Geology and geochemistry of the Águas Claras and Pico Iron Mines, Quadrilátero Ferrífero, Minas Gerais, Brazil

The Águas Claras and Pico Mines are two world-class iron-ore mines hosted within the Lower- Proterozoic banded iron-formations (locally known as itabirites) of the Minas Supergroup located in the Quadrilátero Ferrífero district, Minas Gerais, Brazil. The Águas Claras orebody consists of a 2,500-m-long roughly tabular-shaped lens hosted within the dolomitic itabirite of the Cauê Formation. Dolomitic itabirite is the protore of the soft high-grade iron ore, which is the main ore type of the Águas Claras orebody, representing about 85% of the 284 Mt mined since 1973, with the remaining 15% comprising hard high-grade ore. Hematite is the main constituent of the iron ores. It occurs as martite, granular hematite and locally as specularite. Magnetite appears subordinately as relicts within martite and hematite crystals. Gangue minerals are very rare. These consist of dolomite, chlorite, talc, and apatite, and are especially common in contact with the protore. This virtual absence of gangue minerals is reflected in the chemistry of ores that are characterized by very high Fe contents (an average of 68.2% Fe).The Pico orebody is a continuous ~3,000-m-long body of a lenticular shape hosted within siliceous itabirite, which is the protore of the soft high- and low-grade ores at the Pico Mine. The soft high-grade ores, together with the low-grade ores, called iron-rich itabirite, are the main types of ore, and respectively represent approximately 51 and 29% of the reserves. The remaining 20% consists of hard high-grade ore. The iron oxide mineralogy is the same as that of the Águas Claras Mine, but in different proportions. Gangue minerals are very rare in the high-grade ores, but are slightly more common in the iron-rich itabirite. Quartz is the dominant gangue mineral, and is found with minor quantities of chlorite. The chemistry of the high-grade ores is characterized by high Fe contents (an average of 67.0%) and low P, Al₂O₃, and SiO₂, which are concentrated in the fines. Iron-rich itabirites average 58.6% Fe and 13.5% SiO₂.The genesis of the soft high-grade ores and iron-rich itabirites is related to supergene processes. Leaching of the gangue minerals by groundwater promoted the residual iron enrichment of the itabirites. This process was favored by the tropical climate and topographic situation. The original composition of the itabirites and the presence of structures controlling the circulation of the groundwater have influenced the degree of iron enrichment. The hard high-grade ores are of a hypogene origin. Their genesis is attributed to hydrothermal solutions that leached the gangue minerals and filled the spaces with hematite. This process remains a source of debate and is not yet fully understood.Editorial handling: S.G. Hagemann     

Regional sulfate–hematite–sulfide zoning in the auriferous Mariana anticline, Quadrilátero Ferrífero of Minas Gerais, Brazil

The distribution of mineral deposits, characterised as barite deposits, hematite-rich auriferous deposits and auriferous tourmaline–sulfide deposits, displays a regional sulfate–hematite–sulfide zoning along the thrust-delineated limbs of the Mariana anticline, in the south-eastern part of the Quadrilátero Ferrífero of Minas Gerais, Brazil. Cross-cut relationships of barite veins and sulfide lodes indicate that sulfidation occurred in a late-tectonic context, which is here attributed to the collapse of the ～0.6-Ga Brasiliano thrust front. Reconnaissance S-isotopic data from barite and pyrite (Antônio Pereira barite deposit and its adjacent gold deposit, respectively), and arsenopyrite (Passagem de Mariana gold deposit), suggest a new interpretation for the hydrothermal fluid overprint in the Mariana anticline. The Antônio Pereira barite has Δ³³S values that are near zero, constraining the sulfate source to rocks younger than 2.45 Ga. The barite-δ³⁴S values are between +19.6 and +20.8?‰. The Passagem arsenopyrite and tourmaline have Co/Ni ratios that define a positive linear trend with the Antônio Pereira pyrite. The latter has homogenous δ³⁴S values, between +8.8 and +8.9?‰, which are compatible with thermochemical reduction of aqueous sulfate with the S-isotopic composition of the Antônio Pereira barite.     

