Iron Oxide-apatite Deposits Research Articles

Mineralogy, Chemistry, and Fluid-Aided Evolution of the Pea Ridge Fe Oxide-(Y + REE) Deposit, Southeast Missouri, USA

The Kiruna-type Pea Ridge iron oxide-apatite (IOA) deposit is hosted by a sequence of 1.47 Ga rhyolite tuffs of the St. Francois Mountains, southeast Missouri, USA. It consists of a series of altered zones composed mainly of amphibole, magnetite, hematite, and quartz, together with the presence of several rare earth element (Y + REE)-rich breccia pipes. In many cases, the fluorapatite within these zones is rich in inclusions of monazite, iron oxide, and quartz inclusions, plus minor xenotime. Monazite and minor xenotime are also found intergrown as inclusions in the fluorapatite, as well as in surrounding recrystallized magnetite and hematite in the magnetite ore. Monazite and xenotime typically occur as inclusions within both oxides. Monazite-(Ce) and xenotime-(Y) are both relatively poor (<2 wt %) in ThO2 and UO2. No significant compositional differences exist in the (Y + REE) chemistry between monazite and xenotime inclusions in fluorapatite compared to grains intergrown with magnetite and hematite, suggesting that these two REE-rich minerals are cogenetic. Monazite-xenotime geothermometry and geochronology of monazite inclusions in fluorapatite provide evidence that formation/remobilization of the (Y + REE) phosphates took place at ca. 50° to 400°C, approximately 5 to 10 m.y. after emplacement of the main iron oxide-phosphate orebody. Evidence from field relationships and fluid inclusion chemistry, together with the massive recrystallization and remobilization of fluorapatite, monazite, xenotime, and iron oxides at Pea Ridge, suggest a subvolcanic origin coupled with a strong metasomatic reworking of the IOA deposit.

Iron and Oxygen Isotope Signatures of the Pea Ridge and Pilot Knob Magnetite-Apatite Deposits, Southeast Missouri, USA

New O and Fe stable isotope ratios are reported for magnetite samples from high-grade massive magnetite of the Mesoproterozoic Pea Ridge and Pilot Knob magnetite-apatite ore deposits and these results are compared with data for other iron oxide-apatite deposits to shed light on the origin of the southeast Missouri deposits. The δ 18 O values of magnetite from Pea Ridge ( n = 12) and Pilot Knob ( n = 3) range from 1.0 to 7.0 and 3.3 to 6.7‰, respectively. The δ 56 Fe values of magnetite from Pea Ridge ( n = 10) and Pilot Knob ( n = 6) are 0.03 to 0.35 and 0.06 to 0.27‰, respectively. These δ 18 O and the δ 56 Fe values suggest that magnetite crystallized from a silicate melt (typical igneous δ 56 Fe ranges 0.06–0.49‰) and grew in equilibrium with a magmatic-hydrothermal aqueous fluid. We propose that the δ 18 O and δ 56 Fe data for the Pea Ridge and Pilot Knob magnetite-apatite deposits are consistent with the flotation model recently proposed by Knipping et al. (2015a), which invokes flotation of a magmatic magnetite-fluid suspension and offers a plausible explanation for the igneous (i.e., up to ~15.9 wt % TiO 2 in magnetite) and hydrothermal features of the deposits.

Alteration Facies Linkages Among Iron Oxide Copper-Gold, Iron Oxide-Apatite, and Affiliated Deposits in the Great Bear Magmatic Zone, Northwest Territories, Canada

High-temperature metasomatism driven by ascent of voluminous, saline fluid columns in the upper crust plays a major role in the genesis of iron oxide-alkali alteration ore systems but fundamental questions remain on genetic linkages among iron oxide copper-gold (IOCG), iron oxide-apatite (IOA), albitite-hosted uranium, and skarn deposits that they produce. Excellent surface exposures of such systems in the Great Bear magmatic zone of northernwestern Canada record the depth to paleosurface, prograde evolution of iron oxide-alkali alteration facies, and mineralization. Across the belt, albitite corridors that are tens of kilometers in length record the earliest reactions between highly saline fluids and host rocks along fault zones and subvolcanic intrusions. Pervasive albitization partitioned metals from the host rocks into the ascending fluid column, leaving behind structurally weakened corridors of porous albitite. These corridors were cut, replaced, and overprinted by amphibole- and magnetite-bearing, calcic-iron alteration assemblages. In extreme cases, the discharge of calcium, iron, and specialized metals formed iron oxide-apatite deposits (±vanadium ± rare earth elements) while recharging the outgoing fluids in sodium, potassium, and base and precious metals. As temperatures declined and fluid chemistry evolved through fluid-rock reactions, the formation of potassic-iron alteration assemblages, breccias, and sulfides resulted in magnetite- and hematite-group IOCG mineralization. Within carbonate units, skarns formed prior to, are replaced by, and evolved to calcic-iron alteration facies. Skarns can locally host base metal mineralization. Tectonically uplifted albitite breccias replaced by potassic-iron alteration assemblages became a preferential host for uranium mineralization. The results of this study also illustrate that permutations and cyclical build-up of alteration products can arise from a combination of faulting, differential uplift, and renewed magmatism. Framed within an alteration-facies deposit model, alteration zones and mineral occurrences play a pivotal role in predicting the mineral potential of iron oxide and alkali-altered systems at district to deposit scales.

