Indium Mineralization in the Volcanic Dome-Hosted Ánimas–Chocaya–Siete Suyos Polymetallic Deposit, Potosí, Bolivia

Abstract

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. In this article, 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.

Mineralogy and Distribution of Critical Elements in the Sn–W–Pb–Ag–Zn Huanuni Deposit, Bolivia

“…The three hypogene stages of mineralisation that are characterised in all the studied areas of the Huanuni deposit are in accordance with the basic scheme of mineralisation that was already deduced upon the systematic reconnaissance of Bolivian tin deposits [8,[12][13][14]. This implies two initial stages of mineralisation rich in cassiterite and base-metal sulphides, and a late stage that is characterised by the occurrence of stannite-kësterite and sulphosalts.…”

“…The ore mineralogy and paragenetic sequences in all these deposits are very similar, as already noticed in early works in the region [12][13][14]. The contents in critical elements (i.e., In, Ga, Ge) in these deposits are also variable, normally in the form of minor or trace contents in sphalerite, cassiterite, stannite-kësterite, and a number of sulphides and sulphosalts [5,6,[8][9][10], rarely as minerals in which such elements are stoichiometric constituents like petrukite, sakuraite or argyrodite. In the case of Huanuni, indium also occurs exceptionally as a constituent of a supergene carbonate with unknown mineralogical characteristics ( Figure 6F).…”

“…have also been reported [17]. However, paragenetic sequences that are similar to those determined in the Huanuni deposit have been described to occur as a response to a generalised decrease in temperature of mineralising fluids in other deposits of the Bolivian tin belt [8,14].…”

“…Beyond the relatively limited collection of variables with which it is possible to characterise a given ore deposit as part of a deposit type or family, the Bolivian tin deposits (at least those formed during the Neogene) stand collectively as a mineral system, in the sense of references [51][52][53]. Recent studies in critical element-bearing deposits of the region [4][5][6][7][8][9][10], along with this one, describe deposits whose depositional models vary between xenothermal or mesothermal (i.e., Huari Huari, Huanuni) and epithermal (i.e., Poopó), whereas others (i.e., Ánimas-Chocaya-Siete Suyos) exhibit intermediate characteristics between those. The ore mineralogy and paragenetic sequences in all these deposits are very similar, as already noticed in early works in the region [12][13][14].…”

“…The 2017 production was 8415 tonnes of tin and, despite a fall in production below 7000 tons in 2018, the tin production in Huanuni still accounts for over a half of the total production of Bolivia. These deposits belong to the Bolivian Tin (Sn-W-Ag-Sb-Bi) Belt ( Figure 1) that has significant critical metal resources (In, Ge and Ga [1][2][3][4][5][6][7][8][9][10]). Increasing international demand is envisaged for In, Ge and Ga, particularly in emerging technologies such as the production of photovoltaic cells (In and Ga), optical fibre and infrared optical technologies (Ge).…”