Geologists have established the unusual behavior of gold in the oxidized ores of the Olympiada deposit, one of the largest gold deposits in Russia and in the world.
The Olimpiada deposit is located half a thousand kilometers north of the city of Krasnoyarsk. Since the 80s of the 20th century, more than five hundred tons of gold have been mined at the deposit and, according to the calculations of specialists, about a thousand more tons remained in the bowels.
Primary ores of Olympiada are dense rocks interspersed with sulfide minerals (metal and sulfur compounds) and very fine native gold (Fig. 1).
The gold content in such ores is 3-5 grams per ton.
The deposit was formed about 800 million years ago. During this time, its ores were affected by natural processes, the main components of which are water and oxygen. Under their influence, sulfides and many other minerals were destroyed and oxidized, turning into a brown loose mass - this is how oxidized ores were formed. But the main thing is that this hypergene process can lead to the accumulation of gold.
The maximum gold content in the oxidized ores of the Olympiada deposit reached 450 grams per ton. Previously, it was believed that in such conditions gold behaves like an inert metal - it does not corrode and oxidize., and, unlike most "base" metals, does not react with alkalis and acids (except for "aqua regia"). However, SibFU researchers obtained evidence of its mobility and activity of gold under hypergene conditions, when exposed to special geological and climatic factors.
For this, the research team, consisting of university researchers and specialists from the Norilskgeologiya enterprise (Krasnoyarsk Territory), investigated a 400-meter stratum of oxidized ores of the Olympiada deposit. It turned out that during the oxidation of primary ores, gold contained in sulfide minerals in the form of individual atoms is released, forms complex compounds with other elements and easily migrates.
Aggressive waters dissolve both native gold and its common compound with antimony - the mineral aurostibit (AuSb2). In the process, particles of so-called spongy gold are formed (Fig. 2),
similar to a sea sponge with numerous pores that increase the free surface of the particles. This structure allows aggressive solutions to more intensively dissolve the noble metal.
The stability of complex compounds of gold is different, and when conditions change, they can be destroyed, and the liberated atomic gold is grouped into nano- and micro-segregations, which often take the form of a ball (globules) or even a crystal, which was found at the deposit (Fig. 3).
It is now known that such processes can occur even with the participation of bacteria, which are capable of reducing nanoparticles of metallic gold from its compounds with other elements.
The researchers note that the process of formation of oxidized ores, as well as migration and new formation of gold from the Olympiada deposit are well illustrated in the section of the stratum of oxidized ores. The lower part contains a lot of relict gold of primary ores, as well as a large amount of spongy gold. Higher in the section, the proportion of spongy and relict gold decreases, but the number of newly formed globules and microcrystals increases. But the largest number of globules and microcrystals, as well as the maximum total gold content (up to 60 grams per ton in the studied samples), is observed in the upper layer of the studied section. This zone is also notable for the fact that the mineral cerianite (cerium oxide, CeO2) was found here, which serves as an indicator of a sharp change in environmental conditions. It is with this change in conditions that the new formation of gold is associated, the authors say .
“Although the oxidized ores of the Olympiada deposit were mined back in 2007, the study of the forms of finding, behavior and distribution of gold in them is important for a number of reasons. First of all, oxidized ores are valuable because they do not require complex beneficiation processes and are profitable even at low gold grades in them. On the other hand, oxidation zones of gold deposits are also of fundamental interest associated with exogenous geochemistry and metallogeny of gold, ”noted Sergei Silyanov, leading engineer of the Department of Geology, Mineralogy and Petrography of the Siberian Federal University .
Research co-author, Boris Lobastov , engineer of the analytical laboratory of the Scientific and Technological R&D Center of MMC Norilsk Nickel , in turn, said that the oxidation zone of the studied deposit was a real laboratory in the open air.
“We have studied a lot of technogenic objects - there the processes of transfer of matter and the formation of new minerals occur especially quickly. It was surprising to see how similar the microcrystals of gold and many other minerals from the oxidized ores of the Olympiada deposit to the newly formed minerals of other objects. Such external similarity is not always attributed to convergence - the mechanisms of the formation of new minerals in such different environments are similar, and the study of some processes in this case helps to understand all the others, ”the scientist said.
A study carried out by Siberian scientists showed that the formation of oxidized ores of the Olympiada deposit took place under complex geochemical conditions, in which gold behaved as a mobile element capable of both redistribution and redeposition on geochemical barriers (areas of changing environmental conditions).
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