NATIVE SILVER IN HYPOGENE BARIAN TODOROKITE FROM SAN MIGUEL TENANGO, ZACATLÁN, PUEBLA, MEXICO

The abandoned silver mining district of San Miguel Tenango lies in the northern part of the State of Puebla, Mexico. The ore deposits of this area consist of veins hosted in Mesozoic limestone and shale, and hydrothermal breccias enclosed in early Pliocene dacitic volcanic rocks.

Transmitted- and reflected-light petrography showed that todorokite was precipitated in the first stage of mineralization, adopting a wide variety of habits. A second stage of mineralization was observed, which consist of botryoidal to massive dark brown, reddish, orange and yellow oxides and hydroxides of iron, and ends with brilliant small crystals of specular hematite. Filling open spaces of the minerals of the second stage, a third stage of sulfides is made up of irregular masses of pyrite, chalcopyrite and pentlandite only a few micrometers long; based on this stage, a hypogene origin is assumed for the previous ones.

X-ray diffraction analysis of vein material revealed the presence of todorokite and an amorphous phase probably also of hydrous manganese oxide. Semiquantitative analysis made with SEM-EDS and quantitative analysis with EPMA-SEM showed that the cations in the ore mineral are Ag, Mn, Ba, Ca, Zn, K, Na, Mg, Fe, and Cu plus minor Ni, Co, Sb, and Au; they probably were scavenged from the mineralizing solutions by the highly porous structure of todorokite. A TEM study corroborates the presence of todorokite and of the amorphous phase of hydrous manganese oxide, and shows a nanometric crystalline phase that corresponds with manganosite. It also indicates that silver occurs as nanometric particles of native silver adhered to the external surfaces of todorokite and of the amorphous phase. IR analysis shows that the infrared spectrum corresponds to the reported spectra for todorokite.

The nickel sulfide mineralization is consistent with a magmatic origin for mineralization, but horizons of sedimentary manganese in the host rock, and the possible presence in the underground of the rocks that hold Zn-Cu(Au-Ag) volcanogenic massive sulfides in the near area of Teziutlán, Pue., suggest that the source of metals is both the mantle and the upper crust.