The chemistry of hydrothermal magnetite: A review

The trace element composition of igneous and hydrothermal magnetite from 19 well-studied porphyry Cu ± Au ± Mo, Mo, and W-Mo deposits was measured by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and then classified by partial least squares-discriminant analysis (PLS-DA) to constrain the factors explaining the relationships between the chemical composition of magnetite and the magmatic affinity and porphyry deposit subtypes. Igneous magnetite can be discriminated by relatively high P, Ti, V, Mn, Zr, Nb, Hf, and Ta contents but low Mg, Si, Co, Ni, Ge, Sb, W, and Pb contents, in contrast to hydrothermal magnetite. Compositional differences between igneous and hydrothermal magnetite are mainly controlled by the temperature, oxygen fugacity, cocrystallized sulfides, and element solubility/mobility that significantly affect the partition coefficients between magnetite and melt/fluids. Binary diagrams based on Ti, V, and Cr contents are not enough to discriminate igneous and hydrothermal magnetite in porphyry deposits.Relatively high Si and Al contents discriminate porphyry W-Mo hydrothermal magnetite, probably reflecting the control by high-Si, highly differentiated, granitic intrusions for this deposit type. Relatively high Mg, Mn, Zr, Nb, Sn, and Hf but low Ti and V contents discriminate porphyry Au-Cu hydrothermal magnetite, most likely resulting from a combination of mafic to intermediate intrusion composition, high chlorine in fluids, relatively high oxygen fugacity, and low-temperature conditions. Igneous or hydrothermal magnetite from Cu-Mo, Cu-Au, and Cu-Mo-Au deposits cannot be discriminated from each other, probably due to similar intermediate to felsic intrusion composition, melt/fluid composition, and conditions such as temperature and oxygen fugacity for the formation of these deposits.The magmatic affinity of porphyritic intrusions exerts some control on the chemical composition of igneous and hydrothermal magnetite in porphyry systems. Igneous and hydrothermal magnetite related to alkaline magma is relatively rich in Mg, Mn, Co, Mo, Sn, and high field strength elements (HFSEs), perhaps due to high concentrations of chlorine and fluorine in magma and exsolved fluids, whereas those related to calc-alkaline magma are relatively rich in Ca but depleted in HFSEs, consistent with the high Ca but low HFSE magma composition. Igneous and hydrothermal magnetite related to high-K calc-alkaline magma is relatively rich in Al, Ti, Sc, and Ta, due to a higher temperature of formation or enrichment of these elements in melt/fluids.Partial least squares-discriminant analysis on hydrothermal magnetite compositions from porphyry Cu, iron oxide copper-gold (IOCG), Kiruna-type iron oxide-apatite (IOA), and skarn deposits around the world identify important discriminant elements for these deposit types. Magnetite from porphyry Cu deposits is characterized by relatively high Ti, V, Zn, and Al contents, whereas that from IOCG deposits can be discriminated from other types of magnetite by its relatively high V, Ni, Ti, and Al contents. IOA magnetite is discriminated by higher V, Ti, and Mg but lower Al contents, whereas skarn magnetite can be separated from magnetite from other deposit types by higher Mn, Mg, Ca, and Zn contents. Decreased Ti and V contents in hydrothermal magnetite from porphyry Cu and IOA, to IOCG, and to skarn deposits may be related to decreasing temperature and increasing oxygen fugacity. The relative depletion of Al in IOA magnetite is due to its low magnetite-silicate melt partition coefficient, immobility of Al in fluids, and earlier, higher-temperature magmatic or magmatic-hydrothermal formation of IOA deposits. The relative enrichment of Ni in IOCG magnetite reflects more mafic magmatic composition and less competition with sulfide, whereas elevated Mn, Mg, Ca, and Zn in skarn magnetite results from enrichment of these elements in fluids via more intensive fluid-carbonate rock interaction.

