Formation of magnetite-(apatite) systems by crystallizing ultrabasic iron-rich melts and slag separation

Abstract

Magnetite-(apatite) ore deposits are interpreted as being formed by the crystallization of iron-rich ultrabasic melts, dominantly generated by the interaction of silicate melts with oxidized P-F-SO4-bearing sedimentary rocks. This hypothesis is supported by geologic evidence, experimental studies, numerical modeling, stable and radiogenic isotope geochemistry, mineralogy, and melt- and mineral-inclusion data. Assimilation of crustal rocks during ascent promotes separation from a silicate magma of Fe-rich, Si-Al-poor melts with low solidus temperatures and viscosities, allowing coalescence, migration, and emplacement at deep to subaerial crustal environments. When the iron-rich melt attains neutral buoyancy, fractional crystallization leads to melt immiscibility similar to that observed in industrial blast furnaces, which promotes separation of massive magnetite ore overlain by different types of “slag” containing actinolite or diopside ± phosphates ± magnetite ± feldspar ± anhydrite ± scapolite, commonly enriched in high field strength elements. The mineralogy and morphology of this iron-depleted cap strongly depend on the depth of emplacement and composition of the iron-rich magma. Most of these systems exhibit high oxygen fugacity, which inhibits the precipitation of significant sulfide mineralization. The initially high fO2 of these systems also promotes the formation of low-Ti (< 1 wt%) magnetite: Ti acts as an incompatible component and is enriched in the iron-poor caps and in the hydrothermal aureole. High fluid-phase pressures produced during massive crystallization of magnetite from the melt further facilitate the exsolution of magmatic-hydrothermal fluids responsible for the formation of aureoles of alkali-calcic-iron alteration with hydrothermal replacement-style iron mineralization. On the whole, these systems are dramatically different from the magmatic-hydrothermal systems related to intermediate to felsic igneous rocks; they are more akin to carbonatite and other ultramafic rocks.

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

“…Several IOA deposits and showings are known in the vicinity of the Ildeus intrusion in the SSZ and IOCG-type mineralization occurring NW of the Bakening volcano in the Sredinny Range of Kamchatka (Figure 1). Similar geologic, petrologic and metallogenic relationships have been documented in magmatic terranes hosting IOCG deposits in the Andes of South America and elsewhere [1,3,10,13,15,20,139]. Arc-related magmatic roots of IOCG deposits are further emphasized by the frequent occurrence of magnetite lava (similar to the "classic" El Laco locality in northern Chile) in association with magnetite, hematite, chalcopyrite, bornite and gold mineralization [20,[39][40][41]43,[47][48][49][50], as well as the igneous geochemistry of early generations of magnetite and pyrite in such IOA and IOCG deposits as Carmen, Fresia, El Romeral, El Laco, Los Colorados, Cerro Negro Norte and many others [4,8,[18][19][20][140][141][142][143][144].…”

“…Similar geologic, petrologic and metallogenic relationships have been documented in magmatic terranes hosting IOCG deposits in the Andes of South America and elsewhere [1,3,10,13,15,20,139]. Arc-related magmatic roots of IOCG deposits are further emphasized by the frequent occurrence of magnetite lava (similar to the "classic" El Laco locality in northern Chile) in association with magnetite, hematite, chalcopyrite, bornite and gold mineralization [20,[39][40][41]43,[47][48][49][50], as well as the igneous geochemistry of early generations of magnetite and pyrite in such IOA and IOCG deposits as Carmen, Fresia, El Romeral, El Laco, Los Colorados, Cerro Negro Norte and many others [4,8,[18][19][20][140][141][142][143][144]. We propose that the iron-titanium oxide-apatite-sulfide-sulfate (ITOASS-type) microinclusions in gabbro and adakite from the Russian Far East provide some potential links to the early magmatic stages of formation of IOA and IOCG deposits and offer new insights into the magmatic-hydrothermal evolution of subduction-related mineralized systems in orogenic terranes.…”

“…In several cases, magnetite-apatite intrusions and associated IOA deposits are located in the vicinity of each other (e.g., the Ildeus intrusion is surrounded by multiple IOA metal showings; Figure 1) and even interlayered [11,12,[133][134][135], suggesting close spatial, temporal and, perhaps, even genetic link between subduction-related magmas and Kiruna-type mineralization [5,9,20,49,[135][136][137][138]. Several IOA deposits and showings are known in the vicinity of the Ildeus intrusion in the SSZ and IOCG-type mineralization occurring NW of the Bakening volcano in the Sredinny Range of Kamchatka (Figure 1).…”

“…Iron oxide-apatite (IOA; "Kiruna-type") and iron oxide-copper -gold (IOCG; "Chileantype") deposits contain major resources of a wide range of critical (iron, copper, phosphorous, rare earths, uranium, etc.) and precious (gold, silver) metals [1][2][3][4][5][6][7][8][9][10] and are commonly found in arc-related, orogenic and post-orogenic tectonic settings [1][2][3]7,[11][12][13][14][15][16][17][18][19][20]. The formation of most current models involves multi-stage magmatic-hydrothermal processes and hydrous halogen-rich fluids, which scavenge metals from primary mantle-sourced, metal-rich silicate melts [9,[21][22][23][24][25][26][27][28][29][30][31].…”

“…Most studies tend to agree that IOA-IOCG and related mineral systems reflect a complex interplay of crustal magmatic and hydrothermal processes [1][2][3]5,7,9,10,19,21]. The Minerals 2024, 14, 188 2 of 19 occurrence of magnetite lavas with andesitic cement along with cogenetic pyroclastics and even magnetite volcanic bombs [39][40][41]43,[46][47][48] and the frequent association of IOA-IOCG deposits with plutonic and volcanic rocks of intermediate to felsic compositions (granodiorites and diorites, andesites and dacites) [4,5,10,11,15,20,21] indicate presence of some magmatic component, at least during the early stages of the evolution of IOA, IOCG and related mineral systems. Additional geochemical characteristics such as the Fe isotope signature [29,45] and trace element composition [31] of early-generation magnetite, distribution of F-rich and Cl-rich domains in apatite [28] and Os isotope geochemistry [40] also support petrologic models suggesting that IOA-IOCG deposits can be rooted in evolving lithospheric magmatic systems.…”