Mining and Exploration History of the Southeast Missouri Iron Metallogenic Province

Mining has been a driving force in the development of Missouri for hundreds of years—a history of feast and famine, success and ruin. In southeast Missouri, the mining history spans times from early crude furnaces and small mines to large modern operations working at depths of more than 730 m below the surface. Eight major iron orebodies and myriad minor deposits are known in the southeast Missouri iron metallogenic province, which is hosted in Mesoproterozoic igneous rocks of the St. Francois Mountains terrane. The first iron furnace west of the Mississippi River was opened in Missouri in the early part of the 19 th century, marking the beginning of a prosperous industry that shaped the fortunes of citizens and the state. The industry persevered through the United States Civil War, including the dramatic battle in 1864 that caused extensive damage to the Pilot Knob ironworks. Earlier discovery in 1844 of banded iron formation ores in Michigan and depletion of easily accessible surface iron deposits in southeast Missouri led to eventual downturn in the local mining industry. Magnetic surveys in the 1950s resulted in discovery of several large subsurface deposits, including the Pea Ridge iron oxide-apatite deposit and the Boss iron oxide-copper-gold deposit ca. 52 km to the south. Iron production in Missouri ceased in 2001, when the Pea Ridge mine closed. Great potential nevertheless exists for renewed production of iron and other metals in the province.

In situ elemental and isotopic analysis of fluorapatite from the Taocun magnetite-apatite deposit, Eastern China: Constraints on fluid metasomatism

Metasomatic alteration of fluorapatite has been reported in several iron-oxide apatite (IOA) deposits, but its effect on elemental and isotopic variations has not been well understood. In this study, we present integrated elemental, U-Pb, Sr, and O isotopic microanalytical data on fresh and altered domains in fluorapatite from the Taocun IOA deposit, Eastern China, to evaluate the timing and nature of the metasomatism and its effects on the ore-forming event. Orebodies of the Taocun deposit are spatially associated with a subvolcanic, intermediate intrusion, which displays zonal alteration patterns with albite in the center and increasing actinolite, chlorite, epidote, and carbonate toward the margin. Both disseminated and vein-type ores are present in the Taocun deposit, and fluorapatite commonly occurs with magnetite and actinolite in most ores. Fluorapatite grains from the both types of ores have been variably metasomatized through a coupled dissolution-reprecipitation mechanism. Many trace elements, including Na, Cl, S, Si, Mg, Sr, U, Th, and (REEs+Y), were variably leached from the fluorapatite grains during this process and the Sr and O isotopic signatures of the grains were also modified. The altered fluorapatite grains/domains have in situ 87 Sr/ 86 Sr ratios (0.70829–0.70971) slightly higher than those of the fresh fluorapatite (0.70777–0.70868), and δ 18 O values (−3.0 to +3.4‰) variably lower than the primary domains (+5.3 to +7.5‰). The Sr and O isotopes of the primary fluorapatite are consistent with or slightly higher than those of the ore-hosting intrusion, implying that the early-stage, ore-forming fluids were magmatic in origin but underwent weak interaction with the country rocks. U-Pb dating of the fresh and altered domains of the fluorapatite yielded indistinguishable ages of ~131 Ma, which are the same as the age of the ore-hosting intrusion. In combination with fluid inclusion data, we propose that the metasomatism of fluorapatite was induced by hydrothermal fluids at a late stage of the ore-forming event. The shifts to higher 87 Sr/ 86 Sr ratios and lower δ 18 O values in the altered fluorapatite indicate that the alteration was induced by fluids with more radioactive Sr and lighter O isotope signatures. The metasomatic fluids were likely dominated by meteoric waters that were mixed with the earlier magmatic fluids and interacted with sedimentary rocks. Our study highlights that elemental and isotopic compositions of fluorapatite can be significantly modified by hydrothermal fluids during ore-forming events. Thus, instead of traditional bulk-rock analysis, in situ microanalysis is important to provide accurate constraints on the magmatic and/or hydrothermal evolution of complex ore-forming systems.

