Uchucchacua: A Major Silver Producer in South America
Ulrich Petersen, Oscar Mayta, Luis Gamarra, César E. Vidal, Angel Sabastizagal, 2005. "Uchucchacua: A Major Silver Producer in South America", Andean Metallogeny: New Discoveries, Concepts, and Updates, Richard H. Sillitoe, José Perelló, César E. Vidal
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Although known since at least 1897, Uchucchacua was first explored on a major scale by Compaúía de Minas Buenaventura since 1960. Narrow vein mining started in 1975, but orebodies discovered at depth enabled expansion to today’s 2,000-t/d operation, transforming “Chacua” into the largest primary silver producer in South America.
The ores occur in fractures and faults, as well as in pipes, irregular replacement bodies, and mantos hosted by Late Cretaceous limestone. Porphyritic dacite bodies are probably pre-, syn-, and postore. Most of the ore occurs in distal manganiferous exoskarn and limestone and is mineralogically diverse, consisting mostly of the following.
The grade of the ore mined varies between 16 and 20 oz/t Ag combined with about 10 percent Mn, 1.5 percent Zn, and 0.9 percent Pb. Between 75 and 80 percent of the reserves are high in silver and manganese, whereas about 7 percent contain high zinc and lead grades with only moderate silver and low manganese.
Logarithmic-grade graphs show very good positive linear correlations for zinc versus lead, moderate correlations for silver versus manganese, and arcuate correlation bands for silver or manganese versus zinc or lead. These relationships indicate that the outward zoning sequence is from lead-zinc to silver-manganese or vice versa. The corresponding longitudinal vein sections can generally be contoured unambiguously, showing that the bands of highest grades of lead and zinc coincide very well. The highest silver grades can be contoured convincingly as a band that is zoned outward and/or at a higher elevation than the lead and zinc bands. However, the manganese grades often require two high-grade bands: a main band that mostly coincides with the highest silver grades and a thinner upper band that may represent near-surface manganese enrichment.
Ore intervals in individual veins, pipes, and replacement bodies are up to 200 m in vertical extent. However, the elevations of these intervals change progressively, reflecting the overall geometry of the hydrothermal cell (or cells) responsible for the mineralization. In addition, postore faulting has displaced the ore intervals. As a result, ore has been found to date over a vertical interval of 600 m, between 4,730 and 4,040 m.
At surface, manganese oxide stains in the host limestone and limonite in fractures and faults indicate proximity to ore. Underground, multiple calcite veinlets constitute a guide to nearby orebodies. Geochemical anomalies of 60 to 80 ppm Ag have been documented up to 15 m from an orebody. By extrapolation, 10 ppm Ag anomalies may extend 25 m from ore, and 1 ppm Ag anomalies may attain 40 to 45 m. Ore continues to be found at depth as well as laterally and between known ore zones.