DIRTY GALENA AT PORCO, PROYECTO ARQUEOLOGICO PORCO, BOLIVIA

Porco is a 12 Ma caldera at ~13,000 feet in the Bolivian Altiplano that was exploited for silver by ancient Andeans before nearby Potosí came to symbolize the mineral wealth of 16th century Bolivia. Silver-bearing minerals previously identified at Porco include acanthite (Ag2S), galena (PbS), pyragyrite (Ag3SbS3), and stephanite (Ag5SbS4), and the question of which minerals were preferred by the ancient craftsman is important. Lead oxide precipitated in the chimneys of the pre-contact artisanal smelters or “hornos” indicates that a silver-bearing lead mineral such as galena (PbS), cerussite (PbCO3), or anglesite (PbSO4) was likely used.

Nineteen samples were taken during this study from prospects, veins, and tailings to determine the Ag content and the minerals that may have been used for pre-contact silver smelting. Thirteen of these samples contained 155 to 2650 ppm Ag and would have been considered ore grade (>110 ppm). A hand-selected ore sample from a local, artisanal silver craftsman, contained 3250 ppm Ag. This sample of "sorojche" (Quechua) did not have the large, shiny cleavage cubes and high specific gravity indicative of galena. Instead, it had smaller cubes, irregular texture or “dirty galena” appearance, low specific gravity, and gray streak of silver-bearing tetrahedrite which is the silver ore mineral also identified in the Pb-Zn-Ag ores from the Coeur d'Alene district, Idaho. This sample also contained indium (303 ppm), cadmium (306 ppm), and germanium (16 ppm); elements that are also found with lead-zinc ores. The other samples contained fine-grained, disseminated sulfides, possibly Ag-tetrahedrite/galena, that were too small to identify, in a quartz vein matrix. Identification of this “dirty galena,” lead oxide in artisanal pre-contact smelters, availability, and preference of a modern-day artisanal silver smelter for this mineral suggests that even though a variety of silver minerals were available at Porco, the “dirty galena” was the likely ore preferred by pre-contact silver artisans. Silver is still produced today from Porco and tin is produced from cassiterite found deeper in the volcanic system.