodite using natural magnetite ore as Fe resource. Firstly, the magnetite ore was dissolved in wastewater to provide a certain amount of starting Fe source for the subsequent arsenic precipitation while creating a suitable pH environment for the synthesis of scorodite. Subsequently, the released Fe ions reacted with As ions in wastewater with undissolved magnetite ore as the nuclear site to form amorphous ferric arsenate, which was then transformed into a well-crystallized scorodite precipitate, thus achieving the As removal and immobilization. Under the optimal reaction conditions, including a Fe/As molar ratio of 2.5, a pre-dissolution time of 5 h, an initial pH of 2.0, a reaction time of 24 h, and a reaction temperature of 90 °C, the residual As concentration in the wastewater rapidly reduced from initial 6259.00 mg/L to 3.76 mg/L, corresponding to an arsenic removal efficiency of 99.94%, and the leaching As concentration in scorodite was 0.42 mg/L. This study provides an efficient and environmentally friendly method for the disposal of high As-containing wastewater from nonferrous smelting.

Trace Elements in Magnetite and Origin of the Mariela Iron Oxide-Apatite Deposit, Southern Peru

To better understand the origin of the Andean iron oxide-apatite (IOA) deposits, we conducted a study on the geology and magnetite geochemistry of the Mariela IOA deposit in the Peruvian Iron Belt, central Andes. The Mariela deposit is hosted by gabbroic and dioritic intrusions. The major high-grade massive ores are primarily composed of magnetite and contain variable amounts of apatite and actinolite. Based on textural and geochemical characteristics, three different types of magnetite are recognized: Type I magnetite occurs in the massive magnetite ore, subclassified as inclusion-rich (I-a), inclusion-free (I-b), and mosaic (I-c); Type II magnetite is associated with abundant actinolite and titanite; and Type III magnetite is disseminated in altered host rocks. However, the magnetite geochemistry data for the Mariela deposit plot shows different genetic areas in [Ti + V] vs. [Al + Mn], Ti vs. V, and Fe vs. V/Ti discrimination diagrams, indicating a paradox of magmatic and hydrothermal origins. Our interpretation is as follows: Type I-a magnetite had an initial magmatic or high-temperature magmatic-hydrothermal origin, with slight modifications during transportation and subsequent hydrothermal precipitation (Types I-b and I-c). Type II magnetite is formed from hydrothermal fluid due to the presence of abundant actinolite. Disseminated magnetite (Type III) and veinlet-type magnetite formed after fluid replacement of the host rock. We stress that elemental discrimination diagrams should be combined with field studies and textural observations to provide a reasonable geological interpretation. A clear cooling trend is evident among the three subtypes of Type I magnetite (I-a, I-b, and I-c), as well as Type II and Type III magnetite, with average formative temperatures of 737 °C, 707 °C, 666 °C, 566 °C, and 493 °C, respectively. The microanalytical data on magnetite presented here support the magmatic-hydrothermal flotation model to explain the origin of IOA deposits in the Coastal Cordillera of Southern Peru.

Опыт освоения магнитно-гравитационной сепарации на предприятиях России и стран СНГ для повышения качества железорудного сырья

The paper analyzes the tasks of producing high-quality iron ore concentrates suitable for nonblast-furnace metallurgy and the challenges of processing magnetite ores with the current technological level of dressing equipment and technologies. One of the best methods to improve the quality and the grade of iron ore raw materials is the use of magnetic gravity separation in the technological process, which allows selective separation of the liberated magnetite particles from their aggregates in processing of undersize products of fine vibrating screening. This trend is implemented in the innovative magnetite ore processing technology that uses the principle of the staged output of finished concentrate. The technology makes it possible to produce a high-quality concentrate after the first grinding stage. The magnetic gravity separation has undergone industrial validation, has proven its efficiency in practice, and has become widespread at mining and processing plants not only in Russia, but also in the CIS countries. Over 180 MGS-2.0 magnetic gravity separators produced by Russian manufacturers have already been or are being implemented at various Russian and CIS companies. Industrial separators are equipped with modern automation systems providing high technological separation performance, have no rotating parts and are simple in design. The authors continue R&amp;D works on creation of automated control systems for the magnetic gravity separation process based on designing digital models adapted for introduction into the plant’s automated process control systems.

