RT Journal Article
T1 Formation of magnetite-(apatite) systems by crystallizing ultrabasic iron-rich melts and slag separation
A1 Tornos Arroyo, Fernando
A1 Hanchar, John M.
A1 Steele-MacInnis, Matthew
A1 Crespo Feo, María Elena
A1 Kamenetsky, Vadim S.
A1 Casquet Martín, César
AB 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.
PB Springer
SN 0026-4598
YR 2023
FD 2023-09-02
LK https://hdl.handle.net/20.500.14352/102842
UL https://hdl.handle.net/20.500.14352/102842
LA eng
NO Horizon Europe
NO NSERC Discovery Grants (Canada)
NO MCIN/AEI/10.13039/501100011033
NO Chinese Academy of Sciences
NO CRUE-CSIC
DS Docta Complutense
RD 9 abr 2025