Iron Oxide Copper-gold Systems Research Articles

Magnetotelluric imaging of an iron-oxide copper gold (IOCG) deposit under thick cover

The presence of thick (>100 m) and electrically conductive (< 10 Ω.m) ground cover is a major impediment in mineral exploration of deep resources. Iron-oxide copper gold (IOCG) systems in Australia are often associated with a pronounced potential field anomaly, depending on the oxidation state of iron, but the gravity and magnetic field signatures have little vertical resolution unless constrained by drill hole petrophysics. Additionally, IOCG systems have deep magmatic sources, with their conductivity anomalies extending at least to the lithospheric mantle. The Vulcan IOCG prospect lies about 30 km northeast of the Olympic Dam IOCG mine and is defined by a significant gravity anomaly associated with brecciated haematite beneath 850 m of sedimentary cover sequences. To image the physical properties and structural geometry of the Vulcan IOCG prospect, a 100-site broadband MT and passive seismic array was deployed in a 1 km grid over a 9 by 9 km area. Three-dimensional inversion of MT responses resolve structure in three distinct domains. Firstly, broad limestone-quartzite-shale stratigraphy (1–30 Ω.m) in the 850 m cover is delineated in resistivity and corresponds well with changes in shear-wave velocity. Secondly, the region of brecciated haematite below the cover sequences is shown to have lower resistivity (< 60 Ω.m) than surrounding country rock (> 100 Ω.m). Thirdly, a more electrically conductive (< 30 Ω.m) vertical zone that extends > 5 km is imaged a few kilometers to the northeast of the Vulcan haematite breccia, and appears to be linked by a region of low shear-wave velocity in the depth range 1–2 km. Two-dimensional inversion of more regional MT responses along a 200-km line passing through the Vulcan prospect suggest this vertical region of low resistivity links to the lower-crust with anomalous resistivity of < 60 Ω.m in a similar way as imaged beneath the Olympic Dam mine. It is suggested that this conductive region is associated with graphite precipitated from magmatically derived CO2-rich fluids cooling in a reducing environment.

Hydrothermal alteration and elemental mass balance for the Surda Copper Deposit, Singhbhum Shear Zone, Eastern India: Implications for both copper and magnetite-apatite mineralization

This paper explores the geological relationships, mineralogical, and geochemical characteristics, and quantitative elemental mass balance constraints of the hydrothermal alteration zones within the uraniferous Surda copper deposit in the Singhbhum Shear Zone in eastern India, which is affiliated with an iron oxide copper-gold system. Five distinct alteration zones are identified: a proximal potassic (biotitization) zone containing magnetite- apatite; a central chloritization zone containing copper, uranium, gold, and magnetite; distal silicification (quartz) and albitization zones; and a transitional chlorite-sericitization zone. Evidence suggests these zones originated from mafic precursors linked to a subduction zone. Magnetite mineralization shows both magmatic (δ 18 O +2.0 to +4.8 ‰) and hydrothermal (δ 18 O +0.4 to +0.6 ‰) signatures. Hydrothermal magnetite-apatite formed at temperatures of 440° to 550°C, with continued crystallization at lower temperatures (280° to 360°C) concurrent with copper sulfide and uranium precipitation from an Fe-rich, isotopically heavy fluid (δ 34 S + 4.55 to + 9.66‰). Major elements (Ca, Na, Mg) and some minor and rare earth elements decrease from weakly-altered mafic rocks to late alteration zones, reflecting the breakdown of Ca-bearing hornblende, biotite and plagioclase. Potassium decreases from early to late alteration zones due to mineral transformations. Mass-balanced geochemical data align with hydrothermal alteration mineral assemblages, indicating processes like mineral dissolution, element absorption, and sulfide behavior. Overall, the study highlights the complex hydrothermal alteration processes shaping the Surda copper deposit and provides insights into ore formation mechanisms.

In situ apatite and carbonate Lu-Hf and molybdenite Re-Os geochronology for ore deposit research: Method validation and example application to Cu-Au mineralisation

