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Iron-oxide apatite (IOA) deposits are mined for iron (Fe) and can also contain economically exploitable amounts of Cu, P, U, Ag, Co, and rare earth elements (REE). Recently, it has been proposed based on trace element zonation in magnetite grains from the Los Colorados Kiruna-type IOA deposit, Chile, that ore formation is directly linked to a magmatic source. The model begins with the crystallization of magnetite microlites within an oxidized volatile-rich (H2O+Cl) andesitic magma reservoir, followed by decompression, nucleation of fluid bubbles on magnetite microlite surfaces, segregation of a Fe-Cl-rich fluid-magnetite suspension within the magma reservoir, and subsequent ascent of the suspension from the magma chamber via pre-existing structurally enhanced dilatant zones that act as conduits. Emplacement and precipitation of the suspension results in the formation of magnetite grains with core-to-rim features that record a transition from purely igneous to magmatic-hydrothermal conditions within IOA deposits. Here we test this model by using in situ femtosecond laser ablation MC-ICP-MS measurements of Fe isotopes to determine grain-to-grain and intra-grain Fe isotope variations in magnetite grains from the Los Colorados IOA deposit. All in situ 656Fe values (56Fe/54Fe relative to IRMM-14) plot within the magmatic range (0.06 to 0.50%o), in agreement with previously published bulk Fe isotope analyses in magnetite from the Los Colorados IOA deposit. Different trace element signatures of these magnetite grains indicate an igneous or magmatic-hydrothermal origin, respectively. Although data partly overlap, the assigned igneous magnetites yield on average higher 656Fe values (0.24 ± 0.07%o; n = 33), when compared to magmatic-hydrothermal magnetites (0.15 ± 0.05%o; n = 26). Some magnetite grains exhibit a distinct core-to-rim trend from higher toward lower d56Fe signatures. Furthermore, the 656Fe of the igneous magnetites correlate negatively with trace elements contents typical for igneous formation (Ti, Al, Ga, V, Mn, Zn); igneous magnetites become isotopically heavier with decreasing concentrations of these elements, indicating a trend toward higher d56Fe in the magnetite with magma evolution. Model calculations of the 656Fe evolution in melt, magnetite, and fluid further constrain the magmatic-hydrothermal origin of Kiruna-type IOA deposits. [ABSTRACT FROM AUTHOR]

Interpretation of the mineralizing environment of magnetite-apatite deposits remains controversial with theories that include a hydrothermal or magmatic origin or a combination of those two processes. To address this controversy, we have analyzed the trace element content of magnetite from precisely known geographic locations and geologic environments from the Precambrian magnetite-apatite ore and host rocks in Kiruna, Sweden, and the Pliocene-Holocene El Laco volcano in the Atacama desert of Chile. Magnetite samples from Kiruna have low trace element concentrations with little chemical variation between the ore, host, and related intrusive rocks. Magnetite from andesite at El Laco, and dacite from the nearby Láscar volcano, has high trace element concentrations typical of magmatic magnetite. El Laco ore magnetite have low trace element concentrations and displays growth zoning in incompatible elements (Si, Ca, and Ce), compatible elements (Mg, Al, and Mn), large-ion lithophile element (Sr), and high field strength element (Y, Nb, and Th). The El Laco ore magnetite are similar in composition to magnetite that has been previously interpreted to have crystallized from hydrothermal fluids; however, there is a significant difference in the internal zoning patterns. At El Laco, each zoned element is either enriched or depleted in the same layers, suggesting the magnetite crystallized from a volatile-rich, iron-oxide melt. In general, the compositions of magnetite from these two deposits plot in very wide fields that are not restricted to the proposed fields in published discriminant diagrams. This suggests that the use of these diagrams and genetic models based on them should be used with caution. [ABSTRACT FROM AUTHOR]

