Kori Kollo looks to expand — Battle Mountain sees mine as a model for Bolivian exploration

The prospect of diminishing reserves at the Kori Kollo mine, Bolivia’s largest gold producer, has spurred Battle Mountain Gold (BMG-N) to continue exploring the area for additional deposits.

The company’s efforts are geared toward sustaining present production levels of 20,000 tonnes per day. In addition to the main Kori Kollo pit, mining is now under way at the recently opened Llallagua pit.

The 100,000-ha property is situated in the Altiplano region of western Bolivia, which comprises a series of high plains in the area of Lake Titicaca and La Paz.

Since 1984, Kori Kollo has cranked out more than 1.7 million oz. gold and 7.6 million oz. silver. The mine began on the strength of oxide reserves of just over half a million ounces contained within 10 million tonnes grading 1.62 grams per tonne, as outlined by the Bolivian company Empresa Minera Unificada S.A. (EMUSA).

Bolivia has a long history of mining: at Kori Kollo (which means “Gold Hill” in the native Quechua language), the Spanish mined gold and copper in the 1600s from oxide veins. Bolivians have also been major producers of zinc, tin and silver, though levels have gradually declined.

Seeing the potential for additional sulphide mineralization at Kori Kollo, Battle Mountain first visited the mine earlier in 1988. It ended up purchasing an 88% interest in the mine’s operator, Empresa Minera Inti Raymi S.A., with the remaining 12% held by EMUSA.

In 1993, three years after oxide reserves were exhausted, a carbon-in-pulp plant was commissioned, resulting in a 211,000-oz. increase in gold production. Production peaked at 339,000 oz. in 1995, and has remained in the 300,000-oz. range ever since.

Ore is currently sent through a primary crusher before reporting to the semi-autogenous grinding mill, where it is ground to 70% minus-200 mesh. The crushed ore is leached in tanks with sodium cyanide. Gold recovery is 80%; silver recovery, greater than 95%.

“There is still a good resource of about 1.5 million oz. left in the Kori Kollo pit,” said chief geologist Domingo Ampuero, who spoke to The Northern Miner on a recent visit. The company expects to continue mining from the pit, which measures 1,300 by 700 metres, over the next few years.

By the summer of 1995, exploration had added mineralization from the newly discovered Llallagua-New Esperanza deposits, a few kilometres north of the main pit. At last count, the deposits contained a total resource of about 18 million tonnes averaging 1.9 grams gold and 10 grams silver. Oxide and higher-grade sulphide ores are now being mined at Llallagua, though much of the mineralization is not as high-grade as the main pit, and, moreover, is more refractory. To address this concern, Battle Mountian has been developing a pretreatment step, using bio-oxidation (a bacterial leaching process), to render the ore more amenable to processing.

The company recently began a second test heap, using 5,500 tonnes of ore, to see if the treatment can be used economically in the harsh, high-altitude environment of the Altiplano (the mine is more than 3,800 metres above sea level). The second test, using a finer particle size (no larger than 9 mm), was exposed to the microscopic bacteria for 40 days. Battle Mountain is utilizing strains of three different bacteria, including thiobacillus ferroxidans, cultured from local mines.

The first test heap used 3,900 tonnes over 60 days. The purpose of the tests is to determine the optimum grain size and minimum oxidizing time on the pad. Battle Mountain is sufficiently encouraged with the results that it expects to begin production from bio-oxidation in a little over a year. The company will employ a re-usable pad in which the ore is oxidized, washed and leached.

Determined to find other sources of mineralization, Battle Mountain has completed more than 11,000 metres of drilling south of the main pit and several kilometres north of Llallagua.

Also, the company is searching for sulphide zones beneath thick gravels using geophysics, and much of this work is concentrated southeast of the main pit. Crews are drilling through these gravels into Silurian sandstones and siltstones. Where mineralization is present, the rocks are often metamorphosed to slates and shales by a nearby intrusion. At Kori Kollo, a Miocene dacite porphyry is responsible for mineralization. The finer-grained stocks are the most favorable for mineralization. The gold is associated most commonly with pyrite and, to a lesser extent, stibnite, tetrahedrite, chalcopyrite and arsenopyrite.

Regional geology suggests mineralization at Kori Kollo could be related to emplacement of dacitic intrusives into Silurian siliciclastics in a northwesterly trend across a portion of the central Altiplano region of Bolivia. Principally as a result of its status as the nation’s largest gold producer, Kori Kollo has become a model for exploration, chiefly along a northwest-southeast belt across this part of the country.

Denver-based Orvana Minerals (ORV-T) is using the Kori Kollo model to explore as many as five project areas southeast of the mine. The most advanced of these is the Pederson project, near Challapata. Also, Apex Silver Mines (SIL-X) is capitalizing on the increased activity in Bolivia to develop the San Cristobal lead-zinc-silver deposit in the southern part of the Altiplano.

Vista Gold’s (VGX-X) Amayapampa and Capa Circa deposits, 120 km southeast of Oruro, are hosted in similar Silurian siliciclastics.