ROCK-HEAD SCIENCES is pleased to announce our first ongoing guest blog by a student who will share her field experiences in Peru throughout the Summer of 2014. Anna Bidgood is currently an undergraduate student who is studying Earth Science at Oxford University. She is working for a mining company this summer in the remote Peruvian Andes as an exploration geologist. Follow this guest blog for updates from Anna as she searches for copper porphyry deposits and experiences the local communities. Anna will be updating this blog on an ongoing basis, so stay tuned for more updates!
ANNA’S BLOG (UPDATED SEPTEMBER 1, 2014 – see “The ins and outs of copper porphyry exploration” below):
As a fourth-year undergraduate geologist at Oxford University, I really relish the opportunity to work abroad and gain the experience of working as an exploration geologist. It’s fantastic that there are opportunities for me and my peers that send us half way across the world for three months in the summer. We are not just reading about things that other people are doing, we are involved in the processes that make the world tick. This summer, I am working as an exploration geologist in the Peruvian Andes looking for porphyry copper deposits.
Arriving in Lima
I arrived in Lima in June tired from the 21 hours of travelling and jet lagged from the six hours time difference. I was to spend the first four days in Lima completing my inductions, having medical tests and reading up on copper porphyry deposits in South America. Lima is the metropolis of Peru, buzzing with life and, most noticeably, traffic. There is very limited public transportation in Peru, so as a consequence there are a large number of cars. This is not unusual in a city, but the chaos of Lima is quite unique. However, there are well-defined walkways for pedestrians, and the taxi service is cheap and easy (you just have to choose the least battered looking ones).
Almost as soon as I’d arrived and slept off the jet lag, I was back at the airport for an early morning flight to Tacna in the south. I was extremely excited about seeing Peru. Although Lima was an interesting place, I wanted to get out of the city — after all, that is why I am doing exploration geology. When I got off the plane, all I could see was white! A dense mist sat in the valley, and there was no sun like I had been promised. As soon as the taxi left Tacna to take me to Moquegua, we rose above the mist, and I finally had an amazing view of the Andes.
Despite being mountainous and fairly close to the sea, this area of the Andes is a desert. There are no trees or fauna that I could see, and the mountains look like large sand dunes. The weather is mixed. When it’s sunny, the temperature is in the 20s (degrees Celsius); when out of the sun, it drops by about 10 degrees; and at night, it’s about 10 degrees. Peru is known for its plethora of natural environments which can change over very short distances: the North is typically rainy and warm, and there are rainforests a short distance away from the highest peaks; the south is hot and dry; and the coast is humid. The main difference I have found is that in Lima, it is overcast and grey, whereas in Moquegua, it is clear and sunny all of the time!
Exploration Geology in Peru
There is a lot of exploration geology in Peru, and it is currently at its height. Chile contains some of the world’s largest copper porphyry reserves; however, these have been explored and exploited for a long time. This has shifted the frontier for copper porphyry exploration to Peru. I arrived just as the group of geologists I was to be working with were finishing the preliminary stages of exploration of a project. This means that they have mapped the area, undergone the geophysical surveys and the geochemical analysis of soil samples across the area. A combination of the magnetic and gravity surveys, concentration of elements like molybdenum (Mo), gold (Au), lead (Pb) and copper (Cu) in the soil along with the outcrop maps and cross sections all provide information for the geologist on the location, depth and potential size and quality of the copper porphyry deposit. This then allows them to make educated guesses on where to drill, taking the project to more advanced stages in the exploration process. Drilling is an expensive business, so you really would like to get it right first time. The geologists propose a first, second, and third choice for the location and optimal angle to drill. Then, with their limited budget, they have to decide what is feasible in terms of drilling equipment, etc. This is the bit that takes time. The drilling process provides them with more information on the quality and size of the deposit and whether or not it is economic to mine. Many projects never go on to become mines.
