Beginning of Andean Silver Mining Traced to AD 1000

Geologists turned archeologists have discovered that large-scale silver mining in the Andes Mountains of Bolivia began at least 400 years earlier than researchers had believed, sometime around AD 1000.

Using conventional techniques, researchers had previously concluded that the Incas discovered the large silver deposits at Cerro Rico de Potosi in the mid-15th century, but the evidence was inconclusive because most of the silver artifacts in the area had been removed by looters.

Geologists Mark B. Abbott and Alexander P. Wolfe of the University of Pittsburgh conclude in Friday’s issue of Science that the deposits were exploited much earlier by the Tiwanaku civilization, which preceded the Incan empire.

Abbott and Wolfe were studying the ancient climate of the region using core samples from Lake Lobato, which is upslope from Cerro Rico de Potosi but downwind. During smelting operations at the site to remove silver from ore, small amounts of silver and other metals would be vaporized and deposited on the lake surface, where they would sink into the bottom sediment.

“Sometimes you get lucky and find the perfect site to investigate more than one issue,” Abbott said.

The researchers measured silver, lead, tin, bismuth and antimony concentrations in each layer of the sediment and used conventional radioisotope dating with carbon to determine each layer’s age.

They found that levels of the metals in sediment were low and constant in layers dated up to about AD 1000, then rose suddenly and continued at high levels for 100 to 200 years, during the late stages of the period in which the Tiwanaku held sway over the region. Smelting then continued at a reduced level for another 200 years before it surged again about 1400, when the Incas took over.

The team found distinct spikes in metal concentrations following the Spanish arrival at Potosi in 1545, which they attributed not to increased mining but to the use of less-efficient smelting technology by the colonial occupiers. The Spanish used bellowed Castilian stone furnaces, which overheated the ore and vaporized a larger proportion of it.

Their technology proved so bad that smelting operations were placed back in the hands of the indigenous Incas.

Silver-rich surface ores were largely depleted by 1572 and the miners began using a mercury amalgamation extraction technique imported from Mexico. That produced far fewer volatile metals, and metal levels in the lake declined. Silver production was eventually abandoned about 1930 when the veins were depleted.

As silver levels declined, however, large-scale tin production took its place at Potosi, and that too is recorded in the sediments. Metal levels associated with tin smelting peaked during World War I, when demand was very high, then dropped off abruptly when the industry crashed in 1950.

Abbott speculated that the Potosi findings would provoke a great deal more research, because “now we can do this in other lakes close to ancient mine sites, and there are a lot of those in South America.”