BIF-hosted high-grade magnetite iron ore targeting by hyperspectral wavelength mapping of chlorite: case study of Qidashan Iron Mine, northeast China

There are two kinds of chlorites occurred in Anshan area, among which the hydrothermal Fe-chlorite with longer Fe-OH wavelength is spatially related to the high-grade magnetite ore, meaning that wavelength mapping of chlorites can be used to target the BIF-hosted high grade iron ore. In this article, quadratic polynomial, cubic spline, and quartic polynomial method were used to interpolate the absorption wavelength near 2250 nm of China ZY1-02D satellite hyperspectral image. The result of quadratic polynomial is continuous without data overlapping or intervals, thus most suitable for discrimination of the chlorites. The field truth shows that the spatial distribution of Fe-chlorite, validated by XRD analysis, is not only in accordance with that of high grade magnetite ore bodies, but also consistent with the recently discovered concealed iron bonanza. The study shows that hyperspectral remote sensing techniques can play significant role in the exploration of magnetite iron ore in east Liaoning Province.

CFD study on the influences of size distribution and density of magnetite ore particles on hydrocyclone classification process

Due to the presence of gangue minerals in iron ore, the feed of hydrocyclone in the grinding circuit is a mixture of particles with different densities and sizes. In this study, numerical experiments are performed to investigate the classification characteristics of magnetite ore particles. A significant difference in recovery to underflow of intermediate-sized particles is found due to density effect. By increasing the feed d0.5, the cut size, Ep value, solid concentration and yield of underflow present an increasing trend. Density effect coefficient is proposed and its extreme values are obtained for intermediate-sized particles. Q/M ratio has smaller influences on classification than feed d0.5. The air core diameter, turbulence intensity, equilibrium radius and particle trajectory are analyzed to reveal the migration mechanism of the magnetite and gangue particles under different feed d0.5 and Q/M ratios. This work clarifies the intrinsic classification characteristic of magnetite ore particles.

From Micro to Nano: Grinding Natural Magnetite Ore for Microalgae Harvesting

Microalgae represent a promising feedstock for sustainable biomass and energy. The low cell concentration after cultivation, however, limits the current application fields. Magnetic microalgae harvesting is a recent approach to overcome the economic limitations of exploiting this natural resource. Accordingly, different particle types have been applied, mainly synthetically produced magnetic nanoparticles, though none on an industrial scale. Particle sizes between a few micrometers and a few nanometers have not been tested. We expected 200–500 nm to be advantageous for harvesting and as a compromise between the highly available surface and good separation properties. However, this intermediate magnetite particle size between the micro- and nano-scale cannot be reached via chemical synthesis. Therefore, we ground natural magnetite ore in a planetary ball mill and an agitator bead mill producing particles in the targeted size range. Applying ore particles ground from ~6 µm to 250 nm yields harvesting efficiencies comparable to synthetically produced nanoparticles (Ø ~ 10 nm), with only half the BET surface. Complete harvesting of saline microalgae Microchloropsis salina is possible with ground particles at alkaline pH. We demonstrate the feasibility of a harvesting process with natural, low-cost, easily separable, and readily available magnetite ore particles as a promising step towards exploiting valuable microalgal products in life sciences.

Study of the Distribution of Rare-Earth Elements and Strontium in Apatite from Rocks of the Vuoriyarvi Carbonatite Complex by Total-Reflection X-Ray Fluorescence Spectrometry (TXRF): First Results and Prospects

Abstract —We have studied the trace element composition of apatite from several varieties of carbonatites and associated apatite ores of the Vuoriyarvi alkaline-ultrabasic carbonatite complex (Kola region, northwestern Russia), which hosts several commercial deposits of apatite–magnetite and pyrochlore ores and large-scale unexplored rare-earth mineral occurrences. The composition of apatite was analyzed by total-reflection X-ray fluorescence spectrometry (hereafter, TXRF). Verification of the obtained results was carried out by ICP-MS analysis of weighed portions of apatite monomineral samples. It is shown that during the evolution of the Vuoriyarvi carbonatites and associated apatite–magnetite ores, the contents of Sr and rare-earth elements (REE) and the degree of REE fractionation in apatite synchronously increased. The obtained TXRF data on the composition of apatite in the carbonatite complex demonstrate the efficiency of TXRF and its high potential in both applied and fundamental research concerned with apatite of various geologic objects.

Fluid-Assisted Aggregation and Assembly of Magnetite Microparticles in the Giant El Laco Iron Oxide Deposit, Central Andes

