Torró, and 12 co-authors, reported in 2019 that The Huari Huari deposit, Potosí Department in SW Bolivia, hosts polymetallic stratiform and vein mineralization of Miocene age with significant concentrations of the critical metal indium (In). Vein mineralization records document early crystallization of quartz and cassiterite followed by prominent associations of sulfides and sulfosalts.
The earliest sulfide was arsenopyrite, followed by pyrrhotite, and progressively giving way to pyrite as the main iron sulfide, whereas Cu–Ag–Pb sulfosalts constitute late hypogene associations. Sphalerite is the chief ore mineral, and its crystallization is extended during most of the mineralization lifespan as evidenced by its initial cocrystallization with pyrrhotine, then with pyrite, and finally with Ag–Pb sulfosalts. The composition of sphalerite varies from early to late generations with a continuous decrease in FeS that attests to a decrease in temperature, which is constrained to vary from ~450 to <200 â—¦C, and/or an increase in f(S2), both congruent with the described paragenetic sequence. Indium concentrated mostly in the structure of
Fe-rich sphalerite (up to 3.49 wt. %) and stannite (up to 2.64 wt. %) as limited solid solutions with roquesite in the (Zn,Fe)S–Cu2FeSnS4–CuInS2 pseudoternary system. In sphalerite, In shows a strong positive correlation with Cu at Cu/In = 1, suggesting its incorporation via a (Cu+ + In3+) ↔ 2Zn2+ coupled substitution, and it does not correlate with Fe. In stannite, In shows a moderate, negative correlation with Cu and Sn, and an In3+ ↔(Cu+ + 1 2 Sn4+) coupled substitution is suggested. Coexisting sphalerite and stannite yielded the highest In concentrations and crystallized at temperatures between 350 and 250 â—¦C.
Copper activity probably played a major role in the accumulation of In in the structure of sphalerite since In-bearing sphalerite coexisted with the deposition of stannite, shows high concentrations of Cu (up to 0.13 atoms per formula unit (a.p.f.u.)) in its structure, and hosts exsolutions of stannite and chalcopyrite. Distribution on the district scale of In suggests an input of hydrothermal fluids richer in Cu in the central position of the mineralizing system, represented by the Antón Bravo vein.
The authors concluded that:
1) Ore mineralization in the Huari Huari district included stratiform and (economically more important) polymetallic vein mineralization hosted by Paleozoic metamorphosed shales, quartzites, and Mesozoic sandstones.
2) Ore minerals in the stratiform mineralization included abundant pyrite and lesser amounts of sphalerite, cassiterite, and galena. The concentrations of In in these phases were below the lower detection limit.
3) Mineralization in the polymetallic veins was made up of a cassiterite- and quartz-rich stage followed by a sulfide-rich stage and an Ag–Pb sulfosalt-rich stage. Sphalerite was the most abundant sulfide in the deposit and recorded a protracted crystallization lifespan, initially co-crystallized with pyrrhotite, subsequently with pyrite, and then with sulfosalts in the latest stage. From its early to late generations, sphalerite became progressively depleted in FeS and other minor components. Geothermometric constraints along the paragenetic sequence in the sulfide-sulfosalt stages suggested a continuous decrease in temperatures from an initial 450 â—¦C and an increase in f (S2).
4) Indium occurred preferentially in Fe-rich sphalerite (up to 3.49 wt. % In) and in stannite (up to 2.64 wt. % In) in the form of roquesite solid solutions within the (Zn,Fe)S–Cu2FeSnS4–CuInS2 pseudo-ternary system. The authors suggested that the incorporation of indium in these minerals was produced through the coupled substitution (Cu+ + In3+) ↔ 2Zn2+ in sphalerite and In3+ ↔ (Cu+ + ½ Sn4+) in stannite. Indium and Fe in sphalerite were not correlated, and their concurrent high concentrations were probably circumstantial, at least during the initial mineral deposition and before the occurrence of exsolution phenomena.
5) The highest In values were found in sphalerite with relatively high Cu concentrations and/or sphalerite with conspicuous stannite and chalcopyrite exsolutions as well as in sphalerite interspersed with stannite in zoned crystals. Co-crystallization temperatures between sphalerite and stannite were constrained between 250 and 350â—¦ C.
6) Copper activity was probably the key factor that explained the distribution of In concentrations along the paragenetic sequence and among different structures in this district.
The authors suggested that even the input of fluids with modest quantities of dissolved Cu in Cu-poor mineralizing systems (like Huari Huari) could be instrumental in the concentration of In. Consequently, evidence for high Cu activity during the crystallization of sphalerite (e.g., stannite and chalcopyrite exsolutions) can be prospective for In in xenothermal, Bolivian-type vein systems.
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