The Rb-Sr isochron age of igneous ankerite-calcite and siderite carbonatites in central Tuva is estimated at 118 ± 9 Ma. The following ranges of initial values of O, C, Sr, and sulfide and sulfate S isotopic compositions were established: δ 18 é carb = +(8.8-14.7)‰, δ 13 ë carb = -(3.6-4.9)‰, δ 18 é quartz = +(11.6-13.7)‰, δ 34 S pyrite = +(0.3-1.1)‰, and ( 87 Sr/ 86 Sr) i = 0.7042-0.7048 for ankerite-calcite carbonatite and δ 18 é sid = +(9.2-12.4)‰, δ 13 ë sid = -(3.9-5.9)‰, δ 18 é quartz = +(11.2-11.4)‰, δ 34 S pyrite = -(4.4-1.8)‰, δ 34 S sulfate = +(8.6-14.5)‰, and ( 87 Sr/ 86 Sr) i = 0.7042-0.7045 for siderite carbonatite. The obtained isotopic characteristics indicate that both varieties of carbonatites are cognate and their mantle source is comparable with the sources of Late Mesozoic carbonatites in the western Transbaikal region and Mongolia. The revealed heterogeneity of isotopic compositions of carbonatites is caused by their contamination with country rocks, replacement with hydrothermal celestine, and supergene alteration.
The Ordovician-Lower Carboniferous sequence of slightly metamorphosed gray carbonate-ter rigenous rocks contains the Silurian black cherty shales enriched in carbon (6-9%), pyrite (6-7%), and ura nium (~30 ppm). The uranium ore is localized at the pinch out of areal and linear zones of the Early Permian supergene (exogenic) oxidation of rocks expressed in reddening (hematitization). U, As, Sb, Cu, Ni, Mo, and Ag have been removed from the oxidized black shales and concentrated in the cementation zone in form of pitchblende and sulfides in wall rock disseminations and veinlets largely hosted in carbonate bearing rocks. In the Late Permian, during deposition of the upper Rotliegende and Zechstein, the fractures in the basement were filled with carbonates and sulfates; uranium was partly redeposited along with enrichment in Pb and Zn. Mesozoic and Cenozoic supergene processes altered uranium ore insignificantly.
The geological and mineralogical data on the Chailag-Khem F-Ba-Sr-REE occurrence in the Western Sayan Range, Russia, are discussed. The chemical compositions of rocks, ores, and minerals (ICP-MS, Link) are reported. The occurrence is localized in a tectonic crush zone composed of Cambrian quartz-sericite slates intruded by quartz syenite porphyry. Ore mineralization occurs as veins, cement of tectonic breccia, and metasomatic disseminations in host rocks. Massive ore consists of calcite, strontianite, and quartz; impregnations of euhedral fluorite, ankerite, and bastnaesite crystals; and fine-grained barite aggregate. Accessory minerals include parisite, synchysite, barytocelestine, sulfides, rutile, and uraninite. Late metasomatic calcite and strontianite segregations and veinlets are abundant. In genetic, mineralogical, and geochemical features, the Chailag-Khem occurrence is similar to the Late Mesozoic carbonatite deposits of Central Tuva, of which the Karasug Fe-F-Ba-Sr-REE deposit is the largest and best known. All carbonatite deposits and occurrences are located within a longitudinal zone transverse to the major tectonic elements of the region.
Intrusive fluorite-barite-siderite carbonatite of Karasug deposit (Tuva) crystallized from ionic salt melt is dominated by iron chloride with the participation of alkali metal salts. Melt composition and main reaction of mineral formation: NaFeCl 3 + 4KF + 2Na 2 Ca(CO 3) 2 + Na 2 Ba(CO 3) 2 + Na 2 SO 4 = 6FeCO 3 + 2CaF 2 + BaSO 4 + 14NaCl + 4KCl. The highest temperature (550˚C-650˚C) brine-melt inclusions in minerals are pseudo-primary. Their predominant Na-K-chloride composition does not correspond to the primary melt, but is close to the residual brine-melt.
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