In-situ LA-ICP-MS trace elemental analyses of magnetite: Cu-(Au, Fe) deposits in the Khetri copper belt in Rajasthan Province, NW India

A combination of petrographic observations, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and statistical data exploration was used in this study to determine compositional variations in hydrothermal and igneous magnetite from five porphyry Cu–Mo and skarn deposits in the southwestern United States, and igneous magnetite from the unmineralized, granodioritic Inner Zone Batholith, Japan. The most important overall discriminators for the minor and trace element chemistry of magnetite from the investigated porphyry and skarn deposits are Mg, Al, Ti, V, Mn, Co, Zn, and Ga—of these the elements with the highest variance for (I) igneous magnetite are Mg, Al, Ti, V, Mn, Zn, for (II) hydrothermal porphyry magnetite are Mg, Ti, V, Mn, Co, Zn, and for (III) hydrothermal skarn magnetite are Mg, Ti, Mn, Zn, and Ga. Nickel could only be detected at levels above the limit of reporting (LOR) in two igneous magnetites. Equally, Cr could only be detected in one igneous occurrence. Copper, As, Mo, Ag, Au, and Pb have been reported in magnetite by other authors but could not be detected at levels greater than their respective LORs in our samples. Comparison with the chemical signature of igneous magnetite from the barren Inner Zone Batholith, Japan, suggests that V, Mn, Co, and Ga concentrations are relatively depleted in magnetite from the porphyry and skarn deposits. Higher formation conditions in combination with distinct differences between melt and hydrothermal fluid compositions are reflected in Al, Ti, V, and Ga concentrations that are, on average, higher in igneous magnetite than in hydrothermal magnetite (including porphyry and skarn magnetite). Low Ti and V concentrations in combination with high Mn concentrations are characteristic features of magnetite from skarn deposits. High Mg concentrations (<1,000 ppm) are characteristic for magnetite from magnesian skarn and likely reflect extensive fluid/rock interaction. In porphyry deposits, hydrothermal magnetite from different vein types can be distinguished by varying Ti, V, Mn, and Zn contents. Titanium and V concentrations are highly variable among hydrothermal and igneous magnetites, but Ti concentrations above 3,560 ppm could only be detected in igneous magnetite, and V concentrations are on average lower in hydrothermal magnetite. The highest Ti concentrations are present in igneous magnetite from gabbro and monzonite. The lowest Ti concentrations were recorded in igneous magnetite from granodiorite and granodiorite breccia and largely overlap with Ti concentrations found in hydrothermal porphyry magnetite. Magnesium and Mn concentrations vary between magnetite from different skarn deposits but are generally greater than in hydrothermal magnetite from the porphyry deposits. High Mg, and low Ti and V concentrations characterize hydrothermal magnetite from magnesian skarn deposits and follow a trend that indicates that magnetite from skarn (calcic and magnesian) commonly has low Ti and V concentrations.

Partial least squares-discriminant analysis of trace element compositions of magnetite from various VMS deposit subtypes: Application to mineral exploration

Hydrothermal reequilibration of igneous magnetite in altered granitic plutons and its implications for magnetite classification schemes: Insights from the Handan-Xingtai iron district, North China Craton

Trace elemental modification in magnetite from high-grade metamorphosed BIFs in the southern North China Craton