New Constraints on the Timing of Host-Rock Emplacement, Hydrothermal Alteration, and Iron Oxide-Apatite Mineralization in the Kiruna District, Norrbotten, Sweden

High spatial resolution zircon U-Pb geochronological data obtained directly on the Kiirunavaara iron oxide-apatite (IOA) deposit, related orebodies, and host rocks provide new constraints on the timing of mineralization in these deposits. These data raise new arguments in the debate of a magmatic versus hydrothermal/metasomatic genesis of these major (2,500 Mt, 30–70 wt % Fe) Paleoproterozoic deposits. The main orebody at Kiirunavaara contains Ti-poor magnetite and minor (0.05–5 wt % P) apatite, located between a trachyandesite footwall and a rhyodacite hanging wall, which also hosts smaller orebodies (Nukutus, Rektorn, and Tuolluvaara). The pervasive Na and K metasomatism in the host rock is documented by whole-rock geochemical data and cathodoluminescence (CL) microscopy. Zircon U-Pb data for the metavolcanic rocks in the footwall and hanging wall cluster between 1884 ± 4 and 1880 ± 3 Ma. In the footwall, a syenite-aplite system yields ages of 1880 ± 7 and 1881 ± 4 Ma; a granite pluton exposed underground has an age of 1874 ± 4 Ma. Zircons in two ore samples, never directly dated before this study, yield ages of 1877 ± 4 and 1874 ± 7 Ma. Brecciation at the contacts between the ore and host rocks, the tight age at ca. 1880 Ma for most volcanic and plutonic rocks in the footwall and hanging wall, and the marginally younger age for ore at ca. 1877 to 74 Ma, matching the age of the spatially related granite pluton, suggest a magmatic-hydrothermal emplacement model for the Kiruna area IOA ores.

Metal source and tectonic setting of iron oxide-copper-gold (IOCG) deposits: Evidence from an in situ Nd isotope study of titanite from Norrbotten, Sweden

Titanite occurs as a widespread accessory phase in mineralised zones and alteration associated with iron oxide-copper gold (IOCG) and iron oxide-apatite (IOA) deposits of Norrbotten County, Sweden, and is a major host of the REE in these deposits. In situ analyses of SmNd isotope ratios in titanites previously analysed for UPb geochronology and trace element composition confirms previous interpretations of grain scale isotopic heterogeneity. Initial Nd-isotope ratios expressed relative to CHUR range from ~−3 to −8 in IOA deposits, from ~−1 to −9 in IOCG deposits, and from +2 to −4 in the most Cu-rich, deformed IOCG deposits of the Nautanen Deformation zone. Within individual IOA deposits εNd varies relative to CHUR: from −3.1 to −4.0 at Valkommen (Malmberget), from −1.4 to −5.7 in grain cores, and −7.2 to −8.2 in grain rims rim at Gruvberget; and from −3.0 to −6.0 in grain cores and from −5.8 to −7.1 in grain rims at Luossavaara. In IOCG deposits at Rakkurijärvi εNd varies from −6.1. to −7.1, and in deformed IOCG deposits at Nautanen from −1.3 to −2.3. These values are consistent with the derivation of the REE, and potentially economically enriched metals, from the local volcanic sequence, either via granitic melts, or directly by leaching by metasomatic fluids. The most Cu-rich deposits reflect the involvement of more basic protoliths. The age distribution of these deposits suggest IOA deposit formation during the collisional phase of the Svecofennian orogeny (~1.9–1.8Ga), and IOCG mineralisation during this phase and during post-orogenic collapse (~1.8–1.7Ga), whilst model ages indicate the ultimate enrichment of the continental crust in these metals during pre-collisional extensional and subduction-related basic magmatism. These processes underscore the importance of continental cycles both in producing the preserved geological record of orogenic ore deposition, and in the generation of fertile continental crust, from which metals can be mobilised by subsequent events.

The use of decision tree induction and artificial neural networks for recognizing the geochemical distribution patterns of LREE in the Choghart deposit, Central Iran

Some evidences of rare earth elements (REE) concentrations are found in iron oxide-apatite (IOA) deposits which are located in Central Iranian microcontinent. There are many unsolved problems about the origin and metallogenesis of IOA deposits in this district. Although it is considered that felsic magmatism and mineralization were simultaneous in the district, interaction of multi-stage hydrothermal-magmatic processes within the Early Cambrian volcano-sedimentary sequence probably caused some epigenetic mineralizations. Secondary geological processes (e.g., multi-stage mineralization, alteration, and weathering) have affected on variations of major elements and possible redistribution of REE in IOA deposits. Hence, the geochemical behaviors and distribution patterns of REE are expected to be complicated in different zones of these deposits. The aim of this paper is recognizing LREE distribution patterns based on whole-rock chemical compositions and automatic discovery of their geochemical rules. For this purpose, the pattern recognition techniques including decision tree and neural network were applied on a high-dimensional geochemical dataset from Choghart IOA deposit. Because some data features were irrelevant or redundant in recognizing the distribution patterns of each LREE, a greedy attribute subset selection technique was employed to select the best subset of predictors used in classification tasks. The decision trees (CART algorithm) were pruned optimally to more accurately categorize independent test data than unpruned ones. The most effective classification rules were extracted from the pruned tree to describe the meaningful relationships between the predictors and different concentrations of LREE. A feed-forward artificial neural network was also applied to reliably predict the influence of various rock compositions on the spatial distribution patterns of LREE with a better performance than the decision tree induction. The findings of this study could be effectively used to visualize the LREE distribution patterns as geochemical maps.

