REE and Th minerals in the metasandstones of the Udokan basin (Russia)

Abstract

Based on ICP-MS, optical and electron microscopy, and electron back-scatter diffraction data, the paper considers the distribution and mode of occurrence of REE and Th in metasandstones of the Lower Proterozoic Sakukan Formation, which host mineralization of the Udokan and Unkur copper sandstone deposits. The REE and Th contents increase in the layers enriched in iron oxides and minerals of natural heavy concentrate relatively to the background metasandstones. Detrital allanite is the major REE mineral, which occurs in layers enriched in iron oxides together with authigenic allanite, monazite, and unidentifed Ca and REE carbonates. Thorium is concentrated in an authigenic silicate (thorite or thorogummite), which contains a signifcant amount of Fe, Ca, Al, and Cu and microinclusions of Cu and Pb sulfdes and barite. The diagenetic formation of authigenic REE and Th minerals is suggested as a result of desorption of these elements from siliceous-ferruginous gel-like sediments formed in river deltas and the wave-cut zone of the Proterozoic Sea.

Mineral and S-Isotope Compositions of Cu-Sulfide Deposits in Southern Siberia (Kodar–Udokan Region), Russia

“…It does not depend on a geological setting of samples taken within different sections of the deposit (Medny, Sekuschy, Shumny-Krutoy, etc., Figure 3, Table S2). These variations are due to different mineral associations and their origin, i.e., typical chalcositebornite ores are characterized by δ 34 S = −2.7%-16.0‰, while chalcopyrite-pyrite ores have very light S-isotope composition, where δ 34 S changes from −15 to −28% (Table S2 [50][51][52][53]), and the cross-cutting chalcopyrite-pyrite veinlets are close to −13‰. Some samples located nearby to the gabbro dyke have positive δ 34 S (+4 and +13‰-+14‰) [50].…”

“…The Kodar-Udokan region contains not only separate unique deposits but, in general, it is enriched with copper, which concentrates in different ore types. Its source, mechanisms of concentration, and deposition have been discussed for many decades [1, 10,14,25]. As was shown earlier, the deposits of the Kodar-Udokan region form a wide range of objects varying in geological structure and ore geochemistry.…”

“…It occurs in the exocontact of the Chineysky pluton and it is natural to suppose that its ores were largely formed under its influence. S2 and after [2,25,50,52].…”

“…Maybe, this is the influence of the Luktursky gabbro massif. Therefore, this composition could be the result of S2 and after [2,25,50,52].…”

“…One of the approaches to the solution of the question about the genesis of ores is the study of the sulfur isotope composition in the sulfides of ore deposits and the oxygen isotopes in rocks. Previously obtained data were extremely limited [25,26] and did not allow for solving the problem. The expansion of these works, including the study of objects of "intermediate" type between the two main types of deposits, allows us to approach the problem of the genesis of deposits in the Kodar-Udokan area in a more reasoned manner, indicating more complex relationships between the ores and their complex character, which is shown below.…”