Magnetite (Fe 3 O 4 ) is a common and widespread accessory mineral in many host rocks and mineral deposits. We used electron microprobe analysis (EMPA), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis, and oxygen isotope analysis to test whether magnetite from the five following geologic settings in western Montana and northern Idaho has distinct geochemical signatures: (1) greenschist facies burial metamorphic rocks of the Middle Proterozoic Belt Supergroup, (2) sediment-hosted stratiform Cu-Ag deposits (Spar Lake and Rock Creek) in Belt Supergroup metasedimentary rocks, (3) hydrothermal Ag-Pb-Zn veins of the Coeur d’Alene district, (4) extensively deformed and partially altered Belt Supergroup host rocks from the Coeur d’Alene district, and (5) two Cretaceous postmetamorphic igneous intrusions. EMPA results show that magnetite from each of these five settings is essentially pure Fe 3 O 4 , but LA-ICP-MS analyses results show that magnetite from these five settings has trace element concentrations that generally vary over less than one order of magnitude. These magnetite occurrences show subtle compositional differences that generally correlate with temperatures, as determined by oxygen isotope geothermometry. Burial metamorphic magnetite from the Coeur d’Alene host rocks has the smallest overall trace element contents. Chromium, Co, and Zn are depleted in both hydrothermal and host-rock magnetite from the Coeur d’Alene district. In contrast, magnetite from postmetamorphic igneous rocks in the Belt terrane has relatively large Mg, V, Co, and Mn values, consistent with its formation at relatively high temperatures and subsequent subsolidus reequilibration. Factor analysis was used to trace any underlying or latent relationships among elements that are likely to be incorporated into the magnetite structure. Factor analysis provides geochemical discrimination of at least three types of magnetite in the Belt terrane: (1) Mg-Mn, (2) Ga-Zn-Cr, and (3) Co-Ni-V magnetite. Hydrothermal magnetite from the Gold Hunter siderite vein shows characteristically high values for factor 1. Factor 2 is most pronounced in magnetite from the burial metamorphic host rocks and the sediment-hosted Cu-Ag deposits. Furthermore, factor 2 indicates that Ga, Zn, and Cr concentrations are lower on average in hydrothermal and host-rock magnetite from the Coeur d’Alene district. Factor 3 divides igneous magnetite from other magnetite occurrences. This factor also subdivides magnetite of an alkalic-ultramafic intrusive complex from that of the granitic stock. Hydrothermal magnetite from siderite and calcite veins in the Coeur d’Alene district has consistently low scores for factor 3. The geochemistry of magnetite can be a useful discriminator and pathfinder for hydrothermal deposits. The relatively low formation temperature and the metamorphic history of the Belt terrane led to low trace element concentrations and subtle differences between magnetite from different geologic settings. Nevertheless, by combining LA-ICP-MS analysis and factor analysis, compositional variations between groups of magnetite samples from different geologic settings can be recognized.

Depositional model for banded iron formation host to gold in the Archean Rio das Velhas greenstone belt, Brazil, based on geochemistry and LA-ICP-MS magnetite analyses

The Yamansu iron deposit is hosted in submarine volcanic rocks in the Aqishan–Yamansu belt of Eastern Tianshan, NW China. A geological cross-section for the Carboniferous strata in the ore district shows that ore bodies in the Yamansu deposit are hosted in andesitic crystal tuff of the third cycle of the Carboniferous Yamansu Formation. This indicates an association between mineralization and volcanism. The orebodies are strata bound and lensoid and generally share the occurrence state of the host rocks. Magnetite mineralization mainly occurs asbreccia ores, ores in the mineralized volcanic rocks, massive ores, and sulfide-rich ores according to their structures and sequences of formation. Trace element compositions of magnetite from various types of ores were determined by LA-ICP-MS. The dataset indicates thatdifferent types of magnetite havedistinct trace element contents correlated to their formation environments. Magnetite crystals from breccia ores have high Ti, Ni, V, Cr, and Co and low Si, Al, Ca, and Mg contents, indicating crystallization from a volcanic magmatic eruption, which is consistent with field evidence of coexisting altered volcanic breccia. Magnetite crystals from ores in the mineralized volcanic rocks have moderate Ti, Ni, V, Cr, and Co contents. In contrast, magnetite from massive ores and sulfide-rich ores have low concentrations of Ti, Cr, Ni, and V, high concentrations of Si, Al, Ca, and Mg, and evidence of hydrothermal magnetite. In-situ magnetite compositions imply a magmatic-hydrothermal process. Although δ18O values for magnetite grains fromYamansu vary (+ 1.3 to + 7.0‰), they all plot in the range field of volcanic iron deposits, and they also record a magmatic-hydrothermal process. The compositions of Yamansu magnetites are interpreted as controlled mainly by temperature, fluid, host rock buffering, oxygen fugacity, and sulfur fugacity. The metallogenic conditions of the Yamansu deposit changed from high temperature and low oxygen fugacity to low temperature and high oxygen fugacity. However, more fluid-rock reactions and higher sulfur fugacity were involved during the deposition of massive ores and sulfide-rich ores.