Apatite Trace Element Compositions: A Robust New Tool for Mineral Exploration

Apatites from the major types of mainly magmatic-hydrothermal mineral deposits (30 localities, mostly in British Columbia, Canada) together with apatites from carbonatites (29 intrusive complexes) and unmineralized rocks (11 localities) have been analyzed by electron microprobe and laser ablation-inductively coupled plasma mass spectrometry. Discriminant analysis using Mg, V, Mn, Sr, Y, La, Ce, Eu, Dy, Yb, Pb, Th, and U reveals that apatites from mineral deposits can be distinguished from apatites in carbonatites and unmineralized rocks. Apatites from mineral deposits are characterized by higher Ca and correspondingly lower total contents of trace elements that partition onto the Ca sites (rare earth elements (REEs), Y, Mn, Sr, Pb, Th, and U) than apatites from unmineralized rocks and carbonatites. Apatites from the different deposit types also have distinct trace element compositions that are readily discriminated by the discriminant functions. Apatites from worldwide carbonatites have the most fractionated REE distributions with light REE enrichment (Ce/Yb CN = 35–872), high V (1.6–1,466 ppm), Sr (1,840–22,498 ppm), Ba (1.8–275 ppm), and Nb (0.4–19 ppm) contents, the lowest W contents (0.05–0.55 ppm), and no significant Eu anomalies (Eu/Eu* = 0.9–1.2). Apatites from alkalic porphyry Cu-Au deposits in the North American Cordillera possess high V contents (2.5–337 ppm), whereas apatites from calc-alkaline porphyry Cu-Au and Cu-Mo deposits have high Mn contents (334–10,934 ppm) and typically large negative Eu anomalies (Eu/Eu* = 0.2–1.1). Apatites from iron oxide Cu-Au (IOCG) and related Kiruna-type iron oxide-apatite (IOA) deposits in Canada, China, and Mexico typically have large negative Eu anomalies (Eu/Eu* = 0.2–1.5) and low Mn contents (40–5,753 ppm). Apatites from orogenic Ni-Cu, porphyryrelated Cu-Au breccia, Au-Co skarn, Pb-Zn skarn, and Cu skarn deposits have relatively low abundances of impurity cations. This study demonstrates that detrital apatite grains collected during regional geochemical surveys are effective in identifying specific types of buried mineral deposits in glaciated terranes.

TRACE ELEMENT SIGNATURE OF PYRITE FROM THE LOS COLORADOS IRON OXIDE-APATITE (IOA) DEPOSIT, CHILE: A MISSING LINK BETWEEN ANDEAN IOA AND IRON OXIDE COPPER-GOLD SYSTEMS?

Although studies have proposed that iron oxide-apatite (IOA) deposits may represent the deeper roots of some Andean iron oxide copper-gold (IOCG) systems, their genetic links remain obscure and controversial. A key question when considering an integrated genetic model is whether a magmatic-hydrothermal fluid that precipitates massive magnetite will continue transporting significant amounts of dissolved Fe, Cu, and Au after IOA precipitation. Here we provide new geochemical data for accessory pyrite from the Los Colorados IOA deposit in the Chilean iron belt that confirm the role of this sulfide as a relevant repository for economic metals in IOA deposits. Pyrite occurs at Los Colorados as disseminated grains and as veinlets associated with magnetite and actinolite that postdate the main igneous magnetite stage. Electron probe microanalysis (EPMA) data for pyrite show anomalously high Co and Ni concentrations (up ~3.9 and ~1.5 wt %, respectively) and relatively high As contents (100s of ppm to a maximum of ~2,000 ppm). When combined with results from secondary ion mass spectrometry (SIMS) spot analyses, pyrite data show significant amounts of Cu that range from sub-ppm values (~100 ppb) up to 1,000s of ppm, plus nonnegligible concentrations of Zn, Pb, Cd, Sb, Se, and Te (up to ~100 ppm). The highest contents of Cu measured (wt % level) most likely record the presence of Cu-bearing submicron-sized mineral inclusions. Contents of Au and Ag are up to ~1 and 10 ppm, respectively, with maximum concentrations that can rise up to ~800 ppm Au and ~300 ppm Ag due to the presence of submicron-sized inclusions. The high Co/Ni ratios of pyrite from Los Colorados are consistent with a magmatic-hydrothermal origin associated with a greater mafic affinity, compared to pyrite from porphyry Cu deposits. Furthermore, the geochemical signature of Los Colorados pyrite shares important similarities of composition and microtexture with the few published data for pyrite from IOCG deposits (e.g., Ernest Henry, Australia, and Manto Verde, Chile). These findings, combined with recent geochemical and isotopic studies that support an igneous origin for the dike-shaped magnetite orebodies at Los Colorados, point to a magmatic source of mafic to intermediate composition for the contained metals, and support the hypothesis that IOA systems can source Fe-Cu-Au-rich fluids. Based on experimental studies, these IOA-derived fluids may continue transporting significant amounts of metals to form IOCG mineralization at shallower levels in the crust.

