Ilmenite Exsolution Research Articles

Fe-isotopic evidence for hydrothermal reworking as a mechanism to form high-grade Fe-Ti-V oxide ores in layered intrusions

The genesis of high-grade Fe-Ti-V oxide ores (up to >90 vol%) in layered intrusions remains highly debated. Here, on example of Hongge layered intrusion in China, we show that hydrothermal dissolution and precipitation of Fe-Ti-V oxides played a critical role in forming high-grade massive ore deposits as demonstrated by textural-compositional evidence and in-situ iron isotope data (δ56Fe), analyzed with femtosecond laser ablation multicollector (LA-MC-) ICP-MS. Hongge is a mafic layered intrusion composed of a Lower olivine clinopyroxenite Zone (LZ), a Middle clinopyroxenite Zone (MZ), where thick massive ore layers (with up to 90 % Fe-Ti-V oxides) formed, and an Upper gabbro Zone (UZ). Magnetite in Hongge exhibits two contrasting generations: 1) Mag1, observed in all lithological zones and formed at the magmatic stage, has extensive ilmenite exsolution lamellae and high Ti and Cr content. The δ56Fe of Mag1 shows considerable variations from −0.23 to 0.63 ‰ and strikingly an offset of ∼0.3 ‰ towards lower values in the massive ore zone compared to the zones below and above; 2) Mag2, concentrated mainly in thick massive ore layers in MZ without exsolution lamellae, is almost pure magnetite (with low Ti, Al content) and has extremely low δ56Fe values (−1.24 to −0.09 ‰), indicating precipitation from Fe-enriched hydrothermal fluids. Similarly, the δ56Fe of ilmenite shows significant variations from −1.08 to −0.27 ‰ and is significantly lower than typical values for igneous ilmenite (−0.4–0 ‰). Ilmenite displays a similar Fe isotope variation pattern to Mag1 along the stratigraphic position, i.e., with significantly lower δ56Fe in the massive ore zone. As magnetite and ilmenite together contain essentially all Fe, the isotopic shift of these minerals in the ore zone translates to a bulk isotopic offset of ∼−0.3 ‰ compared to the zones below and above. This requires a bulk flux of isotopically light Fe resulting in Fe enrichment in this zone to form massive or even monomineralic ores. The very light isotopic values, particularly hydrothermal magnetite (Mag2) and petrologic evidence, strongly indicate that the Fe flux into the massive ore layers occurred during hydrothermal reworking. This scenario is furthermore supported by magnetite-ilmenite elemental and isotopic thermometry, according to which Fe-Ti oxides experienced hydrothermal re-equilibration in a temperature range of 400–300 °C. Iron isotopic mass balance calculations imply that ∼20–30 % of the Fe in the thick massive ore layers may result from secondary enrichment through hydrothermal precipitation, significantly increasing the ore tonnages and grades. Potentially, other layered intrusions experienced similar mechanisms of hydrothermal Fe enrichment, which will have to be proven in future investigations.

Iron isotope fractionation during fluid metasomatism and ore-forming processes in magmatic-hydrothermal systems

Iron isotopes of magnetite have been used to unravel the sources and evolutionary processes of magma and ore deposits, but most studies have focused on bulk-rock samples, which possibly provide mixed information. Magnetite is susceptible to fluid-induced metasomatism, e.g., recrystallization and coupled dissolution-reprecipitation (CDR) processes in magmatic-hydrothermal systems, resulting in textural and chemical modification and re-equilibration. The behaviors of Fe isotopes during fluid metasomatism, however, are poorly understood. Magnetite from iron oxide-apatite (IOA) deposits commonly shows multi-generation and metasomatic textures, which have been suggested to form from magmatic melts/fluids to low-temperature hydrothermal processes. In this study, we carried out in situ Fe isotopic and chemical analyses on texturally constrained magnetite from ores and their hosting trachyandesite of the giant Luohe IOA deposit, eastern China, to assess the effect of fluid-assisted metasomatic processes on both chemical and Fe isotopic compositions, and to constrain the ore-forming processes.The ores contain three types of magnetite (Mag1, Mag2 and Mag3), all of which are texturally later than albite and diopside. Pristine Mag1 grains, formed at high temperatures (>700 °C), have ilmenite exsolution lamellae, and have lower Ti (0.61–1.55 wt%) but higher Ni/Cr ratios (mostly > 1) than those of magmatic magnetite (MagM) in the hosting trachyandesite. They also have δ56Fe values (0.12–0.52‰) lower than those of MagM (0.54–0.65‰), which overlap with those of high-temperature hydrothermal magnetite from iron-oxide copper–gold (IOCG), iron skarn, and porphyry Cu deposits, indicating that they were precipitated from high-temperature hypersaline fluids, as recorded by fluid inclusions within early-formed garnet, diopside, and coeval titanite. The heavy Fe isotope compositions of Mag1 are interpreted to be due to fluid exsolution under high temperatures and the preference of isotopically light Fe for earlier diopside (δ56Fe = −0.19 to −0.02‰). Mag2 grains, which show triple junction textures, were formed through a fluid-induced recrystallization process at relatively lower temperatures (∼480 °C). They have δ56Fe values (0.29–0.53‰) similar to those of Mag1 and previously reported high-temperature hydrothermal magnetite in other deposits. In contrast, Mag3 grains, which occur as rims on Mag1 grains, were formed via CDR at much lower temperatures (<300 °C). They have δ56Fe values (−0.15 to 0.22‰) significantly lower than those of pristine Mag1 and overlap those of reported low-temperature hydrothermal magnetite. Decreasing formation temperatures from Mag1 to Mag3 are consistent with the decreasing trace element concentrations (Ti, Al, and V). Our study demonstrates that IOA deposits are formed from high-temperature hypersaline magmatic fluids and that the Fe isotopes of magnetite can be significantly modified during fluid metasomatism and ore-forming processes. Therefore, trace element and in situ Fe isotopic analyses on texturally well-constrained magnetite grains are crucial for determining the origin and evolution of magmatic-hydrothermal systems.