The development of laser ablation inductively coupled plasma quadrupole tandem mass spectrometry (LA-ICP-Q-MS/MS) opens new opportunities to rapidly date a variety of hydrothermal minerals. Here we present in situ Lu-Hf and Re-Os dates for hydrothermal apatite and molybdenite, respectively. We further report the first in situ Lu-Hf dates for bastnäsite, dolomite, and siderite, and assess their potential for constraining ore deposit geochronology. For method validation, we report isotope-dilution Lu-Hf dates for apatite reference material Bamble-1 (1102 ± 5 Ma) and calcite reference material ME-1 (1531 ± 7 Ma), enabling improved accuracy on matrix-matched calibration for LA-ICP-MS/MS Lu-Hf dating. The new methods are applied to the Vulcan Iron-Oxide Copper-Gold (IOCG) prospect in the Olympic Cu-Au Province of South Australia. Such deposits have been difficult to accurately date, given the general lack of reliable mineral geochronometers that are cogenetic with IOCG mineralisation. Hydrothermal apatite Lu-Hf dates and molybdenite Re-Os dates demonstrate that mineralisation at Vulcan largely occurred at ca. 1.6 Ga, contemporaneous with the world class Olympic Dam deposit. Our data also indicates that the Lu-Hf system in apatite is more robust than the U-Pb system for determining the timing of primary apatite formation in an IOCG system. We further demonstrate that dolomite can retain Lu-Hf growth ages over an extended time period (>1.5 billion years), providing constraints on the timing of primary ore mineral crystallisation during brecciation and IOCG mineralisation. Finally, late Neoproterozoic (ca. 589–544 Ma) and Carboniferous (ca. 334 ± 7 Ma) Lu-Hf dates were obtained for texturally late Cu-bearing carbonate veins, illustrating that the carbonate Lu-Hf method allows direct dating of Cu remobilisation events. This has important implications for mineral exploration as the remobilised Cu may have been transferred to younger deposits hosted in Neoproterozoic sedimentary basins overlaying the Olympic IOCG province.

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.

Real-Time Ambient Seismic Noise Tomography of the Hillside Iron Oxide–Copper–Gold Deposit

We conduct an exploration-scale ambient noise tomography (ANT) survey over the Hillside Iron Oxide–Copper–Gold (IOCG) deposit in South Australia, leveraging Fleet’s direct-to-satellite technology for real-time data analysis. The acquisition array consisted of 100 sensors spaced 260 m apart which recorded continuous vertical-component seismic ambient noise for 14 days. High quality Rayleigh wave signals, with a mean signal-to-noise ratio (SNR) of 40, were recovered in the frequency band 1–4 Hz after processing the recorded data between 0.1–9 Hz. Our modelling results capture aspects of the deposit’s known geology, including depth of cover, structures linked to mineralisation, and the mineralised host rock, down to approximately 1 km depth. We compare our velocity model with existing magnetic, gravity, induced polarisation and drilling data, showing strong correlation with each. We identify several new features of the local geology, including the behaviour of key structures down to 1 km, and highlight the significance of a Cambrian-age dolomite that cuts across the main structural corridor that hosts the Hillside deposit. An analysis of model convergence rates with respect to Rayleigh wave SNRs shows that real-time data analysis can reduce recording duration at the site by 65% compared to traditional deployment durations, from ∼14 days to ∼5 days. Finally, we conclude by commenting on the efficacy of the ANT technique for the exploration of IOCG systems more broadly.

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.

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.

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.

Cathodoluminescent, chemical and strontium isotopic characteristics of apatite from Lanniping Fe-Cu deposit, southwestern China: Implications for fluid evolution in IOCG systems

Apatite occurs in diverse types of hydrothermal mineral deposits and has been widely used to constrain the nature and evolution of ore-forming fluid. The origin of ore fluids responsible for formation of iron oxide-copper–gold (IOCG) deposits remains controversial. Here we combine textural observations from optical microscopic cathodoluminescence (OM-CL) and backscattered electron (BSE) images with in situ chemical and strontium isotopic analyses of apatite in the Lanniping Fe-Cu deposit in Kangdian metallogenic province, SW China, to reconstruct the hydrothermal evolution history of this deposit. Paragenetic sequences of the deposit include pre-ore Na-alteration (stage I) of predominant albite, stage II iron-oxide mineralization with the associated Ca-Fe alteration characterized by magnetite and minor apatite, stage III Cu-Fe sulfides and the associated K-Fe alteration, and final stage IV Ca-Mg alteration. Three types of apatite at different paragenetic stages were identified in this deposit. The pristine Type 1 apatite are intergrown with stage II magnetite and show blue colors under OM-CL images. The Type 2 apatite, which are darker than Type 1 apatite under BSE images, show dark yellow CL colors. They commonly occur as metasomatized domains of Type 1 apatite and can contain chalcopyrite mineral inclusions, indicating that they are coeval products during stage III. The Type 3 apatite show bright yellow CL colors, and occur as overgrowth rims of both Type 1 and 2 apatite. The Type 1 apatite have highest REY contents (5342 to 12983 ppm) and primitive 87Sr/86Sr ratios ranging from 0.70630 to 0.71093, which overlap the associated igneous intrusions. In contrast, the Type 2 and Type 3 apatite have gradually lower REY contents (866 to 4255 ppm and 142 to 635 ppm, respectively) but higher 87Sr/86Sr ratios (0.71009 to 0.71698 and 0.71904 to 0.72602, respectively). Our data hence suggest that the ore-forming fluids were mainly magmatic-hydrothermal origin and were gradually enriched in radiogenic Sr isotope by mixing with external fluid. Our case study highlights that texturally constrained apatite could be utilized to effectively trace fluid evolution of IOCG mineralization, and by reference, other hydrothermal mineralizing systems.