Surface water quality and sediment geochemistry in the Gualaxo do Norte basin,eastern Quadrilátero Ferrífero,Minas Gerais,Brazil

This paper describes the geochemistry of water and sediment samples downstream from two iron mines in the Gualaxo do Norte River basin, north of Mariana city, Minas Gerais state, Brazil. Electric conductivity, total dissolved solids, pH, E_h, Na, K, Ca, Mg and trace elements (Fe, Mn, Al, Cd, Ni, Cr, Zn, As, Pb, Co, Sb, Cu, Mo, Rb, Sr, Ti, V, Y, Zr and Ba) have been analysed in water and sediments. The observed Na anomalies are mainly associated with the iron mine processing plants. Enriched concentrations of iron and manganese were only observed in rudimentary gold exploitation areas (prospectors) and where supergenic alteration of the banded iron formation has occurred. Iron and manganese background values are in some places higher than the limits established by Brazilian legislation. Mn, As, Sb and Ba anomalies have been observed in sediments. Their main source is associated with the sulphide-rich mineral deposits in the spring of the Gualaxo do Norte River and not directly related to the present iron exploitation in the region. Background values are proposed for the studied region.     

Using oxygen isotope chemistry to track hydrothermal processes and fluid sources in itabirite-hosted iron ore deposits in the Quadrilátero Ferrífero,Minas Gerais,Brazil

The Quadrilátero Ferrífero, Brazil, is presently the largest accumulation of single itabirite-hosted iron ore bodies worldwide. Detailed petrography of selected hypogene high-grade iron ore bodies at, e.g. the Águas Claras, Conceição, Pau Branco and Pico deposits revealed different iron oxide generations, from oldest to youngest: magnetite → martite (hematite pseudomorph after magnetite) → granoblastic (recrystallised) → microplaty (fine-grained, <100 μm) → specular (coarse-grained, >100 μm) hematite. Laser-fluorination oxygen isotope analyses of selected iron ore species showed that the δ¹⁸O composition of ore-hosted martite ranges between ?4.4 and 0.9?‰ and is up to 11?‰ depleted in ¹⁸O relative to hematite of the host itabirite. During the modification of iron ore and the formation of new iron oxide generations (e.g. microplaty and specular hematite), an increase of up to 8?‰ in δ¹⁸O values is recorded. Calculated δ¹⁸O values of hydrothermal fluids in equilibrium with the iron oxide species indicate: (1) the involvement of isotopically light fluids (e.g. meteoric water or brines) during the upgrade from itabirite-hosted hematite to high-grade iron ore-hosted martite and (2) a minor positive shift in δ¹⁸O_fluid values from martite to specular hematite as result of modified meteoric water or brines with slightly elevated δ¹⁸O values and/or the infiltration of small volumes of isotopically heavy (metamorphic and/or magmatic) fluids into the iron ore system. The circulation of large fluid volumes that cause the systematic decrease of ¹⁸O/¹⁶O ratios from itabirite to high-grade iron ore requires the presence of, e.g. extensive faults and/or large-scale folds.     

Xenotime–hematite aggregates on opaline filaments: evidence for biomineralization in weathered siliciclastic rocks, Capanema, Quadrilátero Ferrífero of Minas Gerais, Brazil

