Replacement Of Magnetite Research Articles

Iron oxidation and porosity generation in serpentinized abyssal peridotite

Serpentinization, the water-driven alteration of olivine-rich rocks, is a fundamental process contributing to planetary habitability and the maintenance of rock-hosted life on Earth. Serpentinization generates molecular hydrogen via the reduction of water and the coupled oxidation of Fe, which partitions into serpentine, brucite and magnetite. Magnetite formation controls the magnetic properties of serpentinites, and researchers have highlighted a link between increasing magnetic susceptibility and decreasing density of peridotites with increasing degree of serpentinization. Several analytical and theoretical studies have suggested that these increases in magnetization and decreases in density are accompanied by increasing porosity. Here, we investigate the potential correlation between porosity generation and serpentinization by analyzing the geochemical composition, Fe redox state, and porosity of 28 abyssal serpentinites recovered via drilling from the Atlantic Ocean subseafloor. Principal component analysis applied to the resulting dataset revealed no direct links between these properties, which we suggest results from the introduction of variability due to low-temperature seafloor weathering processes. Seafloor weathering can modify the porosity of serpentinites by dissolving brucite, and it affects the Fe redox state of serpentinites by inducing the replacement of magnetite by Fe(III) minerals like maghemite, hematite, and goethite. Moreover, we also suggest that ophiolitic serpentinites would not show direct correlations between porosity and Fe redox state, due to geochemical transformations induced by metamorphism and porosity reworking linked to deformation induced by tectonic stresses. Ultimately, our results demonstrate that weathering processes can significantly affect the chemical and physical properties of serpentinites and thus must be considered in geochemical and geophysical evaluations of these rocks.

Optimization of pyroprocessing of zinc sulfide ore to produce ferroalloy and zinc

The article presents the results of thermodynamic analysis and experiments on the electric smelting of Shalkiya ore (5.2% of ZnS, 1.0% of PbS, 50% of SiO2) with the joint producing a silicon-containing ferroalloy and extracting zinc and lead into sublimates. The studies included the thermodynamic modeling using the HSC-6.0 software package and electric smelting in an arc furnace. It was established that the interaction in a ZnS-Fe3O4-2C system under equilibrium conditions is of a stepwise nature with the initial reduction of Fe3O4 to Fe and FeO at 600-1000 °C, then the interaction of ZnS with Fe at 1100-1800 °C with the formation of gaseous zinc, and the interaction of ZnS with FeO with the predominant formation of zinc at temperatures above 1600 °C. The electric smelting a charge containing the Shalkiya ore, coke, magnetite concentrate (85.9% of Fe3O4) and steel shavings allowed us to establish that FeSi45 grade ferrosilicon (41.8-44.6% of Si), with the extraction of 80-85.4% of Si, can be produced in the presence of 24.9-30% of coke with the replacement of magnetite’s iron by steel shavings’ iron from 20.9 to 91.8%. A lower degree of this replacement is associated with intensive development of foaming and a decrease in the extraction of silicon into the alloy. At least 98% of zinc and 96% of lead are extracted into the resulting sublimates containing 29.4% of zinc and 15.1% of lead.

Laboratory evaluation of microwave heating and skid resistance of pavement friction surfacing using calcined bauxite and magnetite aggregates

A high friction surface treatment (HFST) with magnetite (MT) aggregates instead of conventional calcined bauxite (CB) aggregates as microwave absorbers were prepared in this paper. The deicing properties by microwave heating and long-term skid resistance of HFST with different proportions of CB and MT aggregate were evaluated. The deicing properties by microwave heating of HFST were determined by electromagnetic characteristics, heating surface temperature, and microwave deicing time (MDT) tests. The long-term skid resistance of the HFST was characterized by conducting dynamic friction coefficient and mean profile depth (MPD) tests after long-term polishing using a three-wheel polishing device. The results showed that the electromagnetic characteristics of HFST were enhanced greatly by the addition of MT. The microwave heating efficiency of HFST was enhanced by the increase of MT substitution rate, which further enhanced its microwave deicing efficiency. The microwave deicing efficiency of HFST with MT substitution rate of 25% was 1.4 times that of HFST with MT substitution rate of 0%. Long-term skid resistance of HFST was reduced by the addition of MT, however, HFST with MT substitution rate of 25% also maintained a terminal μ40 and μ60 above 0.7, which can provide higher friction comparable with other high friction aggregate such as taconite, basalt, emery and flint. To improve the deicing properties by microwave heating and maintain high skid resistance, 25% MT replacement rate in HFST is recommended.

