Iron Oxide Copper-gold Deposits Research Articles

Contrasting magma chemistry in the Candelaria IOCG district caused by changing tectonic regimes

Iron oxide-copper-gold (IOCG) deposits are a vital source of copper and critical elements for emerging clean technologies. Andean-type IOCG deposits form in continental arcs undergoing extension, and they have a temporal relationship with magmatism although they do not exhibit a close spatial relation with the causative intrusions. The processes required to form IOCG deposits and their potential connections to iron oxide–apatite (IOA)-type mineralization remain poorly constrained, as well as the characteristics of magmatism linked to both deposit types. Here we combine zircon U–Pb geochronology with zircon trace element geochemistry of intrusive rocks associated with the Candelaria deposit, one of the world’s largest IOCG deposits, to unravel distinctive signatures diagnostic of magmatic fertility. Our results reveal a marked transition in the geochemistry of intrusions in the Candelaria district, characterized by changes in the redox state, water content and temperature of magmas over time. The oldest magmatic stage (~ 128–125 Ma), prior to the formation of the Candelaria deposit, was characterized by zircon Eu/Eu* ratios of 0.20–0.42, and redox conditions of ΔFMQ − 0.4 to + 1.0. The earliest magmatic stage related to the formation of Fe-rich mineralization at Candelaria (118–115 Ma) exhibits low zircon Eu/Eu* ratios (0.09–0.18), low oxygen fugacity values (ΔFMQ ~− 1.8 to + 0.2) and relatively high crystallization temperatures. In contrast, the youngest stage at ~ 111–108 Ma shows higher zircon Eu/Eu* (~ 0.37–0.69), higher oxygen fugacity values (ΔFMQ ~ + 0.4 to + 1.3) and a decrease in crystallization temperatures, conditions that are favorable for the transport and precipitation of sulfur and chalcophile elements. We conclude that Candelaria was formed through two distinct ore-forming stages: the first associated with a reduced, high temperature, water-poor magma developed under a low tectonic stress, followed by a more oxidized, water-rich, and low temperature magmatic event related to a compressional regime. The first event led to Fe-rich and S-poor IOA-type mineralization, while the second event with geochemical signatures similar to those of porphyry copper systems, generated the Cu- and S-rich mineralization. This late stage overprinted preexisting IOA mineralization resulting in the formation of the giant Candelaria IOCG deposit.

Paragenetic and geological setting of the Starra iron oxide copper–gold deposits, Mount Isa Inlier, Queensland, Australia: constraints on IOCG deposit models

The five Starra iron oxide copper–gold (IOCG) deposits, which are located in the Cloncurry district of the Mount Isa Inlier, NW Queensland, are hosted in a N-trending zone of deformed iron-rich metasedimentary rocks that are underlain by two mineralogically distinct mafic–intermediate meta-igneous rocks. Copper–Au mineralisation is subdivided into two stages—an early main stage that transitioned from a magnetite-dominated Cu substage comprising chalcopyrite, pyrite, biotite and chlorite, to a hematite–muscovite-bearing Cu–Au substage with chalcopyrite, bornite and rare chalcocite, as well as Au–Ag-tellurides and native Au. This second stage was followed by a subordinate but nonetheless important carbonate–quartz–barite vein stage with minor but locally high-grade Cu–Au mineralisation characterised by abundant chalcocite and bornite, minor native bismuth and wittichenite (Cu3BiS3), together with minor Au–Ag telluride minerals. Bornite in these carbonate–quartz–barite veins contains considerable amounts of Au (>10 ppm), which appears to be deported either as lattice substitution in the bornite crystal structure or as homogeneously distributed Au0 nanoparticles. An earlier, coarse-grained mafic intrusion was affected by early Na–Ca alteration, whereas a second mafic intrusion lacks evidence of Na–Ca alteration but was exposed to K–Fe alteration and subsequent mineralisation at Starra 222. Mushketovite (magnetite after specular hematite) is only observed in ironstone lodes proximal to the mafic–intermediate rocks. The close spatial and mineralogical associations of the mafic–intermediate bodies and the ironstone lodes point to a stronger link between magmatic activity and IOCG mineralisation than has previously been recognised at Starra. Remobilisation of gold by late-stage hydrothermal fluids aided by the relatively low fluid temperatures during the main pulse of Cu–Au mineralisation refined gold grades at Starra, making it a relatively gold-rich IOCG deposit. We show that the Cu–Au deposits of the Starra trend feature mineralogical and geochemical elements common to both Cloncurry IOCG deposits (e.g. abundant magnetite) and IOCG deposits of the Tennant Creek district (e.g. Au–Cu–Bi mineralisation; hematite–bornite–chalcocite assemblages), making them a unique and important member of this complex family of deposits.

