Origin of the El Dragon Selenium Mineralization, Quijarro Province, Potosi, Bolivia

A re-examination of the El Dragon 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) petricekite, krut'aite and penroseite close to endmember composition; (b) a yet unknown mineral of the ideal composition CuNi2Se4; (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, Cu4S5. 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 degrees 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.