Mineralogical and sulfur isotopic characteristics of Archean greenstone belt‐hosted gold mineralization at the Tau deposit of the Mupane gold mine, Botswana

AbstractThe Mupane gold deposit, which is one of the numerous gold occurrences in the Tati Greenstone Belt in the northeastern part of Botswana, consists of four orebodies, namely Tau, Tawana, Kwena, and Tholo deposits. The present research, which focuses on the genesis of the Tau deposit, was based on ore petrography, mineral chemistry of sulfides, and sulfur isotope data. Mineralogical characteristics of the host rocks indicate that banded iron formation at the Tau deposit includes iron oxides (magnetite), carbonates (siderite and ankerite), silicates (chlorite and amphibole), and sulfides (arsenopyrite and pyrrhotite). The deposit features arsenopyrite‐rich zones associated with biotite‐chlorite veins, which are indicative of the precipitation of arsenopyrite concomitant with potassic alteration. The replacement of magnetite by pyrrhotite in some samples suggests that sulfidation was likely the dominant gold precipitation mechanism because it is considered to have destabilized gold‐thiocomplexes in the ore‐forming fluids. Based on textural relationships and chemical composition, arsenopyrite is interpreted to reflect two generations. Arsenopyrite 1 is possibly early in origin, sieve textured with abundant inclusions of pyrrhotite. Arsenopyrite 1 was then overgrown by late arsenopyrite 2 with no porous textures and rare inclusions of pyrrhotite. Gold mineralization was initiated by focused fluid flow and sulfidation of the oxide facies banded iron formation, leading to an epigenetic gold mineralization. The mineralogical assemblages, textures, and mineral chemistry data at the Tau gold deposit revealed two‐stage gold mineralizations commencing with the deposition of invisible gold in arsenopyrite 1 followed by the later formation of native gold during hydrothermal alteration and post‐depositional recrystallization of arsenopyrite 1. Laser ablation inductively coupled plasma mass spectrometric analysis of arsenopyrite from the Tau deposit revealed that the hydrothermal event responsible for the formation of late native gold also affected the distribution of other trace elements within the grains as evidenced by varying trace elements contents in arsenopyrite 1 and arsenopyrite 2. The range of δ34S of gold‐bearing assemblages from the Tau deposit is restricted from +1.6 to +3.9‰, which is typical of Archean orogenic gold deposits and indicates that overall reduced hydrothermal conditions prevailed during the gold mineralization process at the Tau deposit. The results from this study suggest that gold mineralization involved multi‐processes such as sulfidation, metamorphism, deformation, hydrothermal alteration, and gold remobilization.

Replacement of magnetite by hematite in hydrothermal systems: A refined redox-independent model

The replacement of magnetite by hematite is commonly observed in various geologic systems. In contrast to the formation of hematite by a solid-state oxidation, numerous experimental results have demonstrated that it can also occur by a redox-independent dissolution-reprecipitation reaction. However, the orientation relationship and the intermediate products between magnetite and hematite remain unknown during the redox-independent replacement. In this work, high-resolution transmission electron microscopy (TEM) was used to investigate porous hematite from the Baishiya skarn deposit, East Kunlun orogenic belt, aiming to build a refined growth model applicable to the replacement of magnetite by hematite in natural hydrothermal systems. Initially, hydrothermal leaching of Fe2+ from magnetite led to the formation of ferrihydrite that transformed to goethite and hematite nanocrystals successively. Oriented attachment of pseudocubic hematite nanoparticles induced by Cl along specific crystallographic directions formed hematite mesocrystals on the exposed dodecahedral facets of magnetite, leading to an orientation relationship between magnetite and adjacent hematite (i.e., (110)magnetite || (012)hematite), which is different from that of oxidation (i.e., (110)magnetite || (110)hematite). However, oriented attachment can be imperfect in some instances, and dislocations of adjacent nanoparticles result. The dislocations in the hematite mesocrystals have acted as a closed space to capture the remaining ferrihydrite. When the Si concentration in ferrihydrite was sufficient, the solid-state conversion of the remaining ferrihydrite to hematite was blocked. We propose that repeated dissolution and reprecipitation of hematite mesocrystals (i.e., defective crystals) are required to remove Si, and thereby form defect-free hematite crystals, providing a genetic link between the widespread hematitisation and related multistage fluid infiltration in some of the world's richest deposits (e.g., Olympic Dam and Bayan Obo deposits). This is the first time that Si and Cl have been verified to act as key factors to shape the hematite growth pathway during the ore-forming processes, challenging the ‘ion-by-ion’ growth model that has underpinned our knowledge of mineral solubility, nucleation, and mass transfer from nano- to micron-scales in natural hydrothermal systems.

Trace element catalyses mineral replacement reactions and facilitates ore formation

Reaction-induced porosity is a key factor enabling protracted fluid-rock interactions in the Earth’s crust, promoting large-scale mineralogical changes during diagenesis, metamorphism, and ore formation. Here, we show experimentally that the presence of trace amounts of dissolved cerium increases the porosity of hematite (Fe2O3) formed via fluid-induced, redox-independent replacement of magnetite (Fe3O4), thereby increasing the efficiency of coupled magnetite replacement, fluid flow, and element mass transfer. Cerium acts as a catalyst affecting the nucleation and growth of hematite by modifying the Fe2+(aq)/Fe3+(aq) ratio at the reaction interface. Our results demonstrate that trace elements can enhance fluid-mediated mineral replacement reactions, ultimately controlling the kinetics, texture, and composition of fluid-mineral systems. Applied to some of the world’s most valuable orebodies, these results provide new insights into how early formation of extensive magnetite alteration may have preconditioned these ore systems for later enhanced metal accumulation, contributing to their sizes and metal endowment.