A geochemical modeling to predict the different concentrations of REE and their hidden patterns using several supervised learning methods: Choghart iron deposit, bafq, Iran

The Bafq mining district hosts some Kiruna-type iron oxide-apatite (IOA) deposits which are commonly formed as a result of the multistage interaction of hydrothermal-magmatic processes within the Early Cambrian volcano-sedimentary sequence. Rare earth elements (REEs) are potentially concentrated under different physicochemical conditions in IOA deposits. Choghart orebody is one of the main magnetite-apatite deposits in the region. A wide range of primary and secondary geological events have affected the Choghart deposit, causing the behavior of REEs to vary in different zones. This study proposes a suitable exploration technique using various classification methods to identify the different concentrations of REE and their hidden patterns. To provide the required data, a systematic lithogeochemical sampling was performed in the north and NE of the Choghart orebody. The REE contents of samples were transformed into discrete values as distinct classes based on the results of clustering analysis. All possible combinations of features, being the geographical location and the major oxide composition of samples, were selected as subsets of predictors in every classification method. For each REE, 455 prediction models were constructed using these predictors. The performances of the classification methods were evaluated by error criteria with regard to all cases. Having the least amount of errors, the decision tree method was selected as the most suitable method. Based on decision tree results, the best subsets of predictors were chosen for each element. The existence of a significant relationship between the distribution patterns of each REE and the related predictors was assessed by its prediction errors. The assessment illustrated that some REE are reasonably predictable, and others are too irregular to be modeled. The extracted classification rules describe the geochemical relationships among the most important factors influencing the different concentrations of REE in the Choghart orebody. These results can be extended to other similar deposits to predetermine some REE-enriched zones based on major elements analysis. Merely by employing such techniques in REE exploration projects, a great savings in time and cost will be affected.

Fe–O stable isotope pairs elucidate a high-temperature origin of Chilean iron oxide-apatite deposits

Iron oxide–apatite (IOA) ore deposits occur globally and can host millions to billions of tons of Fe in addition to economic reserves of other metals such as rare earth elements, which are critical for the expected growth of technology and renewable energy resources. In this study, we pair the stable Fe and O isotope compositions of magnetite samples from several IOA deposits to constrain the source reservoir of these elements in IOAs. Since magnetite constitutes up to 90 modal% of many IOAs, identifying the source of Fe and O within the magnetite may elucidate high-temperature and/or lower-temperature processes responsible for their formation. Here, we focus on the world-class Los Colorados IOA in the Chilean iron belt (CIB), and present data for magnetite from other Fe oxide deposits in the CIB (El Laco, Mariela). We also report Fe and O isotopic values for other IOA deposits, including Mineville, New York (USA) and the type locale, Kiruna (Sweden). The ranges of Fe isotopic composition (δ56Fe, 56Fe/54Fe relative to IRMM-14) of magnetite from the Chilean deposits are: Los Colorados, δ56Fe (±2σ)=0.08±0.03‰ to 0.24±0.08‰; El Laco, δ56Fe=0.20±0.03‰ to 0.53±0.03‰; Mariela, δ56Fe=0.13±0.03‰. The O isotopic composition (δ18O, 18O/16O relative to VSMOW) of the same Chilean magnetite samples are: Los Colorados, δ18O (±2σ) = 1.92±0.08‰ to 3.17±0.03‰; El Laco, δ18O=4.00±0.10‰ to 4.34±0.10‰; Mariela, δ18O=(1.48±0.04‰). The δ18O and δ56Fe values for Kiruna magnetite yield an average of 1.76±0.25‰ and 0.16±0.07‰, respectively. The Fe and O isotope data from the Chilean IOAs fit unequivocally within the range of magnetite formed by high-temperature magmatic or magmatic–hydrothermal processes (i.e., δ56Fe 0.06–0.49‰ and δ18O=1.0–4.5‰), consistent with a high-temperature origin for Chilean IOA deposits. Additionally, minimum formation temperatures calculated by using the measured Δ18O values of coexisting Los Colorados magnetite and actinolite separates (630°C) as well as Fe numbers of actinolite grains (610–820°C) are consistent with this interpretation. We also present Fe isotope data from magmatic magnetite of the Bushveld Complex, South Africa, where δ56Fe ranges from 0.28±0.04‰ to 0.86±0.07‰. Based on these data and comparison to published Fe and O stable isotope values of igneous magnetite, we propose extending the magmatic/high-temperature δ56Fe range to 0.86‰. Considering that the Chilean IOAs and Kiruna deposit are representative of IOA deposits worldwide, the Fe and O stable isotope data indicate that IOAs are formed by high-temperature (magmatic) processes.