An exploration geologist is not simply a rock lover who spends their time in isolated places making maps. There is a lot more to it than that. For instance, these isolated places may have been the home to communities of people for thousands of years and to begin exploring on their land, you need to ask permission. If you walked into someone’s back garden, would you ask permission? Very often it is the geologists who first come into contact with these communities, and it is the geologists who are there at all times during the exploration. This requires the geologists to be able to communicate well with the communities, provide a friendly face, and answer their questions.
In Peru, there are a lot of social issues making exploration very difficult. The first thing the companies will do when they want to work in an area is to set up a meeting with the leaders, authorities, and the local community to explain what exploration geology involves. It is a common view for people to assume that exploration means that eventually there will be a mine and chaos and disruption will ensue. The aim of this meeting is to benefit both the companies and the community. An exploration company wants to cooperate with the community. The people in the community know the land better than anyone, the best way to get around, and in Peru’s case, are accustomed to the altitude. By working together, jobs are provided for the local community, and a system is put in place to maintain contact with the community. Suggestion boxes are present in the village, allowing the community as a whole to decide what they would benefit from — for example, better water facilities. The mining company will then help the community to accomplish this, making an effort to make a positive difference and not just arrive, look for copper, and leave.
Many mining companies have stopped working in Northern Peru due to the social issues. The local communities are not happy with the mining companies using their water supply for drilling, and until they can be reassured that the water supply is sustainable and this can be proven, exploration in the area has stopped. In order for drilling programs to start, exploration companies must get permission from the authorities to use the water source in the area.
Arequipa and trekking in the Colca canyon
During my days off I visited Arequipa, Peru’s second largest town, one-tenth the size of Lima. Surrounding the town are three looming stratovolcanoes. One of which is El Misti, around 5800m. I didn’t stay long before I’d booked myself on a trekking tour to the Colca canyon. The Colca canyon is the second deepest canyon in the world, measuring around 4200m from top to bottom, twice as deep as the Grand Canyon!
The canyon is quite spectacular, and so is the V-shaped valley connected to the canyon, carved by the river running along its length. The canyon and the valley are quite different places; firstly, they are inhabited by different tribes. These tribes used to shape the heads of young children by strapping sticks to the skull until the age of five. This identified them as belonging to a certain tribe. Nowadays this is not a current practice; instead they wear distinctive hats to identify which tribe they are from. The tribes were prohibited from interacting, marrying, trading or any form of communication. Over the years, this has gradually lessened as trading became a necessity. In the valley, they farm crops on the terraces, particularly corn, barley, wheat and quinoa. At higher altitudes where crops don’t grow, you find the llama and alpaca farms. Alpaca are farmed for their meat which is extremely low in fat and for their wool which is extremely soft. Softer than alpaca wool is the vicuna which is closely related to the llama. However, vicuna are wild animals and are protected so are not eaten.
As you move into the canyon, the slopes get steeper and warmer allowing fruit trees to grow rather than corn. This is what motivated the interaction of the two tribes to start trading. Avocado trees naturally litter the slopes close to the river. Cactus fruits are also farmed and are used to create the valley’s traditional drink known as colca sour. This is similar to pisco sour but made with a cactus fruit that looks like a kiwi and tastes like lemon. Many of the plants have medicinal properties and are gargled to treat the cough. Cactuses are also farmed for the parasite found on its flesh which is used to produce a red dye.
We trekked down into the canyon from around 3500m altitude and descended 1050m. There are no roads for cars down the sides of the valleys, as it is too steep. So instead, mules are used to transport produce and people. At the bottom of the canyon, we stayed in bamboo lodges and swam in the pool which was a blessing after the heat of the sun! Climbing back up the side of the canyon was tough! (1050m of zigzagging up a steep dry track for 2 hours). That’s around the height of Scafell Pike before breakfast. The canyon sides were spectacular for a geologist! They were made of hundreds of metres of basaltic columnar joints, which the path wound above and below, formed from the cooling and contraction of a thick lava flow.