Precise determination of major and trace elements in micrometer-scale ilmenite lamellae in titanomagnetite using LA-ICP-MS technique: application of regression analysis to time-resolved signals.

Laser ablation ICP-MS (LA-ICP-MS) is a powerful microbeam technique capable of rapid and precise determination for a large spectrum of trace elements at ppm or sub-ppm levels. Micrometer-scale minerals and inclusions are very common in geological materials, for which direct measurement is restricted by the spot size using LA-ICP-MS (generally 20-50 μm). In this study, ilmenite lamellae intergrown with magnetite were selected as an example to describe a practical algorithm that applies regression analysis to extract the chemical compositions of binary phases from mixed LA-ICP-MS signals. The method accuracy is confirmed by the agreement between the regressed value for various trace elements in ilmenite exsolutions and their reference values (direct analyses using EPMA and LA-ICP-MS). Results were obtained for most detectable components (Mg, Mn, V, Nb, Ta, Sc, Zr, Hf, Sn, et c.) and their relative deviations are within ±10%, even for those <10 ppm (such as Hf and W). Relative standard errors on the regressed value were calculated to evaluate the precision of the method, which is mostly within 10%, and the worst up to 25%. Therefore, the algorithm described in this contribution provides a solution for precise determination of trace element compositions for micrometer-scale ilmenite lamellae in titanomagnetite using LA-ICP-MS, and is potentially practical for other geological materials.

Geochemistry and magnetite mineral properties in a porphyry copper prospect in A‐type granitoids: A case study from the Karbi Hills of Northeast India

This paper discusses the porphyry copper mineralization in A‐type granitoids from the Precambrian shield of the Karbi Hills (Mikir Hills), NE India. The prospective zone lies in the Kuthori‐Bagori of Kaziranga locality of the north Karbi Hills. The host granitoids are characterized by intermediate to high SiO2 (66.30–72.97 wt.%), Na2O (up to 4.90 wt.%), Ba (up to 1,943 ppm), Zr (up to 410 ppm), Y (up to 70 ppm) and Al2O3 (up to 14.77 wt.%) with low concentrations of CaO (up to 1.53 wt.%), and Cu (up to 32.41 ppm) which suggest a consistent granodiorite‐monzogranite host composition, an intraplate emplacement, A‐type chemistry, shoshonitic to high‐K calc‐alkaline and a predominant metaluminous character. Minerals in the investigated granitoids show hypogene alteration features similar to ideal porphyry style mineralization. Remnants of early chalcopyrite‐bornite‐magnetite (Magnetite‐I) veins have been preserved in the distant propylitic zone. The central core zone of potassic alteration shows the dominance of pyrite‐chalcopyrite‐magnetite (Magnetite‐II) assemblages. Mineral chemistry confirms the presence of two types of magnetites: magmatic (Mag‐I) and hydrothermal (Mag‐II). Mag‐I is octahedral, less commonly cubic, or irregular. It has a porous core with trellis‐type ilmenite exsolution lamellae. Mag‐II is non‐porous, devoid of ilmenite exsolution lamellae, has irregular to distinctly octahedral crystal habit and contains micro‐inclusions of pyrite, chalcopyrite, aluminosilicates, manganite and apatite. The progressive chemical purification of igneous magnetite, exacerbated by hydrothermal re‐equilibration, has resulted in a significant reduction in trace element contents such as Ti, Al, Mg, Zn and Cr, and a significant increase in iron content from 89 to ~94 wt.%. A combination of [Ni/(Cr + Mn) vs. Ti + V], and [(Ca + Al + Mn) vs. Ti + V] variation diagrams and upper threshold concentrations for the Kuthori‐Bagori ore effectively separates Mag‐I and Mag‐II. Our field, petrographic and geochemical data indicate that the Kuthori‐Bagori of Kaziranga granitoid of the Karbi Hills (Mikir Hills) in NE India is an ideal location for porphyry copper mineralization.

The Origin and Discrimination of High-Ti Magnetite in Magmatic-Hydrothermal Systems: Insight from Machine Learning Analysis

AbstractHigh-titanium (high-Ti, more than 1 wt % Ti) magnetite, commonly containing ilmenite exsolution, has long been attributed to an igneous origin and has been used as the most critical factor in previously developed discriminant diagrams. However, recent studies have shown that high-Ti magnetite can be present in high-temperature hydrothermal deposits, suggesting a probable hydrothermal origin. This also calls for reconsideration and necessary modification of the currently available discriminant diagrams. This high-Ti magnetite issue is particularly acute in iron oxide-apatite (IOA) deposits and raises controversy in the discussion of the origin of the high-Ti magnetite.With statistical analysis and machine learning techniques, this study applies two unsupervised dimensionality reduction methods—principal component analysis (PCA) and t-distributed stochastic neighbor embedding (t-SNE)—on a compiled data set consisting of 876 laser ablation-inductively coupled plasma-mass spectrometry analyses of primary high-Ti magnetite from high-temperature ore-forming systems worldwide. Three models are built with different element combinations to identify magnetite of different origins. The models were further evaluated by the support vectors machine (SVM) and receiver operating characteristic (ROC) curves and proved to be able to describe the characteristics of trace element compositions of high-Ti magnetite of different origins. Our models suggest that Mg, Mn, Al, Ti, V, and Co from 59 analyzed trace elements show promising properties as effective discriminators, and on this basis, a new discrimination diagram of lg(Al) + lg(Ti) + lg(V) versus lg(Mn)/[lg(Co) + lg(Mg)] is developed for distinguishing high-Ti magnetite of igneous and hydrothermal origin. Our results also show that the high-Ti magnetite in the IOA deposits has chemical compositions similar to those of high-temperature hydrothermal deposits, including the iron oxide copper-gold and porphyry deposits, but significantly distinct from the igneous magnetite. Our study, hence, implies a magmatic-hydrothermal origin for the magnetite in IOA deposits.

