Tourmalinization at the Darasun goldfield, Eastern Transbaikalia: Compositional, fluid inclusion and isotopic constraints

AbstractZoned tourmaline (schorl-dravite) in the matrix of hydrothermal explosive breccia and ore veins in gold deposits, Chita region, Eastern Transbaikalia, Russia, are associated with Na- and K-rich porphyry-type subvolcanic intrusives. δ18O values of tourmaline from three gold deposits (Darasun, Talatui, Teremkinskoye) are +8.3‰, +7.6‰, and +6.0‰ and calculated δ18O values of fluids responsible for the tourmalinization are +7.3‰, +7.7‰, and +4.2‰, respectively. These data imply an igneous fluid source, except at the Teremkin deposit where mixing with meteoric water is indicated. Wide ranges of Fe3+/Fetot and the presence of vacancies characterize the Darasun deposit tourmaline indicating wide ranges of ƒ(O2) and pH of mineralizing fluids. Initial stage tourmalines from the gold deposits of the Darasun ore district are dravite or high mg schorl. Second stage tourmaline is characterized by oscillatory zoning but with Fe generally increasing towards crystal rims indicating decreasing temperature. Third stage tourmaline formed unzoned crystals with xMg (mole fraction of Mg) close to that of the first stage tourmaline, due to a close association with pyrite and arsenopyrite. From Fe3+/Fetot values, chemical composition and crystallization temperatures, logf(O2) of mineralizing fluids ranged from ca. −25 to −20, much higher than for the gold-bearing beresite–listvenite association, indicating that tourmalinization was not related to gold mineralization

The age of mineralization of Mayskoe gold ore deposit (Central Chukotka): results of Re-Os isotopic dating

The article presents the results of the sulfide mineralization dating of the Mayskoe gold ore deposit using the Re-Os isotope system and isochron age estimation method of the main sulfide minerals: arsenopyrite, pyrite, and antimonite. The complex multistage formation of the studied sulfides, as well as the close intergrowths of genetically different mineral phases, did not allow obtaining a single rhenium-osmium isochron corresponding to the formation time of sulfide mineralization. Isochrones for single minerals, collected from each sulfide sample, turned out to be the result of isotopically distinct components mixture (radiogenic crustal and non-radiogenic mantle) and do not make sense from the geochronological point of view. In terms of geology, the most significant result of the study is an age estimation of 128.8 ± 4.4 Ma, obtained for the sulfide mineralization of Mayskoe deposit using Re-Os isotope dating of single fractions of pyrite and antimonite of the ore mineralization stage. While arsenopyrite is most closely associated with gold mineralization, one of the arsenopyrite varieties corrodes framboidal pyrite of the pre-ore stage, has a maximum of the crust component in the osmium isotopic composition and forms a mixing line in the isochron diagram with an apparent formation age of 458 ± 18 Ma. The initial osmium isotopic composition of the studied sulfides indicates a mixed mantle-crust source of sulfide mineralization. The issue of simultaneous ore genesis and granitoid magmatism in the Mayskoe deposit remained unresolved (the age of granitoids according to the U-Pb zircon system is 108 Ma). However, a possible solution could be the further determination of the Re-Os isochron age of the ore mineralization sulphides from the single paragenesis of a specific sample containing both arsenopyrite and pyrite (+ antimonite) with gold

Morozkinskoye gold deposit (southern Yakutia): age and ore sources

The paper presents the results of the comprehensive isotope geochemical (Re-Os, Pb and δ34S) study of sulfide mineralization of the Morozkinskoye deposit. The ore zones of the deposit are localized in the syenite massif of Mount Rudnaya, which is located within the Central Aldan ore region (southern Yakutia). Gold mineralization is represented by vein-disseminated or vein type mineralization and is manifested in acidic low-temperature metasomatites – beresites (Qz-Ser-Ank-Py). For the first time we obtained an age estimate of the gold mineralization ~ 129 ± 3 Ma, which the synchronism of the hydrothermal ore process in the beresites, which formed the Morozkinskoye deposit, and magmatic crystallization of the syenites of Mount Rudnaya (~130 Ma). The osmium initial isotopic composition of the studied sulfides indicates a mixed mantle-crustal source of sulfide mineralization. New lead isotopic data of syenites indicate the predominance of mantle lead and an insignificant role of the lower – crust lead, while the isotopic composition of pyrite denotes the presence of the upper crustal material in the ore genesis. The sulfide δ34S values vary from –2.3 to +0.6 ‰ and indicate a predominantly magmatic source of sulfur in the ores

U-pb dating of niobium minerals from pyrochlor group (ilmeno-vishnevogorsk carbonatitis-miaskite complex, of the southern urals)

U-Pb dating of the pyrochlore-group minerals from the Nb-rare metal ore deposits of ilmeny-vishnevogorsky carbonatite-miaskite complex of the Ural fold region was carried out. To date the individual pyrochlore crystals were used a new technique of local U-Pb SHRIMP-II dating which was developed at the CIR VSEGEI (St.Petersburg). In the case of high-U pyrochlore (with more than 2.5 wt % UO2) a laser ablation and ICP-MS method was applied for U-Pb-dating. The studied isotope pyrochlore system indicates a multi-stage formation of rare metal niobium mineralization. The earliest age of ore formation (378 ± 4.9 Ma) is fixed by U-pychlore isotope systems of Potanino deposit. This stage of ore formation is probably associated with the final stages of the alkaline-carbonatite magmatic system crystallization. The next stages of ore formation (230 ± 1.5 Ma) are widely manifested in Vishnevogorsk and later on the Potanino deposit (217.2 ± 1.9 Ma) and were probably related to remobilization and redeposition of alkaline-carbonatite and rare metal substances