On route to and from the canyon, we stopped off in some of the local villages and viewpoints. In the first village, I ended up with a full grown Andean Eagle on my head! Soon after, we arrived at the ‘Cruz del Condor’, a fantastic view point and the place where the condors ride on the thermals to gain height. Condors are a type of vulture measuring 1m in height with a wing span of 3m! They are enormous! In this valley, there are only around 50 individuals left and are now a protected species. The locals used to kill the condors, as they thought that they were killing their livestock. However, condors are scavengers, so they only eat dead animals. They are not able to grasp or kill with their talons. They are thought to be the fox’s best friend, as they often eat the remains of a fox’s kill. Condors have been known to live for up to 70 years and only lay one egg every two years.
Working in the field
I’m currently helping out on a couple of exploration projects, based on the land already owned by the mining company, surrounding an operational mine. These areas have been tagged as having potential, and it is our job to map the area at 1:5000 and decide whether they are worth investigating further.
There is a lot of alluvium in the valleys making outcrop scarce. The main areas of outcrop are found in the eroded river valleys, the tops of hills and the occasional vein which is more resistant to erosion.
When we come across an outcrop, we record the location in our notebooks, along with the lithology (rock type), structures (bedding and faults), and any alteration that has occurred.
The Andes Mountains have been formed due to the dense tectonic Nazca plate subducting beneath the South American plate. As the plate is subducted, pressure and temperature start to rise and, in the presence of water from the sediments, the plate will start to melt. This melt, known as magma, rises buoyantly up into the crust, forming volcanoes like those we see today. Volcanic eruptions in the Andes are explosive, forming huge eruption clouds and throwing out clasts of shattered material and lava. The different types of eruptions form different deposits, and it is these deposits that form the rocks which we are currently mapping.
The three main types of rock that we come across are all lavas. This means that they were erupted from a magma chamber to the Earth’s surface, cooling quickly and make up most of the cone-shaped volcanoes in the Andes. These rocks are known as rhyolite, dacite and andesite. The difference between them is the composition, or amount of silica (quartz) within the rock. You can tell this by looking at the larger minerals (phenocrysts) within a fine-grained matrix.
Another rock type is known as ignimbrite. This formed from a hot suspension of particles and gas flowing from an explosive volcano. The key to identifying ignimbrites are the broken fragments of rock distributed throughout.
Finally, we also are mapping intrusive rocks. These are rocks which were never erupted at the surface but cooled and crystalized beneath the ground. This means that the magma was more insulated and lost heat at a slower rate, allowing the crystals to grow larger over time. When you’re identifying an intrusive rock, the crystals are visible with the naked eye and interlock each other showing that they weren’t cooled quickly like lavas.
When mapping, we are not only concerned with the rock type that is outcropping but also the alteration that has occurred. When magma intrudes into the crust and starts to cool and crystallize, the H2O rich fluid is driven out of the magma chamber and into the surrounding rocks. This hot fluid contains a lot of dissolved species, particularly metals like copper and gold. As the fluid moves away from the intrusion it starts to cool. This reduction in temperature reduces the solubility of the metals, meaning that the same amount of fluid can hold a smaller volume of dissolved metals. The rest is precipitated out in fractures and pipes surrounding the intrusion.
The further away you get from the intrusion, the cooler the temperature. The hot fluids catalyze reactions in the rocks, bringing in new elements and forming new minerals. The temperature affects which minerals form; for example, at higher temperatures potassium rich minerals will form like biotite or orthoclase. At slightly lower temperatures sericite (a clay mineral) may form, breaking down plagioclase feldspar. At even lower temperatures, the green minerals epidote and chlorite may form. By mapping these minerals in the field, we are creating a 3D picture of the temperature surrounding an intrusion!
However, it is not quite as simple as that. Fluids from other sources can also be heated by the intrusion and cause alteration in a wide area, which is not directly above the intrusion, but displaced slightly. It is the job of the geologist to try and locate the position and distance above the intrusion by looking at the variation in alteration minerals and therefore temperature.
