The Poopó Polymetallic Epithermal Deposit, Bolivia: Mineralogy, Genetic Constraints, and Distribution of Critical Elements

Abstract

The tin-rich polymetallic epithermal deposit of Poopó, of plausible Late Miocene age, is part of the Bolivian Tin Belt. As an epithermal low sulfidation mineralisation, it represents a typological end-member within the “family” of Bolivian tin deposits. The emplacement of the mineralisation was controlled by the regional fault zone that constitutes the geological border between the Bolivian Altiplano and the Eastern Andes Cordillera. In addition to Sn and Ag, its economic interest resides in its potential in critical elements as In, Ga and Ge. This paper provides the first systematic characterisation of the complex mineralogy and mineral chemistry of the Poopó deposit with the twofold aim of identifying the mineral carriers of critical elements and endeavouring to ascertain plausible metallogenic processes for the formation of this deposit, by means of a multi-methodological approach. The poor development of hydrothermal alteration assemblage, the abundance of sulphosalts and the replacement of löllingite and pyrrhotite by arsenopyrite and pyrite, respectively, indicate that this deposit is ascribed to the low-sulphidation subtype of epithermal deposits, with excursions into higher states of sulphidation. Additionally, the occurrence of pyrophyllite and topaz has been interpreted as the result of discrete pulses of high-sulphidation magmatic fluids. The δ34SVCDT range in sulphides (−5.9 to −2.8‰) is compatible either with: i. hybrid sulphur sources (i.e., magmatic and sedimentary or metasedimentary); or ii. a sole magmatic source involving magmas that derived from partial melting of sedimentary rocks or underwent crustal assimilation. In their overall contents in critical elements (In, Ga and Ge), the key minerals in the Poopó deposit, based on their abundance in the deposit and compositions, are rhodostannite, franckeite, cassiterite, stannite and, less importantly, teallite, sphalerite and jamesonite.

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

“…It strongly suggests a replacement that is either pseudomorphic or driven through particular planes of a former mineral with an original hexagonal or pseudo-hexagonal crystal shape. In Bolivian-type deposits, pyrrhotite is the most likely mineral that forms hexagonal crystals that are to be replaced by other sulfides, since it has been amply described to crystallize early in the mineralization sequence in different deposits (e.g., [33,39,62,63,66]). Pyrrhotite has indeed been observed in samples from the Nueva vein in the Chocaya mine as anhedral relicts and inclusions in pyrite-marcasite-intermediate product aggregates (Figure 11B,C,K).…”

“…The occurrence of In is reported in ore deposits that span a broad range of ages and mineralization styles [11,14]. High concentrations are described prominently in exhalative deposits hosted in volcanic (e.g., [22,23]) and sedimentary sequences (e.g., [24]), granite-hosted (including greisen-type, e.g., [25,26]), vein-stockwork Sn-W, porphyry Sn and xenothermal Sn-W-Cu-Zn-Pb-Ag veins (e.g., [27][28][29][30]), skarn (e.g., [18,31]), ,and epithermal (e.g., [15,16,32,33]) deposits.…”

“…The results of this research should serve as a basis for the exploration of In in other xenothermal vein deposits, in particular those that are hosted in volcanic-dome complexes, and they might be of help for the optimization of metallurgical flowsheets for ore processing with economic concentrations in this metal. This article corresponds to a group of papers of various Bolivian tin deposits with the same collective aim [33,[39][40][41]].…”

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

“…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).…”