Polysomatic intergrowths between amphiboles and non-classical pyriboles in magnetite: smallest-scale features recording a protracted geological history

Abstract Non-classical pyriboles (NCPs) have tetrahedral silicate chains (Ibeam) of multiplicity higher than single (pyroxene) or double (amphibole) Ibeams and are known from amphiboles in altered mafic-ultramafic complexes. NCPs, their polysomatic sequences, and inherent chain-width disorder are petrogenetic tools for interpreting igneous and metamorphic processes. Magnetite, a refractory mineral that can trap and preserve NCPs is a major constituent of iron oxide-copper-gold (IOCG) deposits. We undertook a nanoscale study to show that NCPs and amphiboles are hosted within magnetite cores from the Jatobá Ni-bearing IOCG deposit, Carajás district, Brazil. Monoclinic amphiboles and NCPs form polysomatic intergrowths or occur as sparse inclusions along {111}magnetite. There are two chemical populations of amphiboles: Mg-Fe- and Ca-(Al)-amphiboles, the latter including Ce-bearing Mg-hornblende and (ferro)tschermakite. The occurrence contains one of the widest ranges of chain silicates ever recorded, from simple intergrowths of single to triple Ibeam zippers, including pyroxene slabs, to longer NCP polysomes up to 15-Ibeam chains. Clinojimthompsonite (Cjim) is observed for the first time within magnetite. Although no discrete polysomes could be defined, the NCP-amphibole intergrowths have composition between Mg-Fe amphiboles and Cjim based on Ibeam averages of 2.5-2.7. Relationships between increase in the number of C and A cations from amphibole (2-Ibeam) to n chain silicates (nIbeam) are formulated as nIbeam = T(2+n) = C(5+3n) = A(1+n), n=integer. Empirical models of crystal structures, validated by STEM simulation, are shown for 4- and 5-Ibeam chain silicates. Co-crystallization of double- and triple-chain silicate structures with rhythmic intergrowths as larger blocks along b is often accompanied by rhythmic Ca-Fe zonation along a, supporting primary NCP crystallization via self-patterning during amphibole growth within magnetite in a close-to-equilibrium system. Chain-width disorder is documented from defects including planar faults, derailments, jogs, and swells. Violations of zipper termination rules indicate primary growth rather than replacement. Amphibole-NCPs inclusions support a multi-stage evolution for Jatobá magnetite. They formed during the first cycle of magnetite growth within a mafic/ultramafic lithology that records synshearing events. Subsequent formation of calcic-amphiboles, including Ce-bearing species, indicate IOCG-related fluids at the onset of mineralization. (Ferro)tschermakite formed at ~7.5 kbar during shearing associated with main ore deposition. The multi-stage amphibole-NCPs generations in magnetite revealed by nanoscale study emphasize the interpretive value of magnetite for overprinting events in terranes with protracted geological histories. Analogous NCPs are likely to be abundant in magnetite from magmatic-hydrothermal deposits hosted by greenstone belts and altered mafic/ultramafic complexes. Likewise, discovery of Ce-rich hornblende provides new avenues to understand the early, alkali-calcic alteration stages of IOCG systems and models for REE incorporation into, and subsequent release from chain silicates.

Iron–Titanium Oxide–Apatite–Sulfide–Sulfate Microinclusions in Gabbro and Adakite from the Russian Far East Indicate Possible Magmatic Links to Iron Oxide–Apatite and Iron Oxide–Copper–Gold Deposits

Mesozoic gabbro from the Stanovoy convergent margin and adakitic dacite lava from the Pliocene–Quaternary Bakening volcano in Kamchatka contain iron–titanium oxide–apatite–sulfide–sulfate (ITOASS) microinclusions along with abundant isolated iron–titanium minerals, sulfides and halides of base and precious metals. Iron–titanium minerals include magnetite, ilmenite and rutile; sulfides include chalcopyrite, pyrite and pyrrhotite; sulfates are represented by barite; and halides are predominantly composed of copper and silver chlorides. Apatite in both gabbro and adakitic dacite frequently contains elevated chlorine concentrations (up to 1.7 wt.%). Mineral thermobarometry suggests that the ITOASS microinclusions and associated Fe-Ti minerals and sulfides crystallized from subduction-related metal-rich melts in mid-crustal magmatic conduits at depths of 10 to 20 km below the surface under almost neutral redox conditions (from the unit below to the unit above the QFM buffer). The ITOASS microinclusions in gabbro and adakite from the Russian Far East provide possible magmatic links to iron oxide–apatite (IOA) and iron oxide–copper–gold (IOCG) deposits and offer valuable insights into the early magmatic (pre-metasomatic) evolution of the IOA and ICOG mineralized systems in paleo-subduction- and collision-related geodynamic environments.

Electromagnetic methods for assessing moisture content and density of an iron oxide copper gold deposits (IOCG) tailings sample

Electromagnetic methods for assessing moisture content and density of an iron oxide copper gold deposits (IOCG) tailings sample

Integrated Re-Os, Ar/Ar, and U-Pb geochronology directly dates the timing of mineralization at the Mina Justa and Marcona deposits, Peru

Iron oxide−copper−gold (IOCG) and iron oxide−apatite (IOA) deposits are important sources of Cu and Fe, respectively. They contain abundant Fe-oxides and may contain Au, Ag, Co, rare earth elements (REEs), U, Ni, and V as economically important by-products. In Peru, the Mina Justa IOCG deposit is located next to the giant Marcona IOA deposit. Constraining the timing of Fe and Cu mineralization at Mina Justa is fundamental to understanding the duration and type of processes that generated this mineral deposit, and ultimately to testing the genetic link with other deposits in the area. Previous authors used alteration minerals to indirectly date Cu mineralization at Mina Justa at around 100 Ma. We report Ar/Ar dates of actinolite, U-Pb dates of magnetite, apatite, and titanite collected by in situ laser-ablation−multicollector−inductively coupled plasma−mass spectrometry, and Re-Os thermal ionization mass spectrometry dates for sulfides. These results indicate that Cu mineralization at Mina Justa occurred at ca. 160 Ma and that Fe mineralization is older and coeval with the neighboring Marcona IOA deposit, consistent with Cu mineralization overprinting IOA-style mineralization at Mina Justa.