The El Laco iron oxide mineral deposit in the Central Andes of Chile has attracted significant attention because of its uniquely preserved massive magnetite orebodies, which bear a remarkable similarity to volcanic products. To date, the outcropping highly vesicular and porous massive magnetite orebodies have received little attention from a microtextural point of view, limiting our understanding about the role of volcanogenic processes on iron mineralization. Here, we report the chemical composition of vesicular magnetite at El Laco using EPMA and LA-ICP-MS methods and provide detailed 2D and 3D imaging of the internal structure of these texturally complex magnetite ores by combining SEM observations, synchrotron radiation micro-X-ray fluorescence chemical mapping, and high-resolution X-ray computed microtomography. Our observations reveal the presence of abundant magnetite microspheres with diameters ranging from ∼100 to ∼900 μm, as well as dendritic microstructures forming interconnected networks up to a few millimeters in size. Two-dimensional microtextural and geochemical imaging of the microspheres show that these features are formed by multiple euhedral magnetite crystals growing in all directions and occur immersed within a porous matrix conformed by smaller-sized (∼2–20 μm) and irregularly shaped magnetite microparticles. These types of morphologies have been reported in hydrothermal vents associated with hydrovolcanic processes and commonly described in hydrothermal synthesis experiments of magnetite microspheres, suggesting precipitation from iron-rich fluids. A hydrothermal origin for the magnetite microparticles reported here is further supported by their geochemical signature, which shows a strong depletion in most minor and trace elements typical from magnetite precipitated from hydrothermal fluids in ore-forming environments. We propose that decompression, cooling, and boiling of fluids triggered massive iron supersaturation, resulting in the nucleation of magnetite microparticles or colloids, followed by self-assembly into larger and more complex microstructures. Our data from El Laco deposit agree with models invoking magmatic-hydrothermal fluids to explain the origin of the deposit and provide new insights on the potential role of iron colloids as agents of mineralization in volcanic systems.

In-situ texture and geochemistry of apatite from the Jinling and Zhangjiawa iron skarn deposits, eastern North China Craton: Implications for ore-forming processes and formation of high-grade ores

Apatite commonly contains abundant halogens and trace elements, which can occur in both the magmatic and hydrothermal stages, becoming favorable for recording the magmatic-hydrothermal properties and processes. Skarn-type iron deposits associated with high-Mg diorites are widespread in the Luxi Block, eastern North China Craton. Their ore-forming processes, especially how the different wallrocks (limestones and dolomites) controlled the formation of high-grade iron ores, have been poorly constrained. To unravel the mechanisms, in this contribution, in-situ textural, geochronological (U-Pb dating) and geochemical (halogens and trace elements) analyses by SEM, EPMA and LA-ICP-MS were conducted on apatite from the representative Fe skarn deposits (Jinling and Zhangjiawa deposits) in the Luxi Block. Three generations of apatite were identified in the Jinling deposit, which occur in the feldspathization (Ap1), garnet–pyroxene skarns (Ap2) and massive magnetite ores (Ap3), respectively. One generation of apatite occurring in the massive magnetite ores (Ap-3) was identified in the Zhangjiawa deposit. The Ap2 and Ap-3 apatite grains show patchy textures composed by residual bright zones (Ap2a and Ap-3a) and newly-formed grey to dark zones (Ap2b and Ap-3b), which indicate fluid metasomatism. LA-ICP-MS U-Pb dating shows that the Ap1, Ap2a and Ap3 apatite in the Jinling deposit formed at 130.4 ± 0.9 Ma (2σ), 128.5 ± 2.4 Ma and 128.3 ± 9.2 Ma, respectively, whereas the Ap-3a in the Zhangjiawa deposit formed at 128.1 ± 4.2 Ma. These ages are well consistent with the intrusive ages of the spatially associated high-Mg diorites within errors, corroborating the ore-forming fluids originating from the high-Mg rocks. All the studied apatite grains show F/Cl ratios higher than 1, especially in the early-stage apatite (e.g., 5–19 in Ap1), indicating that the originally exsolved fluids were enriched in F. The high F contents probably played a significant role in leaching Fe from the high-Mg rocks by enhancing rock porosities. The Eu/Eu* ratios of apatite increase from Ap2a to Ap3 while the Ce/Ce* ratios decrease. This suggests an increase of oxygen fugacity. The Sr concentrations in Ap3 are much higher than those in Ap-3a, correlating well with the wallrocks that limestones are developed in the former while dolomites in the latter. The above features indicate that fluid-rock interaction likely led to the increase of oxygen fugacity, which controlled the massive Fe deposition. In the Jinling deposit, the altered apatite grains (Ap2b) show much lower REE + Y but higher F contents than those of the unaltered (Ap2a), not only indicating the REEs being easily mobilized during metasomatism, but also suggesting that the high F contents likely contributed to the formation of high-grade ores by leaching additional Fe from the previously formed Fe-rich skarns. In the Zhangjiawa deposit, the altered apatite grains (Ap-3b) shows lower Cl contents than those of the unaltered (Ap-3a). This probably indicates the mixing of low-salinity fluids (e.g., recycling meteoric water), leading to the loss of Fe and thus lowering the Fe grade. The above results indicate that metasomatism is common in the skarn deposits, which can either elevate or lower the metal grade. The processes can be well recorded by apatite.