Dissolution cavities in weathered pebbly quartzite of the ~2.5-Ga Moeda Formation at Capanema, Quadrilátero Ferrífero of Minas Gerais, Brazil, are decorated with suspended filaments of opaline silica. The filaments sustain xenotime–hematite aggregates in the open space. Xenotime occurs as inclusions in buds and botryoidal aggregates of hematite. The filamentous structures consist of strand-forming buds, hypha-like extensions, and thin strands that compose mat-like arrangements. They resemble microbial filaments that were replaced by opaline silica and fossilized. The occurrence of spherical hematite as protuberances on hematite-free opaline hyphae is interpreted as accretion of dissolved iron onto extracellular polymers. Phosphate sites in polymeric substances expelled from the microbial filaments might have adsorbed yttrium and heavy rare-earth elements from groundwater to the iron-accreting polymers. These would have resulted in botryoidal aggregates of hematite with xenotime inclusions. The presence of authigenic xenotime in the weathering zone opens a new possibility to constrain the evolution of lateritic profiles by xenotime geochronology.     

The chemical composition of specular hematite from Tilkerode, Harz, Germany: implications for the genesis of hydrothermal hematite and comparison with the Quadrilátero Ferrífero of Minas Gerais, Brazil

The black shale-hosted selenide vein-type deposit at Tilkerode, eastern Harz, Germany, has specular hematite enclosed in clausthalite (PbSe). The specular hematite has Ti and V in amounts of up to ～1 wt.% TiO₂ and ～3 wt.% V₂O₅, and subordinate, but important, contents of Mo (22–372 ppm) and B (up to 68 ppm). The Tilkerode hematite serves as a reference for hydrothermal hematite formed at relatively low temperatures (<150 °C). The composition of the Tilkerode hematite is compared with that of two generations of specular hematite from itabirite-hosted iron-ore deposits in the Quadrilátero Ferrífero of Minas Gerais, Brazil. The first generation of specular hematite represents an early tectonic hematitisation of dolomitic itabirite at Águas Claras and occurs as fine-grained crystals. Reconnaissance data indicate that the Águas Claras hematite is poorer in Ti and V, relative to the Tilkerode hematite, but has ～5–10 ppm B and ～7–11 ppm Li. The second generation of specular hematite defines the pervasive tectonic foliation of the Gongo Soco iron ore. This hematite has Ti contents of up to ～2 wt.% TiO₂ and subordinate amounts of V (62–367 ppm); its B and Li concentrations are mostly below <2 ppm B and <1 ppm Li. The presence of Ti and B in the Tilkerode hematite can be explained by highly saline, B-bearing fluids that were capable of mobilising otherwise immobile Ti. The Mo signature of the Tilkerode hematite suggests that Mo was derived from the host black shale. In Minas Gerais, B and Li were incorporated into the early tectonic hematite from saline fluids at relatively low temperatures (Águas Claras) and then released during metamorphic hematite growth at higher temperatures, as suggested by the foliation-defining hematite without B–Li signature (Gongo Soco).     

Structural control and hydrothermal alteration at the BIF-hosted Raposos lode-gold deposit,Quadrilátero Ferrífero,Brazil

In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D₁, D₂ and D₃). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D₃. The orebodies constitute sulfide-bearing D₁-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage.Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H₂O-rich containing CO₂ + Na⁺ and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H₂O is indicated. The CO₂ is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H₂O to CO₂-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration.     

Fluid inclusion and sulfur and oxygen isotope studies on quartz-carbonate-sulfide veins of the Carvoaria Velha deposit,Córrego do Sítio gold lineament,Quadrilátero Ferrífero,Minas Gerais,Brazil

