How modern geologists work

In 1944, Ivan Efremov wrote the story “The Diamond Pipe”, whose heroes, based on the similarity of the geological structure of the Central Siberian and South African plateaus, are looking for diamond deposits in Yakutia: “Here and here colossal eruptions of heavy deep rocks erupted to the surface. ... These explosions pierced many narrow pipes, which are a deposit of diamonds, in the thickness of the rocks. In the space from Kap to Congo, hundreds of such pipes are known, and, of course, a huge number of them are still hidden under the sands of the Kalahari desert. ... So far, no significant deposits have been discovered in the Soviet Union, and if we can find such pipes, you yourself understand how important this is! Everything here is surprisingly similar, except for diamonds, to South Africa - and platinum, and iron, and nickel, and chrome; on this Central Siberian plateau the same type of mineralization. ”


Heavy gaze

Gravity exploration, which allows you to track gravity anomalies, indicating the presence in the depths of rocks with different densities, is one of the important geophysical methods. The rocks saturated with oil and gas have a low density, and metal ores, on the contrary, are high. Three years ago, the European Space Agency launched the GOCE (Gravity field and steady-state Ocean Circulation Explorer) satellite, which compiled the most accurate map of the Earth's gravitational field. True, the satellite is not looking for minerals, but only clarifies the existing fundamental models of our planet.

Efremov’s forecast soon came true: in 1954, the first diamond pipe was discovered in the USSR, and two years later the industrial development of the Mir pipe began. And he didn’t just justify himself, but he turned out to be accurate even in the smallest details: diamonds were found exactly where it was indicated in the story, and precisely by this method. However, this is not surprising: Ivan Efremov graduated from the Leningrad Mining Institute and was not only a writer, but also a well-known geologist and paleontologist (in fairness, it should be noted that he was not the first scientist to make such a prediction: in the 1930s In the years such famous geologists as V.I. Vernadsky and V.S. Sobolev dealt with this issue.

One of the most common geophysical methods is seismic exploration, based on the dependence of the speed of elastic waves on the density of the medium. Low-power explosions are most often used as a source of waves. By tracking the delay of reflected waves with the help of seismic sensors, one can quite accurately restore the structure of the terrain using a computer. Magnetic methods make it possible to estimate the difference in the magnetic properties of rocks, and sometimes they can directly indicate the presence of, say, iron ore - magnetite. Electrical methods measure the polarization of rocks, the piezoelectric effect, or electrical resistance. Radiometric methods make it possible to very quickly analyze the content of specific elements by measuring the scattering or fluorescence of rocks when irradiated with x-ray, gamma or neutron radiation.

Similarly

“The method described by Ivan Efremov, and now remains the main forecasting method in geological exploration, ” says Oleg Ivanovich Guskov, professor at the Department of Methodology for Search and Exploration of Mineral Deposits of the Russian State Geological Exploration University (RGGRU). - There are various models that describe certain tectonic processes leading to the formation of mineral deposits. And the first step in exploration is to establish a correlation between the geological structure and already known deposits. So to speak, by analogy: if the geological structure of two different places is very similar, this may mean that there may be similar minerals. Therefore, it all starts with geological mapping, which helps answer the question of exactly where to look. ”

The scheme of formation of a kimberlite pipe. Kimberlite pipes formed in ancient times during the explosive eruption of volcanoes - the breakthrough of gases and magma from great depths through the crust. Over the past time, the surface of these volcanoes has been destroyed by erosion. Frozen magma in the saw expanding toward the top of the pipe contains a large number of diamonds that formed in the bowels of the earth at high temperatures and pressures.

Drawing up a map that characterizes the geological structure of the terrain is a difficult, time-consuming and expensive task. It requires not only geodetic surveying, but also the use of various methods that allow you to look under the surface of the Earth to a sufficiently large depth. During the Soviet era, it was planned to provide a continuous geological mapping of the country on a scale of 1: 200, 000 by the mid-1990s, but these plans could not be implemented. Now, according to Professor Guskov, the geological map of Russia is like cheese with holes: about 15% of the territory is still occupied by “white spots” - especially in the area south of Taimyr and at the junction of Chukotka and Yakutia.

