Day 3: Carajas background sessions
Sunday, 29 April 2007. Due to visa issues with one member of our group, we are limited to only 4 days at the mine site. So we decided to spent today at the mine offices in town and learn some background information about the mine from the company geologist, Gilberto De Lima Periera Silva. The full name of the mining company is "Companhia Vale do Rio Doce," or CVRD for short. CVRD was a state-run company until 1997, and it is now the 2nd largest diversified mining company in the world. Carajas is one of the biggest, if not the biggest, open pit mining complexes in the world. Production of iron ore was 81.8 million tons in 2006, and the goal for 2007 is 100 million tons. Raw ore is shipped by rail ~900 km to the port of Sao Lius, where is loaded onto massive cargo ships and sent to primarily Germany, China, and Japan for further processing.
Joel, the geologist whose PhD focus on Carajas, next gave us an overview of his research on the geology of Carajas. Carajas is a banded iron formation, or BIF for short. Banded iron formation are chemical sedimentary deposits, meaning that they were formed from minerals that precipitated out of a column of water. In this case, these deposits were laid down in a low energy, deep water (>100 m), oceanic setting. Banded iron formations are so named because they consist of alternating layers of silica and iron. The silica was most likely deposited as an ooze that was lain down from the continuous, slow deposition of amorphous silica gel particles. The iron layers, in contrast, were likely deposited rapidly. Ferrous iron (Fe 2+) dissolved in seawater under reducing conditions will precipitate rapidly when an oxidant, such as oxygen, is introduced.
So if you're thinking about Mars analogs, here's where things get interesting. Banded iron formations at Carajas date back some 2.7 billion years ago. Back when these things were being deposited, there wasn't a lot of oxygen present either in the ocean water or in the atmosphere. Oxygen-producing organisms were not yet abundant on Earth, so these BIFs were deposited under reducing conditions. As oxygen-producing photosynthetic cyanobacteria became more abundant, the oxygen they produced cause ocean-wide deposition of iron layers. A small fraction of these iron layers are preserved as banded iron formations. Perhaps early Mars, if ever clement enough to have standing bodies of water, experienced conditions similar to those recorded in these BIFs.
We spent the rest of the afternoon and evening laying out a field strategy for the coming days. Tomorrow, we'll get to see the mine and lay our hands on some 2.7 billion year old rocks!
Joel, the geologist whose PhD focus on Carajas, next gave us an overview of his research on the geology of Carajas. Carajas is a banded iron formation, or BIF for short. Banded iron formation are chemical sedimentary deposits, meaning that they were formed from minerals that precipitated out of a column of water. In this case, these deposits were laid down in a low energy, deep water (>100 m), oceanic setting. Banded iron formations are so named because they consist of alternating layers of silica and iron. The silica was most likely deposited as an ooze that was lain down from the continuous, slow deposition of amorphous silica gel particles. The iron layers, in contrast, were likely deposited rapidly. Ferrous iron (Fe 2+) dissolved in seawater under reducing conditions will precipitate rapidly when an oxidant, such as oxygen, is introduced.
So if you're thinking about Mars analogs, here's where things get interesting. Banded iron formations at Carajas date back some 2.7 billion years ago. Back when these things were being deposited, there wasn't a lot of oxygen present either in the ocean water or in the atmosphere. Oxygen-producing organisms were not yet abundant on Earth, so these BIFs were deposited under reducing conditions. As oxygen-producing photosynthetic cyanobacteria became more abundant, the oxygen they produced cause ocean-wide deposition of iron layers. A small fraction of these iron layers are preserved as banded iron formations. Perhaps early Mars, if ever clement enough to have standing bodies of water, experienced conditions similar to those recorded in these BIFs.
We spent the rest of the afternoon and evening laying out a field strategy for the coming days. Tomorrow, we'll get to see the mine and lay our hands on some 2.7 billion year old rocks!
posted by Bradley Thomson at 10:15 PM
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