Nanoscale Study of Titanomagnetite from the Panzhihua Layered Intrusion, Southwest China: Multistage Exsolutions Record Ore Formation

Abstract

Titanomagnetite from Fe-Ti-V ores of the Lanjiahuoshan deposit, Panzhihua layered intrusion, Southwest China, was investigated at the nanoscale. The objectives were to establish the composition of exsolution phases and their mutual relationships in order to evaluate the sequence of exsolution among oxide phases, and assess mechanisms of ore formation during magma emplacement. At the micron-scale, titanomagnetite shows crosscutting sets of exsolutions with ilmenite and Al-Mg-Fe-spinel (pleonaste), as well as overprint, both in terms of phase re-equilibration and remobilization of trace elements. Most complex textures were found in titanomagnetite surrounded by ilmenite and this was selected for high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) imaging and STEM energy-dispersive X-ray spectrometry (EDS) spot analysis and mapping on a thin foil prepared in situ on a focused ion beam scanning electron microscope platform. Titanomagnetite revealed two sequential sets of exsolutions, {111} crosscutting {100}, which are associated with changes in phase speciation and trace element distribution patterns. Qandilite is the dominant spinel phase inside titanomagnetite; magnesioferrite is also identified. In contrast, Fe-poor, Al-rich, Mg-bearing spinel is present within ilmenite outside the grain. Vanadium enrichment in newly-formed magnetite lamellae is clear evidence for trace element remobilization. This V-rich magnetite shows epitaxial relationships with ilmenite at the contact with titanomagnetite. Two-fold super-structuring in ilmenite is evidence for non-redox re-equilibration between titanomagnetite and ilmenite, supporting published experimental data. In contrast, the transformation of cubic Ti-rich spinel into rhombohedral ilmenite imaged at the nanoscale represents the “oxy-exsolution” model of titanomagnetite–ilmenite re-equilibration via formation of a transient ulvöspinel species. Nanoscale disorder is encountered as vacancy layers in Ti-rich spinel, and lower symmetry in the Fe-poor, Al-Mg phase, suggesting that slow cooling rates can preserve small-scale phase equilibration. The cooling history of titanomagnetite ore can be reconstructed as three distinct stages, concordant with published models for the magma plumbing system: equilibrium crystallization of Al-rich, Mg-bearing titanomagnetite from cumulus melts at ~55 km, with initial exsolutions occurring above 800 °C at moderate fO2 conditions (Stage 1); crosscutting {111} exsolutions resulting in formation of qandilite, attributable to temperature increase due to emplacement of another batch of melt affecting the interstitial cumulus during uplift. Formation of 2-fold superstructure ilmenite + V-rich magnetite exsolution pairs representing non-redox equilibration indicates resetting of the cooling path at this stage (Stage 2); and ilmenite formation from pre-existing Ti-rich spinel and ulvöspinel, illustrative of redox-driven cooling paths at <10 km (Stage 3). HAADF STEM provides direct imaging of atomic arrangements, allowing recognition of processes not recognizable at the micron-scale, and can thus be used to constrain exsolution models during ore formation.

High‐Latitude Geomagnetic Secular Variation at the End of the Cretaceous Normal Superchron Recorded by Volcanic Flows From the Okhotsk‐Chukotka Volcanic Belt

“…We thus speculate that the remanent magnetization of this flow is mainly carried by titanomagnetite grains, exsolved from host pyroxene grains during the cooling of lava. These titanomagnetite inclusions likely experienced high‐temperature oxidation (e.g., Gao et al., 2019), as revealed by the presence of fine ilmenite lamellae and titanium‐poor areas. On the contrary, the remanence carriers associated with the low‐temperature phase ( T C < 300°C) from groups #2 and #4 were likely exposed to low‐temperature oxidation, as revealed by the presence of shrinkage cracks (Figure 6f; e.g., Petersen & Vali, 1987).…”

Editorial for Special Issue “Minerals Down to the Nanoscale: A Glimpse at Ore-Forming Processes”

“…Changes in mineral signatures, phase associations, and zoning patterns are interpreted in terms of partitioning and cooling paths in magmatic systems [24,28]. The presence of critical metals, invisible gold in sulphides, and fluid-rock interactions in magmatic-hydrothermal systems are addressed for single deposits or ore provinces [23,25,27,[29][30][31][32].…”

“…The morphology of pyrite from sedimentary deposits is suggested as an indicator of the hydrothermal influence on the deposit. Gao et al [28] report a nanoscale investigation of titanomagnetite from Fe-Ti-V ores of the Lanjiahuoshan deposit, Panzhihua layered intrusion, Southwest China. The research uses HAADF STEM imaging and STEM EDS spot analysis and mapping to establish the composition of exsolution phases and their mutual relationships in order to evaluate the sequence of exsolution and to assess mechanisms of ore formation during magma emplacement.…”

Cation Inversion in Slag Magnetite: Energy Loss Measurements of Fe-L3 Edge Shift between Atom Columns

“…High-temperature magnetite in magmatic-hydrothermal ore deposits or as an accessory in igneous rocks formed from natural silicate melts contains micron-to nanoscale inclusions of other spinels, among which ulvöspinel, hercynite and spinel sensu stricto are the most common [26][27][28]. For example, hercynite nanoparticles (NPs) are identified in magnetite formed at 670-510 ± 50 • C [28].…”