MINERALOGICAL FEATURES AND SUBSOLIDUS STRUCTURES OF IZRANDITES FROM ALEXANDROVSK COMPLEX

This article presents the results of studying the compositions of rock-forming and accessory ultramafic minerals of izrandite samples from Alexandrovsk metamorphic complex. It was found that petrographical, mineralogical and compositional features indicate igneous cumulative origin of studied rocks but amazing freshness of izrandite contradict with their “oldest age”. Together with igneous, numerous occurrences of subsolidus structures were found in studied rocks. These are following: reaction rims on the olivine-plagioclase boundaries, orthopyroxene-pargasite symplectite intergrowths on the olivine-augite and olivine-plagioclase boundaries, ilmenite and Ti-magnetite lamellae in clinopyroxene and amphibole grains, Ti-magnetite solid-solution breakdown structures. It is shown that spinel group mineral grains have complex structure including different blocks: Ti-magnetite with ilmenite exsolution lamellae, homogenous ilmenite and high-Al spinel, and these were probably formed by subsolidus processes. We have considered critically some scenarios of izrandite origin. We infer that izrandites are similar to cumulative rocks from some cratonic layered intrusions, but it is necessary to note that there is no hard evidence of that izrandite is relict of the oldest protolith of metamorphic rocks.

Magnetite geochemistry and iron isotope signature of disseminated and massive mineralization in the Kalatongke magmatic Cu[sbnd]Ni sulfide deposit, northwest China

Magnetite is a common accessory mineral and an indicator to magma evolution and sulfide fractional crystallization in magmatic NiCu sulfide deposits. The Kalatongke magmatic sulfide deposit at the southern margin of the Central Asian Orogenic Belt is composed of 11 mafic intrusions. The sulfide orebodies (Y1, Y1 deep, and Y2) are hosted by three small mafic intrusions, and magnetite is common in all mineralized samples. Based on ilmenite exsolution texture, composition, and type of mineralization, three types of magnetite are identified, (1) primary Fe-rich magma-related magnetite (type 1), (2) sulfide liquid-related magnetite (type 2), and (3) hydrothermal magnetite (type 3). We speculate that Fe isotope equilibrium fractionation between early or co-crystallized light 54Fe-enriched sulfide and magnetite may account for the ultra-high magnetite δ56Fe value (0.70‰ to 1.35 ‰) in the magmatic magnetite (types 1 and 2). Enriched compatible elements in type 1 magnetite (TiO2 = 1.52 wt% to 7.89 wt%, V = 775 ppm to 2420 ppm, Cr = 2697 ppm to 14,658 ppm, and Ni contents = 478 ppm to 1256 ppm), reflect the characteristics of primary Fe-rich magmatic magnetite crystallized at relatively high temperature and low oxygen fugacity. Type 2 magnetite in massive ores has Ni (762 ppm to 1769 ppm) and δ56Fe (0.89 ‰ to 1.10 ‰) values similar to type 1 magnetite, but significantly lower TiO2, V and Cr contents (TiO2 = 0.07 to 3.72 wt%, V = 61 to 2314 ppm, Cr = 5 to 2937 ppm), indicating co-crystallization with an evolved sulfide liquid across a wide span of temperature and oxygen fugacity. Type 3 magnetite is of hydrothermal fluid origin, with low δ56Fe (0.29 ‰ to 0.41 ‰) and Ni (177 ppm to 377 ppm).Magnetite composition and Fe isotope signatures at the Kalatongke magmatic sulfide deposit can reveal the genesis of magnetite and the evolution of the melt and sulfide. The Y1 orebody was completely crystallized over a short period of time, while the Y1 deep and Y2 east orebodies underwent a protracted evolution during which magnetite co-crystallized from primitive Fe-rich melt, Fe-rich evolved melt and Cu-rich intermediate sulfide solution. Magnetite and sulfide in the Y2 west massive orebody likely formed last. The compatibility of both lithophile and chalcophile elements, combined with Fe isotopes in magnetite, makes magnetite a good indicator of silicate and sulfide melt evolution.

An investigation of Fe‐Ti, V in the north‐east Karbi Hills, Shillong Plateau, north‐east India: Implication for mineralization

The Kulamati gabbro‐anorthosite complex in the Karbi Hills of the Shillong Plateau, north‐east India is a newly discovered stratified Fe‐Ti‐V‐bearing magmatic body that has been classified as a titano‐magnetite ore. The host gabbro‐anorthosite is found intruded into the Mesoproterozoic Shillong Group. The stratified Fe‐Ti‐V oxide body consists mostly of magnetite‐rich and magnetite‐poor layers. The magnetite‐rich bands are composed of titanomagnetite, haematite, ilmenite, and coulsonite with minor Al‐spinel, while the mineral constituents of silicate bands contain plagioclase, clinopyroxene, olivine, apatite, and chlorite. Titaniferous magnetites display a wide variety of subsolvus features, including Al‐spinel–magnetite–ulvöspinel exsolutions and crystallographically oriented ilmenite exsolutions, but magnetites are exclusively vanadiferous. Comprehensive chemical analyses show that the ore is rich in TiO2 (10.98–12.78 wt%) and V2O5 (1.32–1.47 wt%). Ti is attributed by the presence of rutile, titanite, and ilmenite, while V is attributed by coulsonite as a solid solution component in the titanomagnetite. It records moderate to high Al2O3 (4.51–5.46 wt%), with enrichment of FeO and Fe2O3 (26.22–28.32 wt% and 39.34–42.86 wt%, respectively), which can be attributed to the presence of magnetite‐spinel‐ulvospinel. Higher Fe2O3 and lower FeO are indicative of oxidizing conditions of the ore‐forming environment. The rare earth element (REE) patterns have (La/Yb)N and (Ce/Yb)N ratios comparable to an evolved basaltic melt, with light REE being enriched compared to heavy REE and a positive Eu anomaly. The electron probe micro analyser data of magnetite is conformable with the bulk chemistry, and shows consistently high FeO (70.87–88.06 wt%), Cr2O3 (1.02–2.44 wt%), TiO2 (0.16–6.20 wt%), and V2O3 (1.47–2.26 wt%), with trace amounts of Al, Si, Mg, Mn, and PGE. As per targeted parameters, the ore occurrence distinctly belongs to a stratified Fe‐Ti‐V class, where alternate thick (&gt;1.0 m) and thin (&lt;20 cm) strata resulted from the late magmatic crystallization of the original tholeiitic composition.