Postmagmatic magnetite–apatite assemblage in mafic intrusions: a case study of dolerite at Olympic Dam, South Australia

An assemblage of magnetite and apatite is common worldwide in different ore deposit types, including disparate members of the iron-oxide copper–gold (IOCG) clan. The Kiruna-type iron oxide-apatite deposits, a subtype of the IOCG family, are recognized as economic targets as well. A wide range of competing genetic models exists for magnetite–apatite deposits, including magmatic, magmatic-hydrothermal, hydrothermal(-metasomatic), and sedimentary(-exhalative). The sources and mechanisms of transport and deposition of Fe and P remain highly debatable. This study reports petrographic and geochemical features of the magnetite–apatite-rich vein assemblages in the dolerite dykes of the Gairdner Dyke Swarm (~0.82 Ga) that intruded the Roxby Downs Granite (~0.59 Ga), the host of the supergiant Olympic Dam IOCG deposit. These symmetrical, only few mm narrow veins are prevalent in such dykes and comprise besides usually colloform magnetite and prismatic apatite also further minerals (e.g., calcite, quartz). The genetic relationships between the veins and host dolerite are implied based on alteration in the immediate vicinity (~4 mm) of the veins. In particular, Ti-magnetite–ilmenite is partially to completely transformed to titanite and magmatic apatite disappears. We conclude that the mafic dykes were a local source of Fe and P re-concentrated in the magnetite–apatite veins. Uranium-Pb ages for vein apatite and titanite associated with the vein in this case study suggest that alteration of the dolerite and healing of the fractures occurred shortly after dyke emplacement. We propose that in this particular case the origin of the magnetite–apatite assemblage is clearly related to hydrothermal alteration of the host mafic magmatic rocks.

Posht-e-Badam Metallogenic Block (Central Iran): A suitable zone for REE mineralization

One of the most important ores for REE mineralization are iron oxide–apatite (IOA) deposits. The Posht-e-Badam Block (PBB) is a part of the Central Iranian geostructural zone which is the host of most important Fe deposits of Iran. Exploration studies of the IOA deposits within the PBB (e.g. Esphordi, Gazestan, Zarigan, Lak-e-Siah, Sechahoun, Chahgaz, Mishdovan, Cheshmeh Firouzi and Shekarab) demonstrate that these deposits contain high contents of REE. Concentrations of ΣREE in the most important IOA deposits of the PBB include the following: the Esphordi deposit varies between 1.2 and 1.88%, the Gazestan deposit between 0.17 and 1.57%, the Zarigan deposit between 0.5 and 1.2% and the Lak-e-Siah deposit varies between 0.45 and 1.36%. Concentrations of ΣREE within the apatite crystals present within the IOA ores in the Esphordi, Lak-e-Siah and Homeijan deposits have ranges between 1.9–2.54%, 1.9–2.16% and of 2.55%, respectively. These elements are mainly concentrated in apatite crystals, but other mineral...

KIRUNA-TYPE IRON OXIDE-APATITE DEPOSITS, BAFQ DISTRICT, CENTRAL IRAN: FLUID-AIDED GENESIS OF FLUORAPATITE-MONAZITE-XENOTIME ASSEMBLAGES

In the Bafq region, Central Iran, apatite from Kiruna-type iron oxide-apatite deposits is enriched in LREE (up to 2 wt.%), Na, Si, and Cl (up to 15% of the halogen component), but altered apatite is strongly depleted in these elements. Monazite and xenotime inclusions with low Th and U and associated with a distinct pervasive porosity are found in the altered areas of the apatite and are a direct product of fluid-aided coupled dissolution-reprecipitation processes. Monazite-xenotime thermometry indicates that these inclusions formed below 400–300 °C, which agrees well with available fluid inclusion data. Comparison of these iron oxide-apatite deposits with other Kiruna-type iron oxide-apatite deposits suggests that they formed from highly differentiated volcanic magmas which interacted with both magmatic and externally derived fluids shortly after crystallization and during cooling. The concentrations of Sr, Mn, and Y in the apatites ranges from 40–824 ppm, 10–440 ppm, and 66–3700 ppm, respectively, which strongly supports a granitoid source for genesis of the iron ores. An extensive Na-Ca metasomatic alteration zone, with magnetite-apatite mineralization, existence of magmatic bodies grading to breccias, and fluid inclusion data all point to the involvement of both magmatic and hydrothermal processes in the ore formation.

