In Coyhaique, Fabien helped us to get in touch with an officer of the El Toqui mine, located 90 km further north. We got an appointment with a geologist from the Coyhaique office before going on with our bikes. Two days later, we were at Villa Mañihuales, a small village close to the mine. We had originally planned to get to the mine by bike, but we learnt that a daily bus carries the minors and that we could also take it. We then seeked a safe place to leave the tent and our equipment during a whole day, because the bus left at 5 am to only come back as 20:30 pm. We now have our tricks and it was among the carabinieros that we asked for some help. As always here, every problem has a solution, and it required a few minutes only to the police officer to propose us the dry open garage under the building while the rain continued to fall for hours.
4:30: We quickly prepared our stuff and walked to the bus stop under a starry sky. We had an hour's rest in a very comfortable bus before arriving at the cafeteria of the mine, where we enjoyed a breakfast with the workers. Around 9:30 am, two young geologists, Victor and Franco, received us.
"Do you know that this is the only zinc mine in Chile?" They said. "The first zinc anomaly was discovered in 70’s by a prospector, but it was only in the 80’s that the deposit was discovered. Since then, several mining companies have owned it, and it is now a Canadian company, Breakwater Resources, that owns the mine."
Victor and Franco explained us the system used for exploration. Once a mineral anomaly (here zinc) was detected, the geologists made a systematic sampling of river sediments, with a frequency of one sample per km2, and a total of 50 samples. Geochemical analyses of these samples were done to determine if there was a significant mineral content. A coarse mapping was performed and the main ore anomaly was located. The second sampling campaign was more precise and focused around to the anomaly detected earlier. A sample was taken every ¼ km2, and corresponded to approximately 100 samples in total. A more detailed map could identify one or more areas with strong anomaly of ore. Finally, soil analyses were carried out by following a regular grid of 20x20 m (1000 samples). At the end of this exploratory study, the deposit was located on a map and its zinc content was known. The extension in depth of the deposit was determined by geophysical measurements (gravimetric, resistivity). Gravimetry is the measurement of the gravity field that depends on the distribution of rock density and can detect heavy minerals. The resistivity is the measurement of electrical conductivity of rocks, which depends on their chemical composition.
Victor took stones from a bag and placed them on the table. These were not ordinary rocks because some were metallic gray, others rather golden yellow. These were minerals from the deposit. Victor explained: "The gray minerals are galena, which contains lead. Yellow minerals are chalcopyrite and pyrhotite, containing copper and iron. The black mineral is blende, zinc concentrate. The zinc ore is the most concentrated in the deposit, with a content of 8%. But other metals are associated with zinc, such as iron or lead, but they are not exploited." Some of El Toqui deposits also contain gold and silver. Gold has a content of 12 grams per ton and is contained in the matrix of the rock. Then Victor told us that in the limestone, the fossils were replaced by the ore and we could see minerals that exhibit exactly the form of fossil that they replaced.
"But how was formed the ore deposit?" We asked. Franco replied: "Magmatic fluids rich in zinc and other elements rose to the surface along faults. These fluids were blocked by an impermeable rock level and interacted chemically with the rocks beneath the impermeable level. Depending on the temperature and composition of the rock, some minerals were deposited and concentrated, in particular zinc, while others were dissolved, such as calcite”.
"And how could it be decided that a deposit can be exploited?”. Victor replied that several criteria are involved, in particular the content of ore, the international supply and the cost of production. For example, in the case of zinc, currently a deposit is considered profitable if the ore contains more than 4% zinc.
Finally we asked what the role of mining geologist was. Geologists could work either in exploration or production. In exploration, the geologist must determine the deposits and their geometry at depth. It deals with sampling campaigns, geophysical analysis, and description of carrots. All data are correlated to build a cross-section or a geological profile. All data are compiled and compared in order to build a geological model of the deposit. Franco added: "The geologist has a heavy responsibility because the industrial activity of the mine depends of his work and his intuition. In addition, the mine shall ensure the economic survival of the region since the 300 people who work in the mine are from Mañihuales Villa and the surrounding areas.” In production, the geologist daily monitors of the extraction front. For reasons of safety, we did not have the permission to enter at the bottom of the galleries and we could not illustrate the work of the minors. Franco and Victor returned to their post, and we settled in the office during the afternoon until 19h, when the bus took us back to Villa Mañihuales.