Petrography of martite–goethite ore and implications for ore genesis, South Flank, Hamersley Province, Western Australia

Martite–goethite (M–G) ores are characterised by the pseudomorphic textural replacement of gangue phases (carbonate, silicate and quartz) in the primary banded iron formation (BIF) by fine-grained goethite. This differentiates them petrographically from rocks enriched in Fe as a result of lateritic weathering and leaching of gangue. A preliminary petrographic study of martite–goethite mineralisation from the Grand Central deposit at BHP’s South Flank in the central Hamersley Province reveals that pseudomorphic textures are abundantly preserved. The mineralisation occurs in the Marra Mamba Iron Formation. Magnetite is converted to martite, and the quartz–silicate–carbonate bands are pseudomorphed by goethite. Of the gangue phases, carbonate and silicate minerals are more susceptible to replacement than is quartz. The replacement of quartz by goethite does not always continue to completion, and relict quartz grains are typically preserved on the lower margin and down-dip extremity of the body of mineralised BIF. The mineralisation appears to take place in three stages. The first stage involves replacement of magnetite by hematite (martitisation) and of the carbonate–silicate–quartz gangue by very fine-grained goethite that pseudomorphs and thus preserves the original texture of the non Fe-oxide portion of the BIF protolith. This ‘Stage 1’ goethite generation appears to be ochreous. Micro-porosity development, most likely developing through dissolution of any remaining quartz that has escaped replacement by goethite, although possibly through the dissolution of goethite after quartz, forms the second stage. The third and final stage is characterised by infill of the micro-porosity to varying degrees by a second generation of coarser-grained goethite that typically rims pores, giving them a comb-textured, drusy appearance. This ‘Stage 3’ goethite generation appears to be brown goethite. This tripartite paragenesis has not been explicitly recognised in previous petrographic studies of M–G mineralisation. Subsequent lateritic weathering affects material in the vadose zone above the pre-mining water-table, introducing additional complexity to the mineralisation. Further dissolution takes place, resulting in meso-scale porosity that is clearly visible to the naked eye. Voids are rimmed by a colloform intergrowth of goethite and hydrohematite. This ‘Stage 4’ goethite is likely to be vitreous goethite. Previous studies have shown that the three different types of goethite (ochreous, brown and vitreous) have distinctly different chemical and physical characteristics, and behave differently during downstream processing of Fe ore, so it is important to understand their genesis and distribution in time and space. KEY POINTS The process of M-G mineralisation involves a tripartite paragenesis: Stage 1—replacement of magnetite by hematite (martitisation) and of the carbonate–silicate–quartz gangue by very fine-grained goethite that pseudomorphs the original texture, Stage 2—dissolution of remaining gangue phases (principally quartz), and Stage 3—infill of the micro-porosity to varying degrees by a second generation of coarser-grained goethite. Overprinting effects of weathering form Stage 4—further dissolution and reprecipitation of Fe as colloform goethite-(hydro)hematite layers lining vugs. Goethites formed during the three different stages are likely to have different chemical and physical properties and this has implications for geometallurgical and materials handling behaviour during processing.

Dating hypogene iron mineralization events in Archean BIF at Weld Range, Western Australia: insights into the tectonomagmatic history of the northern margin of the Yilgarn Craton

This study is the first to constrain the absolute timing of hypogene iron mineralization in Archean BIF located in the Yilgarn Craton. In situ SHRIMP U–Th–Pb geochronology on xenotime and monazite grains has been used to constrain the age of hypogene magnetite replacement ores at the Beebyn deposit and hypogene magnetite vein ores at the Madoonga deposit in the Weld Range study area. The Beebyn magnetite replacement ores (c. 2627 Ma) are younger than the published maximum depositional age of the BIF hosts of the Wilgie Mia Formation (2792 ± 9 Ma) and partially overlaps crystallization ages of granitic rocks (2757–2606 Ma) that intrude supracrustal rocks throughout the study area. These plutons, and their probable subvolcanic expressions, are considered to be likely sources of energy and fluids responsible for magnetite replacement ores. In contrast, Madoonga magnetite veins record multiple dates: the first at 2857 ± 41 Ma, followed by events at c. 2775 Ma through 1812 Ma. The two oldest monazite dates of 2857 ± 41 and 2832 ± 51 Ma are interpreted to be mineralization ages for magnetite veins, possibly related to subseafloor volcanism and base metal VMS systems; whereas the younger dates (i.e. 2775–1812 Ma) probably represent multiple episodes of phosphate mineral precipitation related to reactivation of structures and overprinting by discrete pulses of hydrothermal fluids. The Madoonga BIFs are genetically distinct from the Beebyn BIFs and are the oldest dated BIFs at Weld Range, being older than c. 2857 Ma, but younger than the c. 2970 Ma felsic volcanic rocks in the stratigraphic footwall to the Madoonga BIFs. Hydrothermal events at the Madoonga deposit (2215–2120 Ma) coincide with published dates for rifting, mafic–ultramafic magmatism, and basin development along the northern margin of the Yilgarn Craton (2215–2145 Ma), whereas the younger phosphate mineral dates correspond with the Glenburgh (2002–1947 Ma) and Capricorn Orogenies (1817–1772 Ma). Tectonic activity along the northern margin of the Yilgarn Craton coincides with reported ages for hematite mineralization in BIF-hosted iron-ore deposits in the Hamersley Basin and Pilbara Craton, suggesting the far-reaching effects of tectonism along paleocratonic boundaries as drivers for iron mineralization in BIF.