Regional structural setting of late-orogenic IOCG mineralization along the northern Nautanen deformation zone, Norrbotten, Sweden

The northern Norrbotten ore province in Sweden is one of the most mineralized areas in Europe. Iron, gold and/or copper deposits occur as iron oxide-apatite-style mineralization (IOA, Kiruna-type) as well as iron oxide-copper–gold (IOCG) style. Regardless of mineralization style, most deposits appear to be spatially controlled by a set of crustal-scale Palaeoproterozoic shear zones which share similar structural characteristics and deformation histories.Reappraisal of regional geological and geophysical data, coupled with structural mapping, suggests crustal-scale shear zones form continuous c. N-S-trending zones extending from the Skellefte district in the south into the northern Norrbotten ore province. One example from Norrbotten is a zone that extends SSW from Karesuando in the north towards Svappavaara. While this structure has traditionally been inferred to continue SW towards Arjeplog (i.e. the Karesuando – Arjeplog Deformation Zone; KADZ), we favour its deflection SSE into the Nautanen-Aitik trend, making it a continuous, IOCG-bearing, crustal-scale deformation zone. Similar shear zone geometries can be observed in analogous zones to the west. Most of these crustal scale structures record at least two time-separated deformation events of regional significance. IOA and IOCG deposits form in different tectonic environments, separated in time and overprinting each other.

Metasomatism and depletion of the southern Gawler Craton from combined mantle xenocryst and AusLAMP magnetotelluric data

Metasomatised mantle is considered a prerequisite for the formation of many major ore deposits. Since magnetotellurics (MT) is strongly sensitive to mantle metasomatism, MT is a useful tool for analysis of mineral systems that incorporate a metasomatised mantle as a key ingredient. In this work, we combine recently reported kimberlite-hosted mantle xenocryst data from the southern Gawler Craton and Adelaide Superbasin with AusLAMP MT models. We find good correlations between mantle MT conductors and geochemical indicators of mantle metasomatism from clinopyroxene xenocrysts, with the most metasomatically enriched lithospheric mantle beneath the Adelaide Superbasin and more depleted mantle beneath the Gawler Craton. There is a significant step in lithospheric thickness across the Adelaide Superbasin, and particularly around the Terowie kimberlite cluster. Combined with the fertile mantle, this helps explain this region’s prospectivity for sediment-hosted copper deposits. High mantle resistivities and depleted geochemical compositions towards the central Gawler Craton on and north of the Eyre Peninsula are consistent with melting of previously metasomatised mantle, including during production of the Gawler Range Volcanics and Hiltaba Suite Large Igneous Province. Most of the major iron oxide copper gold (IOCG) and gold deposits in the region lie on the edge of this zone of mantle melting, consistent with mineral systems models that require melting of previously metasomatised mantle in the generation of these deposits.

Pb-bearing Cu-(Fe)-sulfides: Evidence for continuous hydrothermal activity in the northern Olympic Cu-Au Province, South Australia

Lead-isotope data were obtained for a population of Pb-chalcogenides (galena, clausthalite and altaite) and Cu-(Fe)-sulfides in Cu-(Fe)-sulfide mineral separates from the Prominent Hill iron-oxide copper gold deposit, Mt Woods Inlier, South Australia. Each mineral displays wide variability in 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios, ranging from moderately radiogenic Pb signatures to strongly radiogenic. Any contribution from non-radiogenic (common) lead at the time of initial deposit formation (1600–1585 Ma) is minor. Lead is overwhelmingly derived from the decay of U, with a minor contribution from the decay of Th also present within the ores. The heterogenous nature of the recorded Pb isotope values suggests a mixing of isotope signatures during repeated cycles of fluid-assisted dissolution, remobilisation and reprecipitation of U- and Pb-bearing phases. Hydrothermal experiments showed that the incorporation of Pb-chalcogenide (galena) occurs rapidly (days) at relatively mild conditions (≤150 °C), via precipitation in reaction-induced porosity formed during coupled dissolution reprecipitation reactions of the host Cu-(Fe)-sulfides. Hence, even minor hydrothermal activity may result in some trapping of radiogenic Pb by pre-existing Cu-(Fe)-sulfides. Construction of Pb-Pb pseudo-isochrons from the data supports these assessments, returning a range of possible (theoretical) ages (0–0.535 Ga) for Pb-chalcogenide formation. An additional (two-stage) Pb evolution model is preserved by the data with separation of each reservoir from the bulk silicate earth at ∼ 830 Ma and mixing of the two reservoirs at ∼ 500 Ma. Although the Pb-isotope data can be interpreted as resulting from a number of discrete events that generated regional-scale fluid flow, near-continuous Pb (re)-mobilisation at the nm- to deposit-scales is an equally valid interpretation, and is also consistent with observations of modern radionuclide mobility in the ores. The observations made within this study should be considered when discussing the mobility of elements within and between mineral phases in ore-forming systems. They show that many mineral phases, particularly the sulphide mineral phases discussed here, should not be considered as closed systems, but potentially, as systems in which chemistry and mineral stoichiometry evolve continuously over geological time.