Trace elements in magnetite from massive iron oxide-apatite deposits indicate a combined formation by igneous and magmatic-hydrothermal processes

Iron oxide-apatite (IOA) deposits are an important source of iron and other elements (e.g., REE, P, U, Ag and Co) vital to modern society. However, their formation, including the namesake Kiruna-type IOA deposit (Sweden), remains controversial. Working hypotheses include a purely magmatic origin involving separation of an Fe-, P-rich, volatile-rich oxide melt from a Si-rich silicate melt, and precipitation of magnetite from an aqueous ore fluid, which is either of magmatic-hydrothermal or non-magmatic surface or metamorphic origin. In this study, we focus on the geochemistry of magnetite from the Cretaceous Kiruna-type Los Colorados IOA deposit (∼350Mt Fe) located in the northern Chilean Iron Belt. Los Colorados has experienced minimal hydrothermal alteration that commonly obscures primary features in IOA deposits. Laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) transects and electron probe micro-analyzer (EPMA) wavelength-dispersive X-ray (WDX) spectrometry mapping demonstrate distinct chemical zoning in magnetite grains, wherein cores are enriched in Ti, Al, Mn and Mg. The concentrations of these trace elements in magnetite cores are consistent with igneous magnetite crystallized from a silicate melt, whereas magnetite rims show a pronounced depletion in these elements, consistent with magnetite grown from an Fe-rich magmatic-hydrothermal aqueous fluid. Further, magnetite grains contain polycrystalline inclusions that re-homogenize at magmatic temperatures (>850°C). Smaller inclusions (<5μm) contain halite crystals indicating a saline environment during magnetite growth. The combination of these observations are consistent with a formation model for IOA deposits in northern Chile that involves crystallization of magnetite microlites from a silicate melt, nucleation of aqueous fluid bubbles on magnetite surfaces, and formation and ascent of buoyant fluid bubble-magnetite aggregates. Decompression of the fluid-magnetite aggregate during ascent along regional-scale transcurrent faults promotes continued growth of the magmatic magnetite microlites from the Fe-rich magmatic-hydrothermal fluid, which manifests in magnetite rims that have trace element abundances consistent with growth from a magmatic-hydrothermal fluid. Mass balance calculations indicate that this process can leach and transport sufficient Fe from a magmatic source to form large IOA deposits such as Los Colorados. Furthermore, published experimental data demonstrate that a saline magmatic-hydrothermal ore fluid will scavenge significant quantities of metals such as Cu and Au from a silicate melt, and when combined with solubility data for Fe, Cu and Au, it is plausible that the magmatic-hydrothermal ore fluid that continues to ascend from the IOA depositional environment can retain sufficient concentrations of these metals to form iron oxide copper–gold (IOCG) deposits at lateral and/or stratigraphically higher levels in the crust. Notably, this study provides a new discrimination diagram to identify magnetite from Kiruna-type deposits and to distinguish them from IOCG, porphyry and Fe–Ti–V/P deposits, based on low Cr (<100ppm) and high V (>500ppm) concentrations.

Giant Kiruna-type deposits form by efficient flotation of magmatic magnetite suspensions

Kiruna-type iron oxide-apatite (IOA) deposits are an important source of Fe ore, and two radically different processes are being actively investigated for their origin. One hypothesis invokes direct crystallization of immiscible Fe-rich melt that separated from a parent silicate magma, while the other hypothesis invokes deposition of Fe-oxides from hydrothermal fluids of either magmatic or crustal origin. Here, we present a new model based on Fe and O stable isotopes and trace and major element geochemistry data of magnetite from the ~350 Mt Fe Los Colorados IOA deposit in the Chilean iron belt that merges these divergent processes into a single sequence of events that explains all characteristic features of these curious deposits. We propose that concentration of magnetite takes place by the preferred wetting of magnetite, followed by buoyant segregation of these earlyformed magmatic magnetite-bubble pairs, which become a rising magnetite suspension that deposits massive magnetite in regionalscale transcurrent faults. Our data demonstrate an unambiguous magmatic origin, consistent with the namesake IOA analogue in the Kiruna district, Sweden. Further, our model explains the observed coexisting purely magmatic and hydrothermal-magmatic features and allows a genetic connection between Kiruna-type IOA and iron oxide-copper-gold deposits, contributing to a global understanding valuable to exploration efforts.

A new genetic model for the Triassic Yangyang iron-oxide–apatite deposit, South Korea: Constraints from in situ U–Pb and trace element analyses of accessory minerals