In-situ element geochemical and sulfur isotope signature of pyrite and chalcopyrite: Constraints on ore-forming processes of the Laoshankou iron oxide-copper (-gold) deposit, northern East Junggar

The Laoshankou deposit is an iron oxide-copper–gold (IOCG)-like deposit in the northern margin of East Junggar, Central Asian Orogenic Belt (CAOB), in which pyrite is ubiquitous and present from stage II magnetite mineralization (Py1a and Py1b), stage III-A pyrite mineralization (Py2a and Py2b), to s tage III-B chalcopyrite mineralization (Py3a and Py3b). Despite of many previous studies, the source(s) of ore-forming fluids in the Laoshankou deposit are still controversial. Whether non-magmatic fluids were involved in the IOCG systems has also been highly debated in recent years. To further characterize the nature of ore-forming fluids in Paleozoic IOCG deposits in basin inversion settings, we conducted in-situ secondary ion mass spectrometry (SIMS) sulfur isotope and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace element analysis in a set of pyrite/chalcopyrite-bearing samples in the Laoshankou deposit. Sulfur isotope results of the sulfides in all stages (−3.5 to 2.6‰) fit in the magmatic sulfur range, indicating the contribution of magmatic sulfur. However, the weakly negative δ34S values of − 3.5‰ to − 1.5‰ (Py3), possibly indicating the addition of organic sulfur, which can also be proved by organic-rich fluid inclusions and strata. The low Se/S (<5 × 10−5), the volcanogenic-like Co/Ni ratios (3.9–28, average 16), and the varying Ni contents (81–978 ppm, average 315 ppm) of Py1 indicate that its magmatic sulfur was leached from mafic volcanic rocks by heated seawater in a reducing condition at stage II. The elevated Ni (433–3736 ppm, average 1401 ppm) and Se (39–143 ppm, average 80 ppm) contents, and the Se/S (∼1.5 × 10−4) ratios of Py2 indicate a more oxidizing condition. We suggest that the magmatic sulfur of Py2 in stage III-A was also leached from mafic rocks by seawater, but magmatic hydrothermal fluid input cannot be excluded. The high Co/Ni (7.7–415, average 142) and low Ni/Se (0.43–3.4, average 1.7) ratios, and the low Ni contents (31–381 ppm, average 133 ppm) of Py3 show that such organic sulfur was likely leached from organic-rich sequence by relatively low-temperature and reduced fluid, with significant contribution from the felsic rocks in stage III-B. Combined with previous halogen and C–H-O isotopes results, external non-magmatic fluids can be proved in the Laoshankou deposit. Stage III-B chalcopyrite grains replaced early-stage pyrites, during which some alleged less-mobile elements (e.g., Co, and As) in the replaced pyrites were released into the Cu ore-forming fluids and formed extremely low-Co, porous/fractured, and inclusion-rich Py1b and Py2b, together with high Co-As contents in the Py3 and the Py3-chalcopyrite boundary. We propose that external seawater and organic bittern brine may have an effect on the mineralization at Laoshankou. Combined with previous researches in the Andean and East Tianshan, external fluids may play a key role for the IOCG deposits in arc-related basin inversion setting.

Distinguishing IOCG and IOA deposits via random forest algorithm based on magnetite composition

Iron oxide-copper-gold (IOCG) and iron oxide-apatite (IOA) are two significant mineral deposit types with similar tectonic settings and hydrothermal alteration characteristics. There are huge differences in the geological setting, alteration system, and ore-forming fluid composition among IOCG and IOA deposits, leading to controversial genesis. Distinguishing between these two deposit types is significant to reveal the origin of IOCG and IOA systems. In this study, random forest (RF) was employed to classify IOCG and IOA deposits based on the chemical composition of magnetite measured by the electron probe microanalyzer (EPMA) and laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The obtained results show that (1) a relatively high overall classification accuracy (0.76 for EPMA data and 0.91 for LA-ICP-MS) was obtained via the RF, indicating that the elemental composition of magnetite can effectively distinguish IOCG and IOA deposits; (2) the performance of the RF model based on LA-ICP-MS data is better than that of EMPA data, indicating that the application of more geochemical variables is helpful in distinguishing IOCG and IOA deposits; and (3) the elements V, Mg, and Mn in EPMA data, and Si, Mg, and V in LA-ICP-MS data are identified as the key elements for distinguishing IOCG and IOA deposits.