We report on the mineralogy, petrology, and O‐isotope compositions of magnetite and fayalite (Fa90−100) from several metasomatically altered and weakly metamorphosed carbonaceous (Y‐81020 [CO3.05], EET 90043 [CO3.1], MAC 88107 [CO3.1‐like], and Kaba [oxidized Bali‐like CV3.1]) and unequilibrated ordinary chondrites (UOCs; Semarkona [LL3.00], MET 00452 [LL3.05], MET 96503 [LL3.05], EET 910161 [LL3.05], Ngawi [LL3.0−3.6 breccia], and Vicência [LL3.2]). In MAC 88107, EET 90043, and Kaba, nearly pure fayalite (Fa98−100) associates with phyllosilicates, magnetite, Fe,Ni‐sulfides, and hedenbergite (Fs~50Wo~50), and occurs in all chondritic components—chondrules, matrices, and refractory inclusions. In UOCs, nearly pure fayalite (Fa95−98) associates with phyllosilicates and magnetite, and occurs mainly in matrices and fine‐grained chondrule rims. Oxygen‐isotope compositions of fayalite and magnetite in UOCs, COs, CVs, and MAC 88107 are in disequilibrium with those of chondrule olivine and low‐Ca pyroxene phenocrysts, and plot along mass‐dependent fractionation lines with slope of ~0.5, but different Δ17O (~+4.3 ± 1.4‰, −0.2 ± 0.6‰, −1.5 ± 1‰, and −1.8 ± 0.8‰, respectively). Based on the mineralogical observations, thermodynamic analysis, O‐isotope compositions, and recently reported experimental data, we infer that (1) fayalite and magnetite in COs, CVs, MAC 88107, and UOCs resulted from aqueous fluid–rock interaction on the chondrite parent asteroids that occurred at low local water‐to‐rock mass ratios (0.1−0.4) and elevated temperatures (~100−300 °C), and (2) Δ17O of fayalite and magnetite reflects O‐isotope compositions of aqueous fluids on the host meteorite parent bodies. The observed differences in Δ17O of fayalite–magnetite assemblages in UOCs, CVs, COs, and MAC 88107 suggest that water ices that accreted into the ordinary chondrite and carbonaceous chondrite parent asteroids had different Δ17O, implying spatial and/or temporal variations in O‐isotope compositions of water in the protoplanetary disk.

Mineralogical and microfabric characteristics of magnetite in the Wuyang Precambrian BIFs, southern North China Craton: Implications for genesis and depositional processes of the associated BIFs

The resurgence of Banded Iron Formations (BIFs) during the Neoproterozoic, following a billion-year hiatus, reflects significant geodynamic and climatic transition. Newly discovered Neoproterozoic BIFs in the central Anti-Atlas region of Morocco provide key insights into these processes. The studied BIFs units are exposed within the Bou Azzer-El Graara inlier (Central Anti-Atlas), an oceanic paleo-suture zone between the Paleoproterozoic West African Craton and remnants of a Neoproterozoic magmatic arc. This inlier comprises 750 to 680 Ma magmatic arcs and ophiolitic remnants, both intruded by ~650 Ma dioritic plutons and overlain by Ediacaran metasedimentary sequences. The studied BIFs are hosted in meta-volcano-sedimentary units, intercalated between magmatic arc and ophiolitic complexes, and locally intruded by igneous bodies. Neither the BIFs nor their host volcano-sedimentary schists are associated with glacio-derived sediments.Petrological, geochemical, and geochronological analyses were conducted to reconstruct the paleo-depositional environment and identify the mechanisms of BIF formation. In situ U-Pb dating on hematite yielded a crystallization age of 641 &amp;#177; 41 Ma. Hematite dating could be interpreted as an early diagenetic age probably close to BIF deposition.The whole-rock major and trace element composition of the Bou Azzer BIFs exhibits a high correlation among terrigenous proxies (e.g., Al, Zr, Hf) and silica content, with trends strongly aligning with the felsic host rocks. This suggests that the BIFs&amp;#8217; whole-rock geochemical signature, specifically the siliceous layers, is predominantly controlled by detrital inputs. Multi-element geochemistry, (e.g. mean La/YbSN ratio of 0.36, low TiO&amp;#8322; content of 0.24 wt%, Y/Ho ratio of 26, Nb-Ta depletion) combined with Nd-Sr isotopic data from the host rocks (&amp;#949;Nd&amp;#7511; +4.0 to +4.5), indicates a juvenile arc source, consistent with presence of igneous minerals, such as feldspar, epidote, and amphibole, in both the host rocks and BIF samples.Hematite in BIFs show two habitus: large euhedral grains surrounded by platy hematite. Petrographic evidence suggests that euhedral hematite precipitated at a more precocious stage, while platy hematite is distinctly aligned with the foliation of the host sediments. In situ LA-ICP-MS analyses of hematite from the two habitus reveal distinct geochemical signatures from each other and from the whole-rock compositions. Overall, hematite exhibits significantly lower &amp;#931;REE and superchondritic Y/Ho ratios up to 42, with a median value of ~28. Large euhedral hematite displays a pronounced negative Ce anomaly, indicative of precipitation from oxygenated seawater and distant from hydrothermal sources, as shown by low positive Eu anomaly (~1.06). The chemical composition of platy hematite shows no Eu or Ce&amp;#160;anomalies, suggesting anoxic conditions during diagenetic crystallization.&amp;#160;The Bou Azzer BIFs are Cryogenian and were deposited in an arc-bounded basin, with no evidence of glaciogenic influence. This paleo-depositional context emphasizes the role of limited arc-related basins during the Neoproterozoic, which facilitated the development of unique suboxic conditions.