Mesozoic magmatic-hydrothermal iron oxide (MHIO) deposits in Mexico have been identified either (1) in regions close to the paleo-Pacific convergent margin, in Baja California and southwestern-southern Mexico, and (2) in regions several hundreds of kilometers inland from that margin, in Chihuahua and Sonora. The formation of these deposits started during the transition from dominantly oceanic extensional tectonomagmatic environments (Aptian–Albian; see Camprubí, 2013) towards the continental thickening by means of subduction-related magmatic processes into the Cenozoic. Despite their need for further studies, in this paper we suggest that MHIO deposits in the type-1 regions, on the one hand, formed possibly in intra-arc continental settings, similarly to those that occur in the Coastal Andes Cordillera in South America. On the other hand, deposits in the type-2 regions are likely to have formed in back-arc continental settings, which are analogous to those in eastern Mexico during the Cenozoic (collectively included in the Eastern Mexico Alkaline Province). MHIO deposits in Mexico are typically associated with intrusive sequences that include gabbros, diorites and monzonites within plutonic ensembles that are largely granitic, where calc-alkaline- and tholeiitic-affinity intrusions display synchronicity at some degree and complex interactions (magma mixing or mingling). The oldest MHIO deposits known in Mexico are those in the northern half of the Baja California Peninsula (Early Cretaceous), in association with the Peninsular Batholits, and delineating a relatively ephemeral metallogenic epoch. Contrastingly, MHIO deposits in southwestern and southern Mexico started forming during the Late Cretaceous and continued forming during the Cenozoic, in association with the Silicic Large Igneous Province (SLIP) of the Western and Southern Sierras Madres and the related back-arc magmatic processes in the Eastern Mexico Alkaline Province. The age spans for the formation of individual MHIO deposits in southwestern and southern Mexico appear to be wider than those in the Baja California Peninsula, a feature that is consistent with the ages of the associated intrusions but may also explain the fact that MHIO deposits in southwestern-southern Mexico tend to be larger than those in Baja California. The case of the late Cretaceous to Eocene (ca. 71 to 48 Ma) multi-stage Peña Colorada deposit in Colima, the largest iron deposit in Mexico, exemplifies a sort of telescoping process, in which (1) stages of mineralization that formed proximal to magmatic sources (skarns and skarnoids) were postdated by (2) distal to source stages (Kiruna-type stages and hydrothermal veins). It may be speculated that such evolution may occur in other MHIO deposits elsewhere that were associated with a steady magmatism or ‘magmatic supply’ unless rapid exhumation ‘quenched’, and ultimately stopped, this evolution. Such might be the case of MHIO deposits in the Baja California Peninsula, where intrusions migrated eastwards between 140 and 80 Ma before being rapidly exhumed. The formation of MHIO deposits in regions close to the paleo-Pacific convergent margin started forming once the accretion of oceanic terranes was complete by the Early-Late Cretaceous boundary, and such formation is likely to have been terminated once the continental thickening reached a critical point during the Laramide orogenic pulse. As such pulse showed a north-to-south progression, it resulted in the termination of the formation of ‘coastal’ MHIO deposits earlier in the Baja California Peninsula than in southwestern and southern Mexico (Late Cretaceous and Eocene, respectively). [ABSTRACT FROM AUTHOR]

[Display omitted] • The BIMB hosts several large Kiruna-type IOA deposits. • Higher REEs concentration in phosphate zone than the host rock and iron zones. • Similarities in REEs suggest a co-magmatic source for iron-apatite ores. • LA-ICPMS U-Pb dating of apatite yielded ages of 543 to 437 Ma. • The first apatite mineralization was related to the intrusions (547 to 525 Ma) The Bafq iron metallogenic belt in Central Iran hosts several large Kiruna-type iron oxide-apatite deposits (including Chadormalu, Choghart, Chahgaz, Esfordi, Lake-Siah, Gazestan, Se-Chahoun, Mishdovan, Anomaly 14 and 21A). These deposits have rare earth element (REE) potential, with the REE mainly concentrated in apatite. The Chadormalu iron-apatite deposit is hosted by igneous rocks that experienced various degrees of alteration and metamorphism. Enrichment in the LREEs relative to the HREEs ((La/Yb) N = 7.4–25.5; (La/Sm) N = 1.9–3.6, and (Ce/Yb) N = 7.4–25.5) and the gentle HREE slope ((Gd/Yb) N = 2.4 to 4.6) are significant features of the Chadormalu apatite. The similarities in REE patterns across the Bafq iron metallogenic belt (Chadormalu, Anomaly 14; Gazestan, Esfordi, and Lake-Siah deposits) suggest a cogenetic evolution for the iron-apatite ores. LA-ICPMS U-Pb dating of apatite yielded ages of 529 ± 25 Ma (Chadormalu), 543 ± 32 (Chahgaz), 517 ± 25 Ma (Mishdovan) and 533 ± 19 Ma (Anomaly 14). These ages are contemporaneous with Proto-Tethyan continental-arc magmatism from 547 Ma to 525 Ma, suggesting the synchroneous formation of the magnetite-apatite mineralization with its Late Proterozoic to Early Cambrian host rocks. Younger apatite U-Pb ages include an age of 437 ± 21 Ma from Chadormalu, an age of 457 ± 140 Ma from Anomaly 14, and an age of 215 ± 11 Ma for a sample from Anomaly 21A. The younger ages are assumed to reflect resetting of the U-Pb system due to hydrothermal events related to two regional extensional episodes, i.e. Early Silurian syenite magmatism at 440–420 Ma, and Late Triassic granite-tonalite magmatism at about 210–220 Ma. [ABSTRACT FROM AUTHOR]