Development of Heavyweight Self-Compacting Concrete and Ambient-Cured Heavyweight Geopolymer Concrete Using Magnetite Aggregates.

Heavyweight self-compacting concrete (HWSCC) and heavyweight geopolymer concrete (HWGC) are new types of concrete that integrate the advantages of heavyweight concrete (HWC) with self-compacting concrete (SCC) and geopolymer concrete (GC), respectively. The replacement of natural coarse aggregates with magnetite aggregates in control SCC and control GC at volume ratios of 50%, 75%, and 100% was considered in this study to obtain heavyweight concrete classifications, according to British standards, which provide proper protection from sources that emit harmful radiations in medical and nuclear industries and may also be used in many offshore structures. The main aim of this study is to examine the fresh and mechanical properties of both types of mixes. The experimental program investigates the fresh properties of HWSCC and HWGC through the slump flow test. However, J-ring tests were only conducted for HWSCC mixes to ensure the flow requirements in order to achieve self-compacting properties. Moreover, the mechanical properties of both type of mixes were investigated after 7 and 28 days curing at an ambient temperature. The standard 100 × 200 mm cylinders were subjected to compressive and tensile tests. Furthermore, the flexural strength were examined by testing 450 × 100 × 100 mm prisms under four-point loading. The flexural load-displacement relationship for all mixes were also investigated. The results indicated that the maximum compressive strength of 53.54 MPa was achieved by using the control SCC mix after 28 days. However, in HWGC mixes, the maximum compressive strength of 31.31 MPa was achieved by 25% magnetite replacement samples. The overall result shows the strength of HWSCC decreases by increasing magnetite aggregate proportions, while, in HWGC mixes, the compressive strength increased with 50% magnetite replacement followed by a decrease in strength by 75% and 100% magnetite replacements. The maximum densities of 2901 and 2896 kg/m3 were obtained by 100% magnetite replacements in HWSCC and HWGC, respectively.

Combined igneous and hydrothermal source for the Kiruna-type Bafq magnetite-apatite deposit in Central Iran; trace element and oxygen isotope studies of magnetite

The main deposits of the Bafq district (Choghart, Se Chahun and Chadormalou deposits) are magnetite-apatite ore bodies with Early Cambrian age, hosted by Early Cambrian volcano-sedimentary rocks, showing lava flow structure with miarolitic cavities and occasional brecciation. The texture and geochemistry (trace elements and O-isotope) of the studied magnetite crystals show three types of magnetite with different proportions in different parts of the district and the deposits. These are primary ignious magnetite and high-temperature hydrothermal magnetite in the massive parts of the deposits and foam-like magnetite in the veins or in the fractures of the massive part. The lack of zonation, high Ti/Fe ratios and δ18O values (2‰<) indicate that massive magnetites formed at high temperature condition, while foam-like magnetite recrystallized at lower temperature. Textures and Co, V, Ti, Al, and Pb contents are used to distinguish ignious magnetites from the high-temperature hydrothermal magnetites. Foam-like magnetites contain lower REE and has lower δ18O values (2‰>) than the massive magnetites.The magmatic origin for the main part of the ores in the Bafq is more plausible. Contamination of the primary magmas of the Bafq magmatic rocks with crustal phosphorus and evaporate units was the major agent for triggering iron-rich melt immiscibility from the original magma. Concentration of volatiles in the iron-rich melt involving in crystallization of the primary magnetite and nucleation of the aqueous fluid bubbles on magnetite surfaces led to ascending the buoyant fluid bubble-magnetite aggregates. This caused continuation of the magmatic magnetite microlites growth from the iron-rich magmatic-hydrothermal fluid and formed high-temperature hydrothermal magnetite. Formation of hypersaline fluid due to crystallization of iron-rich melt at shallow depth led to crystallization of titanite (and Th-U-REE-bearing minerals) and sodic-calcic alteration related to magnetite mineralization. Microscopic evidence (replacement of magnetite by hematite and filling of fractures of thorite by magnetite) show that degassing continued at shallow depth, which led to re-crystallization of the primary magnetites and formation of the foam-like magnetites.

