Abstract
A comprehensive nanoscale study on magnetite from samples from the outer, weakly mineralized shell at Olympic Dam, South Australia, has been undertaken using atom-scale resolution High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF STEM) imaging and STEM energy-dispersive X-ray spectrometry mapping and spot analysis, supported by STEM simulations. Silician magnetite within these samples is characterized and the significance of nanoscale inclusions in hydrothermal and magmatic magnetite addressed. Silician magnetite, here containing Si–Fe-nanoprecipitates and a diverse range of nanomineral inclusions [(ferro)actinolite, diopside and epidote but also U-, W-(Mo), Y-As- and As-S-nanoparticles] appears typical for these samples. We observe both silician magnetite nanoprecipitates with spinel-type structures and a γ-Fe1.5SiO4 phase with maghemite structure. These are distinct from one another and occur as bleb-like and nm-wide strips along d111 in magnetite, respectively. Overprinting of silician magnetite during transition from K-feldspar to sericite is also expressed as abundant lattice-scale defects (twinning, faults) associated with the transformation of nanoprecipitates with spinel structure into maghemite via Fe-vacancy ordering. Such mineral associations are characteristic of early, alkali-calcic alteration in the iron-oxide copper gold (IOCG) system at Olympic Dam. Magmatic magnetite from granite hosting the deposit is quite distinct from silician magnetite and features nanomineral associations of hercynite-ulvöspinel-ilmenite. Silician magnetite has petrogenetic value in defining stages of ore deposit evolution at Olympic Dam and for IOCG systems elsewhere. The new data also add new perspectives into the definition of silician magnetite and its occurrence in ore deposits.
Highlights
Magnetite (Fe2+ Fe3+ 2 O4 ), an abundant mineral in ore deposits of various genetic types, is one of the ‘2-3 oxide spinels’, A2+ B3+ 2 O4 ; Table 1 ([1] and references therein)
Magnetite from the outer shell of the Olympic Dam deposit is characterized by the presence of μm- to nm-scale inclusions of various composition and with variable distributions
Crystal zoning is well-expressed in silician magnetite (Figure 2a) throughout all samples whereas magmatic magnetite from Roxby Downs Granite (RDG) shows lamellar networks of Ti-bearing oxides attributable to
Summary
Magnetite (Fe2+ Fe3+ 2 O4 ), an abundant mineral in ore deposits of various genetic types, is one of the ‘2-3 oxide spinels’, A2+ B3+ 2 O4 ; Table 1 ([1] and references therein). The oxide spinel structure has a cubic close packing of anions (O), with the cations hosted either in tetrahedral (T) or octahedral (M) sites (e.g., [2]). Of particular petrogenetic significance are the magnetite-ulvöspinel and magnetite-ahrensite solid solution series. These are two examples of paired “2-3” and “4-2” spinel series that involve coupled substitutions between A and B cations to accommodate the charge balance in the T and M sites.
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