The Yangyang iron-oxide–apatite deposit in South Korea has undergone multiple episodes of igneous activity, deformation, hydrothermal alteration, and iron-oxide–apatite (IOA) mineralization. The iron orebodies occur as concordant- to discordant-layered lenticular or massive magnetite and/or magnetite–pyrite ores. The iron mineralization occurs along a N–S-trending shear zone within the Yangyang syenite, which experienced both early ductile and later brittle deformations. Alteration was caused mainly by the injection of hydrothermal fluid through the shear zone, leading to Fe–P mineralization. We recognize multiple stages of alteration in the Yangyang deposit, based on a paragenesis that is defined by distinct mineral assemblages including Na–Ca–K alteration phases (e.g., albite, diopside, actinolite, and biotite) and accessory minerals containing high field strength elements (e.g., apatite, sphene, allanite, and monazite). The alteration around the magnetite ore body shows an evolutionary trend from Ca (–Na) alteration, through K to phyllic alterations. The Fe–P mineralization is associated with the Ca–K and K alteration products. The iron orebodies are hosted by deformed and altered syenite, which intruded the Paleoproterozoic gneiss complexes at 233±1Ma (SHRIMP U–Pb zircon age) in a post-collisional tectonic setting. LA-ICP-MS U–Pb dating of REE-rich sphene and apatite from the iron ores and alteration products yields Fe mineralization ages of 216±3Ma (sphene) and 212±13Ma (apatite). This is the first time, which IOA-type mineralization in the Korean Peninsula was dated as Triassic age related to post-collisional magmatism within the Gyeonggi Massif, South Korea. The U–Pb system was subsequently reset (208±3Ma–sphene and 151±13Ma–apatite) by Jurassic and Cretaceous magmatism. This unique geological evolution was responsible for Mesozoic metal enrichment and remobilization into suitable structural traps in the Yangyang district.

Alteration, oxygen isotope, and fluid inclusion study of the Meishan iron oxide–apatite deposit, SE China

The Meishan deposit (338 Mt at 39 % Fe) comprises massive ores in the main orebody and stockwork and disseminated ores along the main orebody. Four stages of mineralization and related alteration have been identified. The second stage of mineralization, which was the main stage of iron mineralization, formed stringer, disseminated iron ores, as well as the main Meishan orebody. The fourth stage formed small pyrite and/or gold orebodies above or alongside the main magnetite orebody. Stage 2 apatites have homogenization temperatures of 257–485 °C and salinities of 7.3–11 wt% NaCleq. Calculated δ18Ofluid values of magnetite and apatite from the disseminated ores vary between 7.7 and 14.9 ‰, which is similar to values observed in the massive ores (8.1–12.9 ‰). The high-18O fluids at Meishan have been interpreted as being of magmatic–hydrothermal origin. These fluids are indicative of the boiling of ore-forming fluids. Quartz, occurring as cavity fillings, gives homogenization temperatures from 202 to 344 °C, with most values lying between 250 and 330 °C. Corresponding salinities are ∼5 wt% NaCleq. Calculated δ18Ofluid values are +6.4 to +6.8 ‰. These values indicate that the lower-temperature (250–330 °C) quartz was deposited from a cooling magmatic–hydrothermal fluid. Stage 3 siderites contain fluid inclusions that homogenized between 190 and 310 °C, mainly between 210 and 290 °C. Corresponding salinities are 4–8 wt% NaCleq. Stage 4 quartz–carbonate veinlets contain fluid inclusions that homogenized at moderate to low temperatures (150–230 °C) and exhibit low salinities (2–10 wt% NaCl eq). δ18Ofluid values of the mineralizing fluids for the quartz and calcite can be calculated to vary from −0.7 to +5.6 ‰ and +6.3 to +10.2 ‰, respectively. While there is some overlap, the δ18O values of the fluids are generally lower than those observed in the massive and disseminated magnetite ores. δD values for the quartz and calcite vary between −154 and −123 ‰ and −123 and −111 ‰, respectively. These values suggest late-stage input of a shallow-sourced, isotopically light meteoric fluid at the temperature of pyrite and gold deposition. The fluid inclusion and stable isotope data indicate a cooling magmatic–hydrothermal system that progressed from isotopically heavy to isotopically depleted fluids as it cooled. Such fluid evolution is comparable with those of other Kiruna-type deposits worldwide.

Geology, geochemistry, and some genetic discussion of the Chador-Malu iron oxide-apatite deposit, Bafq District, Central Iran

The Chador-Malu iron oxide-apatite system (Bafq District, Central Iran) contains the largest known iron ore deposit in Iran (pre-mining reserve of 400 Mt @ 55 % Fe), and comprises the pipe-like northern (this study) and the sill-like southern orebodies of predominantly massive ore, and a sodic-calcic alteration envelope. The geology and geochemistry of the Chador-Malu deposit demonstrates its similar characteristics to the Kiruna-type deposits. There is circumstantial evidence for rare earth elements (REE) mobilization during apatite leaching by high-temperature fluids and associated monazite nucleation. Pervasive actinolitization of the rhyolitic country rocks led to the formation of actinolite-rich metasomatic host rocks, which represent another evidence for high-temperature fluids at Chador-Malu. Hydrothermal mineralization is suggested by small iron ore veins (2–3 cm thick) and breccias cemented by iron oxides, as well as a Fe-metasomatism which overprints all types of host rock alteration. Based on REE geochemistry and spatial relationships, it is proposed that a potential source for metals and P could be late-stage Fe-P melt differentiates of the Cambrian magmatism, which is consistent with the late Fe-metasomatism of the host rocks. The proposed Fe-P melts and the mineralization would be linked by hydrothermal media through the zones of ring fracture at Chador-Malu and similar parts of the Bafq district.