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

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-SO 4 -bearing sedimentary rocks. This hypothesis is supported by geologic evidence, experimental studies,...

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Bibliographic Details
Published in:Mineralium deposita 2024, Vol.59 (1), p.189-225
Main Authors: Tornos, Fernando, Hanchar, John M., Steele-MacInnis, Matthew, Crespo, Elena, Kamenetsky, Vadim S., Casquet, Cesar
Format: Article
Language:English
Subjects:
Anhydrite
Apatite
Calcium magnesium silicates
Coalescence
Crystallization
Diopside
Earth and Environmental Science
Earth Sciences
Feldspars
Field strength
Fluids
Fractional crystallization
Fugacity
Furnaces
Geochemistry
Geology
Hydrothermal systems
Igneous rocks
Immiscibility
Iron
Isotopes
Lava
Magma
Magnetite
Melts (crystal growth)
Mineral deposits
Mineral inclusions
Mineral Resources
Mineralization
Mineralogy
Miscibility
Numerical models
Phosphates
Radiogenic materials
Sedimentary rocks
Separation
Silicates
Silicon
Slag
Solidus
Sulphates
Sulphides
Ultramafic rocks
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Summary: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-SO 4 -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 f O 2 of these systems also promotes the formation of low-Ti (
ISSN:0026-4598
1432-1866
DOI:10.1007/s00126-023-01203-w