HYPOGENE Ba-RICH TODOROKITE AND ASSOCIATED NANOMETRIC NATIVE SILVER IN THE SAN MIGUEL TENANGO MINING AREA, ZACATLAN, PUEBLA, MEXICO

Abstract

The San Miguel Tenango area, in the northern part of the State of Puebla, Mexico, is an old, now-abandoned silver mining district worked intermittently since Colonial times until the 1970s. Ores were high in grade, but had a low metallurgical recovery. We characterize the main ore mineral of this mining area, a Ba-rich manganese oxide with associated native silver, to investigate the cause of the low recovery. The ore deposits of this area consist of veins in Mesozoic limestone and shale, and hydrothermal breccias in early Pliocene dacitic volcanic rocks. Because of the present inaccessibility of the mine workings, only one vein was available for sampling. The ore mineral is Ba-rich todorokite with associated native silver, both occurring as cavity fillings. The vein-type ore deposits are related to adjacent early Pliocene dacitic volcanic rocks consisting of ash-fall tuffs, ash flows, volcanic necks, and a volcanic dome. Transmitted- and reflected-light petrography show that todorokite precipitated during the first stage of mineralization in a wide variety of habits. A second stage of mineralization consists of botryoidal to massive oxides and hydroxides of iron, ending with specular hematite. Filling open spaces in minerals of the second stage are minerals of a third stage consisting of irregular masses of pyrite, chalcopyrite, and pentlandite a few μm across. On the basis of this stage, a hypogene origin is assumed for the previous two stages. An XRD analysis of the vein material revealed the presence of todorokite and an amorphous phase, and probably also a hydrous manganese oxide. Semiquantitative analysis made with SEM–EDS and quantitative analysis with EPMA–SEM showed that the cations in both Mn oxide phases are Ag, Mn, Ba, Ca, Zn, K, Na, Mg, Fe, and Cu, with lesser amounts of Ni, Co, Sb, and Au, the absence of Pb being noteworthy. A TEM study corroborates the presence of todorokite and of an amorphous phase of hydrous manganese oxide, and shows a nanometric crystalline phase that corresponds to manganosite. Also, TEM analyses show that Ag occurs as nanometric particles of native silver adsorbed on the external surfaces of todorokite (not in the tunnels of its structure) and of the amorphous phase. In addition, IR analysis confirms the presence of todorokite. We attribute the low recovery of silver to scavenging of the dissolved silver by todorokite during the metallurgical process.