Mercury isotopic compositions of iron oxide‑copper‑gold (IOCG) hydrothermal systems: Deep Hg cycling in intracontinental settings

Mercury isotopes display unique mass-independent fractionation (MIF), typically expressed as Δ199Hg, which relates to photochemical reactions occurring at Earth's surface. Pronounced Hg-MIF signals have been observed in various hydrothermal systems in both convergent margins and intracontinental settings, highlighting the recycling of Hg from marine or terrestrial reservoirs into shallow continental hydrothermal systems. However, the geochemical fate of Hg in deep continental environments of intracontinental settings remains poorly understood. Iron oxide‑copper‑gold (IOCG) hydrothermal systems typically involve the circulation of fluids from both deep magmatism and continental basins, providing an opportunity to investigate this issue. Here, we present the Hg isotopic compositions of ore minerals from five representative IOCG deposits in the Kangdian region, South China. All the studied ore samples display large mass-dependent fractionation with δ202Hg of −3.23 to 1.06‰, but a relatively narrow range of Δ199Hg of −0.17 to 0.11‰. These isotopic signatures support the hypothesis of binary mixing of Hg from two sources: (1) a magmatic endmember (∼two thirds) derived from the lithospheric mantle with low δ202Hg (−3 to −1‰) and near-zero Δ199Hg, and (2) a crustal source (∼one third) originating from the basement rocks with relatively higher δ202Hg (−1 to 1‰) and negative Δ199Hg (−0.2 to 0‰). Compared to the shallow Pb-Zn-Au-Sb hydrothermal systems which formed in intracontinental basins through the circulation of fluids in the upper crust, the Hg components (and by analogy, the ore metals) in the Kangdian IOCG system are dominated by a lithospheric mantle source. Our study sheds light on deep Hg cycling and confirms the Hg isotope tracer as a tool for revealing metal sources in hydrothermal ore systems.

THE Fe-Cu DISCONNECT: UNRAVELING A COMPOSITE IRON OXIDE COPPER-GOLD DEPOSIT IN THE OLYMPIC Fe-Cu-Au PROVINCE, GAWLER CRATON

Abstract The northern Olympic Cu-Au province, Gawler craton, Australia, includes a series of magnetite-dominated deposits/prospects associated with minor Cu-Au mineralization such as the 8.37 million tonne Cairn Hill deposit. Cairn Hill has long been considered a deep, magnetite end member of the iron oxide copper-gold (IOCG) family that is largely represented in the southern Olympic province by the 1590 Ma hematite-dominated Olympic Dam, Carrapeteena, and Prominent Hill deposits. In contrast to the southern district, the deposits in the northern Olympic Cu-Au province are hosted in rocks that experienced multiple phases of high-temperature metamorphism and deformation. New U-Pb zircon geochronology shows the magnetite-hornblende lodes at Cairn Hill were formed at ca. 1580 Ma at amphibolite facies conditions. The magnetite lodes are crosscut by ca. 1515 Ma granitic dikes. A second high-temperature event is recorded by U-Pb monazite geochronology at ca. 1490 Ma and involved deformation and metamorphism along the Cairn Hill shear zone at conditions of 4.6 to 5.3 kbar and 740° to 770°C. The 1490 Ma event reworked the iron lodes and 1515 Ma granitic dikes. However, Cu mineralization at Cairn Hill occurs in brittle fractures and quartz-biotite veins, overprinting the 1490 Ma deformation and metamorphism. Despite a spatial association between magnetite and Cu, the long thermal history that affected magnetite mineralization and the clear petrographic links between magnetite and high-temperature granulite facies minerals contrast with the late, low-temperature hydrothermal Cu mineralization and indicate the two are not paragenetically related. Therefore, the spatial but not temporal association between magnetite and Cu has effectively overlain two distinct episodes of mineralization to create the Fe-Cu deposit observed today. Although this fits within the broad IOCG deposit family, exploration strategies for Cairn Hill-style composite deposits should be distinct from IOCG deposits with cogenetic Fe and Cu.

Combining flipped class sessions with traditional lectures in a non-computational upper level economic geology class

Flipped classrooms have been shown to be useful in both introductory and advanced computational Earth Science courses. However, to date they have not been implemented in advanced level non-computational courses. Here we assess a three-year study into the use of flipped classroom techniques in a fourth year undergraduate Mineral Deposits class. One to two flipped classrooms were used to teach porphyry deposits, iron oxide copper gold and/or volcanogenic massive sulfide deposits. The effectiveness of the technique was assessed using a combination of student feedback forms, comparison of students’ ability to answer exam questions from topics taught by flipped classroom versus other topics taught by traditional methods, and interviews with students 6 to 36 months after completion of the course. The students’ ability to answer lecture exam questions was slightly higher in topics taught as flipped classrooms compared to traditional techniques. Whereas the students’ ability to answer laboratory exam questions was slightly lower between topics taught as flipped classrooms. However, most students did think that the flipped classrooms were useful and aided in their learning of the material. The use of video lectures was particularly appreciated by some who found that it increased their flexibility and ability to absorb the material at their own pace. As such we determined that flipped classrooms are an effective technique for teaching upper-level non-computational Earth Science courses and they increased behavioral, emotional, and cognitive engagement.

