Origin of the El Dragón Selenium Mineralization, Quijarro Province, Potosí, Bolivia

Abstract

Abstract:A re-examination of the El Dragón vein-type Se deposit using polarized light microscopy and scanning-electron microscopy combined with electron-microprobe analyses revealed the following results: the detection of (a) petříčekite, krut'aite and penroseite close to endmember composition; (b) a yet unknown mineral of the ideal composition CuNi 2 Se 4 ; (c) intermediate members of the vaesite-pyrite solid-solution series; and (d) a mineral with a composition intermediate between athabascaite and its yet unknown S-equivalent, Cu 4 S 5 . Triggered by volcanic-hydrothermal activities around the Porco caldera, formation of the mineralization involved five episodes of fault formation and re-opening, fluid-mediated metal mobilization, metalliferous fluid infiltration, and mineral precipitation, re-deposition, and alteration that probably extended from 12 Ma until today. The origin of the Se-vein was accomplished by the multiple injection of highly oxidized, metal-rich fluids into a fault at the interface between black shale and siltstone. These low-T solutions (estimated max. temperature 100 • C, max. pressure 1 bar) possessed high to exceptionally Se fugacities (log f Se2 fluctuating between of −14.5 and −11.2) that prevailed for most of the evolution of the deposit, only interrupted once, during the episode of deposition of sulfides of Cu and Fe and resulting partial alteration of the pre-existing selenides. Formation of end-member krut'aite and native selenium implies a minimum log f Se2 of −11.2 at the final stage of vein formation. The likely source of Se and the accompanying metals (Cu, Ag, Ni, Co, Au, Pb, Hg, Cd, Fe and Bi) is the neighboring Kupferschiefer-type (possibly Devonian) black shale rich in framboidal pyrite, Cu-sulfide aggregates, and organic matter. Deposition of Se-minerals occurred where the oxidized metal-bearing solutions became in contact with a reductant, associated with the reduction of mobile selenate to immobile selenide or elemental Se.

Selenium Minerals: Structural and Chemical Diversity and Complexity

“…Selenium minerals sensu stricto (e.g., minerals with independent crystallographic sites dominated by Se) occur mainly in hydrothermal deposits [51], oxidation zones of ore mineral deposits [40,41], and as products of fumarolic activity [33]. Selenides are also known from magmatic sulfide ores associated with ultrabasic rocks, where they also have a hydrothermal origin.…”

The Sierra de Cacheuta Vein-Type Se Mineralization, Mendoza Province, Argentina

“…In the course of later replacement, cobalt, copper and nickel liberated during dissolution of krut'aite-(g1) were concentrated in tyrrellite. The low Ni content was insufficient to crystallize independent nickel selenides as, for example, in El Dragón, Bolivia [19]. Late-formed krut'aite-(g3) ( Table 2, ana# 4) has almost end-member composition (Cu 0.99 Se 2.00 ) (n = 3).…”

“…Plotting the compositions of trogtalite, krut'aite, and tyrrellite in the Co-Ni-Cu ternary diagrams representing the systems (Co,Ni,Cu)Se 2 and (Co,Ni,Cu) 3 Se 4 reveals chemical signatures that mostly deviate from those reported from other Se occurrences (Figures 17 and 18) and, thus, constitute diagnostic features for this site. [1,2], this paper), Tumiñico [1,2], Trogtal [2,20], Petrovice [21], El Dragón [4,19,22,23], Pacajake [19,[22][23][24], and Hope's Nose [24,25].…”

“…Note that bitumen is also noted as constituent of the Se veins ( [11], this paper). Suggesting these shales as elemental source is speculative, because we [1,2], this paper), Tumiñico [1,2], Trogtal [2,20], Petrovice [21], El Dragón [4,19,22,23], Pacajake [19,[22][23][24], and Hope's Nose [24,25]. , this paper), Tumiñico [1,2], Trogtal [2,20], Petrovice [21], El Dragón [4,19,22,23], Pacajake [19,[22][23][24], and Hope's Nose [24,25].…”

A Calorimetric and Thermodynamic Investigation of the Synthetic Analogue of Mandarinoite, Fe2(SeO3)3·5H2O

“…For this purpose, an Eh-pH diagram of the Fe-Cu-Se-CO 2 -H 2 O system has been constructed to obtain the thermodynamic properties of chalcomenite [4] and the average contents of these elements in acidic waters of the oxidation zones of sulfide deposits ( Figure 6). This system attracts interest because it allows the estimation of the physical-chemical parameters of the formation of hydrous Fe and Cu selenites (mandarinoite and chalcomenite), which are associated in the oxidation zones of some selenium-bearing deposits (e.g., Virgen de Surumi mine [6], El Dragón mine [25], Baccu Locci Mine [26]). As can be seen, copper selenides and oxides (klockmannite CuSe, krut'aite CuSe 2 , umangite, Cu 3 Se 2 , cuprite Cu 2 O, and tenorite CuO) are largely stable at lower concentrations of copper and selenium.…”