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.
In weakly metamorphosed massive sulfide deposits of the Urals (Dergamysh, Yubileynoe, Yaman-Kasy, Molodezhnoe, Valentorskoe, Aleksandrinskoe, Saf’yanovskoe), banded sulfides (ore diagenites) are recognized as the products of seafloor supergene alteration (halmyrolysis) of fine-clastic sulfide sediments and further diagenesis leading to the formation of authigenic mineralization. The ore diagenites are subdivided into pyrrhotite-, chalcopyrite-, bornite-, sphalerite-, barite- and hematite-rich types. The relative contents of sphalerite-, bornite- and barite-rich facies increases in the progression from ultramafic (=Atlantic) to bimodal mafic (=Uralian) and bimodal felsic (=Baymak and Rudny Altay) types of massive sulfide deposits. The ore diagenites have lost primary features within the ore clasts and dominantly exhibit replacement and neo-formed nodular microtextures. The evolution of the mineralogy is dependent on the original primary composition, sizes and proportions of the hydrothermal ore clasts mixed with lithic serpentinite and hyaloclastic volcanic fragments together with carbonaceous and calcareous fragments. Each type of ore diagenite is characterized by specific rare mineral assemblages: Cu–Co–Ni sulfides are common in pyrrhotite-rich diagenites; tellurides and selenides in chalcopyrite-rich diagenites; minerals of the germanite group and Cu–Ag and Cu–Sn sulfides in bornite-rich diagenites; abundant galena and sulfosalts in barite- and sphalerite-rich diagenites and diverse tellurides characterize hematite-rich diagenites. Native gold in variable amounts is typical of all types of diagenites.
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