skip to main content
Explore Scholarly Publications and Datasets in the NSF-PAR

  • NSF Public Access
  • Search Results
  • Accepted Manuscript: A review of magnetite geochemistry of Chilean iron oxide-apatite (IOA) deposits and its implications for ore-forming processes
Title: A review of magnetite geochemistry of Chilean iron oxide-apatite (IOA) deposits and its implications for ore-forming processes
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 compositional re-equilibration during magmatic and superimposed hydrothermal events. Based on the data presented here, more » 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
Authors:
Award ID(s):
1924142
Publication Date:
NSF-PAR ID:
10294834
Journal Name:
Ore geology reviews
ISSN:
0169-1368
Sponsoring Org:
National Science Foundation
More Like this
  1. ( , Economic geology)
    The textures of outcrop and near-surface exposures of the massive magnetite orebodies (>90 vol % magnetite) at the Plio-Pleistocene El Laco iron oxide-apatite (IOA) deposit in northern Chile are similar to basaltic lava flows and have compositions that overlap high- and low-temperature hydrothermal magnetite. Existing models— liquid immiscibility and complete metasomatic replacement of andesitic lava flows—attempt to explain the genesis of the orebodies by entirely igneous or entirely hydrothermal processes. Importantly, those models were developed by studying only near-surface and outcrop samples. Here, we present the results of a comprehensive study of samples from outcrop and drill core that require a new model for the evolution of the El Laco ore deposit. Backscattered electron (BSE) imaging, electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were used to investigate the textural and compositional variability of magnetite and apatite from surface and drill core samples in order to obtain a holistic understanding of textures and compositions laterally and vertically through the orebodies. Magnetite was analyzed from 39 surface samples from five orebodies (Cristales Grandes, Rodados Negros, San Vicente Alto, Laco Norte, and Laco Sur) and 47 drill core samples from three orebodies (Laco Norte, Laco Sur, andmore »Extensión Laco Sur). The geochemistry of apatite from eight surface samples from three orebodies (Cristales Grandes, Rodados Negros, and Laco Sur) was investigated. Minor and trace element compositions of magnetite in these samples are similar to magnetite from igneous rocks and magmatic-hydrothermal systems. Magnetite grains from deeper zones of the orebodies contain >1 wt % titanium, as well as ilmenite oxyexsolution lamellae and interstitial ilmenite. The ilmenite oxyexsolution lamellae, interstitial ilmenite, and igneous-like trace element concentrations in titanomagnetite from the deeper parts of the orebodies are consistent with original crystallization of titanomagnetite from silicate melt or high-temperature magmatic-hydrothermal fluid. The systematic decrease of trace element concentrations in magnetite from intermediate to shallow depths is consistent with progressive growth of magnetite from a cooling magmatic-hydrothermal fluid. Apatite grains from surface outcrops are F rich (typically >3 wt %) and have compositions that overlap igneous and magmatic-hydrothermal apatite. Magnetite and fluorapatite grains contain mineral inclusions (e.g., monazite and thorite) that evince syn- or postmineralization metasomatic alteration. Magnetite grains commonly meet at triple junctions, which preserve evidence for reequilibration of the ore minerals with hydrothermal fluid during or after mineralization. The data presented here are consistent with genesis of the El Laco orebodies via shallow emplacement and eruption of magnetite-bearing magmatic-hydrothermal fluid suspensions that were mobilized by decompression- induced collapse of the volcanic edifice. The ore-forming magnetite-fluid suspension would have rheological properties similar to basaltic lava flows, which explains the textures and presence of cavities and gas escape tubes in surface outcrops.« less
  2. ( , 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
  3. ( , Ore geology reviews)
