Abstract:
Trace element concentrations in magnetite are dictated by the petrogenetic environment and by the physico-chemical conditions
during magmatic, hydrothermal, or sedimentary processes. This makes magnetite chemistry a useful tool in the exploration
of ore-forming processes. We describe magnetite compositions from Ni-Cu-(PGE)-sulfide mineralized rocks from seven
mafic–ultramafic intrusions peripheral to the Mesoproterozoic AMCG (anorthosite-mangerite-charnockite-granite) suite of
the Kunene Complex of Angola and Namibia to investigate metallogenic processes through the geochemical characterization
of Fe-oxides, which were analyzed in-situ via Electron Probe Microanalysis (EPMA), and Laser Ablation-Inductively
Coupled Plasma-Mass Spectrometry (LA-ICP-MS). We identified magmatic magnetite, segregated from both a silicate
liquid and an immiscible sulfide liquid. Elements like Cr, Co and V suggest that the sulfide-related magnetite segregated
from a relatively primitive Fe-rich monosulfide solid solution (MSS). Secondary Cr-rich magnetite appears in intrusions
with abundant chromite or Cr-spinel. Two types of hydrothermal magnetite were identified, related to the pervasive replacement
of sulfides and a late-stage, low-T fluid circulation event. Magnetite replacing sulfides is associated with serpentinized
ultramafic rocks and is preferentially observed in the intrusions with the highest base and precious metal tenors. The high
concentration of Ni, Co, Cu, Pd, As and Sb in these grains is corroborated by the identification of micron-size PGE mineral
inclusions. We infer that serpentinization during hydrothermal fluid circulation was accompanied by desulphurization of
sulfides with metal remobilization and reconcentration to generate magnetite carrying Pd microinclusions. We suggest that
the highly serpentinized ultramafic rocks in the Kunene Complex region may become a possible target for economic Ni-
Cu-(PGE) mineralization.
Description:
SUPPLEMENTARY FILE 1. Analytical techniques and settings.
SUPPLEMENTARY FILE 2. EPMA data (as wt%) for the analyzed oxides. Abbreviations: Mt = magnetite; Cr-Mt = chromium magnetite; Ferritchr = ferritchromite; Cr-Sp = Cr-spinel; Sp = spinel. According to the Cr content, the Fe-oxides are classified as magnetite (Cr2O3: 0–6 wt%), Cr-magnetite (Cr2O3: 6–13 wt%), ferritchromite (Cr2O3 >13 wt%, e.g., Hodel et al. 2020).
SUPPLEMENTARY FILE 3. Analytical results of the reference materials used to monitor the quality of the data. SUPPLEMENTARY FILE 4. Summary of LA-ICP-MS results (in ppm) for magnetite (Mt), Cr-magnetite (Cr-Mt) and ferritchromite (Ferritchrom). <… = below detection limit. 57Fe was assumed with stoichiometric values of 72.4 wt% in magnetite. In Cr-magnetite and ferritchromite 57Fe is according to EPMA average data (KSAT310-125: 57Fe in Cr-Mt = 46.5 wt%, 57Fe in Ferritchr = 38.8 wt%; KSAT280-149: 57Fe in Ferritchr = 37.4 wt%). Note that silica might be overestimated due to the high background signal. The analyses with Ni > 0.5 wt%, Cu > 0.3 wt%, and Zn > 0.3 wt%, likely affected by sulfide contamination, are in italics, and were not considered in the discussion. Chondrite REE normalization values after Boynton (1985).