Geochemical signatures of the Neoarchean and Paleoproterozoic copper systems in the Carajás Mineral Province, NW Brazil: Implications for metal endowment

The main copper deposits of the Carajás Mineral Province are grouped into: i) iron oxide-copper-gold (IOCG), and ii) granite-related Cu-Au. They were formed during multiple mineralizing events in the Neoarchean (ca. 2.7; ca. 2.5 Ga) and Paleoproterozoic (ca. 2.1; ca. 1.8 Ga). A combination of non-composition and composition data analysis, molar element ratio, and principal component analysis using a lithogeochemical dataset (n = 1040) has been applied to recognize geochemical associations in these deposits. The Neoarchean IOCG deposits in the Northern and Southern Copper Belts of the Carajás Domain are controlled by ductile structures and comprise high-temperature hydrothermal associations within sodic (albite-scapolite), sodic-calcic (albite-actinolite-scapolite±hastingsite), calcic-iron (magnetite-actinolite-apatite), potassic (biotite-tourmaline±scapolite), potassic-iron (biotite-magnetite; almandine-grunerite-biotite) and iron-rich alteration zones. These zones envelop chalcopyrite-magnetite-actinolite-apatite (e.g., Sequeirinho orebody), magnetite-chalcopyrite-siderite (i.e., Igarapé Bahia deposit) breccias, or massive magnetite bodies with bornite-chalcocite (i.e., Salobo). The Fe-Cu-Au-P-REE association is common to all Neoarchean IOCG deposits of the province. Nevertheless, the IOCG deposits from the northern sector (ca. 2.5 Ga) are also enriched in Ba-Pb-Zn-Ag-Mo-REE ± Sn ± W ± Bi±U ± Nb ± Ta ± Y. Conversely, the chemical signature of Neoarchean (ca. 2.7 Ga) IOCG deposits from the southern sector display enrichment in V-Ni-Co-Cr. Part of these chemical variations may be due to differences in the nature of 7host rocks, including greenstone belt sequences in the south and metasedimentary exhalative rocks in north. Both in the northern and southern sectors, the sodic alteration is marked by Na, Ga, Ti, Sr, Th and Mo. The sodic-calcic displays Na, Ca, Mo, Mn, Mg, Pb, Zn and Sn enrichment. The iron-rich alteration zones (grunerite-almandine-magnetite) from the northern sector display a Fe, Mg, Ba, V, Ni, Co, Cr, Cu association. The calcic-iron alteration within the southern sector is marked by enrichment in Ca, Fe, V, Ni, Co, Cr, Mn, Mg, Th, P, HREE and Cu. The Fe-V-Ni-Co signature typical of the iron-rich and calcic-iron alteration stages is thus a lithogeochemical footprint of early Neoarchean IOCG mineralization, providing a useful proxy for exploration. Significant amounts of K, Rb, Ba, Al, Si, Nb, Ta, Th, U, Zr, Pb, Zn and Cu are common in potassic-iron alteration zones from Neoarchean deposits in both northern and southern sectors. The Paleoproterozoic IOCG and granite-related deposits are controlled by brittle structures and represent oxidized (muscovite-potassium feldspar-pyrite-hematite-chlorite), low temperature, sulfur-rich (pyrite-rich) and iron-deficient hydrothermal systems. The Paleoproterozoic IOCG deposits display potassic or potassic-iron alteration (potassium feldspar-biotite±magnetite), generally overprinted by chlorite alteration with late chalcopyrite-rich vein/breccia containing calcite-quartz-albite-chlorite-epidote-sericite. The Paleoproterozoic IOCG mineralized zones display enrichment in granitophile elements, defined by Sn-Mo-REE ± W ± Be±Bi±Nb ± Y. The granite-related deposits are marked by chlorite-rich or greisen alteration types characterized by enrichment in K, Sn, W, Mo, and Bi. The ore zones within these deposits are related to the Cu-Au-Mo-W-Sn-Bi-Be-Nb-U-Y signature. The recurrence and superposition of multiple hydrothermal events at a deposit-scale is evidenced by hybrid geochemical signatures, marked by an intermediate composition between the Neoarchean (Fe-V-Ni-Co-Ba-Pb-Zn) and Paleoproterozoic (Sn-W-Mo-Bi) signatures. The results obtained provide insights on the chemical evolution of the Carajás copper systems and demonstrate the influence of the multiple hydrothermal episodes on the geochemical signature of these deposits.

Formation of giant iron oxide-copper-gold deposits by superimposed episodic hydrothermal pulses

Iron oxide-copper-gold (IOCG) deposits are a globally important source of copper, gold and critical commodities. Despite their relevance, IOCG deposits remain an ill-defined clan, with a range of characteristics that has complicated development of the general genetic model. Here we focus on the Candelaria IOCG deposit in Chile and reveal that by using micro-textural and compositional variations in actinolite, a common alteration mineral found in many IOCG deposits, we can constrain the evolution of these systems. We demonstrate that Candelaria formed by the superposition of at least two pulses of mineralization with a late Cu-rich event overprinting and superimposed over an early, and probably higher temperature, iron oxide-apatite (IOA) mineralization event. These distinct pulses were likely caused by episodic injections of magmatic-hydrothermal fluids from crystallizing magmas at depth. Our data provide empirical evidence of grain-to-deposit scale compositional and potentially temperature changes in an IOCG system. The results support the use of actinolite chemistry as a novel approach to understand the formation of IOCG deposits and a potential tool for vectoring in exploration.