When the residual fluid from an intrusion is forced into the surrounding rocks, this exerts a huge pressure which may cause the rocks to fracture, providing a pathway for the fluid to move through. These fractures often contain broken up bits of rock known as breccia. The passage of fluids also causes mineralization. This is evident to us in the field in the form of tourmaline breccias.
The copper porphyry deposits are formed at high temperatures close to the intrusion, so if we can locate the intrusion, then we have a higher chance of finding any deposits.
Faults can be very obvious in the field or can be very subtle and hard to identify. Key observations include the presence of minerals such as bright blue copper oxide or green epidote lining fractures in the rock. A fault is a type of fracture with displacement, so it can provide a pathway for fluid movement as discussed above. Occasionally the fault plane itself is exposed; the surface of the fault plane can record the direction of movement on the fault by the presence of slickenslides, scratches in the rock parallel to fault movement. This allows us to identify the major stress directions acting on the area.
In some areas, the fault has caused displacement and erosion has produced large pits or holes in the ground oriented with the same angle as the fault.
Working with the locals (**NEW!)
Whilst the health and safety coordinator was giving an induction to the new geophysical contractors, I spent some time wandering around the office, looking at all of the displays. In the centre of the room was a large 3 dimensional topographical map of the project and beyond. It detailed the major rivers as well as all of the mines in the area. Surrounding this display, covering the walls were a series of photographs explaining the company’s different initiatives and projects.
It is important to work closely with the local people as much as possible. This not only involves employing people from Moquegua to assist in the field and eventually work on the mine but also working with the people who live in the immediate area to the project. For example, our local llama farmer has his house in the middle of the area that we have been mapping. He is a very friendly man and always comes to say hello, he even told us that he thinks that “there is definitely copper here.” This man knows the land well as he spends most of his days walking all over and caring for his llamas and donkeys. We hired him to show us some locations where he had identified a large vein and faults with copper oxide mineralisation. These are important structures for us to map; a lot of time was saved by our cooperation. In addition to this, he told us about certain plants and even showed us a large boulder field on a hill which is potentially an old burial ground.
As well as working with the locals, we are also trying to work for the locals but providing them with education and facilities which will benefit their lives and income.
Water supply system: This explained where the mine would get their water from. They will be using a local river with high levels of boron and arsenic which makes it unsuitable for human consumption and farming. Good quality water will then be channelled back to a river which will be particularly beneficial during the dry season, allowing a regular flow throughout the year.
Alpaca’s fibre chain production: This project aims to improve the quality of life for high Andean shepherds, improving their income through improved pasture and breeding. They are also training local people as to the conservation of wetlands.
Technical assistance genetic improvement program: Animal breeding can produce better fibres for making fine fabrics and influence positively on the household income. They are also developing a training programme in textile handicraft production, providing access to markets.
In another room was another display detailing the production of the Peru’s national drink known as Pisco. Pisco is made from grapes which are crushed and distilled and stored in pottery barrels. This region is known as the ruta del pisco as it is surrounded by pisco farms. Although this area is not frequented by tourists, the pisco farms will hopefully start to attract tourist as the industry grows.
Animals and Vegetation
As I mentioned before, this area of the Andes is a desert, so there is very limited vegetation and wildlife. The main plant to grow in the desert areas are cacti. These range in all shapes and forms from large branching forms, to small spherical balls. It is these balls which litter the ground and make mapping difficult. They grow on top of each other in a tower but detach easily onto you as you walk past. Their spines seem to hook beneath your skin which makes them painful to remove. They are also covered in very tiny spines which are impossible to remove without tweezers as they’re hard to find! Because of this, we have to wear gloves and use our geological hammer to remove any cactuses that flick up onto our legs. However, the gloves didn’t help me so much when I sat in a pile of cactuses.
In the more fertile valleys there are a lot more plants and even a few trees! Most of this land is farmed on terraces and barley, corn, grapes, quinoa, etc. are grown.
There is a wider variety of wildlife around these areas: Alpaca and llamas are farmed for their meat and wool. Alpaca wool is softer than llama’s, and they can be identified by the thicker coats, shorter legs and necks and snouts. Llamas have long necks and snouts.