The Córrego do Sítio lineament is defined as a 16-km long, NE-SW-trending ductile shear zones, which controls fourteen gold deposits, including the Carvoaria Velha deposit. The dominant lithotypes of this deposit are metagrewackes with subordinate carbonaceous phyllites lenses of the Archean Rio das Velhas greenstone belt, which host gold mineralization. Metamafic dikes and sills are parallel and crosscut the host metasedimentary sequence. All these rocks exhibit alteration to quartz, carbonate and sericite, besides sulfides and sulfosalts. The main gold mineralization styles at the Carvoaria Velha deposit, and at Córrego do Sítio as a whole, are quartz-carbonate-sulfide ± sulfosalt veins of varied distribution. The veins are classified as: V1 veins – quartz-ankerite-pyrite-berthierite-gold – parallel to the main regional foliation S_n,; V2 veins – quartz-ankerite-pyrite – developed at extensional crenulation cleavage S_n + 1, and rarely gold mineralized; V3 veins – quartz-ankerite – filling S_n + 3 fractures, usually free of sulfides and sulfosalts; V4 veins – quartz-calcite – of restricted occurrence in metamafic dikes and sills. The latter crosscut the metasedimentary sequence, are extensional and display no preferential orientation. The most common ore minerals in all vein types are arsenopyrite, pyrite, berthierite, and pyrrhotite. Microprobe analyses reveal the presence of metallic phases rich in Sb, Pb and Co, such as stibnite, ullmanite, tetrahedrite, galena, cobaltite, which commonly overgrow the sulfides. Fluid inclusion studies trapped in quartz from V1, V2 and V4 veins have identified a H₂O-CO₂ ± CH₄-NaCl mineralizing fluid that may contain KCl, MgCl₂ e FeCl₂. The presence of CH₄ in fluid inclusions of the V1 and V2 veins reflect interaction of the fluid with the Córrego do Sítio and Santa Quitéria carbon-bearing metapelitic host units.Based on the microthermometric data, the hydrothermal fluid is interpreted to have evolved in at least two stages: i) an early fluid stage, trapped in smoky quartz, of moderate salinity (~ 8.5 eq. wt% NaCl), and minimal trapping temperature of 330 ± 17 °C; and ii) a late-stage fluid trapped in recrystallized quartz with lower salinity (~ 4.6 eq. wt% NaCl), and a minimal trapping temperature of 365 ± 33 °C. Isotopic values of δ¹⁸O_fluid (+ 7.9 and + 13.0‰), Δ³³S (+ 0.3 to + 3.5‰) and δ³⁴S (− 2.9 to + 6.1‰) suggest that indeed the hydrothermal fluid responsible for the gold mineralization at the Carvoaria Velha deposit must have had a metamorphic origin, and interacted with metasedimentary sequences during its ascending path. The addition of CH₄ during fluid-rock interaction may have caused some decrease in ƒO₂ of the fluid which, as a consequence, destabilized gold-bearing sulfur complexes, liberating S^− 2 for the formation of Fe sulfides and sulfosalts (arsenopyrite, pyrite, pyrrothite and berthierite, etc.), and outcome gold deposition.     

Hydrothermal fluid source constrained by Co/Ni ratios in coexisting arsenopyrite and tourmaline: the auriferous lode of Passagem, Quadrilátero Ferrífero of Minas Gerais, Brazil

The auriferous lode of Passagem de Mariana is characterised by abundant tourmaline, which is intergrown with arsenopyrite. Spot measurements using laser ablation–inductively coupled plasma–mass spectrometry show that Co and Ni are the most abundant trace elements in the arsenopyrite (45–538?ppm Co, 246–828?ppm Ni), with Co/Ni ratios consistently <1. The coexisting tourmaline also has Co/Ni <1, with Co and Ni contents that are ~2 orders of magnitude lower than those in the arsenopyrite. The Co/Ni ratios of tourmaline and arsenopyrite are tightly distributed along a positive linear trend, the angular coefficient of which represents the Co/Ni of the hydrothermal fluid from which these minerals precipitated. The fluid Co/Ni ratio is close to the average Co/Ni value for the upper continental crust. In conjunction with the abundance of lode tourmaline and its B-isotope data (from the literature), the Co/Ni ratios of tourmaline and arsenopyrite fingerprint a continental evaporitic source of B.     

