Torró, and 12 co-authors, reported in 2019 that a volcanic dome complex of Miocene age hosts the In-bearing Ánimas–Chocaya–Siete Suyos district in SW Bolivia. Ore mineralization occurs as banded and massive infillings in sub-vertical, NE-SW striking veins. The authors conducted a detailed petrographic study is combined with in situ mineral geochemistry determinations in ore from the Arturo, Chorro and Diez veins in the Siete Suyos mine, the Ánimas, Burton, Colorada, and Rosario veins in the Ánimas mine and the Nueva vein in the Chocaya mine.
A three-stage paragenetic sequence is roughly determined for all of them, and includes:
1) an early low-sulfidation stage that is dominated by cassiterite, pyrrhotite, arsenopyrite, and high-Fe sphalerite (FeS > 21 mol. %);
2) a second intermediate-sulfidation stage dominated by pyrite + marcasite ± intermediate product, sphalerite (FeS < 21 mol. %), stannite, and local famatinite; and,
3) a late intermediate-sulfidation stage dominated by galena and Ag-Pb-Sn sulfosalts.
Electron-probe microanalyses reveal high indium enrichment in stage-2 sphalerite (up to 9.66 wt.% In) and stannite (up to 4.11 wt.% In), and a moderate enrichment in rare wurtzite (up to 1.61 wt.% In), stage-1 sphalerite (0.35 wt.% In), cassiterite (up to 0.25 wt.% In2O3), and ramdohrite (up to 0.24 wt.% In). Therefore, the main indium mineralization in the district can be associated to the second, intermediate-sulfidation stage, chiefly in those veins in which sphalerite and stannite are more abundant.
Atomic concentrations of In and Cu in sphalerite yield a positive correlation at Cu/In = 1 that agrees with a (Cu+ + In3+) ↔ 2Zn2+ coupled substitution. The availability of Cu in the mineralizing fluids during the crystallization of sphalerite is, in consequence, essential for the incorporation of indium in its crystal lattice and would control the distribution of indium enrichment at different scales.
The highest concentrations of indium in sphalerite, which is found in the Diez vein in the Siete Suyos mine, occur in crustiform bands of sphalerite with local “chalcopyrite disease” texture, which has not been observed in the other studied veins. In stannite, the atomic concentrations of In are negatively correlated with those of Cu and Sn at Cu + In = 2 and Sn + In = 1. Thus, atomic proportions and correlations suggest the contextualization of the main indium mineralization in the sphalerite–stannite–roquesite pseudoternary system.
The authors concluded that detailed textural observations in ore mineralization from the Ánimas–Chocaya–Siete Suyos district revealed a relatively complex mineralogy that can be associated with a three-stage mineralization sequence. In general terms, a first stage rich in cassiterite + arsenopyrite + pyrrhotite ± Fe-rich sphalerite was followed by a second stage rich in pyrite + sphalerite + stannite ± famatinite and a third stage rich in galena and Ag-Pb-Sn sulfosalts. The mineralogy records a shift from the low- to intermediate-sulfidation stages of mineralization.
Intermediate-sulfidation assemblages are dominant in vein mineralization in the Ánimas and Siete Suyos mines, in the SE sector of the district. Indium occurs in exceptionally high concentrations in sphalerite (up to 9.66 wt.% In) and stannite (up to 4.11 wt.% In) from the Ánimas and Siete Suyos mines, and in lower, although still anomalous, amount in wurtzite (up to 1.61 wt.% In), cassiterite (up to 0.25 wt.% In2O3), and ramdohrite (up to 0.24 wt.% In).
In sphalerite, the atomic concentrations of In and Cu yield positive correlations at Cu/In = 1 thus pointing to a (Cu+ + In3+) ↔ 2Zn2+ coupled substitution. In stannite, the atomic concentrations of In, Cu, and Sn yield negative correlations at In + Cu = 2 and In + Sn = 1. The incorporation of In in the structure of these two minerals can be contextualized in the sphalerite–stannite–roquesite pseudoternary system. The availability of Cu during the crystallization of sphalerite is necessary for In enrichment in sphalerite.
In this study, the highest values of In in sphalerite were found in the Diez vein in the Siete Suyos mine; the sphalerite richest in In occurs as micrometer-sized crustiform micro-bands that are also enriched in Cu, and that contain local chalcopyrite disease texture that is suggestive of a high activity of Cu in the mineralizing fluids.
The authors recognized intermediate-sulfidation-state assemblages with abundant sphalerite and stannite as the chief host for In in the Ánimas–Chocaya–Siete Suyos district and probably in other similar, dome-hosted Bolivian-type deposits.
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