The Bahariyeh iron oxide copper–gold deposit, Khaf-Khashmar-Bardaskan magmatic belt, NE Iran: Constraints from geochemical, fluid inclusions, and O-S isotope studies

The Bahariyeh deposit is located in the central segment of the Khaf-Kashmar-Bardaskan magmatic belt (KKBMB), northeast Iran. Previous U-Pb data on zircon revealed an age of 38–41 Ma for the felsic-intermediate intrusions. The stockwork like vein orebodies are hosted in Eocene monzodiorite porphyry rocks which have strong structural controls imposed by the Great Dorouneh Fault System. Hydrothermal alteration is dominated by silicification, propylitic (chloritization), argillic, and sericitic zones. The mineralized veins in the Bahariyeh deposit can be divided into seven types: 1) quartz ± pyrite; 2) quartz + specular hematite + pyrite ± gold; 3) quartz + chalcopyrite + pyrite; 4) quartz + specular hematite + chalcopyrite + pyrite ± sphalerite ± galena ± gold; 5) chalcopyrite + specular hematite + pyrite ± gold; 6) specular hematite ± pyrite; and 7) chalcopyrite + pyrite. The various vein-style orebodies contain high concentrations of Cu, Au, Pb, Zn, and Ag. Microthermometric measurements of fluid inclusions in mineralized veins demonstrate homogenization temperatures in the range of 271–500 °C (mode of 300 °C), and corresponding salinities in the range of 7.9–26.3 wt% NaCl equivalent. The δ18OWater values calculated from δ18O of quartz, and the δ34S values of chalcopyrite reveal a dominantly magmatic origin for the ore-forming fluids. The fluid inclusions indicate that mixing between magmatic fluids and meteoric water, together with cooling, are the dominant mechanisms for metal deposition in the Bahariyeh district. The combination of ore deposit geology, alteration type, fluid evolution processes, and O–S isotope data suggest that the Bahariyeh deposit be classified as a Cu ± Au-rich iron oxide-copper–gold (IOCG) system.

The Montecristo mining district, northern Chile: the relationship between vein-like magnetite-(apatite) and iron oxide-copper–gold deposits

The Montecristo district, northern Chile, is one of the few places worldwide where there is a direct relationship between magnetite-(apatite) (MtAp) mineralization and iron oxide-copper–gold (IOCG) mineralization. The MtAp mineralization includes Ti-poor magnetite, fluorapatite, and actinolite and is crosscut and partially replaced by a younger IOCG mineralization that includes a second generation of actinolite and magnetite with quartz, chalcopyrite, pyrite, and molybdenite. The MtAp stage at Montecristo is interpreted as the crystallized iron-rich melts that used the pre-existing structures of the Atacama Fault System as conduits. These rocks later acted as a trap for hydrothermal IOCG mineralization. Geochronology data at Montecristo indicate that the host diorite (U–Pb zircon 153.3 ± 1.8 Ma, 2-sigma), MtAp mineralization (40Ar-39Ar in actinolite, 154 ± 2 Ma and 153 ± 4 Ma, 2-sigma), and the IOCG event (Re-Os on molybdenite, 151.8 ± 0.6 Ma, 2-sigma) are coeval within error and took place in a time span of less than 3.4 Ma. The εHfi and εNdi values of the host diorite are + 8.0 to + 9.8 and + 4.3 to + 5.4, respectively. The whole-rock 87Sr/86Sri values of the IOCG mineralization (0.70425 to 0.70442) are in the lower end of those of the MtAp mineralization (0.70426–0.70629). In contrast, εNdi values for the IOCG mineralization (+ 5.4 and + 5.7) fall between those of the MtAp rocks (+ 6.6 to + 7.2) and the host diorite, which suggests that the IOCG event was related to fluids having a more crustal Nd (εNdi < + 5.7) composition than the MtAp mineralization. This likely reflects the mixing of Nd from the MtAp protolith and a deep magmatic-hydrothermal source, very likely an unexposed intrusion equivalent to the host diorite. Sulfur isotope compositions (δ34S, + 0.3 to + 3.4‰) are consistent with a magmatic source.

Trace elements and sulfur isotopic compositions of sulfides in the giant Dahongshan Fe-Cu-(Au-Co) deposit, SW China: Implications for fluid evolution and Co enrichment in IOCG systems

