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1. The Mina Justa Iron Oxide-Copper-Gold (IOCG) Deposit, Peru: Constraints on Metal and Ore Fluid Sources

   Rodriguez-Mustafa, M.A. Simon ( October 2021 , Economic geology)

   Iron oxide-copper-gold (IOCG) deposits are major sources of Cu, contain abundant Fe-oxides and may contain Au, Ag, Co, rare earth elements (REE), U and other metals as economically important byproducts in some deposits. They form by hydrothermal processes, but the source of the metals and ore fluid(s) is still debated. We investigated the geochemistry of magnetite from the manto and breccia ore bodies at the Mina Justa IOCG deposit in Peru to assess the source of the iron oxides and their relationship with the economic Cu mineralization. We identified three magnetite types: Type Inclusion (I) is only found in the manto, is the richest in trace elements, and crystallized between 459 - 707 °C; Type Dark (D) has no visible inclusions and formed at around 543 °C; and Type Bright (B) has no inclusions, has the highest Fe content, and formed at around 443 °C. Magnetite samples from Mina Justa yielded an average δ56Fe ± 2σ value of 0.28 ± 0.05‰ (n=9), an average δ18O ± 2σ value 2.19 ± 0.45‰ (n=9), and Δ’17O values that range between -0.075‰ and -0.047‰. Sulfide separates yielded δ65Cu values that range from -0.32‰ to -0.09‰. The trace element compositions and textures of magnetite,more »along with temperature estimations for magnetite crystallization, are consistent with the manto magnetite belonging to an IOA style mineralization that was overprinted by a younger, structurally-controlled IOCG event that formed the breccia ore body. Altogether, the stable isotopic data fingerprint a magmatic-hydrothermal source for the ore fluids carrying the Fe and Cu at Mina Justa and preclude the input from meteoric water and basinal brines.« less
2. The Mina Justa Iron Oxide Copper-Gold (IOCG) Deposit, Peru: Constraints on Metal and Ore Fluid Sources

   https://doi.org/10.5382/econgeo.4875  

   Maria A. Rodriguez-Mustafa, Adam C. ( September 2022 , Economic geology)

   Iron oxide copper-gold (IOCG) deposits are major sources of Cu, contain abundant Fe oxides, and may contain Au, Ag, Co, rare earth elements (REEs), U, and other metals as economically important byproducts in some deposits. They form by hydrothermal processes, but the source of the metals and ore fluid(s) is still debated. We investigated the geochemistry of magnetite from the hydrothermal unit and manto orebodies at the Mina Justa IOCG deposit in Peru to assess the source of the iron oxides and their relationship with the economic Cu mineralization. We identified three types of magnetite: magnetite with inclusions (type I) is only found in the manto, is the richest in trace elements, and crystallized between 459° and 707°C; type Dark (D) has no visible inclusions and formed at around 543°C; and type Bright (B) has no inclusions, has the highest Fe content, and formed at around 443°C. Temperatures were estimated using the Mg content in magnetite. Magnetite samples from Mina Justa yielded an average δ56Fe ± 2σ value of 0.28 ± 0.05‰ (n = 9), an average δ18O ± 2σ value of 2.19 ± 0.45‰ (n = 9), and D’17O values that range between –0.075 and –0.047‰. Sulfide separates yieldedmore »δ65Cu values that range from –0.32 to –0.09‰. The trace element compositions and textures of magnetite, along with temperature estimations for magnetite crystallization, are consistent with the manto magnetite belonging to an iron oxide-apatite (IOA) style mineralization that was overprinted by a younger, structurally controlled IOCG event that formed the hydrothermal unit orebody. Altogether, the stable isotopic data fingerprint a magmatic-hydrothermal source for the ore fluids carrying the Fe and Cu at Mina Justa and preclude significant input from meteoric water and basinal brines.« less
3. A review of magnetite geochemistry of Chilean iron oxide-apatite (IOA) deposits and its implications for ore-forming processes

   Pamla, G. ( September 2020 , Ore geology reviews)

