Tellurium mineralization at the La Moctezuma mine, near Moctezuma, SonoraStudies

Abstract

The La Moctezuma mine, locally known as "La Bambolla," is located southwest of Moctezuma, Sonora. It was worked for its gold ore with sufficient grade to allow shipment from 1936 to 1945, although it contained native tellurium and other tellurium minerals that were not considered commercially significant at the time, and there are no records of them in the El Paso, Texas smelter. In recent years, re-exploration of these deposits has shown that they are richer in quantity and variety of tellurium minerals than any other known mine and has provided a preliminary idea of the general geology of the area.

The veins containing tellurium, in which gold is the only economically significant constituent besides the former, occupy series of subparallel fractures that traverse sedimentary rocks of early Tertiary or possibly Late Cretaceous age. These sedimentary rocks were likely originally tuffs deposited in water and have been intensely hydrothermally altered with the introduction of silica, smaller amounts of barite, and metallic minerals. The sedimentary rocks have been extensively tilted and faulted, with dip angles ranging from horizontal to vertical, with local reclining.

The primary metallic minerals are gold, tellurium, and pyrite, while the gangue minerals consist of quartz and barite. Oxidation has extended to depths of at least 80 meters, resulting in the formation of a complete series of secondary tellurium minerals, some of which are new mineral species. This series consists of tellurites and tellurates of iron, zinc, manganese, calcium, uranium, and lead. The ore is very erratic in distribution and presents "nails" with very little continuity, both vertically and horizontally, containing variable percentages of tellurium and gold of good grade. The "nails" are separated by low-grade quartz vein bands, with contents ranging from 500 g to 2 kg of tellurium per ton and a few grams of gold. Based on the mineral association and texture of the ore, the deposit is classified as epithermal.