Geochronology and thermochronology of gold mineralization in the Turmalina deposit,NE of the Quadrilátero Ferrífero Region,Brazil

The Turmalina gold deposit comprises three epigenetic domains whose development is related to the propagation/reactivation events of the NW–SE Pitangui Shear Zone (PSZ). The lodes are hosted in a Late Archaean sedimentary sequence on top of a strongly deformed (mafic-dominated) metavolcanic pile metamorphosed under 3.5–4 kbar and 540–610 °C; the association forms the upper part of a lithostratigraphic succession (Pitangui Group) that overlies an older TTG gneissic basement. According to field evidence and petrographic observations, the ore-forming process is polyphasic, starting at the time when the PSZ crossed the metamorphic quartz + staurolite + biotite + almadine + hornblende isograde; the main evolving stages, however, mostly took place throughout the metamorphic retrogression path. Fluid inclusion microthermometry also shows that metamorphogenic aqueous–carbonic solutions (initially with ≈ 16–20 eq. wt.% NaCl and circulating at approximately 4 kbar and 550 °C) were subjected to repeated boiling and mixing with cooler aqueous solutions at approximately 1–2 kb and 300–350 °C. These boiling events, which were triggered by depressurization, were contemporaneous with gold (and later sulfide) deposition, preceding a late stage of hydrothermal activity under lower PT conditions (< 1 kbar and ≈ 130–230 °C). To constrain both the mineralization age and the source/pathways of the ore-forming fluids in the Turmalina deposit, a multi-system isotope (Pb–Pb, Rb–Sr and Sm–Nd) study was carried out using various whole-rock and mineral samples. The main results are as follows: (i) the onset of the ore-forming process took place at ca. 2.2–2.1 Ga; (ii) the critical timing for gold formation was confined to ca. 2–1.9 Ga; and (iii) the late hydrothermal influxes occurred after 1.75 Ga. Therefore, the ore-forming process can be envisaged as a result of successive physical–chemical processes that took place during two major, long-lasting (≈ 250 Ma) periods under initial cooling rates of approximately 1 °C/Ma and, after ca. 2–1.9 Ga, approximately 2.5 °C/Ma. Moreover, the rejuvenation episodes at ca. 2–1.9 Ga and ca. 1.75 Ga probably indicate reactivation events in the PSZ, which triggered new fluid inflows into the system and revitalized the ore-forming process. That was the case for fluids that circulated deeply through both the older basement rocks and the mafic volcanic pile, allowing either metal enrichment from multistage leaching processes of various reservoirs or possible U/Th decoupling during ore remobilization. The Turmalina ore-system lifetime is consequently confined to the Rhyacian period (Paleoproterozoic), which is compatible with the age constraints presented by other studies regarding the fold-thrust belt's development (ca. 2.125 Ga) and orogenic collapse (ca. 2.095 Ga).     

Paleobiogeoclimatic scenarios of the Late Quaternary inferred from fluvial deposits of the Quadrilátero Ferrífero (Southeastern Brazil)

