Rodolfo Marin Rivera
Andacollo, a town on the fringes of Chile’s Atacama Desert, has a long pre-Hispanic history, and it is today known not only for its massive festival honouring the Virgin of the Rosary, but also for its gold mining history.
About 90% of the population is involved in mining activities. Many are generations of miners committed to extract hope from the deep earth day after day.
The mining history of Andacollo is related to the invasion of the Incas that mainly happened before that the Spaniards were conquering Chile. 500 years ago, the Incas were the first to extract gold from the mountains close to the town. Nowadays, the mining activity is carried out by the so-called “pirquineros” (miners) that work independently in small and artisan operations using ancient techniques and tools that are not so different from the ones used by their ancestors. The minerals continue being extracted from the mountains not so far from the town, where the gold grade is very low and it is located in “veins” very deep in the earth, making the work difficult and extremely dangerous. The pirquineros appeal to their faith and devotion to the Virgin, and ask for protection and health to extract the precious metal from the bowels of the Earth.
Nowadays, the scanty pirquineros that still remain in the town, still continue performing the process of grinding minerals in “maray” and “trapiches”. Gold is recovered by the amalgam formed with mercury during or after continuous milling.
Both milling devices are pre-hispanic techniques that have been adapted to recover gold. The “maray” are handcrafted systems used by the Incas to grind corn, wheat and barley, adapted by the Spaniards to grind minerals. It is driven by hand using a grinding cylindrical stone that rests on a cup, which in the past was made of stone and now concrete. It is still possible to see it in the courts of some houses.
The “trapiches”, similarly to the “maray”, are a sophisticated mechanical system used to grind gold minerals, which are in use since the end of the XIX century. It is a kind of circular tray full of water, on which the miners turn two heavy wheels to grind the mineral. Coarser particles of gold are recovered at the back of the “trapiche” caught by the amalgam formed with mercury. Middle size particles of gold are collected at the edges of metallic sheets made of copper and mercury alloy. The finest gold particles are collected and re-washed by a flotation process.
Mercury is used extensively during the rustic gold mining, so that a mercury-gold amalgam is formed. The gold is produced by boiling away the mercury from the amalgam. Mercury is effective in extracting small gold particles, but the process is hazardous due to the toxicity of the vapours. Today, new technological processes based on nitric acid or aqua regia have been developed, but their application is only economically feasible at larger scales.