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The kimberlite pipe Mir in Yakutia is one of the largest in the world. The quarry has a depth of 525 m and a diameter of 1200 m. The scale allows you to judge the ledges on the wall, which are expensive for mining trucks.

Restore history

On the geological map, igneous rocks are depicted in different colors, depending on the composition, and sedimentary rocks, depending on age. Measuring the absolute age of rocks using radio dating is quite expensive, so for the initial assessment, you can use simpler paleontological methods (that is, dating fossil fossils). The map, which shows the structure of several "underground floors", also displays various tectonic features, disturbances and folds of the earth's crust. “Such a map makes it possible to restore the entire geological history of the area, ” Oleg Ivanovich explains, “and thereby trace the processes that lead to the emergence of mineral deposits. Then we check the predictions that make the models of these processes, we take samples for analysis. Moreover, increased concentrations of the necessary elements do not mean at all that we discovered a deposit: it could very well turn out to be a halo, that is, traces of ore manifestation scattered on the surface as a result of erosion. Resistant minerals are often found in the sand of rivers. For example, geologists use this method to search for diamonds: following the traces of the accompanying pyrope mineral in river sand, they find native diamondiferous rocks (kimberlite). Well, then the assessment begins - the deposit is this or just a separate ore occurrence, which occurs several hundred times more. ”

As part of the large-scale experiment URSEIS'95 (Urals Reflection Seismic Experiment and Integrated Studies), seismic sounding of the Southern Urals was carried out, which made it possible to clarify its geological history and structure. The Urals formed during the Paleozoic at the junction of the East European Platform and the West Siberian Plate, while this territory was covered by the sea. In the Carboniferous period, a chain of islands formed here, and not cracks formed along the bottom, along which metal-rich lava rose to the surface sedimentary layers. All this determines the wealth of the region with various minerals.

Inaccurate predictions

However, as in any other science, theoretical models in geology do not always give correct predictions. For example, in South Africa, in addition to diamonds, the largest uranium reserves in ancient conglomerates have been explored. But in Yakutia, despite the very close similarity of the structure of the Central Siberian and South African platforms, uranium deposits of a similar type were not found. So process models have yet to be clarified.

Magmatic intrusions into rocks can be very diverse in form — ring intrusions (dikes, subvolcanoes), formation intrusions (sills), mushroom-like (laccoliths). To the right are shown plutons - magma bubbles floating up in the surrounding rock.

Another typical example of partially justified predictions is large copper-nickel deposits in the Norilsk region. “The geological structure in this region has been studied quite well, even on a large scale (1:50 000), ” explains Professor Guskov. - The processes taking place here are also known. During the introduction (intrusion) of red-hot magma from the depths of the Earth into the rocks lying above, segregation occurs: minerals containing iron, copper and nickel (pyrhotite, pentlandite, chalcopyrite) are separated into droplets in the lower part of the intrusion. These minerals also contain a significant amount of platinum group metals - palladium and platinum, which account for (at cost) 55%. Very similar deposits of copper-nickel ores formed by intrusions were discovered in the Pechenga area on the Kola Peninsula (Kiev, Vuruchuaivench). There is only one difference: until recently, it was believed that platinum was not there. A few years ago, geologists found platinum, but not where they expected: not in the lower part of the intrusion, as in Norilsk, but on higher horizons, in low-sulfide platinum-ore ores. The existing models are still unable to explain why, with a very similar geological structure, which indicates similar processes of formation of deposits, such a difference is observed in these two places. "

Generalists

Of course, modern geophysical methods and remote sensing methods greatly facilitate the work of geological prospectors. However, their task is not only to find out the details of the terrain structure and discover the field, but also to give a preliminary economic assessment. And for this we need knowledge of both geology proper and the technology of the process of extraction and enrichment of various minerals, mining, and also the fundamentals of the economy. “And not only, ” adds Oleg Ivanovich. - Once it was believed that geologists did not need mathematics, and now we have a special course "Mathematical Methods in Geology." However, the motto of geologists is relevant today - the mind remains the main one. And a hammer. ”

The article “Between the mind and the hammer” was published in the journal Popular Mechanics (No. 7, July 2012). Do you like the article?

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