    The Chahgaz iron oxide-apatite (IOA) deposit is one of the main IOA deposits in the Bafq metallogenic province, Central Iran. The Chahgaz mineral deposit is hosted by Early Cambrian felsic to intermediate, altered subvolcanic to effusive rocks that range compositionally from granite to diorite. Geochemical, geochronologic and tectonomagmatic investigations of various host rock types in the Bafq province indicate that mineralization was the product of Early Cambrian active continental margin processes that evolved calc-alkaline felsic igneous rocks followed by formation of diabase dykes in a back-arc basin environment. Magnetite is present in massive magnetite-rich ore bodies and veinlets that cut the massive ore bodies. Detailed macro- and micro-scopic characterization of mineralized samples and host rocks reveals a paragenetic sequence containing three generations of magnetite that are distinguished from one another compositionally and texturally. The massive ores contain apatite in trace amounts, consistent with IOA deposits globally, and locally exhibit textures that are visually similar to lava flow structures, as described for the El Laco IOA deposit, Chile. The ore bodies contain miarolitic cavities that are filled by calcite, hematite and quartz. The host rocks for the Chahgaz deposit have undergone widespread hydrothermal metasomatism including Na-Ca, K-, Mg-, Si-, sericitic, argillicmore »and carbonatization alteration. The compositions of two generations of magnetite, referred to as Mag1 and Mag2, in massive ore overlap compositions reported for igneous and high-temperature magmatic-hydrothermal magnetite. The third generation of magnetite, which is referred to as Mag3 and is present in veinlets cross-cutting the massive magnetite ore bodies, overlaps compositions reported for low to moderate temperature magmatichydrothermal magnetite. Pyrite is present as disseminated grains coeval with Mag1 and micro-fracture filling in the massive magnetite-rich ore bodies. The δ18O values obtained for magnetite from representative samples of massive magnetite Mag1 ore range between 2.18 and 6.32‰ and are consistent with δ18O values reported for igneous and magmatic-hydrothermal magnetite from other deposits in the Bafq district and globally. The δ34S values for pyrite range from 22.54 to 24.94‰ and are consistent with an evaporitic sulfur source; plausibly by magma contamination with evaporitic rocks of the Early Cambrian Volcano-Sedimentary Sequence (ECVSS). The data presented here are consistent with formation of the massive magnetite-rich ore bodies in the Chahgaz IOA deposit by an iron-rich magmatichydrothermal fluid.« less
  4. ( , 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
  5. ; ( , SEG Meeting)
    Societal demand for critical metals used in the high-tech and green industries has led to an increased interest in REEs associated with ore deposits. Hydrothermal mineralization of monazite (CePO4) in various REE deposits can display significant variations in REE mineralogy and rock chemistry. Monazite displays textural and REE compositional variations, such as those observed in the giant Bayan Obo REE deposit in China and in the Pea Ridge iron-oxide-apatite (IOA) deposit in Missouri. The coupling of compositional variations of monazite with thermodynamic modeling of fluid-rock interaction processes may provide a useful vectoring tool in these ore deposits. However, interpreting these geochemical fingerprints requires building an internally consistent thermodynamic dataset for REE minerals and their relevant aqueous complexes. In this study, a series of hydrothermal solubility experiments were carried out using synthetic monazite crystals (i.e., LaPO4, PrPO4, NdPO4, and EuPO4) to assess the consistency of reported mineral calorimetric data and the thermodynamic data of the aqueous REE complexes. The solubility experiments were conducted in aqueous HClO4-H3PO4–bearing solutions at temperatures between 100° and 250°C and at saturated water vapor pressure. Equilibrium constants (Ks0) for the dissolution reaction of monazite end members were retrieved as a function of temperature and extrapolated to standardmore »conditions of 25°C and 1 bar. Results indicate significant differences between the new solubility data and those reported in the literature. We demonstrate the impact of these new thermodynamic data in a series of fluid-rock interaction models using the GEMS code package (http://gems.web.psi.ch) and the MINES thermodynamic database (http://tdb.mines.edu). The simulated monazite stability can be correlated to field observations and allows for the prediction of the behavior of REE in hydrothermal fluids and their association to alteration observed in ore deposits.« less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email