Advances in geochronology in the Suwałki anorthosite massif and subsequent granite veins, northeastern Poland

The Mesoproterozoic Suwałki anorthosite massif (SAM) in NE Poland, intruded into a late Palaeoproterozoic domain in southeastern Fennoscandia, comprises anorthosite, Fe-Ti-V oxide ore deposits, gabbroic rocks and rapakivi-like granitoids. In the present study, a new set of age determinations on zircon grains from AMCG components (anorthosites and rapakivi granites) from different parts of the SAM massif, from granite veins cutting the AMCG rock suite, and from micro-coronas formed on Fe-Ti oxides, is presented. The new age results obtained using a SHRIMPIIe/MC change the previous interpretation (Re-Os NTIMS method on sulfides) that the SAM anorthosites were emplaced at ca 1.55 Ga. The new zircon SHRIMP ages indicate that anorthosites were intruded in several magmatic pulses culminating at ca. 1515 Ma and 1507 Ma. This is confirmed by single spot data and concordia ages of 1513 ± 6 Ma, 1509 ± 3 Ma and 1507 ± 3 Ma obtained for anorthosites. Similar single-spot peaks at 1515 Ma, 1509 Ma and 1496 Ma characterize rapakivi-type granites which also yielded concordia ages of 1513 ± 6 Ma, 1507 ± 6 Ma and 1499 ± 8 Ma. The SHRIMP data from the SAM anorthosites indicate an age of emplacement significantly younger than the ca 1.55 Ga age determined previously, and coeval to ages obtained for rapakivi granite. Both rock types contain entrained antecrysts, reflecting a two-step crystallization in a deep-seated magma chamber and continuous crystallization during the rising of the melt and incremental emplacement of the SAM. Abundant granitic aplites and pegmatites in sharp contact with the host AMCG rocks define a time frame for the entire SAM solidification. The crystallization ages of individual veins ranging between 1489 ± 6 Ma and 1475 ± 5 Ma are clearly older than cooling ages previously obtained by Ar-Ar and K-Ar methods. Microgranites with numerous inherited zircon cores were generated by re-melting of the local sources, including late Paleoproterozoic ca 1.83 Ga crust and SAM related rocks, within a shallow felsic magma chamber. Finally, zircon coronas on Fe-Ti oxides ilmenite grains record subsolidus ilmenite exsolution processes (from ilmenite) occurring simultaneously with the crystallization of the aplite and pegmatite veins.

Paleointensity and Rock Magnetism of Martian Nakhlite Meteorite Miller Range 03346: Evidence for Intense Small‐Scale Crustal Magnetization on Mars

AbstractThe martian dynamo’s strength and duration are essential for understanding Mars' habitability and deep interior dynamics. Although most northern volcanic terranes were likely emplaced after the martian dynamo ceased, recent data from the InSight mission show stronger than predicted crustal fields. Studying young volcanic martian meteorites offers a precise, complementary method to characterize the strength of the martian crustal field and examine its implications for past dynamo activity. We present the first rock and paleomagnetic study of nine mutually oriented samples from the martian Nakhlite meteorite Miller Range (MIL) 03346, which is well‐suited for paleomagnetic analysis due to its well‐known age (1,368 ± 83 Ma) and lack of significant aqueous, thermal, and shock overprinting. Rock magnetic analysis, including quantum diamond microscope imaging, showed that the natural remanent magnetization (NRM) is carried by Ti‐magnetite crystals containing µm‐scale ilmenite exsolution lamellae, which can accurately record ancient magnetic fields. Demagnetization of the NRM revealed a high coercivity magnetization interpreted to date from the age of eruption based on its intensity, unidirectionality, and a passing fusion crust baked contact test. Paleointensities of four samples reveal a 5.1 ± 1.5 µT paleofield, representing the most reliable martian paleointensity estimates to‐date and stronger than the 2 µT surface fields measured by InSight. Modeling shows that the observed fields can be explained by an older subsurface magnetized layer without a late, active dynamo and support a deeply buried, highly magnetized crust in the northern hemisphere of Mars. These results provide corroborating evidence for strong, small‐scale crustal fields on Mars.

Lamellar magnetism and exchange bias in billion-year-old metamorphic titanohematite with nanoscale ilmenite exsolution lamellae – III. Atomic-magnetic basis for experimental results