The Quadrilátero Ferrífero is an important mineral province in Southeastern Brazil and has one of the largest iron ore reserves in the world. Previous work in this region has indicated that the formation of fluvial successions with duricrusts coincided with drier/cooler climatic phases alternating with moister/warmer periods during which the formation of fluvial successions without duricrusts occurred. For the construction of this proposal, ages of fluvial sediments obtained through Optically Stimulated Luminescence (OSL) were associated with data from the literature on paleobioclimatic scenarios. Therefore, using these observations as a starting point, this paper aims to investigate evidence of bioclimatic oscillations obtained directly from the fluvial successions and discuss its influence on the geomorphogenis of local river valleys. For this purpose, phytolith, carbon isotope and granulometric analyses were carried out, as well as dating of sediments using OSL and of soil organic matter through radiocarbon. The results show that in the oldest depositional succession (DS1 — about 34ka) the predominant phytoliths are those of bulliform polyedric, elongate, acicular and globular granulate types and δ¹³C values are typical of C3 plants. On the other hand, despite having a similar phytolith assemblage (abundance of bulliform polyedric, elongate, bulliform cuneiform, acicular, globular psilate and bilobate flat/concave types), the fluvial successions associated with significant conglomeratic duricrusts (DS2 and DS3) present a dominance of δ¹³C values characteristic of C4 plants. The Bi index indicates water stress in all the successions, and the Ic index suggests decreasing temperatures with depth in DS3. Thus, the three fluvial successions indicate a savanna-like environment, but depositional successions DS2 (∼27ka) and DS3 show drier/cooler climatic conditions when compared to DS1 and to the present-day regime. Both scenarios evolved under conditions of the Last Glacial period, but DS2 and DS3 were formed closer to the Last Glacial Maximum, and therefore under the strong influence of the lower temperatures during this period. These drier/cooler conditions in steep valleys with unprotected hillslopes may have been decisive for the formation of relatively thicker layers of gravel and sand, which later became duricrusts. The results indicate that climate has also played an important role in the regional hydrosedimentological dynamics, given the variations in vegetation influencing the abandonment of fill terraces and formation of nested floodplains.     

Metamorphism and exhumation of basement gneiss domes in the Quadrilátero Ferrífero:Two stage dome-and-keel evolution?

The presence of dome-and-keel provinces in Archean cratons has been connected with the initiation of plate tectonics on Earth as these features are most commonly observed in Archean rocks.The Quadrilátero Ferrífero in Brazil has been identified as a Paleoproterozoic dome-and-keel province for more than three decades.The prevailing model suggests that it formed during the Rhyacian Transamazonian orogeny,making it unique among dome-and-keel provinces.However,a lack of appropriate lithologies,datable minerals and the metamorphic overprint of later orogenesis has resulted in a cryptic metamorphic record for the formation of this dome-and-keel province.A clinopyroxene-bearing migmatite from the core of the Ba??o dome has peak P-T conditions of 5-7 kbar and 700-750 ℃ and a published age of ca.2730 Ma based on U-Pb ages of zircon from leucosomes,suggesting that this age represents the migmatisation event.A fine-grained epidote-albite-titanite assemblage overprints the coarse-grained clinopyroxene and amphibole,giving P-7 conditions of 8-9 kbar and 550 ℃ with an associated titanite age of ca.2050 Ma.A garnet-bearing amphibolite sample also from the core of the dome has peak P-T conditions of 7-8 kbar and 650-700 ℃,and texturally late titanite from this sample produces an age of ca.2060 Ma.Three additional samples were collected from the edges of the dome.A garnet-gedrite bearing felsic schist produces peak P-T conditions of 8-9 kbar and 650-700℃ on a clockwise P-T evolution.This sample has a U-Pb zircon age of ca.2775 Ma,which could date metamorphism or be the age of its volcaniclastic protolith.Texturally unconstrained titanite from the sample gives an age of ca.2040 Ma.A garnet-bearing amphibolite that occurs as a boudin within the felsic schist gives both zircon and titanite ages of ca.2050 Ma and has peak P-T conditions of 5-6 kbar and 650-700 ℃ on a near isobaric P-T path.An amphibolite dike,observed to cross-cut the felsic schist produces a zircon U-Pb age of ca.2760 Ma.Altogether this data suggests that the samples were metamorphosed in the Archean(ca.2775-2730 Ma)and again during the Transamazonian event.The most plausible explanation for this data is that dome-and-keel formation occurred in the Archean with migmatisation and high-temperature metamorphism occurring at this time.The Paleoproterozoic event is interpreted as a reactivation of the dome-and-keel formation structures,with Paleoproterozoic keels crosscutting Archean keels and producing metamorphic aureoles.The high radiogenic heat production and the presence of dense sedimentary successions in Archean terranes make dome-and-keel provinces a uniquely Archean feature,but they are susceptible to reworking,resulting in an enigmatic record of formation.     