Evidence of iron oxide-copper–gold mineralization in the Torud-Chahshirin Magmatic Belt, northern Iran: Insight from the Robaie area

The Torud-Chahshirin Magmatic Belt in northern Iran is defined by Tertiary porphyry, IOCG, Skarn, Manto, MVT, and Au, Ag and base metal-rich epithermal systems, and it is a part of the eastern Alborz Orogenic Belt of Iran. Mineralization in the Torud-Chahshirin Magmatic Belt occurs as veins, veinlet, and disseminated mineralization hosted by the Eocene-Oligocene volcanic, subvolcanic, and volcanoclastic rocks. Mineralization prospects of the Torud-Chahshirin Mgmatic Belt are spatially and temporally accompanied with magmatic rocks of early to middle Eocene (∼55–37 Ma.) calc-alkaline and early to late Oligocene (∼34–24 Ma.) alkaline rocks that have continually evolved through assimilation and fractional crystallization (AFC) processes at shallow crustal levels. The Robaie area is located in the Torud-Chahshirin Magmatic Belt. The dominant stratigraphic unit is composed of Eocene volcanic rocks (trachyandesitic to andesitic in composition), which are intruded by subvolcanic rocks. The subvolcanic rocks have features typical of shoshonitic affinity and are metaluminous to weakly peraluminous. These rocks are characterized by enrichment in large-ion-lithophile elements (LILEs) and light rare-earth-elements (LREEs), depletion of heavy rare-earth-elements (REEs, LaN/YbN ≈ 9.65–24.34), and high-field-strength-elements (HFSEs). Values of εNd(i) commonly show a small variation between +0.2 and +0.7, whilst 86Sr/87Sr (i) goes from 0.7056 to 0.7063. The whole set of evidence agrees with derivation of intrusive rocks of Robaie from primitive magma resulting from partial melting in a mantle wedge above a subduction zone. A sample from a dyke of biotite-hornblende diorite porphyry, with a negative εNd(i) value (−1.7), probably corresponds to a magma that underwent assimilation of crustal material. U-Pb data of zircons from dioritic stock and dyke show ages of 49.99 Ma and 50.43 Ma, respectively. Mineralization in Robaie is related to the biotite-hornblende diorite intrusions and is associated with hydrothermal alterations including argillic, argillic-silicification, argillic-sericitic, and propylitic alteration. The ore-bearing veins, veinlets, and disseminated mineralization are controlled by structures. There are two types of veins and veinlets mineralization in the Robaie area including: 1) quartz-chalcopyrite-pyrite-specularite ± gold veins, and 2) quartz-chalcopyrite-pyrite-specularite-magnetite ± gold veinlets. The highest amount of copper (>1%) and gold (>5 g/t) is associated with quartz-chalcopyrite-pyrite-specularite ± gold veins. Microthermometric study of fluid inclusions shows that homogenization temperatures are 181–422 °C and salinities of ore-forming fluids ranged from 11.1 to 17 wt% NaCl equivalent. The temperature and salinity of ore-forming fluids were lower in the formation of the quartz-chalcopyrite-pyrite-specularite ± gold veins than in the quartz-chalcopyrite-pyrite-specularite-magnetite ± gold veinlets, and fluid mixing was the most important mechanism for mineralization in the area. The mineralogy, alteration, geochemistry, fluid inclusion studies, and petrogenesis of subvolcanic rocks of the Robaie area indicate that the Robaie deposit could be classified as a member of the Iron Oxide-Copper-Gold (IOCG) deposits.

Geochemical and Isotopic Signature of Pyrite as a Proxy for Fluid Source and Evolution in the Candelaria-Punta del Cobre Iron Oxide Copper-Gold District, Chile