SUMMARY Lamellar magnetism is a source of remanent magnetization in natural rocks different from common bulk magnetic moments in ferrimagnetic minerals. It has been found to be a source for a wide class of magnetic anomalies with extremely high Koenigsberger ratio. Its physical origin are uncompensated interface moments in contact layers of nanoscale ilmenite lamellae inside an hematite host, which also generate unusual low-temperature (low-T) magnetic properties, such as shifted low-T hysteresis loops due to exchange bias. The atomic-magnetic basis for the exchange bias discovered in the hematite-ilmenite system is explored in a series of papers. In this third article of the series, simple models are developed for lamellae interactions of different structures when samples are either cooled in zero-field, or field-cooled in 5 T to temperatures below the ordering temperature of ilmenite. These models are built on the low-temperature measurements described earlier in Paper II. The important observations include: (i) the effects of lamellar shapes on magnetic coupling, (ii) the high-T acquisition of lamellar magnetism and low-T acquisition of magnetization of ilmenite lamellae, (iii) the intensity of lamellar magnetism and the consequent ilmenite magnetism in populations of randomly oriented crystals, (iv) lattice-preferred orientation of the titanohematite host crystal populations and (v) the effects of magnetic domain walls in the host on hysteresis properties. Based on exemplary growth models of lamellae with different geometries and surface couplings we here provide simple models to assess and explain the different observations listed above. Already the simplified models show that the shapes of the edges of ilmenite lamellae against their hematite hosts can control the degree of low-T coupling between ilmenite, and the lamellar magnetic moments. The models also explain certain features of the low-T exchange bias in the natural samples and emphasize the role of lattice-preferred orientation upon the intensity of remanent magnetization. The inverse link between ilmenite remanence and exchange-bias shift in bimodal low-T ilmenite lamellae is related to different densities of hematite domain walls induced by the clusters of ilmenite lamellae.

ID-TIMS zircon U–Pb geochronology of the Camel Donga eucrite

Eucrite meteorites are considered to originate from the upper crust of asteroid 4-Vesta. These differentiated basaltic meteorites commonly contain accessory zircons with grain sizes smaller than 20–30 μm in diameter. U–Pb zircon dating using secondary ion mass spectrometry (SIMS) revealed that initial magmatism and global thermal metamorphism occurred in the Vestan crust ~4550–4555 million year ago (Ma). To improve the temporal resolution in order to distinguish zircons grown during different thermal events in the early stage of crustal evolution of Vesta, we have applied a chemical abrasion-based method for extracting small zircon crystals from bulk meteorite samples and obtained high-precision isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U–Pb dates for zircons from the Camel Donga eucrite. Some of the Camel Donga zircons occur in contact with ilmenite, implying that they have formed by exsolution of ilmenite followed by reaction with silica during high temperature metamorphism. Raman measurements indicate that the zircons are moderately radiation damaged and that high temperature annealing (900 °C for 48 h) is likely to improve their crystallinities. Zircons separated from acid residues after dissolving other minerals in a mixture of hydrofluoric and nitric acid experienced U–Pb and Pb isotope fractionation during the acid treatment. High temperature annealing prior to acid extraction efficiently suppresses Pb isotope fractionation resulting in accurate and precise 207Pb–206Pb dates. The weighted mean 207Pb–206Pb date of the annealed zircons is 4552.56 ± 0.69 Ma (2σ), which is significantly older than that of Camel Donga zircons previously obtained by SIMS (4531 ± 10 Ma) but is consistent with 207Pb–206Pb dates of zircons in the other eucrites. Thus, the Camel Donga eucrite zircons formed during initial magmatism or global thermal metamorphism in the early stage of Vestan crustal formation.

A Continuum from Iron Oxide Copper-Gold to Iron Oxide-Apatite Deposits: Evidence from Fe and O Stable Isotopes and Trace Element Chemistry of Magnetite

AbstractIron oxide copper-gold (IOCG) and iron oxide-apatite (IOA) deposits are major sources of Fe, Cu, and Au. Magnetite is the modally dominant and commodity mineral in IOA deposits, whereas magnetite and hematite are predominant in IOCG deposits, with copper sulfides being the primary commodity minerals. It is generally accepted that IOCG deposits formed by hydrothermal processes, but there is a lack of consensus for the source of the ore fluid(s). There are multiple competing hypotheses for the formation of IOA deposits, with models that range from purely magmatic to purely hydrothermal. In the Chilean iron belt, the spatial and temporal association of IOCG and IOA deposits has led to the hypothesis that IOA and IOCG deposits are genetically connected, where S-Cu-Au–poor magnetite-dominated IOA deposits represent the stratigraphically deeper levels of S-Cu-Au–rich magnetite- and hematite-dominated IOCG deposits. Here we report minor element and Fe and O stable isotope abundances for magnetite and H stable isotope abundances for actinolite from the Candelaria IOCG deposit and Quince IOA prospect in the Chilean iron belt. Backscattered electron imaging reveals textures of igneous and magmatic-hydrothermal affinities and the exsolution of Mn-rich ilmenite from magnetite in Quince and deep levels of Candelaria (&amp;gt;500 m below the bottom of the open pit). Trace element concentrations in magnetite systematically increase with depth in both deposits and decrease from core to rim within magnetite grains in shallow samples from Candelaria. These results are consistent with a cooling trend for magnetite growth from deep to shallow levels in both systems. Iron isotope compositions of magnetite range from δ56Fe values of 0.11 ± 0.07 to 0.16 ± 0.05‰ for Quince and between 0.16 ± 0.03 and 0.42 ± 0.04‰ for Candelaria. Oxygen isotope compositions of magnetite range from δ18O values of 2.65 ± 0.07 to 3.33 ± 0.07‰ for Quince and between 1.16 ± 0.07 and 7.80 ± 0.07‰ for Candelaria. For cogenetic actinolite, δD values range from –41.7 ± 2.10 to –39.0 ± 2.10‰ for Quince and from –93.9 ± 2.10 to –54.0 ± 2.10‰ for Candelaria, and δ18O values range between 5.89 ± 0.23 and 6.02 ± 0.23‰ for Quince and between 7.50 ± 0.23 and 7.69 ± 0.23‰ for Candelaria. The paired Fe and O isotope compositions of magnetite and the H isotope signature of actinolite fingerprint a magmatic source reservoir for ore fluids at Candelaria and Quince. Temperature estimates from O isotope thermometry and Fe# of actinolite (Fe# = [molar Fe]/([molar Fe] + [molar Mg])) are consistent with high-temperature mineralization (600°–860°C). The reintegrated composition of primary Ti-rich magnetite is consistent with igneous magnetite and supports magmatic conditions for the formation of magnetite in the Quince prospect and the deep portion of the Candelaria deposit. The trace element variations and zonation in magnetite from shallower levels of Candelaria are consistent with magnetite growth from a cooling magmatic-hydrothermal fluid. The combined chemical and textural data are consistent with a combined igneous and magmatic-hydrothermal origin for Quince and Candelaria, where the deeper portion of Candelaria corresponds to a transitional phase between the shallower IOCG deposit and a deeper IOA system analogous to the Quince IOA prospect, providing evidence for a continuum between both deposit types.