Mineralogy and trace-element geochemistry of the high-grade iron ores of the Águas Claras Mine and comparison with the Capão Xavier and Tamanduá iron ore deposits, Quadrilátero Ferrífero, Brazil

Several major iron deposits occur in the Quadrilátero Ferrífero (QF), southeastern region of Brazil, where metamorphosed and heterogeneously deformed banded iron formation (BIF) of the Cauê Formation, regionally called itabirite, was transformed into high- (Fe >64%) and low-grade (30%?2O₃, with a higher amount of detrimental impurities, especially MnO, in the soft ore. Both hard and soft ores are depleted in trace elements. The high-grade ores at the Águas Claras Mine have at least a dual origin, involving hypogene and supergene processes. The occurrence of the hard, massive high-grade ore within “fresh” dolomitic itabirite is evidence of its hypogene origin. Despite the contention about the origin of the dolomitic itabirite (if this rock is a carbonate-rich facies of the Cauê Formation or a hematite–carbonate precursor of the soft high-grade ore), mineralogical and geochemical features of the soft high-grade ore indicate that it was formed by leaching of dolomite from the dolomitic itabirite by meteoric water. The comparison of the Águas Claras, Capão Xavier and Tamanduá orebodies shows that the original composition of the itabiritic protore plays a major role in the genesis of high- and low-grade soft ores in the QF. Under the same weathering and structural conditions, the dolomitic itabirite is the more favorable to form high-grade deposits than siliceous itabirite. Field relations at the Águas Claras and Capão Xavier deposits suggest that it is not possible to form huge soft high-grade supergene deposits from siliceous itabirite, unless another control, such as impermeable barriers, had played an important role. The occurrence in the Tamanduá Mine of a large, soft, high-grade orebody formed from siliceous itabirite and closely associated with hypogene hard ore suggests that large, soft, high-grade orebodies of the Quadrilátero Ferrífero, which occur within siliceous itabirite, have a hypogene contribution in their formation.     

Hydrothermal and metamorphic fluid-rock interaction associated with hypogene “hard” iron ore mineralisation in the Quadrilátero Ferrífero,Brazil: Implications from in-situ laser ablation ICP-MS iron oxide chemistry

In-situ laser ablation ICP-MS analyses on iron oxides in itabirite and iron ore from the Quadrilátero Ferrífero (Brazil) reveal a wide range in trace element abundances (e.g., average concentrations in hematite: Al = 40–2200 ppm, Mg = 1–930 ppm, Mn = 5–540 ppm, Ti = 3–500 ppm, V = 2–390 ppm, Cr = 1–98 ppm, As = 0.5–60 ppm). The chemistry of early hematite stages is mostly inherited from host rock and precursor magnetite, e.g., Mn concentrations correlate with bulk Mn content in itabirite. With progressive iron ore formation and modification, external fluids play a more prominent role. This is reflected by REE-Y switching from seawater-like Y/Ho ratios (> 44) in early-, to more chondrite-like Y/Ho ratios (< 34), in late-hematite stages, likely due to fluid–rock reactions with country rocks (e.g., phyllites) or exchange with magmatic hydrothermal fluids.The following ore formation stages and key processes, supported by mineral scale mass balance calculations, are constrained: (1) martitisation, cogenetic with gangue leaching, is driven by large volumes of oxidising, Si-undersaturated fluids resulting in an absolute depletion of Mg, Mn, Al, Ti, Ni and Zn, and enrichment of Pb, As, LREE and Y; (2) the formation of granoblastic hematite and locally microplaty hematite represents a largely isochemical recrystallisation of magnetite and/or martite accompanied by a depletion of Mg and Y and an elevated Ti mobility at the mineral scale; and (3) precipitation of schistose and vein-hosted specular hematite along shear and fracture zones is driven by an external Fe–Si-rich hydrothermal fluid likely under high fluid/rock ratios.