Abstract Pyrite is ubiquitous in the world-class iron oxide copper-gold (IOCG) deposits of the Candelaria-Punta del Cobre district, documented from early to late stages of mineralization and observed in deep and shallow levels of mineralized bodies. Despite its abundance, the chemical and isotopic signature of pyrite from the Candelaria-Punta del Cobre district, and most IOCG deposits worldwide, remains poorly understood. We evaluated in situ chemical and isotopic variations at the grain scale in a set of pyrite-bearing samples collected throughout the district in order to characterize and further understand the nature of mineralization in this IOCG system. Our multianalytical approach integrated synchrotron micro-X-ray fluorescence (μ-XRF) mapping of pyrite grains with electron probe microanalysis and laser ablation-inductively coupled plasma-mass spectrometry data, and sulfur isotope determinations using secondary ion mass spectrometry (SIMS) complemented with bulk sulfur isotope analyses of coeval pyrite, chalcopyrite, and anhydrite. Synchrotron μ-XRF elemental concentration maps of individual pyrite grains reveal a strong zonation of Co, Ni, As, and Se. The observed relationships between Ni and Se are interpreted to reflect changes in temperature and redox conditions during ore formation and provide constraints on fluid evolution. Co and Ni concentrations and ratios suggest contributions from magmas of mafic-intermediate composition. Pyrite chemical concentrations reflect potential stratigraphic controls, where the sample from the upper part of the stratigraphy diverges from trends formed by the rest of the sample set from lower stratigraphic levels. The SIMS δ34S values of pyrite (and chalcopyrite) range between –2 up to 10‰, and bulk δ34S values of pyrite range between 4 up to 12‰. The majority of the δ34S analyses, falling between –1 and 2‰, indicate a magmatic source for sulfur and, by inference, for the hydrothermal ore fluid(s). Variation in the δ34S signature can be explained by changes in the redox conditions, fluid sources, and/or the temperature of the hydrothermal fluid. The Se/S ratio combined with δ34S values in pyrite is consistent with mixing between a magmatic-hydrothermal fluid and a fluid with a probable basinal signature. The results of this study are consistent with the hydrothermal fluids responsible for mineralization in the Candelaria-Punta del Cobre district being predominantly of magmatic origin, plausibly from mafic-intermediate magmas based on the Ni-Co content in pyrite. External fluid incursion, potentially from a basinal sedimentary source, occurred late in the evolution of the system, adding additional reduced sulfur as pyrite. There is no evidence to suggest that the late fluid added significant Cu-Au mineralization, but this cannot be ruled out. Finally, the data reveal that trace element ratios coupled with spatially resolved sulfur isotope data in pyrite are powerful proxies to track the magmatic-hydrothermal evolution of IOCG systems and help constrain the source of their contained metals.

OPENING THE MAGMATIC-HYDROTHERMAL WINDOW: HIGH-PRECISION U-Pb GEOCHRONOLOGY OF THE MESOPROTEROZOIC OLYMPIC DAM Cu-U-Au-Ag DEPOSIT, SOUTH AUSTRALIA

Abstract Establishing timescales for iron oxide copper-gold (IOCG) deposit formation and the temporal relationships between ores and the magmatic rocks from which hydrothermal, metal-rich fluids are sourced is often dependent on low-precision data, particularly for deposits that formed during the Proterozoic. Unlike accessory minerals routinely used to track hydrothermal mineralization, iron oxides are dominant components of IOCG systems and are therefore pivotal to understanding deposit evolution. The presence of ubiquitous, magmatic-hydrothermal U-(Pb)-W-Sn-Mo–bearing zoned hematite resolves a range of geochronological issues concerning formation of the ~1.6 Ga Olympic Dam IOCG deposit, South Australia, at up to ~0.05% precision (207Pb/206Pb weighted mean; 2σ) using isotope dilution-thermal ionization mass spectrometry (ID-TIMS). Coupled with chemical abrasion-ID-TIMS zircon dates from host granite and volcanic rocks within and enclosing the ore-body, a confident magmatic-hydrothermal chronology is defined. The youngest zircon date from the granite intrusion hosting Olympic Dam indicates magmatism was occurring up until 1593.28 ± 0.26 Ma. The orebody was principally formed during a major mineralizing event following granite uplift and during cupola collapse, whereby the hematite with the oldest age is recorded in the outer shell of the deposit at 1591.27 ± 0.89 Ma, ~2 m.y. later than the youngest documented magmatic zircon. Hematite dates captured throughout major lithologies, different ore zones, and the ~2-km vertical extent of the deposit support ~2 m.y. of hydrothermal activity. New age constraints on the spatial-temporal evolution of the formation of Olympic Dam are considered with respect to a mantle to crustal continuum model. Cyclical tapping of magma reservoirs to maintain crystal mushes for extended time periods and incremental building of batholiths on the million-year scale prior to main mineralization pulses can explain the ~2-m.y. temporal window temporal window inferred from the data. Despite the challenge of reconciling such an extended window with contemporary models for porphyry deposits (≤1 m.y.), formation of Proterozoic ore deposits has been addressed at high-precision and supports the case that giant IOCG deposits may form over millions of years.