Mechanism and kinetics of hydrothermal replacement of magnetite by hematite

The replacement of magnetite by hematite was studied through a series of experiments under mild hydrothermal conditions (140–220 °C, vapour saturated pressures) to quantify the kinetics of the transformation and the relative effects of redox and non-redox processes on the transformation. The results indicate that oxygen is not an essential factor in the replacement reaction of magnetite by hematite, but the addition of excess oxidant does trigger the oxidation reaction, and increases the kinetics of the transformation. However, even under high O2(aq) environments, some of the replacement still occurred via Fe2+ leaching from magnetite. The kinetics of the replacement reaction depends upon temperature and solution parameters such as pH and the concentrations of ligands, all of which are factors that control the solubility of magnetite and affect the transport of Fe2+ (and the oxidant) to and from the reaction front. Reaction rates are fast at ∼200 °C, and in nature transport properties of Fe and, in the case of the redox-controlled replacement, the oxidant will be the rate-limiting control on the reaction progress. Using an Avrami treatment of the kinetic data and the Arrhenius equation, the activation energy for the transformation under non-redox conditions was calculated to be 26 ± 6 kJ mol−1. This value is in agreement with the reported activation energy for the dissolution of magnetite, which is the rate-limiting process for the transformation under non-redox conditions.

Petrography and trace element signatures of iron-oxides in deposits from the Middleback Ranges, South Australia: From banded iron formation to ore

The Middleback Ranges is a major iron ore belt in the southeastern region of the Gawler Craton, South Australia, interpreted to be of BIF origin. Iron ore deposits are hosted within ∼2550 Ma metasedimentary rocks of the Middleback Group and occur as a series of N-S trending hills, forming a ∼60 km-long magnetic anomaly. A petrographic-geochemical study of iron-oxides from BIFs and iron ores was undertaken on samples from thirteen locations spanning the strike of the belt. Iron-oxides are texturally diverse due to multiple processes accompanying and postdating ore formation. Primary magnetite features preserved in the southern segment of the belt display distinct overprinting features (e.g., increased porosity, reworked grain boundaries) and multiple generations of growth associated with deposition of trace minerals, including native gold. Northwards along strike, this overprint is expressed by the pseudomorphic replacement of magnetite by hematite (martite) and is locally associated with brecciation, the presence of rare earth element (REE)-minerals, and replacement by iron-hydroxides. Whereas evidence of microplaty hematite accompanying martitization and predating iron-hydroxides is observed throughout the belt, distinct iron-oxide generations postdating the iron-hydroxides are inferred based on compositional zoning with respect to Si and intimate relationships between iron-oxides and -hydroxides. Chondrite-normalized fractionation trends obtained from the various iron-oxides and from different BIF types generally display LREE-enrichment and distinct positive Eu- and Y-anomalies. An increasing ΣREY- and LREE-trend accompanies martitization. The presence of specific element groups, e.g., granitophile elements (U, W, Sn, Mo), or transition metals (Cr, Mn, Co, Ni, Ti, V, Nb) within the lattice of iron-oxides suggests their formation in evolving environments associated with the emplacement of felsic and mafic lithologies, respectively. The impact of local setting on iron-oxide formation is highlighted by complex trace element signatures of iron-oxides; the northern segment of the belt is relatively enriched in As and Sb, while the southern segment is enriched in granitophile elements and REY. Further complexity is shown in the local variation of Mn and Zn in iron-oxides in the southern segment of the belt, where the Iron Magnet deposit shows the strongest correlation between the two elements. Various depositional settings for BIFs are inferred based upon Post-Archean Australian Shale-normalized REY fractionation trends of iron-oxides and various water types. These settings include environments where iron-minerals precipitate from mixtures of 1) anoxic seawater and high-temperature hydrothermal fluids, 2) anoxic seawater and low-temperature hydrothermal fluids, and 3) oxygen-richer seawater and low-temperature hydrothermal fluids. A genetic model for ore formation is proposed based upon textural and compositional variations observed in iron-oxides throughout the belt and includes formation resulting from supergene fluids rich in elements leached from local granites penetrating BIF, interaction with granite-derived hydrothermal fluids, and heat generated during emplacement of younger dikes. Recognition of petrographic features linked to changes in composition demonstrates the utility of iron-oxides to trace iron ore formation in a temporal and spatial context, with implications for ore genesis and models of mineral exploration.

EARLY, DEEP MAGNETITE-FLUORAPATITE MINERALIZATION AT THE OLYMPIC DAM Cu-U-Au-Ag DEPOSIT, SOUTH AUSTRALIA*

The Olympic Dam iron oxide copper-gold (IOCG)-uranium-silver deposit (South Australia) is hosted in the large Olympic Dam breccia complex within the ~1.59 Ga Roxby Downs Granite. This breccia complex formed through multiple stages of hydrothermal activity and texturally destructive brecciation that affected the granite. The deepest diamond drill hole to date (RD2773, end at ~2,329 m) intersected weakly altered, in situbrecciated granite (~370–2,329 m) and a quartz-phyric felsic unit (~2,010–2,265 m). These two rock units host coarse-grained hydrothermal minerals, from ~2,150 m to the end of the drill hole. The main minerals in this assemblage are magnetite (± hematite), pyrite, fluorapatite, and quartz, with minor disseminated chalcopyrite, sericite, chlorite, rare earth element (REE)-fluorcarbonates, monazite, uraninite, thorite, galena, sphalerite, anhydrite, schorl, rutile, and pyrrhotite. The assemblage is cut by abundant multiphase veinlets and calcite (± fluorite ± barite) veins. A zircon U-Pb age for the felsic unit (1591 ± 11 Ma) implies that this unit is broadly coeval with the granite, whereas U-Pb ages for hydrothermal uraninite (1593.5 ± 5.1 Ma), fluorapatite (1583.3 ± 6.5 Ma), and hematite (1592 ± 15 Ma) indicate that deposition of the U-REE–rich hydrothermal magnetite-fluorapatite-pyrite-quartz assemblage and replacement of magnetite by hematite occurred soon after emplacement of the granitic host rocks. Sm-Nd dating of ubiquitous calcite veins suggests formation at ~1.54 Ga. The deep ~1.59 Ga magnetite-fluorapatite-pyrite-quartz assemblage at Olympic Dam resembles those characteristic of iron oxide-apatite deposits and many other sensu stricto IOCG deposits. This study confirms that the ~1.59 Ga event involved significant and widespread IOCG mineralization in the Olympic Cu-Au province.