Related to llamas and alpacas are vicuna and Guanaco, two wild South American camelids which live in the high alpine areas of the Andes. Vicuna are smaller and are wild animals and endangered. They are not hunted although their wool is used as it is even softer than alpaca! However, they only produce small amounts of fine wool which can only be shoven every three years making it very expensive! In Inca times, it was against the law for anyone but royalty to wear vicuna garments.
Other domesticated animals include horses, donkeys, cows and mules. Mules are expensive and hard to come by as they are bred from a horse and a donkey and as a consequence are infertile. Not only this but the horse and donkey must have supposedly been raised together since infancy in order to mate. However, mules combine the best characteristics of both of these animals and are by far the strongest pack animal in the Andes.
Peru is home to 1,879 species of birds although the majority of these are found in the Amazon. Despite this, the bird life in the Andes extremely varied. I have seen a multitude of nectar-feeding birds feeding from the flowers of cactuses. There are many eagles and falcons around including the peregrine falcon and Andean eagle.
Vultures perch by the side of the road looking for carrion, and china linda are a common site in the mountains. In the fields, there are more birds than I can identify, but a particularly common one is the ibis. By the coast are numerous pelicans hanging around the fish market looking for scraps.
Moving to a New Project
After spending a few days in Lima, enjoying the sights by bike and paraglide I moved to a new area, completely different from the south. This area is closer to Lima, so we were able to drive there; however, road works had closed a section of road meaning that we had to wait for three hours before we could pass and consequently arrived in the dark.
The village I am staying in is at around 3800m altitude and is much smaller than any places I’ve previously stayed at in Peru. It is away from the main roads, along a dirt track, up a steep hill. There are no hotels. The accommodation for us here is a camp consisting of a few buildings and a courtyard. The dorms are sectioned off with false walls, the toilets and showers are in an outhouse outside. We even have a kitchen and a cook! It was nowhere near as cold as I expected it to be; the car said it was 10^C at 8pm. Although it did feel slightly colder than this, it wasn’t extremely cold overnight.
In the morning, I woke up to blue skies and an amazing view! I couldn’t believe how different it was compared to the south. There were trees, vegetation, big mountains and rivers: everything that we’d been lacking before. After breakfast we drove up to the project just to have a look around and to start to acclimatise to the 4800m of altitude. The drive was only an hour, but we kept stopping to let me take photos of the views!
It was quite obvious when we arrived at the project because the vegetation became scarcer and the mountain turned red from the alteration.
The ins and outs of copper porphyry exploration (Updated September 1, 2014)
Copper porphyry systems contain deposits of copper in mineral form, concentrated by hydrothermal processes. Hydrothermal systems consist of hot fluids containing dissolved compounds which are transported from the hot igneous intrusion through the colder host rock. This migration through the host rock and through fractures causes the rock to be altered, producing characteristic mineral assemblages relating to the temperature of the fluids. The temperature decreases away from the intrusion and therefore produces zones consisting of specific minerals which can be mapped on the surface to locate the porphyry centre.
The potassic zone consists of secondary biotite and an intense quartz stockwork. It is present at the top of the porphyry intrusion.
Surrounding the potassic zone is a widespread biotite hornfels. This rock formed from contact metamorphism in a high temperature environment close to the intrusion. Closer to the intrusive centre and directly above it you can find sheeted veins.
Surrounding this zone is a lower temperature zone known as the prophylitic zone. This zone is often zoned itself with epidote forming at higher temperatures than chlorite. However, oxidation of pyrite causes the production of acidic fluids which can bleach the rock, making the chlorite hard to spot.
FeS2 + 2H2O + 3.5O2 à 2H2SO4 + FeO
By identifying this mineral zoning, we can determine the zoning vector which points up temperature, towards the intrusive centre.