Metamorphic Response to Alpine Thrusting of a Crustal-scale Basement Nappe in Southern Calabria (Italy)

Abstract Structural, geophysical and age data indicate that the tilted cross-section of the Variscan continental crust exposed in the Serre of southern Calabria forms the uppermost Alpine nappe (‘Serre nappe’) of three Variscan basement slices derived from the southern European margin. This Alpine nappe stack is a fragment of the western Mediterranean Alps and rests now, after Miocene emplacement, on top of the Apennine carbonate platform. We report for the first time a P–T path for prograde Alpine metamorphism, which is restricted to the two lower nappes (Castagna and Bagni nappes) that are squeezed between cooler tectonic units, the Serre nappe above and the Apennine platform below. Therefore, we attribute their metamorphism to tectonic loading and concomitant shear heating during Eocene south-directed overthrusting of the crustal-scale Serre nappe. In the underlying Castagna nappe, Alpine metamorphism is only locally recorded, mainly by new growth of garnet, forming at the expense of retrogressed Variscan biotite dated at 43 Ma. The local existence of Alpine besides relict Variscan mineral assemblages in the strongly but heterogeneously overprinted rocks allows for characterization of metamorphic evolutions during both the Alpine and Variscan orogeneses in the former intermediate level of the Variscan crust of Calabria. The metamorphic evolutions have been reconstructed through P–T pseudosection modelling for Al-rich metasediments. In the Castagna nappe, rarely preserved Variscan garnet–sillimanite–biotite–ilmenite–plagioclase–quartz (± K-feldspar ± Si-poor white mica) assemblages formed under amphibolite-facies subsolidus conditions (650 ± 60 °C, 4·0 ± 0·5 kbar). During subsequent decompression and cooling to greenschist-facies conditions garnet was replaced by biotite–sillimanite and later by white mica–chlorite intergrowths. Retrogression of Variscan biotite is evidenced by the exsolution of ilmenite along grain boundaries and cleavages, textures that were subsequently overgrown by Alpine garnet coexisting with Si-rich white mica, rare chloritoid (in metapelites), and hornblende (in metagreywackes). Alpine garnet shows prograde zoning and is Ca-rich, and thus is distinct from unzoned and Ca-poor Variscan garnet porphyroblasts. Estimated conditions (520 ± 40 °C, 8·0 ± 1·0 kbar) record elevated pressures during Alpine lower amphibolite-facies metamorphism. In the lowermost Bagni nappe, rare prograde-zoned, Ca-rich garnet in strongly retrogressed mylonitic quartz-phyllites allows isopleth thermobarometry, which indicates lower amphibolite-facies conditions (555 ± 10 °C, 7·4 ± 0·3 kbar) resembling those for Alpine garnet growth in the Castagna nappe. The similar clockwise P–T paths for prograde Alpine metamorphism and the consistent peak pressures of 7–9 kbar in the Castagna and Bagni nappes point to a joint short-lived metamorphism during overthrusting of the crustal-scale Serre nappe within the south European margin during the north-directed subduction of the Alpine Tethys. South-directed overthrusting of the now tilted Variscan crustal section of the Serre along the up to 500 m thick mylonite horizon of the Curinga–Girifalco Line is in agreement with seismic data indicating an extended, few kilometres thick low-velocity zone (Bagni and Castagna nappes and mylonites of the Curinga–Girifalco Line) below the exposed lower crustal section of the Serre nappe. Alpine tectonic transport direction, timing and metamorphic conditions described here are consistent with those reported from the Aspromonte area in southernmost Calabria, suggesting a coeval Alpine history characterized by metamorphism owing to nappe loading and concomitant shear heating. The Alpine subduction–erosion–accretion processes inferred here for the Calabrian basement nappes resemble those proposed for the Dent Blanche nappe system in the Western Alps.

Petrography, microthermometry, and isotopy of the gold veins from Vetas, Santander (Colombia)

The gold mineralization located in Vetas, Santander, consists of auriferous quartz veins hosted in Bucaramanga gneiss rocks, intrusive Jurassic rocks, and intrusive to porphyritic Miocene rocks. This study identified four mineralizing events: (1). Sericite, carbonate (ankerite and calcite?), massive and microcrystalline quartz, sphalerite, adularia, albite, galena, thin pyrite, pyrrhotite, chalcopyrite. The age for this stage is 10.78 ±0.23Ma (Ar/Ar on sericite). (2). Molybdenite, magnetite with exsolution of ilmenite, As-pyrite, sphalerite, fine-grained pyrite and little chalcopyrite quartz with huge, feathery, fine mosaic, flamboyant and microcrystalline textures and, tourmaline and sericite. (3). Gold and tennantite associated with sphalerite, fine- and coarse-grained pyrite, As-pyrite, chalcopyrite like inclusions, and quartz with flamboyant, mosaic, massive and “comb” textures, and tourmaline. Stage 2 and 3 happened from 7.58 ±0.15 Ma to 6,89±0,41Ma (Ar/Ar on sericite). (4). Thick, thin, and pyrite with arsenic, hematite and microcrystalline quartz (forming breccia texture), and sericite. The age for this stage is 5.24 ±0.10 (Ar/Ar on sericite). Post-mineral: quartz comb, alunite, halloysite, kaolinite, and ferrum hydroxides. The stable isotopes, ∂18O, ∂D, and ∂34S and fluid inclusions analysis infer that fluids were producing a mixture of meteoric and magmatic fluids with low salinity and minimum trapping temperatures between 200°C to 390°C. The mineralogy association, and fluid inclusions, in the first event show characteristic of low sulfidation epithermal. The second stage was hottest and with more magmatic signature over printed an intermediate sulfidation system; show a little more salinity on the fluids and more mineralogical diversity, the third and four events, could show an evolution of this fluid, where it was cooling and impoverishing on metals. Two initials stages are contemporaneous with two magmatic Miocene pulses on the area: the first one of granodiorite composition 10, 9± 0.2 Ma (U/Pb zircon), and the other one rhyodacite with 8.4 ±0.2 y 9.0 ± 0.2 Ma.