Iron-oxide-copper–gold (IOCG) deposits are characterized by Cu-Au polymetallic mineralization and may contain large amounts of critical metals (e.g. Co, REE, and U) as by-products. Sources of ore-forming fluids and metals of the IOCG deposits remain controversial, either a magmatic or basinal source has been advocated. The ∼1660 Ma Dahongshan Fe-Cu-(Au-Co) deposit in the Kangdian region in southwestern China has spatially separated Fe and Fe-Cu orebodies, in which Co-rich pyrite has been identified in different types of ores and host rocks. Here, we integrate in-situ sulfur isotopic compositions and trace element geochemistry of sulfides to characterize the nature of ore fluids and the Co enrichment processes.Paragenetic sequences of sulfides in the Dahongshan deposit include: Pyrite (Py I) associated with magnetite mineralization in Fe ores, which has rims (Py II-1) intergrown with hematite-chalcopyrite assemblage; Pyrite and chalcopyrite (Sulfide II) in the Fe-Cu ores with protolith of arenaceous dolostone (Sulfide II-2) and interbedded argillaceous siltstone and dolostone (Sulfide II-3); and Sulfide III in post-ore reworking veins formed during regional metamorphism. Py I shows a magmatic sulfur isotopic signature (−2.2 to 1.2‰) and relatively high Au-Cu-Zn contents, indicating a predominance of magmatic-hydrothermal component during magnetite formation. By contrast, the combined sulfur isotopes and trace element compositions of Sulfides II in Fe-Cu ores indicate the mixing of two distinct fluid endmembers: Sulfides precipitated from one endmember show magmatic affinity with low δ34S values (as low as −2.1‰), high Se/S ratios (∼11.8 × 10-4), high Co contents (∼10600 ppm) and Co/Ni ratios (∼110), likely from a magmatic-hydrothermal source of mafic magmas affinity; whereas sulfides from the other endmember have overall higher δ34S values (up to 15.8‰), relatively lower Se/S ratios (∼0.09 × 10-4), lower Co contents (∼900 ppm) and Co/Ni ratios (∼13). Together with the published B-O isotopic data, the high-δ34S fluid is attributed to externally-derived basinal brine at Dahongshan. The clear negative correlations between δ34S values and Se/S ratios, Co contents, as well as Co/Ni ratios of Py II highlight that the incursion of exchanged basinal brines may be vital for triggering economic sulfide mineralization in Dahongshan. Furthermore, when assuming a similar predominance of magmatic fluids, Py I associated with magnetite ores is less enriched in Co (with lower Co/Ni ratios) than Py II from Fe-Cu ores, which can be tentatively ascribed to the decreasing Co solubility limits and formation of cattierite (CoS2) solid solution in pyrite as the fluid evolved. Finally, sulfide III in reworking veins show comparable δ34S values and Se/S ratios, but higher Co contents (up to 3.1%) than sulfide II, implying that Co can be further enriched during later remobilization events. This study exemplifies the utility of sulfide as a robust monitor of ore-forming process in IOCG system.

Unravelling magmatic-hydrothermal processes at Salobo and GT-46 IOCG deposits, Carajás mineral province, Brazil: Constraints from whole-rock geochemistry and trace elements in apatite

The iron oxide-copper-gold (IOCG) deposits of the northern portion of the Carajás Province, Amazonian Craton, Brazil, were primarily formed at the 2.5 Ga IOCG event. These deposits commonly exhibit wide Fe–K hydrothermal alteration halos (i.e., almandine-grunerite-biotite-magnetite) and display a temporal and spatial association with the 2.5 Ga granitic bodies, particularly in the GT-46 and in the world-class Salobo deposit. However, this association still lacks investigation concerning the metallogenic influence of the 2.5 Ga granitic bodies in the IOCG formation. Within these deposits, ca. 2.5 hydrothermally altered granites are recognized and include the Old Salobo, at the Salobo deposit, and G1 and G2, at the GT-46 deposit. Whole-rock geochemistry combined with petrography has pointed out that the G1 and Old Salobo are represented by granites to monzogranites, ferroan, metaluminous to weakly peraluminous with negative Eu anomaly. On the contrary, the G2 granite seems to represent a distinct body with granite composition, ferroan, metaluminous to weakly peraluminous with positive Eu anomaly. This granite exhibit pegmatitic textures, high Sr/Y ratios and listric-shaped normalized REE patterns, which suggest crystallization under high water contents. These granites primarily plots in the field of the volcanic arc granites, though some shifts towards to syn-collisional granites are observed. Effects of hydrothermal alteration are commonly seen in these granites both in textural aspects and geochemistry. Apatite mineral chemistry studies shows that magmatic signature in apatite is preserved in the G2 granite (G2core apatite), Old Salobo granite (OSA) and in the GT-46 ore zone (OZGT-A apatite), represented by enrichment in LREE and negative Eu anomaly. Hydrothermally altered apatite is recognized in the Old Salobo (OSB) and G2 granite (G2rim) and in the ore zone of GT-46 deposit and (OZGT-B) with depletion in LREE, which suggest that the same fluid might have affected both deposits. Truly hydrothermal apatite crystals from the Salobo and GT-46 ore zones shows high (Eu/Eu*)N values, indicative of high oxygen fugacity and alkaline fluids. Strong HREE depletion of the apatite grains from the central zone of the deposits suggest coeval almandine crystallization in the Fe–K alteration halos and evolved hydrothermal fluids. These results indicate that magmatic and hydrothermal process coexist in the IOCG deposits from the northern sector of Carajás Province. This might suggest that the 2.5 Ga IOCG deposits from the Carajás Province can have formed during the development of wide magmatic-hydrothermal systems.