Il'meno-Vishnevogorsky Alkaline Complex (IVAC) from Urals, Russia : age, sources and origin

The IVAC is one of the largest alkaline complexes of miaskites, fenites and carbonatites with REE-Zr-Nb mineralization located within the Urals Fold Belt. We have dated by different isotope methods including Rb-Sr and Sm-Nd isochrons on WR and rock-forming minerals, SHRIMP U-Pb zircon and U-Pb pyrochlore dating, the main rock types of IVAC and studied Lu-Hf isotope systematics and trace element patterns of various components of the complex. The isotope dating has detected numerous age clusters for the IVAC rocks -- 446-410, 390-360, 335-325 and 280-230 Ma. The isotope signatures of the IVAC main rock types: epsilon Sr = -6. . .-10, epsilon Nd = +3. . .+6, epsilon Hf = +4. . .+6, are similar to those for platform ultramafic-alkaline complexes (UACP) and require a depleted mantle source. IVAC miaskites and carbonatites differ by high Sr while carbonatites show slightly depletion in Ba, Nb, Ti, Zr, Hf relatively to UACP rocks. The obtained age data specify the magmatic intrusion of miaskites (O3 -S) and long-lasted metamorphic reworking during formation of the IVAC at collision (D2-3 , C1 ) and post-collision (P-T) stages of the Urals Belt evolution. Close resemblance of the IVAC and UACP complexes according to isotope and geochemical characteristics are fixed, but IVAC carbonatites have much in common with carbonatites localized in linear post-collision zones. The IVAC origin is supposeded to be connected with a mantle source, but the melting of the UACP rocks or oceanic crust rocks with the same isotope signatures is not ruled out.1 page(s

(Table 1) Isotopic composition of Sm and Nd in mantle restites of the Central Atlantic and in associated plutonic and volcanic rocks

Ultrabasic rock samples collected from two areas of the crustal zone of the Mid-Atlantic Ridge (MAR): (1) 13-17°N (near the intersection of the ridge axis with the 15°20'N prime fracture zone), and (2) 33°40'N prime (the western intersection of the MAR crest with the Heis fracture zone) were objects of this study. Samples of peridotite and of plutonic and volcanic rocks associated with it were used to measure their Sm/Nd, 143Nd/144Nd, and 147Sm/144Nd ratios, which allowed to test time and genetic relationships between evolution of mantle material under the ridge crest and products of its magmatic activity. Results of this work proved ubiquitous discrepancy between melting degree values of extremely depleted mantle peridotites in the MAR area between 14°N and 16°N, obtained using petrologic and geochemical methods. This discrepancy suggests large-scale interaction between mantle material and magmatic melts and fluids enriched in incompatible elements or fluids. The results obtained suggest that repeated melting of the mantle under the axial MAR zone is an universal characteristic of magmatism in low-velocity spreading centers. The results of this study also proved the crestal MAR zone in the Central Atlantic region show distinct indications of isotope-geochemical segmentation of the mantle. It is suggested that the geochemically anomalous MAR mantle peridotite in the zone of the MAR intersection with the 15°20'N prime fracture zone can be interpreted as fragments of mantle substrate, foreign for the Atlantic mantle north of the equator

Ore and Geochemical Specialization and Substance Sources of the Ural and Timan Carbonatite Complexes (Russia): Insights from Trace Element, Rb–Sr, and Sm–Nd Isotope Data

The Ilmeno–Vishnevogorsk (IVC), Buldym, and Chetlassky carbonatite complexes are localized in the folded regions of the Urals and Timan. These complexes differ in geochemical signatures and ore specialization: Nb-deposits of pyrochlore carbonatites are associated with the IVC, while Nb–REE-deposits with the Buldym complex and REE-deposits of bastnäsite carbonatites with the Chetlassky complex. A comparative study of these carbonatite complexes has been conducted in order to establish the reasons for their ore specialization and their sources. The IVC is characterized by low 87Sr/86Sri (0.70336–0.70399) and εNd (+2 to +6), suggesting a single moderately depleted mantle source for rocks and pyrochlore mineralization. The Buldym complex has a higher 87Sr/86Sri (0.70440–0.70513) with negative εNd (−0.2 to −3), which corresponds to enriched mantle source EMI-type. The REE carbonatites of the Chetlassky complex show low 87Sr/86Sri (0.70336–0.70369) and a high εNd (+5–+6), which is close to the DM mantle source with ~5% marine sedimentary component. Based on Sr–Nd isotope signatures, major, and trace element data, we assume that the different ore specialization of Urals and Timan carbonatites may be caused not only by crustal evolution of alkaline-carbonatite magmas, but also by the heterogeneity of their mantle sources associated with different degrees of enrichment in recycled components