Genesis history of iron ore from Mesoarchean BIF at the Wodgina mine, Western Australia

ABSTRACTSeveral iron-ore deposits hosted within Mesoarchean banded iron formations (BIFs) are mined throughout the North Pilbara Craton, Western Australia. Among these, significant goethite±martite deposits (total resources >50 Mt at 55.8 wt% Fe) are distributed in the Wodgina district within 2 km of the world-class pegmatite-hosted, tantalum Wodgina deposits. In this study, we investigate the dominant controls on iron mineralisation at Wodgina and test the potential role of felsic magma-derived fluids in early alteration and upgrade of nearby BIF units. Camp-scale distribution and geochemistry of iron ore at Wodgina argue against any significant influence of identified felsic intrusions in the upgrade of BIF. Whereas, the formation of BIF-hosted goethite±martite iron ore at Wodgina involves: (i) early (ca 2950 Ma) metamorphism of BIF causing camp-scale recrystallisation of pre-existing iron oxides to form euhedral magnetite, with local enrichment to sub-economic grades (∼40 wt% Fe) within or proximal to metre-wide, bedding-parallel shear zones, and (ii) later supergene lateritic enrichment of the magnetite-bearing BIF and shear zones, forming near-surface goethite±martite ore. The supergene alteration sequence includes: (i) downward progression of the oxidation front and replacement of magnetite by martite, (ii) local development of silcrete at ∼40 m below the modern surface caused by the lowering of the water-table, (iii) intensive replacement of quartz by goethite, resulting in the goethite±martite ore bodies at Wodgina, and (iv) late formation of ferricrete and ochreous goethite. Goethitisation most likely took place within the hot and very wet climate that prevailed from the Paleocene to the mid-Eocene. Goethite precipitation was accompanied by the incorporation of trace elements P, Zn, As, Ni and Co, which were likely derived from supergene fluid interaction with nearby shales. Enrichment of these elements in goethite-rich ore indicates that they are potentially useful pathfinder elements for concealed ore bodies covered by trace element-depleted pedogenic silcrete and siliciclastic rocks located throughout the Wodgina mine.

The Tasiast deposit, Mauritania

The Tasiast gold deposits are hosted within Mesoarchean rocks of the Aouéouat greenstone belt, Mauritania. The Tasiast Mine consists of two deposits hosted within distinctly different rock types, both situated within the hanging wall of the west-vergent Tasiast thrust. The Piment deposits are hosted within metasedimentary rocks including metaturbidites and banded iron formation where the main mineral association consists of magnetite-quartz-pyrrhotite±actinolite±garnet±biotite. Gold is associated with silica flooding and sulphide replacement of magnetite in the turbidites and in the banded iron formation units. The West Branch deposit is hosted within meta-igneous rocks, mainly diorites and quartz diorites that lie stratigraphically below host rocks of the Piment deposits. Most of the gold mineralisation at West Branch is hosted by quartz–carbonate veins within the sheared and hydrothermally altered meta-diorites that constitute the Greenschist Zone. At Tasiast, gold mineralisation has been defined over a strike length>10km and to vertical depths of 740m. All of the significant mineralised bodies defined to date dip moderately to steeply (45° to 70°) to the east and have a south–southeasterly plunge. Gold deposits on the Tasiast trend are associated with second order shear zones that are splays cutting the hanging wall block of the Tasiast thrust. An age of 2839±36Ma obtained from the hydrothermal overgrowth on zircons from a quartz vein is interpreted to represent the age of mineralisation.

Hematite replacement of iron-bearing precursor sediments in the 3.46-b.y.-old Marble Bar Chert, Pilbara craton, Australia