Fluorite as indicator mineral in iron oxide-copper-gold systems: explaining the IOCG deposit diversity

Hydrothermal iron oxide-copper-gold (IOCG) ore deposits are globally important sources of Cu and Au. IOCG systems show many common features, but also considerable diversity in terms of geological setting, mineralization style and ore fluid characteristics. The key factors that control ore formation are controversially debated and no general ore deposit model has been able to explain the diversity of IOCG deposits on the global scale. Building on previous work that characterized four distinct fluid types at the Prominent Hill IOCG deposit, South Australia, we have analyzed the rare earth element (REE) composition of fluorite from the Prominent Hill, Olympic Dam and Ernest Henry IOCG deposits and from two prospects close to Prominent Hill. The REE data of fluorite from Prominent Hill show four distinct chondrite-normalized patterns, which reflect the four fluid types identified as metalliferous volcanic lake water derived fluid, magmatic-hydrothermal fluid, sedimentary basin brine, and wall rock-buffered basement brine. Two strikingly similar REE pattern types were found in fluorite from the Olympic Dam deposit and one in an exploratory data from the Ernest Henry deposit, demonstrating the involvement of the same principal fluid types. We therefore conclude that world-class IOCG deposits can form by interaction of host rocks with different characteristic types of ore fluids and their combinations, thus IOCG deposits do not necessarily form by a single sequence of processes.

The link between hydrothermal nickel mineralization and an iron oxide-copper–gold (IOCG) system: Constraints based on mineral chemistry in the Jatobá deposit, Carajás Province

The Jatobá deposit represents a specific portion of an Archean-Paleoproterozoic iron oxide–copper–gold system, notable due to its hydrothermal nickel-rich zones. Neoarchean mafic and acid volcanic rocks of the Itacaiúnas Supergroup host the mineralized zones. Pre-, syn- and late shearing hydrothermal alteration, associated with the Canaã shear zone development, was pervasive and intense. Early distal hydrothermal alteration encompasses potassic-iron-silica (quartz–biotite–magnetite–apatite), sodic (albite, scapolite), and sodic-calcic (ferro-pargasite) alteration. The main syn-tectonic hydrothermal alteration stage resulted in scapolite–hastingsite–biotite controlled by the mylonitic foliation. Syn-deformation massive magnetite bodies represent proximal envelopes of the mineralized zones. They were intercepted by actinolite-apatite-magnetite and potassic-iron (biotite–magnetite) alteration fronts. Late tectonic hydrothermal alteration comprises chlorite–(epidote–quartz–calcite), fibrous scapolite veins, and green biotite–scapolite–F-Cl-apatite veinlets. The copper–(nickel) mineralized zones in the Jatobá deposit were formed in four stages, coeval to ductile and ductile-brittle deformational events. The mineralization stage (I), spatially related to massive magnetite–(apatite–actinolite) bodies, has the highest nickel content. It is characterized by Ni-pyrrhotite, Ni-pyrite, and subordinately, Co-chalcopyrite, Ce-allanite, Co-pentlandite, and Ce-monazite. The mineralization style evolved from replacement fronts controlled by the mylonitic foliation to hydraulic breccia zones. The mineralization stage (II) was synchronous to the development of syn-tectonic potassic-iron alteration and Co-chalcopyrite (±Ni-pyrite ± Ni-pyrrhotite). The mineralization stage (III), controlled by ductile-brittle and brittle structures, enabled the formation of veins with typical open-space filling textures infilled by paler brown or green biotite, Co-chalcopyrite, and siegenite (±Co-pyrite, ±Co-magnetite, ± cassiterite). The late mineralization stage (IV), coeval to widespread chlorite alteration, formed the most important copper mineralization at Jatobá. It is represented by branching veinlets and breccias with chalcopyrite, Co-chalcopyrite, Co-pyrite, sphalerite, molybdenite, uraninite, monazite, W-bearing hematite, and rare earth carbonates (bastnäsite, coskrenite, and sahamalite). The high chlorine contents of halogen-rich minerals (e.g., scapolite, biotite, apatite, and amphibole) imply the participation of highly saline, supersaturated fluids in the system evolution. Metalliferous fluids were highly focused, and mineralized zones represent the hotter hydrothermal centers. The high V and Ti contents in magnetite and its coexistence with hercynite and ilmenite indicate temperatures above 600 °C. These conditions would be similar to that of magnetite formed in the transition from magmatic to deep magmatic-hydrothermal systems, including the IOA deposits, especially on the onset of the early nickel-enriched mineralization stage. Such conditions enabled nickel leaching from mafic and mafic-ultramafic rocks, its transport in Cl-rich hydrothermal fluids, and precipitation related to episodic decompression. Temperature decrease (<380 °C), indicated by chlorite geothermometers, favored the bulk of chalcopyrite precipitation in late mineralization stages. The Jatobá deposit has attributes comparable to those of relatively deep portions of IOCG mineral systems formed from the channeling of overpressured superheated deep-seated fluids of probable magmatic origin.