   Magnetite is the most important iron ore in iron oxide-apatite (IOA) deposits which represent the Cu-poor endmember of the iron oxide-copper–gold (IOCG) clan. Magnetite chemistry has been used as a petrogenetic indicator to identify the geological environment of ore formation and as a fingerprint of the source reservoir of iron. In this study, we present new textural and microanalytical EPMA and LA-ICP-MS data of magnetite from Carmen, Fresia, Mariela and El Romeral IOA deposits located in the Cretaceous Coastal Cordillera of northern Chile. We also provide a comprehensive summary and discussion of magnetite geochemistry from Andean IOAs including Los Colorados, Cerro Negro Norte, El Romeral (Chilean Iron Belt) and the Pliocene El Laco IOA deposit located in the Central Volcanic Zone of the Chilean Andes. Microtextures coupled with geochemical data were used to define and characterize the occurrence of different magnetite types. Magnetite exhibits a variety of textural features including oscillatory zoning, colloform banding, re-equilibration textures, exsolution lamellae and symplectites. The magmatic vs. hydrothermal origin of the different magnetite types and the evolution of IOA deposits can be assessed using diagrams based on compatible trace elements. However, magnetite is very susceptible to hydrothermal alteration and to both textural and compositionalmore »re-equilibration during magmatic and superimposed hydrothermal events. Based on the data presented here, we conclude that V and Ga are possibly the most reliable compatible elements in magnetite to trace ore-forming processes in the Andean IOA deposits. Magnetite chemistry reveals different conditions/events of formation for each IOA deposit ranging from high-temperature, low-oxygen fugacity (ƒO2), purely magmatic (> 600 °C) conditions; to lower temperature and higher ƒO2 magmatic-hydrothermal (300–600 °C) to low-temperature hydrothermal (< 200–300 °C) conditions. Specifically, a continuous transition from high-temperature, low- ƒO2 conditions in the deepest portions of the deposits to low-temperature, relatively higher ƒO2 conditions towards surface are described for magnetite from El Laco. The new and compiled magnetite data from IOA deposits from the Chilean Iron Belt and El Laco are consistent with a transition from magmatic to hydrothermal conditions. The flotation model plausibly explains such features, which result from the crystallization of magnetite microlites from a silicate melt, nucleation and coalescence of aqueous fluid bubbles on magnetite surfaces, followed by ascent of a fluid-magnetite suspension along reactivated transtensional faults or through fissures formed during the collapse of the volcanic structure (El Laco). The decompression of the coalesced fluid-magnetite aggregates during ascent promotes the continued growth of magnetite microlites from the Fe-rich magmatic-hydrothermal fluid. As with any general genetic model, the flotation model allows variation and the definition of different styles or subtypes of IOA mineralization. The deeper, intrusive-like Los Colorados deposit shows contrasting features when compared with the Cerro Negro Norte hydrothermal type, the pegmatitic apatite-rich deposits of Carmen, Fresia and Mariela, and the shallow, subaerial deposits of El Laco. These apparent differences depend fundamentally on the depth of formation, the presence of structures and faults that trigger decompression, the composition of the host rocks, and the source and flux rate of hydrothermal fluids.« less
4. Triple Oxygen (δ18O, Δ17O), Hydrogen (δ2H), and Iron (δ56Fe) Stable Isotope Signatures Indicate a Silicate Magma Source and Magmatic-Hydrothermal Genesis for Magnetite Orebodies at El Laco, Chile

   https://doi.org/10.5382/econgeo.4760  

   Childress, T. ( November 2020 , Economic geology)