The history of atmospheric oxygen prior to the Great Oxidation Event (2.45–2.2 Ga) is not well understood. Hematite in the Marble Bar Chert from a NASA-funded drill hole (ABDP1) in the Pilbara craton, Australia, has been cited as evidence for an oxygenated ocean 3.46 b.y. ago. However, isotopic data from the same drill hole have been used to argue for an anoxic ocean. It is generally agreed that the hematite is primary, representing either a direct hydrothermal precipitate or a dehydration product of iron oxyhydroxides that formed during anoxygenic photosynthesis. Here we present new petrographic evidence from the Marble Bar Chert (in drill hole ABDP1) that shows that hematite in jasper bands formed via mineral replacement reactions. The hematite mostly occurs as sub-micron–sized inclusions within chert (so-called “dusty” hematite) that are typically arranged into polygonal clusters surrounded by a rim of clear quartz, resembling shrinkage structures. The lateral transition from laminated chert enclosing minute inclusions of greenalite, siderite, and magnetite to chert dominated by dusty hematite provides evidence for in situ replacement of iron-bearing minerals. The presence of hematite-rich bands containing octahedral crystals with residual cores of magnetite indicates that some of the hematite was derived from the replacement of magnetite. This interpretation is supported by the widespread occurrence of magnetite in jasper displaying progressive stages of replacement, from unaltered octahedral inclusions in quartz to hematite pseudomorphs along quartz grain boundaries. The occurrence of dusty hematite in fractures, sedimentary laminae, and the outer margins of polygonal clusters containing greenalite is consistent with fluid-mediated oxidation of iron-rich precursor minerals. The presence of syn-sedimentary chert breccias comprising rotated fragments of laminated chert indicates that the precursor sediment was silicified shortly after deposition. The abundance of “dusty” greenalite inclusions, which are texturally the earliest components of the laminated chert, suggests that the precursor sediment contained an iron-rich clay mineral. Our results show that hematite has replaced ferrous-rich minerals after deposition and provide a mechanism to explain the origin of hematite in the Marble Bar Chert, which is consistent with the origin of hematite in adjacent basalts. A secondary origin for hematite invalidates arguments for an oxygen-bearing ocean ∼3.46 b.y. ago and provides a viable explanation for the formation of Archean jasper bands. Our findings show that misinterpretations about the origin of hematite in early Precambrian cherts could lead to false conclusions about the chemistry of the ancient ocean and atmosphere.

Mineral chemistry and geothermometry using relict primary minerals in the La Cocha ultramafic body: A slice of the upper mantle in the Sierra Chica of Córdoba, Sierras Pampeanas, Argentina

Abstract The La Cocha ultramafic body, in the Sierra Chica of Cordoba (Sierras Pampeanas, Argentina), is formed by serpentinized spinel harzburgites, with lenses of spinel pyroxenites and hornblendites. The associated metamorphic rocks are garnet sillimanite gneisses, intercalated with tabular bodies of pyroxene amphibolites and forsterite marbles. Mineral chemistry of relict primary phases (olivine, orthopyroxene and spinel) from samples of the spinel harzburgites and pyroxenites was determined, and several geothermometers were applied to estimate the temperature conditions under which these rocks may have been equilibrated. In the spinel harzburgites, the primary spinel is Al–chromite [Cr# = Cr/(Cr + Al) = 0.48–0.57], which is replaced by ferrichromite and chlinoclore by metamorphism. Orthopyroxene is enstatite (En 92 ) and olivine is classified as forsterite (Fo 92 ); this last one shows a homogeneous and constant composition along the ultramafic body. Using geothermometric calibrations of the pair olivine–spinel, the highest temperature of 1157 °C would correspond to the primary conditions of formation of the harzburgites. The spinel pyroxenites show a mineral composition defined by orthopyroxene (En 85 , enstatite), olivine (Fo 86 , chrysolite), spinel ( s. s. ) and magnetite. Serpentine and clinochlore were produced by metamorphism. Spinel has high concentrations in Al and very low in Cr, and is classified as spinel sensu stricto ; magnetite replacement was produced by metamorphism. Orthopyroxene and olivine are depleted in MgO regarding these minerals in the harzburgites. Temperatures of 785–734 °C calculated using geothermometers with orthopyroxene are interpreted to be produced by metamorphism in amphibolite to granulite facies conditions. Cumular textures were not observed in outcrops and thin sections of the studied rocks. The narrow composicional range and high forsterite content in olivine, the high Cr# in spinel, and the low concentrations of Ni and Cr in whole rock analyses indicate a mantle residual origin for these peridotites, which would exclude a cumular origin. The association of peridotites with mafic bodies formed in an N-type MORB environment and a relict mantle fabric showed by the elongated crystals/aggregates of olivine (preserved in pseudomorphic replacements), indicating a high temperature flow, allow to interpret the La Cocha ultramafic body as a slice of oceanic mantle, belonging to basal tectonites of an ophiolite complex. This body shows similar petrological, geochemical and structural features than the other ultramafic bodies in the Sierras de Cordoba, therefore the origin proposed here could be applied to the other bodies.