Another important tool to use is geochemistry and geochemical zoning. This is important because it tells us whether or not the intrusion is rich in copper, the desired mineral; you can have mineral zoning without copper. Key elements include lead, zinc and silver at the periphery of the system and copper, molybdenum and gold closer to the intrusion. By looking at anomalies in the soil and veins in the rocks we can identify if these elements exists in high concentrations.
Overprinting much of the biotitic hornfels is quartz-sericite alteration. This alteration occurs in a cone shape above the intrusion (see diagram above). It is known to remobilise copper and so could concentrate or deplete the deposit. It is often accompanied by D veining. D veining consists of a quartz vein containing a suture, often of pyrite, and a halo of sericite surrounding the vein. The width of the halo and extent of overlap relates to the intensity of alteration.
Overprinting everything with a strong structural influence is the advanced argillic alteration which destroys all minerals and texture in the rock making it difficult to identify the parent rock, leaving quartz and pyrophylite minerals only. It is confined to ledges parallel to the fractures, which are thought to be the feeders of the lithocap (see diagram above).
Widespread overprinting by quartz-sericite and advanced argillic alteration can be caused by a process called telescoping. This is when erosion rates are high or sector collapse occurs, causing the porphyry system to approach the surface. The near surface epithermal environment now causes overprinting, by advanced argillic alteration, onto the earlier higher temperature alteration. By juxtaposing two mineralising environments, we have the potential for a very high grade deposit at shallower depths.
Diagram showing the process of telescoping where later alteration overprints earlier, higher temperature alteration
High sulphidation events can cause hypogene enrichment. Acidic fluids from the intrusion, which are rich in sulphur, destroy copper sulphides such as chalcopyrite. The fluid migrates and precipitates this copper as bornite, pyrite and energite.
The orientation of the porphyry system below ground is important for drilling. You can infer the potential orientation from the orientation of fracture systems and veins at the surface. We saw a strong preferential orientation of veins and fractures which allowed us to infer that the porphyry system is elongated in the same direction. For the size of porphyry deposits that we are looking for, a few kms squared on the surface must be altered to a propylitic grade. The minimum size deposit for a large company is around 500MT of 1% copper equivalent (can be made up by Au and Mo). This would provide us with a mine of 20 years lifespan.
An important part of mapping our copper porphyry systems is to define whether or not the intrusions are precursors to the porphyry system, early intrusions, inter-mineral or late. These intrusions have different characteristics and tell us whether there is copper at depth or whether there is a chance it could have been diluted. Porphyry intrusions have been known to span millions of years and consist of multiple phases which replenish and raise the temperature of the intrusion, increasing the timespan.
Precursor intrusions show evidence of all the phases of alteration. They can be equigranular in texture rather than porphyritic, showing that it cooled slowly at deeper crustal levels. They often cover a wider area than the porphyry so can be mapped below ground using a magnetic survey depending on their magnetic content. A high magnetite content shows a high magnetic signal.
Early intrusions have potassic alteration and strong stockwork veining as described above. Whereas intermineral intrusions also have intense alteration but less quartz veining. They may also have formed breccias during emplacement which could contain blocks with quartz stockwork indicating that this intrusion formed later. Late intrusions have less intense alteration, maybe quartz sericite, and no quartz veining.
Regional scale faults are often pathways for the hydrothermal fluids to reach the surface. They are also important in determining the orientation of the porphyry body at depth. In the photo we can see two orientations of faulting; in the region of the porphyry centre that we have mapped the faults are almost vertical (red line), allowing us to infer that the porphyry body is directly below the drill point. However, there are also some more angled faults (blue) in the region of some other identified copper porphyry system centres. These faults indicate that the intrusion may be offset at depth. This is proven by the photo showing a dipping intrusion parallel to the angled faults, showing that they have an influence on the igneous systems.
In the following photo, the underlying Jurassic sediments can be seen on the right of the photo, whereas on the left is a thick volcanic sequence. It is thought that there is a fault running through this area which has been uplifted and downthrown along its length, accounting for the exposed Jurassic sediments and thick volcanic basin.