   The Plio-Pleistocene El Laco iron oxide-apatite (IOA) orebodies in northern Chile are some of the most enigmatic mineral deposits on Earth, interpreted to have formed as lava flows or by hydrothermal replacement, two radically different processes. Field observations provide some support for both processes, but ultimately fail to explain all observations. Previously proposed genetic models based on observations and study of outcrop samples include (1) magnetite crystallization from an erupting immiscible Fe- and P-rich (Si-poor) melt and (2) metasomatic replacement of andesitic lava flows by a hypogene hydrothermal fluid. A more recent investigation of outcrop and drill core samples at El Laco generated data that were used to develop a new genetic model that invokes shallow emplacement and surface venting of a magnetite-bearing magmatic-hydrothermal fluid suspension. This fluid, with rheological properties similar to basaltic lava, would have been mobilized by decompression- induced collapse of the volcanic edifice. In this study, we report oxygen, including 17O, hydrogen, and iron stable isotope ratios in magnetite and bulk iron oxide (magnetite with minor secondary hematite and minor goethite) from five of seven orebodies around the El Laco volcano, excluding San Vicente Bajo and the minor Laquito deposits. Calculated values of δ18O, Δ17O, δD,more »and δ56Fe fingerprint the source of the ore-forming fluid(s): Δ17Osample = δ17Osample – δ18Osample * 0.5305. Magnetite and bulk iron oxide (magnetite variably altered to goethite and hematite) from Laco Sur, Cristales Grandes, and San Vicente Alto yield δ18O values that range from 4.3 to 4.5‰ (n = 5), 3.0 to 3.9‰ (n = 5), and –8.5 to –0.5‰ (n = 5), respectively. Magnetite samples from Rodados Negros are the least altered samples and were also analyzed for 17O as well as conventional 16O and 18O, yielding calculated δ18O values that range from 2.6 to 3.8‰ (n = 9) and Δ17O values that range from –0.13 to –0.07‰ (n = 5). Bulk iron oxide from Laco Norte yielded δ18O values that range from –10.2 to +4.5‰ (avg = 0.8‰, n = 18). The δ2H values of magnetite and bulk iron oxide from all five orebodies range from –192.8 to –79.9‰ (n = 28); hydrogen is present in fluid inclusions in magnetite and iron oxide, and in minor goethite. Values of δ56Fe for magnetite and bulk iron oxide from all five orebodies range from 0.04 to 0.70‰ (avg = 0.29‰, σ = 0.15‰, n = 26). The iron and oxygen isotope data are consistent with a silicate magma source for iron and oxygen in magnetite from all sampled El Laco orebodies. Oxygen (δ18O Δ +4.4 to –10.2‰) and hydrogen (δ 2H ≃ –79.9 to –192.8‰) stable isotope data for bulk iron oxide samples that contain minor goethite from Laco Norte and San Vicente Alto reveal that magnetite has been variably altered to meteoric values, consistent with goethite in equilibrium with local δ18O and δ2H meteoric values of ≃ –15.4 and –211‰, respectively. The H2O contents of iron oxide samples from Laco Norte and San Vicente Alto systematically increase with increasing abundance of goethite and decreasing values of δ18O and δ2H. The values of δ2H (≃ –88 to –140‰) and δ18O (3.0–4.5‰) for magnetite samples from Cristales Grandes, Laco Sur, and Rodados Negros are consistent with growth of magnetite from a degassing silicate melt and/or a boiling magmatic-hydrothermal fluid; the latter is also consistent with δ18O values for quartz, and salinities and homogenization temperatures for fluid inclusions trapped in apatite and clinopyroxene coeval with magnetite. The sum of the data unequivocally fingerprint a silicate magma as the source of the ore fluids responsible for mineralization at El Laco and are consistent with a model that explains mineralization as the synergistic result of common magmatic and magmatic-hydrothermal processes during the evolution of a caldera-related explosive volcanic system.« less
5. Formation of the Mantoverde iron oxide-copper-gold (IOCG) deposit, Chile: insights from Fe and O stable isotopes and comparisons with iron oxide-apatite (IOA) deposits

   https://doi.org/10.1007/s00126-019-00936-x  

   Childress, T.M. ( January 2020 , Mineralium deposita)

   The Mantoverde iron oxide-copper-gold (IOCG) deposit, Chile, contains hundreds of millions of tonnes (Mt) of mineable iron oxide and copper sulfide ore.While there is an agreement that mineralization at Mantoverde was caused by hydrothermal fluid(s), there is a lack of consensus for the role(s) that non-magmatic vs. magmatic fluid(s) played during the evolution of the mineralized system. In order to overcome the hydrothermal overprint at Mantoverde, which is known to disturb most conventional stable isotope systems (e.g., oxygen), we report the first δ56Fe and δ18O pairs for early-stage magnetite and late-stage hematite that provide information on the source reservoir of the hydrothermal fluids. Magnetite δ56Fe values range from 0.46 ± 0.04 to 0.58 ± 0.02‰and average 0.51 ± 0.16‰(n = 10; 2σ). Three hematite δ56Fe values were measured to be 0.34 ± 0.10, 0.42 ± 0.09, and 0.46 ± 0.06. Magnetite δ18O values range from 0.69 ± 0.04 to 4.61 ± 0.05‰ and average 2.99 ± 2.70‰ (n = 9; 2σ). Hematite δ18O values range from − 1.36 ± 0.05 to 5.57 ± 0.05‰and average 0.10 ± 5.38‰(n = 6; 2σ). These new δ56Fe and δ18O values fingerprint a magmatic-hydrothermal fluid as the predominant ore-forming fluid responsible for mineralization inmore »the Mantoverde system.« less