Evolution of brines and CO2-rich fluids and hydrothermal overprinting in the genesis of the Borrachudo copper deposit, Carajás Province

The Carajás Province is one of the world’s major domains of iron oxide-copper-gold (IOCG) deposits, which formed during the Neoarchean (ca. 2.70–2.68 Ga and ca. 2.57 Ga). The province also hosts other copper-gold deposits of Paleoproterozoic age (ca. 1.88 Ga). The Borrachudo copper deposit is located about 6 km southwest of the world-class Cristalino IOCG deposit. Porphyritic rhyolite and amygdaloidal andesite of the ca. 2.76–2.74 Ga Grão Pará Group, formed in a volcano-plutonic event during the Neoarchean, are the main host to the Borrachudo deposit. Copper mineralization was related to two distinct hydrothermal events. The first, controlled by the development of ductile structures, was likely linked to the Carajás ca. 2.70 Ga IOCG system. It evolved from sodic alteration, silicification, and iron metasomatism through to calcic and potassic alteration. Chalcopyrite strings along foliation, breccias, and massive ore represent early copper mineralization. During the early hydrothermal event, aqueous (165° to >400 °C; 0.23 to >38.32 wt% NaCl equiv.), aqueous-carbonic and carbonic fluid inclusions (TmCO2 = −60.1 to −59.4 °C; CO2 density of 0.82 to 0.97 g/cm3) were trapped in quartz formed during the silicification. Immiscibility of a magmatic fluid or heterogeneous trapping of brines and CO2-bearing fluids, followed by mixing with diluted colder fluids, are suggested. The late event, controlled by brittle structures, records recurrence of hydrothermal alteration stages, such as silicification, sodic alteration, and potassic alteration, as well as chlorite and carbonate alteration. The copper mineralization is represented by chalcopyrite in branched breccia pipes and veins with minor magnetite, pyrite, siegenite, cobaltite, galena, anglesite, uraninite, and thorite. Titanite in veins of albite-chalcopyrite-apatite yielded an LA-ICP-MS U-Pb titanite age of 2011 ± 6.8 Ma. During the late hydrothermal event, aqueous fluids (140 to >320 °C; 6.7 to 40 wt% NaCl equiv.) reflected a more advanced mixing process involving diluted external fluids. The Borrachudo deposit points to the recurrence of multiple hydrothermal events responsible for the copper metallogenesis, which were intrinsically controlled by the magmatic and tectonic evolution of the Carajás Province.

Hematite geochemistry and geochronology resolve genetic and temporal links among iron-oxide copper gold systems, Olympic Dam district, South Australia

The Wirrda Well and Acropolis prospects, ~25 km south from the Olympic Dam deposit, are among several dozen examples of iron-oxide copper gold (IOCG) mineralisation forming a 600 km-long province in the eastern Gawler Craton, South Australia. IOCG systems across the province differ in terms of host lithology, iron-oxide mineral associations, intensity of brecciation, and grade of Cu-(U)-Au mineralisation. Previous efforts to provide a geochronological framework for mineralisation, and correlate this with igneous activity, were inhibited by a lack of clear crosscutting features (particularly in breccia-hosted systems) and a scarcity of reliable hydrothermal mineral geochronometers. The earliest generation of hematite, characterised by enrichment in U-W-Sn-Mo and previously reported from Olympic Dam, provides confident U-Pb dates and insights into hydrothermal fluid signatures. Similar hematite is recognised from Wirrda Well and Acropolis, allowing direct comparison between the three IOCG systems. Relationships between early, silician (Wirrda Well) and titaniferous (Acropolis) magnetite and U-bearing hematite at the two prospects differ. Inferred martitisation of magnetite at Acropolis has generated zoned hematite with U-W-Mo-enrichment at constant Sn concentration, but with a marked loss of REE + Y (REY) and Ti, whereas at Wirrda Well, single-crystal zoned hematite, resembling that from Olympic Dam, retains high REY and Sn concentrations. Although morphologies, textures and compositions of U-bearing hematite at Olympic Dam, Wirrda Well and Acropolis vary significantly, a common U-Pb (laser ablation-inductively coupled plasma-mass spectrometry) age of ~1590 Ma is obtained from the most reliable data (207Pb/206Pb: 1598.9 ± 6.3 Ma at Wirrda Well and 1590.6 ± 6.5 Ma at Acropolis). The geochemical signatures of iron-oxides from the two prospects share common trends for U, W, Sn, Mo, high field strength and siderophile elements, comparable with signatures of Fe-oxides from the ‘outer shell’ of the Olympic Dam deposit. Given that datable iron-oxides would seem to be ubiquitous phases throughout the eastern Gawler Craton, the commonality in geochemistry and age of U-bearing hematite from the Olympic Dam district is significant, as it provides a single, traceable mineral to compare IOCG systems across the province.

