I got to tag along on the Economic Geology field trip this past week! My friends Andrea and Cinthia are taking the class, taught by Dr. Barra, and I was lucky enough to be the plus one. We took a bus from Santiago to La Serena, which is about a 6 hour drive north on the coast. We camped in an enclosed campsite within the city and had a chef cook us breakfast and dinner as well as packed us lunches for the whole trip! Andrea, Cinthia, and I had never had a chef for geology trips but it is something they always have at the University of Chile. Us three shared two borrowed tents for the 6 day trip (Nov 6-11). We visited 4 different types of mines in the area around La Serena and on the last day we went to a mineral museum and the beach before the drive back to Santiago. At each mine we had the obligatory security/geology talk before doing various tours and examining cores.
El Pleito Mine
The first mine we went to was the El Pleito iron mine. It is an IOA (Iron Oxide Apatite) deposit and is part of the Chilean iron belt located in the Coastal Mountain Range. The ore was emplaced into Cretaceous volcanic rocks and has various stages of alteration. The picture to the right shows a view of the pit cut into the side of a mountain. The principle minerals of this deposit are magnetite, actinolite, and less apatite. The magnetite and actinolite form layers in the veins of ore that grade into smaller veins of magnetite and stockwork further from the main site of mineralization. The rocks are really pretty, with layers of green actinolite, metallic gray/black magnetite, and small areas of white apatite. They are also really heavy, with an average grade of 28% Fe! The formation of these types of deposits are not well known, with two end-member ideas: a magmatic fluid mode of formation and a hydrothermal fluid mode of formation. El Pleito also has small veins of pyrite, which suggests that this deposit could have formed through a mixture of these two fluid processes in which an initial magmatic fluid fractured the rock and deposited magnetite/actinolite while a later, shallower fluid event induced hydrothermal alteration and deposited the pyrite.
The second mine was a stratabound copper-silver deposit: Talcuna. This deposit consisted of layers of sedimentary rocks (the Quebrada Marquesa formation) that have copper mineralization in certain members of the Qda. Marquesa Fm. Faulting plays a huge role in the Cu-Ag mineralization, with Cu-rich zones between areas barren of ore mineralization associated with the many faults cutting the region. Permeability also plays a large role in the formation of this deposit, as ore mineralization only occurs below an impermeable metamorphosed shale layer called the Techo Rojo (red roof). Low temperature hydrothermal fluids are thought to have circulated through these rock layers and deposited Cu-minerals in the rocks that were permeable enough for the fluids to spread out and precipitate Cu-rich minerals. The main ore minerals are bornite and chalcopyrite, with minor calcocite. The ore is disseminated in the host rock and there is also minor magnetite. The copper grade of the mine is 25% and the Cu mineralization occurred in the Cretaceous while a separate Ag-rich fluid deposited silver minerals in the Miocene. At this mine we hiked up a hill and saw all the members of the Quebrada Marquesa formation up to the Techo Rojo. The picture to the left shows a view of the small processing plant at Talcuna as well as a competitor mine literally right next door. Our geologist guide told us that the two companies don’t get along well and therefore a complete geological picture of the area is not available because both companies guard their findings.
The third mine was The Panulcillo Mine. They mine copper from an open pit and also have a large processing facility that processes sulfides and oxides for smaller mines. The formation of this deposit is also very controversial given the mineral assemblages and alteration patterns. Skarns form during contact metamorphism when a large igneous intrusion comes into contact with a limestone layer and alters it due to the high temperatures. Typical minerals include garnet, pyroxenes, and amphiboles, as well as epidote. This mine had a lot of garnet, and secondary fluid events, leaching, and supergene alteration have enriched certain areas in chalcopyrite, calcocite, pyrite, and bornite. This deposit also has strong potassic alteration and zonations, providing evidence for a mixed skarn/hydrothermal fluid formation model. Walking around the pit area was really cool because there was a ton of garnet/bornite/pyrite-rich rock near the bottom and beautiful blue/green oxidized copper minerals near the top. We got to explore practically all of the mine and it was fun looking for different minerals.
Carmen de Andacollo
The final tour was at the “smallest of the biggest mines in Chile”, as described by our guide. It is a copper-gold porphyry deposit very similar to the Los Pelambres mine I described in my last post so I will forego another explanation on its formation. Here we noticed the difference in security and layout between the small mines we had visited during the week and a larger mine. We examined the cores and then drove to the lookout to get a picture of the massive scale of this deposit. The photo to the right shows the view; note the semi truck and trailer driving on the road between the big and small pits to get a sense of the scale. This mine reminded me a lot of the Hull-Rust iron mine in Hibbing, MN, because a huge open pit mine sits right next to a town. In this case the city of Andacollo hasn’t had to move due to mining activities (yet) but plays a big role in the development of the mine.