Summary Iron ore deposits of the Lake Superior type within the Archaean and Proterozoic banded iron formations of Western Australia show consistent and stable directions of remanent magnetization. The palaeomagnetic poles of the ores do not correlate with poles of Post-Cambrian Australian rocks, which suggests a Pre-Cambrian age for the ore formation. Magnetic anisotropies of the ores are low and the distribution of the principal susceptibility axes tends to reflect the magnetic fabric of the host rocks. In a previous publication (Chamalaun & Porath 1967) we reported the results of a palaeomagnetic study on the hematite ore bodies of the Middleback Ranges in South Australia. It was found that these were consistently and stably magnetized with a Pre-Cambrian direction. We therefore extended the study to the Western Australian ore bodies, the results of which are reported here. A large number of high-grade hematite ore bodies are known to occur in the Archaean and Proterozoic banded iron formations of Western Australia, but in order to obtain fresh subsurface samples we have sampled only those that are actively mined. The localities of the ore bodies sampled are shown in Fig. 1. The Mt Goldsworthy and Koolyanobbing deposits occur in Archaean banded iron formations, whilst the host rocks for the Mt Tom Price and Mt Newman ore bodies are Proterozoic iron formations. 2. Results (a) Mt Goldsworthy. The Mt Goldsworthy deposit is located at the northern extremity of the Western Australian Archaean shield. A folded and faulted banded iron formation, which consists of alternating layers of silica and hematite, forms the host rock of a number of high-grade hematite ore bodies of varying origins. Brandt (1964, 1966) makes a distinction between lode ore and crust ore deposits. Lode ores are deep lenses of massive finely crystalline hematite (average grain size about 20p). The major deposit at Mt Goldsworthy consists of lode ore and is controlled by a steep angled transcurrent fault. Pebbles of massive hematite ore occur in the basal Proterozoic conglomerate indicating that the ore formed in Pre-Proterozoic times (Brandt 1964). Brandt (1966) favours a hypogene origin of the lode ore along the lines suggested by Dorr (1965) for the Brazilian ores. Crust ores form shallow cappings of hematite and goethite on the upturned