Formation of As(II)-pyrite during experimental replacement of magnetite under hydrothermal conditions

A ‘new’ type of arsenian pyrite was formed during experimental replacement of magnetite under hydrothermal conditions (T=125 and 220°C; Psat) and in the presence of S(-II) and various As-containing species. The amount of As in pyrite depended on the As-source, with sources containing cationic As (As(II), As(III) and As(V)) resulting in considerably higher amounts of As in the product arsenian pyrite than anionic sources. The highest As content was 23.83±0.20wt%, corresponding to a S:Fe:As molar ratio of 2:0.58:0.42. Electron probe micro-analyses revealed an inverse correlation between the Fe and As contents in the arsenian pyrite, indicating that As is substituting for Fe. Arsenic concentrations were highly inhomogeneous within the pyrite rim; in general, higher As contents were found within solid pyrite growing on the outer rim, compared to the highly porous and texturally complex pyrite found close to the reaction boundary. This likely reflects different uptake mechanisms for As during the pyrite nucleation and growth stages. X-ray Absorption Near Edge Structure (XANES) analyses showed that the As in the arsenian pyrite was predominantly in the form of As(II). Cross-sectional X-ray photoelectron spectroscopy (XPS) analysis of the arsenian pyrite confirmed the presence of As(II), but also showed evidence for more oxidized species (As(III) and As(V) oxides), as well as small amounts of polymeric As–As bonding. This indicates a large difference between As in the bulk (XANES measurements) and at the pyrite surface (XPS). Ab initio XANES calculations are consistent with As replacing Fe in pyrite in the form of As(II). Our experimental study suggests that the formal oxidation state of As in this type of arsenian pyrite is close to +2, and that in addition to fluid composition and oxidation state, the reaction path leading to pyrite formation plays a significant role in controlling the chemistry of arsenian pyrite.

Geochemical Evolution of the Banded Iron Formation-Hosted High-Grade Iron Ore System in the Koolyanobbing Greenstone Belt, Western Australia

The banded iron formation (BIF)-hosted iron ore deposits in the lower greenstone succession of the Koolyanobbing greenstone belt, 50 km north of Southern Cross in Western Australia, are a ∼200 Mt high-grade Fe (>58%) pre-mining resource and represents one of the most important iron ore districts in the Yilgarn craton. Four hypogene alteration (ore-forming) stages and one supergene upgrading event took place: (1) During ore stage 1, LREE-depleted, transition metal-enriched, Mg-Fe (±Ca) carbonates replaced quartz in BIFs. The deposit-scale alteration was most likely induced by devolatilization of sea-floor-altered, Ca-Si-depleted mafic rocks in the vicinity of the BIF during early regional (syn-D 1), very low to low-grade metamorphism and was most strongly developed on reactivated BIF-basalt contacts. (2) Ore stage 2 involved the formation of patchy magnetite ore by a syn-D 2 to-D 4 dissolution of early carbonate. Enrichment of Fe 2O 3total in magnetite iron ore was by a factor of 2 to 2.4, and compatible trace elements in magnetite, such as Ga, V, and Al, were immobile. A subdeposit-scale ferroan talc-footprint proximal to magnetite iron ore in the largest deposit (K deposit) was associated with ore stage 2 and resulted from dissolution of magnesite due to reaction with silica in the BIF under greenschist facies conditions and potentially high fluid/rock ratio. (3) Magnetite growth, during ore stage 3, forming granular magnetite-martite ore is related to a subsequent hydrothermal event, occurring locally throughout the belt, especially in D 2b faults. (4) Ore stage 4 was associated with Fe-Ca-P-(L)REE-Y-enriched hydrothermal fluids, possibly from a magmatic source such as the postmetamorphic Lake Seabrook granite that crops out about 10 km west of the Koolyanobbing deposits and at the southern margin of the greenstone belt. These Ca-enriched fluids interacted with distal metamorphosed mafic rock and influenced the BIF-ore system in a small number of deposits. They were channelled through regional D 4 faults and caused specularitedolomite- quartz alteration, resulting in Fe grades of up to 68%. (5) Supergene upgrade (ore stage 5) by (further) gangue leaching in the weathering zone was most effective in carbonate-altered BIFs and magnetite ore. This process, together with supergene martitization and goethite replacement of magnetite, led to the formation of high-grade, locally (at K deposit) high P goethite-martite ore. At Koolyanobbing, the two geochemically distinct stages of Archean carbonate alteration clearly controlled the formation of hypogene magnetite-specularite- martite-rich ore and recent supergene modification, including the further upgrade of Fe ore. © 2012 by ECONOMIC GEOLOGY.

Rock magnetic characterization of early and late diagenesis in a stratigraphic well from the Llanos foreland basin (Eastern Colombia)

Abstract We have carried out rock magnetic characterizations of different lithofacies along the stratigraphic well Saltarín 1A, in order to learn about the various diagenetic events that could have affected the Miocene sequence of the Llanos foreland basin (Colombia). Thermomagnetic and low-temperature susceptibility measurements performed on some selected samples were complemented with analyses of Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) and isothermal remanent magnetization (IRM) acquisition curves. The identification of the magnetic mineral assemblages at each depth level analysed, as well as their relative concentrations, were determined from a direct signal analysis (DSA) of the IRM curves. Samples from the top of the Guayabo formation reveal the presence of hydrocarbons-related microscopic framboids of pyrite with partial replacement of magnetite. The bottom of the Guayabo formation shows hematite and goethite and appears to record a thoroughly documented Middle Miocene global regression. In samples from the León formation, pyrrhotite could have resulted from an early diagenesis that took place in a lacustrine environment via sulphate reduction. Traces of crude oil in samples from the Carbonera formation, and the additional occurrence of hematite and magnetite, suggest that a hydrocarbons-mediated late diagenesis could also have affected the lowermost levels of Saltarín 1A.