The Milluachaqui epithermal silver district of northern Peru

Abstract

The mines in the Milluachaqui district, located near Salpo in the northern part of Peru, show a vertically zoned, epithermal silver-gold mineralization. Colonial production was obtained from oxidized, high-grade ores, consisting mostly of native silver and cerargyrite. More modern development exposed, from the top of ore shoots to their base, the "bonanza" ores (the pyrargyrite-polybasite-acanthite zone), silver-copper ores (the tetrahedrite zone), silver-lead ores (the galena-sphalerite zone), and the basal zone ores (the sphalerite zone). The ratio of gold to silver in the "bonanza" ores was about 1 to 100, by weight. Interpretation of mineralogical textures and the composition of minerals in the sulfide zone suggests that supergene movement and enrichment of silver at Milluachaqui was not a significant factor. Silver tended to be fixed in place during weathering as the chloride and native mineral. The "bonanza" ores are interpreted to be hypogene sulfide accumulations. These findings contrast to hypotheses proposed for some epithermal districts in the U.S. which assume supergene activity to have been significant in the accumulation of high grade, near-surface silver ores.