Numerous iron ore deposits are hosted within the Meso to Neo-Archean banded iron formations (BIFs) extending across the Singhbhum-Orissa Craton, eastern India. Despite the widespread distribution of BIFs, which forms part of the iron ore group (IOG), heterogeneity in their grade and mineral composition is occasionally observed even within a single ore deposit. Kiriburu-Meghahatuburu iron ore deposit (KMIOD), west Singhbhum district, Jharkhand, eastern India is characterized by a dominant hematite (often martitized) occurrence with a total resource of &gt;150 million tonnes (MT) at 62.85 wt % Fe. Very high-grade blue dust ore (friable and powdery hematitewith~67% Fe), high-grade massive, hard laminated hematitic ores (~66% Fe) and medium to low grade goethitic/lateritic ores (50%–60% Fe) are the common iron-ore lithologies in KMIOD. These ores can be distinguished in the field from their physical appearance, meso-scale texture and spatial occurrences with the host rocks along with the variation in chemical composition. The high-grade ores are characterized by high Fe (&gt;62 wt %), low Al2O3 (1.5–2.5 wt %), low SiO2 (2.0–4.5 wt %) and low P (&lt;0.06 wt %). Detailed field studies and laboratory investigations on the ore mineral assemblages suggest that the mineralization of high-grade iron ores at KMIOD is controlled by three major parameters, i.e., lithological, paleoclimatic and structural controls. High-grade iron ores such as blue dust seem to be formed during leaching processes through inter-bedded ferruginous shale and banded hematite jasper (BHJ) occurring within BIFs. Structural elements such as folds, joint network, fracture arrays, local faults and steeply dipping bedding planes are surmised as strong controls for the evolution of different iron ore types from the BHJ. Most of the high-grade ores are concentrated at the hinge portions of second generation folds (F2) owing to the easy access for circulation of meteoric solution along the fractures developed due to release of stresses at the hinge portions aided by supergene ore enrichment processes. The BHJ and interbedded ferruginous shale seem to have been given a significant contribution for the formation of different grades of iron ores over the area. Lithologically, the BIFs are governed by rheological features providing channel ways in the ore enrichment process. The variation in the iron ore mineralogy is caused by the variation in depositional and paleoclimatic environment, structural setting and lithological attributes. Hence, these parameters could be used for future exploration and grade recovery of iron ore resources in the region and in the adjoining areas.

• Chlorite chemistry reveals distinct signatures across ore deposit types. • Principal Component Analysis classifies chlorite by deposit type. • Host-rock normalization improves chlorite-based classification accuracy. • Tennant Creek chlorite confirms IOCG affinity. Minerals belonging to the chlorite mineral group commonly form as alteration products of primary ferromagnesian minerals in and around hydrothermal ore deposits, where the chemistry of chlorite species can be used as a powerful tool for mineralization vectoring. In this study, we present a compilation and evaluation of chlorite-group major and trace element data from ninety-six localities, including samples from porphyry Cu, epithermal Au, IOCG, orogenic Au, and VHMS deposits, to assess the potential of chlorite chemistry for classifying ore deposit types. Our results show that a variety of elements, including Al, Co, Cr, Fe, Ga, Li, Mg, Mn, Ni, Pb, Sr, Th, Ti, U, V, Zn, and REE, are controlled primarily by hydrothermal fluid chemistry and metal enrichment rather than whole-rock chemistry, which allows for discrimination of the different hydrothermal ore-forming environments in our sample suite. Host rock composition and precursor minerals play an important role in controlling the Cr, Co, Ni, V, and Pb (and to a lesser extent, Fe, Mg, and Ga) contents in chlorite. These elements can mask the hydrothermal signal related to the ore deposit type, and we propose a normalization method to account for the influence of the host rock composition. Fluid temperature exerts a major control on Si, Al, Li, and Ti contents in chlorite, but the impact of temperature on deposit classification is relatively limited. Principal Component Analysis (PCA) shows that the assemblage of Mg, Fe, Sr, U, and Zn in chlorite is sufficient to classify samples into specific deposit types. Comparison of Tennant Creek chlorite to a global database indicates that major and some trace element contents (e.g., Zn, Sn, Cu) from the Hermitage and Mauretania deposits are similar to IOCG deposits globally. However, Tennant Creek chlorites exhibit higher Co, Bi, REE, and U concentrations than chlorite from other IOCG systems, reflecting unique metal composition of the hydrothermal fluids that formed these deposits. PCA places Tennant Creek clearly within the IOCG field, providing new insights into deposit classification. Overall, this study highlights the potential of chlorite chemistry as a tool for ore deposit classification and exploration targeting.