Mapping iron oxide Cu-Au (IOCG) mineral potential in Australia using a knowledge-driven mineral systems-based approach

Iron oxide copper-gold (IOCG) deposits are consequences of lithospheric- to deposit-scale earth processes, and form where there was a coincidence of ore-forming processes in space and time. Building on previous conceptualisations we view a ‘mineral system’ as an ore-forming geological system in which four components are required to have operated efficiently and coincidentally, namely: (1) available sources of ore metals (i.e., copper, gold, uranium, rare-earth elements) and hydrothermal fluids; (2) energy sources to drive fluids in the ore-forming system; (3) active crustal and mantle lithospheric architecture, representing hydrothermal fluid and/or magma flow pathways; and (4) physico-chemical gradients along which ore metals were deposited to form ore bodies.This holistic multi-scale mineral systems framework has been used to develop a practical, knowledge-based yet data-rich, prospectivity mapping method applicable at regional to continental scales for hydrothermal and orthomagmatic ore-forming systems. We demonstrate how the mineral system components can be translated into mappable criteria and show how maps of mineral potential are generated by integrating diverse and rich input data sets. The method enables prediction of mineral potential not only in brownfields areas but also in greenfields and covered terranes with no previously known mineralisation. Here we report the application of this methodology in regional-scale mapping of the potential for IOCG deposits in Australia, using examples from five studies over the last decade in northern Queensland, eastern South Australia, and southern and central-eastern Northern Territory. Uncertainties in the results arising from assignment of weightings to input data layers were investigated by the application of Monte Carlo-type probabilistic simulations. The results of 500 iterations using randomly assigned weightings overall support the deterministic results but also show that modelled prospectivity is controlled mainly by variations in intrinsic values of the input geoscientific data sets (e.g. highs and lows of gravity values) rather than by the weightings.The results of the knowledge-driven data-rich analyses of IOCG potential have been validated against known IOCG deposits (not used directly in the analysis). We find in all five studies (Queensland, South Australia and Northern Territory) a good spatial correspondence, with few exceptions. Statistical analysis of prospectivity mapping results from the Tennant Creek – Mt Isa study area demonstrate that 15 of 16 IOCG deposits occur in the highest modelled prospectivity areas within 4.2% of the study area, representing an area reduction of 95.8%. Moreover, several new discoveries of Cu-Au mineralisation have been made within areas previously highlighted as highly prospective. This success and validation support the utility of Geoscience Australia’s approach as a decision-support tool to assist exploration companies and governments in craton- to regional-scale area selection for discovery of IOCG and other mineral systems.

Silician Magnetite: Si–Fe-Nanoprecipitates and Other Mineral Inclusions in Magnetite from the Olympic Dam Deposit, South Australia

A comprehensive nanoscale study on magnetite from samples from the outer, weakly mineralized shell at Olympic Dam, South Australia, has been undertaken using atom-scale resolution High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF STEM) imaging and STEM energy-dispersive X-ray spectrometry mapping and spot analysis, supported by STEM simulations. Silician magnetite within these samples is characterized and the significance of nanoscale inclusions in hydrothermal and magmatic magnetite addressed. Silician magnetite, here containing Si–Fe-nanoprecipitates and a diverse range of nanomineral inclusions [(ferro)actinolite, diopside and epidote but also U-, W-(Mo), Y-As- and As-S-nanoparticles] appears typical for these samples. We observe both silician magnetite nanoprecipitates with spinel-type structures and a γ-Fe1.5SiO4 phase with maghemite structure. These are distinct from one another and occur as bleb-like and nm-wide strips along d111 in magnetite, respectively. Overprinting of silician magnetite during transition from K-feldspar to sericite is also expressed as abundant lattice-scale defects (twinning, faults) associated with the transformation of nanoprecipitates with spinel structure into maghemite via Fe-vacancy ordering. Such mineral associations are characteristic of early, alkali-calcic alteration in the iron-oxide copper gold (IOCG) system at Olympic Dam. Magmatic magnetite from granite hosting the deposit is quite distinct from silician magnetite and features nanomineral associations of hercynite-ulvöspinel-ilmenite. Silician magnetite has petrogenetic value in defining stages of ore deposit evolution at Olympic Dam and for IOCG systems elsewhere. The new data also add new perspectives into the definition of silician magnetite and its occurrence in ore deposits.