This polymetallic orebody is the most northerly VHMS-type (volcanic-hosted massive sulphide) deposit of the Iberian Pyrite Belt (IPB) known so far (Figure 1). It occurs underneath approximately 130 m of sediment from the Sado Tertiary basin, limiting interpretation to drillhole data. The orebody is folded, faulted, and interpreted to occur mostly on the sub vertical overturned and intensely faulted limb of a southwest-verging anticline. Lagoa Salgada is further offset by an east-west-trending Alpine-age fault in the north, with a 50 m down throw of the northern block, but whose horizontal amount and sense of displacement remain unknown [10].

The deposit comprises a Central Stockwork Zone and a Northwestern Massive Sulfide Lens. The Central Stockwork zone is 700 m thick and comprises sulfide veins and semi massive sulfide lenses and is mainly hosted by an Upper Devonian (Fammenian) age, is thick (up to 250 m) and a strongly chloritized quartz-phyric rhyodacite unit. Additionally, there is a feldspar- and quartz-phyric rhyodacite that dominates the sequence hosting the massive sulfide lens in the northwest. These two rhyodacites are clearly distinguished by their phenocryst content; geochemically, the former corresponds to a more evolved series than the latter. The Central Stockwork is characterized by minor Pb, Zn and Sn, and Cu values can reach 1.4 wt % [11].

The known part of the Northwestern Massive Sulfide Lens averages 20 to 25 m in thickness and displays a large variation in metal contents, including significant concentrations of Zn (maximum 20 wt %) and Pb (maximum 23 wt %) associated with the feldspar- and quartz-phyric rhyodacite. Lesser quantities of Sn, Cu, Hg, As, Sb, Au, Ag and In were also detected [12]. Average grades of 0.35% Cu, 3.22% Pb, 4.43% Zn, 0.40% Sn, 72.52 g/t Ag, and 0.95 g/t Au have been estimated [13].

The deposit has an inferred mineral resource of 3.7 Mt from both the Central Stockwork and the Northwestern Massive Sulfide Lens. Setting Lagoa Salgada aside from other nearby VHMS deposits is the presence of indium as a trace element within the base metal element suite. This very scarce metal is preferentially hosted by sphalerite (see Figure 10 from [11]), with contents attaining an average maximum of 0.62% In. The fact that until now discrete inclusions of In-bearing minerals have not been observed reinforces the idea that indium occurs possibly within nanophases [12] as postulated before [1]. The preponderance of sphalerite-associated indium in the Lagoa Salgada deposit suggests that it was formed at relatively low temperatures with intermittent pulses of higher temperatures that have originated the Cu-bearing ores.

Indium is the element with atomic number 49, having the electronic structure [Kr] 4d¹⁰ 5s² 5p¹. Accordingly, a trivalent state is frequently assumed by indium ions, suggesting an inertness of the 5s² electron-pair. In Nature indium is mainly carried by zinc sulphide - the mineral sphalerite [14].

Stable In-compounds are diversified, ranging from the hydride (InH) and the nitride (InN) that configure the two stable formal valences (1+) and (3+), to the phosphide (InP) and the arsenide (InAs) that have been extensively applied in semiconductor technologies and where indium behaves formally as a trivalent cation. Other synthetic In-compounds include halides, oxides and chalcogenides, the latter being worthy of a particular interest for the understanding of indium geochemical behavior.

The recovery of indium stands mostly on zinc extraction from sphalerite, the common cubic form of zinc sulphide which is the prototype of the so-called “tetrahedral sulphides” where the metal ions fill half of the available tetrahedral sites within the cubic closest packing (ccp) of sulphur anions, leaving interstices still accessible for further in-filling (Figure 2).

The crystal-chemical formula of sphalerite can then be written Zn^t [S]^c where t specifies the tetrahedral coordination of cations and c quotes the ccp of anions. Indium is also carried in trace (but noteworthy) contents by excess-metal copper-rich sulphides [16] like the mineral bornite, with structural formula Cu₅^t Fe^t [S₄]^c. Very seldom does indium form specific minerals; beyond the already mentioned natural sulphides (roquesite and indite), another example is sakuraiite [17,18], also a "tetrahedral sulphide" with approximate formula (Cu,Zn,Fe)₃(In,Sn)S₄. However, it is still not clear if sphalerite carries indium mainly in solid solution through a diadochic replacement of zinc or if indium concentrates alternatively (or simultaneously) in nanodomains by filling interstitial sites available in the structural array of sphalerite. The fact that discrete inclusions of In-bearing sulphides—such as sakuraiite and roquesite—could not yet be observed in sphalerite from Lagoa Salgada ore samples reinforces the idea that indium occurs mainly within nanophases [1,19].

Excess-metal indium chalcogenides—namely In₂Se—were first quoted fifty years ago [20]. Pure In-chalcogenides have been synthesized since then – In₆Se₇, In₇Te₁₀, InTe and In₄Se₃ plus In₄Te₃, most of them with excess metal – and their structural characterization has revealed the occurrence of polymetallic cations: [In₂]⁴⁺ dimers and/or [In₃]⁵⁺ trimers [8,21].

Therefore, the occurrence of In-In interactions capable of leading to the formation of metallic polycations in nanodomains within the sphalerite host mineral is a possibility to be considered, thus contributing to improve the understanding of indium crystal chemistry in natural chalcogenides.

Irradiated materials were samples of polymetallic chalcogenide ores from the Lagoa Salgada deposit with an indium bulk content of about 90 ppm and which phase constitution plus chemical composition were previously characterized using laboratory methodologies, respectively X-ray diffraction and EPMA (as described in [11]). Figure 3 reproduces the micrograph of the studied sample surface where sphalerite is well represented.

Metallic indium was used for energy calibration (In L₃-edge at 3730.1 eV). Commercial products were used as model compounds displaying distinct bonding situations of indium in the formal valence state 3+: InF_3, In₂O₃ and In₂S₃. These synthetics were also previously checked by X-ray diffraction in the laboratory and the sulphide demonstrated as poorly crystallized.

The instrumental set-up of ID21 beamline at the ESRF is equipped with a Scanning X-ray Microscope that can be operated in the energy range 2.1–9.2 keV, thus enabling the analysis of both relevant absorption edges when studying indium chalcogenides: the In L₃-edge (ideally at 3730.1 eV for the metal) and the S K-edge (at 2472 eV for elemental sulphur).

In L₃-edge XANES spectra were collected in fluorescence yield (FY) mode using a photodiode detector mounted in the horizontal plane perpendicular to the X-ray beam and performing the energy scanning between 3.71 and 3.80 keV. A fixed-exit Si (111) monochromator was used, assuring a good energy resolution (0.4 eV) at the edge.

Small sample fragments were directly irradiated with a beam-size of 1 × 0.3 μm² and the fluorescence yield was detected using a high-purity germanium solid state detector.

Access to the user-friendly program PyMCA 4.3.0 (Python multichannel analyzer, [23]) is assured at the beamline, thus enabling the selection of most suitable points to irradiate in the ore samples. A preliminary tracing of topochemical maps was a further significant aid in this selection (Figure 4).