Textural and chemical variations of magnetite from porphyry Cu-Au and Cu skarn deposits in the Zhongdian region, northwestern Yunnan, SW China

The Zhongdian region in northwestern Yunnan (SW China) hosts a number of important porphyry and skarn deposits. In this study, we integrate petrographic observations, SEM-EDS elemental mapping, EPMA and LA-ICP-MS geochemical analyses on magnetite from several key deposits in the region. Magnetite grains from the Pulang porphyry Cu-Au, Lannitang porphyry Au-Cu, Langdu Cu skarn, Disuga porphyry, and andesite of Tumugou Formation were analyzed to unravel the magnetite formation mechanism, and the relative oxygen fugacity of the porphyry/skarn ore-related hydrothermal fluids in this region.Two types of magnetites (Magnetite I and II) have been identified. Magnetite I is commonly distributed in the chlorite and/or sericite alteration zone(s). Magnetite I grains are characterized by being textually inhomogeneous. The Magnetite I core (with local ilmenite exsolution lamellae) is Ti-Cr rich, indicative of a dominantly igneous origin. The Magnetite I core and its surrounding primary igneous minerals (biotite, amphibole and plagioclase) underwent late alteration. Magnetite I grains are characterized by core-rim texture, sharp core-rim contact, and irregular grain boundaries. These textural features indicate that Magnetite I may have undergone dissolution-reprecipitation processes, during which the Magnetite I core was chemically purified via losing Ti and gaining Fe. Magnetite II (in potassic and sericite alteration zones) is textually homogeneous with relatively low contents of Ti, Mg, Al, V, Ni, and Mn. We suggest that Magnetite II was formed by hydrothermal alteration. Compared to Magnetite II grains from the Pulang and Langdu deposits, those from the Lannitang deposit have lower Mn and V but higher Sn contents, indicative of higher oxygen fugacity of the hydrothermal fluids. We propose that binary plots such as Ti vs. Sc, V vs. Sn and Ga vs. Sn can distinguish the different origins of magnetites.From an exploration perspective, Magnetite II from the altered Disuga porphyries has similar texture and composition to its counterpart from the potassic-altered Pulang porphyries. This, together with the presence of Cu-sulfide inclusions in the Disuga Magnetite I, suggests significant mineralization potential in these porphyries.

Episodic mafic magmatism in the Eyre Peninsula: Defining syn- and post-depositional BIF environments for iron deposits in the Middleback Ranges, South Australia

Banded iron formation (BIF)-derived iron ore deposits of the Middleback Ranges, Eyre Peninsula, South Australia, are hosted within Archean to Paleoproterozoic basement in the southeastern Gawler Craton. Mafic sills and dikes are associated with the orebodies throughout the belt and although described previously, have never been studied in detail. Two main types of mafic rocks, amphibolites and dolerites, are distinguishable from field occurrence and mineralogy. Amphibolites interbedded with BIFs from the southern Iron Magnet deposit are characterized by extensive overprinting. These amphibolites contain metamorphic assemblages of coexisting Mg-Fe- and Ca-amphiboles, garnet, corundum, biotite and relict titaniferous magnetite displaying ilmenite exsolution. Amphibolite petrography and geochemistry in the southern part of the belt indicate formation in a complex BIF depositional environment, with possible detrital input of titaniferous magnetite from igneous rocks. In contrast, NW-trending dolerite dikes occur throughout the belt and contain assemblages consisting of igneous magnesiohornblende and plagioclase. SHRIMP U–Pb zircon ages for amphibolites and dolerites yield two age groups. Amphibolite from Iron Magnet yields a 207Pb/206Pb weighted mean age of 2542 ± 15 Ma. The most robust 206Pb/238U weighted mean age of 782 ± 23 Ma for dolerites comes from the Iron Queen deposit in the central part of the Middleback Ranges. LA-ICP-MS U–Pb geochronology of U-bearing hematite from the Iron Knight South deposit yields ages of ~680 Ma. Taken together, these results suggest mafic magmatism contemporaneous with Archean BIF deposition followed by a high-grade metamorphic event (~2.47–2.41 Ga Sleafordian Orogeny). Dikes from Cape Donington, the southernmost point on the Eyre Peninsula are, instead, associated with the 1.73–1.69 Ga Kimban Orogeny, as evidenced by zircon from a pyroxene-rich dolerite, which yields a 206Pb/238U weighted mean age of 1711 ± 14 Ma. The dolerite ages from the Iron Queen deposit provide the first evidence for the presence of the ~800 Ma craton-scale Gairdner Large Igneous Province in the Middleback Ranges. The young U–Pb hematite age from Iron Knight South and post-emplacement alteration of dikes of Gairdner age from the central part of the Middleback Ranges belt provide additional support for multiple events leading to upgrading of iron ores.

Atom Probe Tomography Analysis of Exsolved Mineral Phases.