Multivariate statistical analysis of trace elements in apatite: Discrimination of apatite with different origins

Partial least squares-discriminant analysis (PLS-DA) was used to construct the link between trace element contents of magmatic or hydrothermal apatite and deposit and rock types. Apatite with different origins is discriminated by characteristic chemical composition. In average, ore magmatic apatite has higher light REE and Th, and lower Sr than barren rock apatite, independent of deposit types. Hydrothermal apatite has lower La, Ce, Pr, and Nd contents than magmatic apatite, probably because fluids prefer migrating light rare earth element out of apatite. Hydrothermal apatite from iron oxide copper gold (IOCG) and iron oxide apatite (IOA) deposits is discriminated by high Mn and Sr relative to magmatic apatite, probably because these elements are fluid mobile. Barren magmatic apatite from granitoid-related deposits is well separated from ore magmatic apatite by higher Sr, Eu, Gd, Tb, Dy, and lower Mn contents, which are likely related to the relatively low degree of differentiation and high aluminum saturation index of parental magma.Magmatic apatite from IOA deposits is discriminated by relatively high Nd, Sm, Gd, Tb, Dy, and low Mn and U contents, which are possibly related to the crystallization of Mn-compatible magnetite microlites and peraluminous magmas. Magmatic apatite from granitoid-related W deposits is relatively rich in Y, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The relative enrichment of Sr in magmatic apatite from granitoid-related Cu-Pb-Zn and Pb-Zn deposits is likely due to less fractionation of Sr-compatible plagioclase. Melt of sediment component accounts for high Mn and Th in magmatic apatite from granitoid-related Mo deposits. Magmatic apatite from porphyry deposits is discriminated by relatively high V, Sr, and Eu contents. Magmatic apatite from porphyry Mo deposits has relatively high Pr, Nd, and Sm and low Sr contents compared to other types of porphyry deposits due to the more evolved magma. Hydrothermal apatite from IOA deposits is discriminated by high V and Sr, whereas those from IOCG deposits have high Sm, Eu, Gd, Tb, and U contents, because the lower temperature of IOCG cause precipitation of REE in Cl-rich fluid. Relatively low Gd and Tb in hydrothermal apatite from skarn deposits are possibly due to the crystallization of amphibole.Apatite in carbonatite has high contents of trace elements possibly because high contents of Ca and P in melts cause high apatite-melt partition coefficients of trace elements. Apatite from sedimentary and metamorphic rocks is discriminated by relative enrichment of Sr and Eu. Compared to igneous rocks, apatite from sedimentary rocks has low REE contents, reflecting low REE budget in surface waters. Breakdown of some minerals to release and remobilize REE and U during metamorphism can interpret high heavy REE and U contents in apatite from high-grade metamorphic rocks relative to those from low- and medium-grade metamorphic rocks. The ability to discriminate apatite from different types of deposits and rocks indicates the application potential of apatite chemistry in mineral exploration. A flowchart was proposed to identify apatite with unknown origins.

The distribution of trace elements in sulfides and magnetite from the Jaguar hydrothermal nickel deposit: Exploring the link with IOA and IOCG deposits within the Carajás Mineral Province, Brazil

The Jaguar nickel deposit represents an important and unconventional discovery of hydrothermal Ni resources (58.9 Mt @ 0.95% Ni) associated with magnetite and apatite within the Carajás Mineral Province, Northern Brazil. The Jaguar deposit shares several similarities with iron oxide copper–gold (IOCG) deposits of the Carajás Mineral Province, especially regarding the structural control, and the hydrothermal alteration. Although IOCG and Kiruna-type iron oxide-apatite (IOA) deposits are closely associated in several geological provinces, this association has not yet been fully constrained in the Carajás Mineral Province. The Jaguar deposit occurs near mafic–ultramafic intrusions, along a regional fault zone, and is hosted either by granitic (northern portion) or felsic subvolcanic (southern portion) rocks. It is characterized by sub-vertical zones hosting a biotite-chlorite (Bt-Chl) hydrothermal alteration with ductile structures and disseminated sulfides (pyrite and millerite) and magnetite. These zones are overprinted by sulfide-magnetite-apatite-bearing breccias, which represent the main mineralization bodies. The sulfide assemblage in the mineralized zones is dominated by pyrite, pentlandite and millerite, with minor sphalerite and chalcopyrite.We have investigated the concentration of trace elements in the magnetite and sulfides from the host rocks, different alteration facies and the mineralization at the Jaguar deposit by LA-ICP-MS. Magnetite composition ranges from higher Al, Mn, Ti and V contents, for at least part of those in host rocks (granitic and felsic subvolcanic) and Bt-Chl alteration, into lower contents in the magnetite-apatite alteration, associated with the main Ni mineralization. Magnetite with higher Ti and V contents in host rocks is associated with discrete hydrothermal alteration pockets, have high Ni/Cr ratios and plot in high temperature hydrothermal fields in multi-element diagrams. Therefore, we support that these features are compatible with a hydrothermal origin. Anomalously high-Ni contents in magnetite point that the Neoarchean mafic–ultramafic layered intrusions of the Carajás Province represent potential Ni sources for hydrothermal remobilization. Anomalously high V contents in magnetite suggest lower fO2 conditions upon the formation of the deposit. The concentration of trace elements in magnetite associated with the Jaguar deposit is similar to those from other IOCG deposits from the Carajás Province (but with higher V and Ni contents), especially the Sossego mine in the Southern Copper Belt, indicating that it could represent a Ni-rich member of the regional-scale IOCG mineral system at Carajás. Pyrite and chalcopyrite have a composition similar to those from magmatic deposits, when using previously proposed discriminant diagrams, but this is also the case for other IOA and IOCG deposits. However, low PGE contents (Ru, Rh, Ir and Os) in pyrite from the Jaguar deposit support a hydrothermal origin. Our results highlight that the classifications provided by available discriminant diagrams are not unequivocal. We suggest that multi-element diagrams represent a complementary approach to binary plots and provide a more comprehensive classification for the use of indicator minerals.