Element diffusion rates and temperature/pressure control a range of fundamental volcanic and metamorphic processes. Such processes are often recorded in lamellae exsolved from host mineral phases. Thus, the analysis of the orientation, size, morphology, composition and spacing of exsolution lamellae is an area of active research in the geosciences. The conventional study of these lamellae has been conducted by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and more recently with focused ion beam (FIB)-based nanotomography, yet with limited chemical information. Here, we explore the use of atom probe tomography (APT) for the nanoscale analysis of ilmenite exsolution lamellae in igneous titanomagnetite from ash deposits erupted from the active Soufrière Hills Volcano (Montserrat, British West Indies). APT allows the precise calculation of interlamellar spacings (14-29 ± 2 nm) and reveals smooth diffusion profiles with no sharp phase boundaries during the exchange of Fe and Ti/O between the exsolved lamellae and the host crystal. Our results suggest that this novel approach permits nanoscale measurements of lamellae composition and interlamellar spacing that may provide a means to estimate the lava dome temperatures necessary to model extrusion rates and lava dome failure, both of which play a key role in volcanic hazard mitigation efforts.

Geology, petrography and mineral chemistry of iron oxide-apatite occurrences (IOA type), western sector of the neoproterozoic Santa Quiteria magmatic arc, Ceará northeast, Brazil

Iron oxide-apatite (IOA) occurrences have recently been identified in the western sector of the Neoproterozoic Santa Quitéria magmatic arc (Ceará state, northeast Brazil), close to the boundary with the Paleozoic Parnaíba Basin. The IOA mineralization is hosted by albitized metadiorites and metavolcano-sedimentary rocks, which are crosscut by a late- to post-Brasiliano/Pan-African biotite granite intrusion with a U-Pb age approximately 548 ± 4 Ma. Within the metavolcano-sedimentary sequence, metavolcanic rocks are bimodal and consist of albitized metabasalt-andesite and metarhyolites with a U-Pb age of 554 ± 6 Ma, whereas the metasedimentary units are represented by calc-silicate, marble, and pelitic gneisses. The iron oxide-apatite mineralization occurs as (i) banded–stratabound lenses/layers composed of magnetite (50–70%), with or without ilmenite exsolution; apatite (≤8%); monazite; and locally subordinate copper sulfides (~2%); (ii) massive magnetite-(specular hematite)-apatite bodies; (iii) disseminated and vein magnetite-apatite within albitized metadiorite; (iv) massive rhombohedral hematite bodies; and (v) garnet-magnetite type. The host rocks also display marialite, diopside, albite, and epidote (sodic-calcic alteration), biotite and K feldspar (potassic alteration), chlorite and epidote in lower-T hydrothermal alteration assemblages. Mineral chemistry data reveal that (i) except for vein types, apatite in all the other occurrences is F rich (2.4 ≤ F ≤ 4.4%) and low in Cl (<0.5%), and (ii) magnetite contains variable concentrations of Ti, V, Cr and Ni attributed to both igneous and hydrothermal environments. In addition, the magnetite of the stratabound occurrence is low in TiO2 (≤1.6%); however, its oxy-exsolutions are rich in TiO2 (14–52%), indicating a titanium-rich original iron oxide. The low sulfide content and high concentration of apatite indicate that the investigated Fe-P occurrences are of the IOA type, similar to those of IOA provinces elsewhere, such as Kiruna, El Laco, Gushan, Pea Ridge, Pilot Knob, and Bafq. Hence, the discovery of these occurrences in the Neoproterozoic Santa Quitéria magmatic arc has a twofold metallogenic significance: (i) these are the first records of IOA-type deposits in Brazil; and (ii) open a favorability potential for the exploration of IOCG-type deposits in this tectonic domain.

Formation of chromitites and ferrogabbros in ultramafic and mafic members of the Variscan Ślęża ophiolite (SW Poland)

Ultramafic and mafic rocks of the Variscan Ślęża ophiolite (Bohemian Massif, Sudetes, SW Poland) contain abundant Cr and Fe-Ti mineralizations. The occurrence of chromitites is related with serpentinites of refractory harzburgitic protoliths. Chromite I from chromitites has variable Cr and Mg numbers (Cr# = 0.41–0.58, Mg# = 0.62–0.83) and low TiO2 contents (<0.20 wt%). Chromite II surrounds chromite I or fills fissures in it and is enriched in Cr and depleted in Al relative to chromite I (Cr# = 0.54–0.68). Chromite I grains contain spherical grains of diopside, phlogopite, pargasite, pyrite and chalcopyrite, occurring as mono- or polyphase solid inclusions. Metagabbros with isotropic fabric contain locally up to 14.6 vol% of magnetite and ilmenite. Magnetite has variable TiO2, Al2O3 and V2O3 contents (0.19–9.82 wt%, 0.13–1.67 wt% and 1.08–1.61 wt%, respectively). It forms xenomorphic grains with trellis or sandwich exsolutions of ilmenite, as well as aggregates with individual ilmenite grains.The moderate Cr# of chromite I points to its origin from a MORB-type melt. The occurrence of hydrous inclusions and nodular textures indicates chromitite formation and alteration under low pressure conditions. Primary sulfide, phlogopite and pargasite inclusions suggest that the parental melt was moderately hydrous and sulphur-rich. Water in the magmatic system originated supposedly from interaction of primary MORB melt with hydrothermal fluids and a significant influence of fluids on chromite composition is expressed by its very low Ti content.Metagabbros containing abundant Fe-Ti oxide mineralization may be classified as ferrogabbros. High V and Ti contents in magnetite indicate a magmatic origin. Its xenomorphic and intragrain mode of occurrence suggest crystallization between pre-existing gabbroic minerals like pyroxene and plagioclase. Ilmenite exsolutions indicate its growth at the expense of magnetite under oxidizing conditions. Occurrence of chromitites in harzburgites and ferrogabbros related to isotropic gabbros may suggest various spreading conditions during formation of the mafic and ultramafic members of the Ślęża ophiolite.