A. V. Chugaev

High-precision Pb isotope analysis by multicollector-ICP-mass-spectrometry using 205Tl/203Tl normalization: Optimization and calibration of the method for the studies of Pb isotope variations

The development of the MC-ICP-MS method, which was launched about one decade ago and was largely stimulated by the need to solve geological problems, has opened a new avenue in isotope mass spectrometry. One of the advantages of this method is the possibility of applying a newly developed approach to the correction of analytical results for the effect of mass discrimination by normalizing the measured isotope ratios of an element to a reference (standard) isotope ratio of another element. This makes it possible to overcome the main disadvantage of conventional thermal ionization mass spectrometry (TIMS), in which the effect of mass discrimination cannot be fully taken into account during isotope analysis, and thus to implement a highly accurate method for the analysis of Pb-isotope composition. In application to the capability of the NEPTUNE MC-ICP mass spectrometer, we optimized and calibrated a method for high-accuracy Pb isotope analysis in solutions spiked with Tl, with all currently measured Pb-isotope ratios normalized to the standard 205Tl/203Tl ratio (TLN-MC-ICP-MS). The factors affecting the random and systematic analytical errors were examined, and the optimal operating regime and analytical conditions were determined. Much attention was paid to the correlation of the measurement results and the mass discrimination effect determined from the 205Tl/203Tl ratio. The value of the 205Tl/203Tl normalizing ratio was analytically determined through isotope analyses of the NIST SRM 981, and SRM 982 standard samples of Pb-isotope composition. The data obtained for two mixtures Tl + Pb (SRM 982) and Tl + Pb (SRM 981) in ten replicate analyses were 2.38898 ± 12 and 2.38883 ± 20, respectively. These results are in good mutual agreement, and their general mean 205Tl/203Tl = 2.3889 ± 1 coincides (within the error) with the recently published values of 2.3887 ± 7 [Collerson et al., 2002] and 2.3889 ± 1 [Thirlwall, 2002]. The precision of the method (±2SD), which was assayed by the long-term reproducibility of the results of replicate analyses of SRM 981 and seven galena samples (90 analyses) was 0.016–0.018% for the 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios and 0.005 and 0.009% for the 207Pb/206Pb and 208Pb/206Pb ratios, respectively. The precision of the isotope analysis of common Pb was significantly improved (by factors of 6–10 for various isotope ratios) compared with the precision of TIMS techniques acceptable in isotope studies during three decades. The described method was applied to examine the Pb-isotope composition of approximately 250 samples of galena, scheelite, and pyrite from a number of well known (including large) gold, sulfied, and base-metal deposits. The precision of the method (0.01–0.02%) makes it possible to study small inter-and intra-phase differences in Pb-isotope ratios in hydrothermal and magmatic rocks, to assay the scale of regional and variations in the isotope composition of ore Pb, and to correlate the Pb-isotope composition of rocks and ores and reveal its evolutionary trends.

Geochronology of eruptions and parental magma sources of Elbrus volcano, the Greater Caucasus: K-Ar and Sr-Nd-Pb isotope data

Complex geochronological and isotope-geochemical studies showed that the Late Quaternary Elbrus volcano (Greater Caucasus) experienced long (approximately 200 ka) discrete evolution, with protracted periods of igneous quiescence (approximately 50 ka) between large-scale eruptions. The volcanic activity of Elbrus is subdivided into three phases: MiddleNeopleistocene (225–170 ka), Late Neopleistocene (110–70 ka), and Late Neopleistocene-Holocene (less than 35 ka).Petrogeochemical and isotope (Sr-Nd-Pb) signatures of Elbrus lavas point to their mantle-crustal origin. It was shown that hybrid parental magmas of the volcano were formed due to mixing and/or contamination of deep-seated mantle melts by Paleozoic upper crustal material of the Greater Caucasus. Mantle reservoir that participated in the genesis of Elbrus lavas as well as most other Neogene-Quaternary magmatic rocks of Caucasus was represented by the lower mantle “Caucasus” source. Primary melts generated by this source in composition corresponded to K-Na subalkali basalts with the following isotopic characteristics: 87Sr/86Sr = 0.7041 ± 0.0001, ƒNd = +4.1 ± 0.2, 147Sm/144Nd = 0.105–0.114, 206Pb/204Pb = 18.72, 207Pb/204Pb = 15.62, and 208Pb/204Pb = 38.78. The temporal evolution of isotope characteristics for lavas of Elbrus volcano is well described by a Sr-Nd mixing hyperbole between “Caucasus” source and estimated average composition of the Paleozoic upper crust of the Greater Caucasus. It was shown that, with time, the proportions of mantle material in the parental magmas of Elbrus gently increased: from ∼60% at the Middle-Neopleistocene phase of activity to ∼80% at the Late Neopleistocene-Holocene phase, which indicates an increase of the activity of deep-seated source at decreasing input of crustal melts or contamination with time. Unraveled evolution of the volcano with discrete eruption events, lacking signs of cessation of the Late Neopleistocene-Holocene phase, increasing contribution of deep-seated mantle source in the genesis of Elbrus lavas with time as deduced from isotope-geochemical data, as well as numerous geophysical and geological evidence indicate that Elbrus is a potentially active volcano and its eruptions may be resumed. Possible scenarios were proposed for evolution of the volcano, if its eruptive activity were to continue.

Geochronology and Genesis of Subalkaline Basaltic Lava Rivers at the Dzhavakheti Highland, Lesser Caucasus: K-Ar and Sr-Nd Isotopic Data

The paper reports newly obtained K-Ar isotopic-geochronological data on the age of three lava flows (Khrami, Mashavera, and Kura), which begin at the Dzhavakheti volcanic highland in southern Georgia. All of the dated rocks, including those from the Kura Flow, which was previously considered as the Pleistocene, are demonstrated to have a Pliocene age. The lavas of the longest Khrami Flow were erupted at 3.25–3.10 Ma, and those of the Kura and Mashavera Flows at 2.20–2.05 Ma, a fact testifying to two pulses of volcanic activity at the Dzhavakheti Highland. The petrogeochemical and isotopic characteristics of the rocks (87Sr/86Sr = 0.7039–0.7042; ∈Nd = 3.4–5.1) indicate that they are subalkaline within-plate basalts formed by the fractional crystallization of a basic mantle melt with the usually discontinuous selective or rarely continuous contamination with material that was not in geochemical equilibrium with the melt. The volcanics of the Khrami Flow are characterized by the less radiogenic Sr isotopic composition and the highest ∈Nd values, while the younger rocks of the Mashavera and Kura Flows have similar and more “crustal” isotopic signatures. The 87Sr/86Sr ratios of the Dzhavakheti subalkaline basalts are close to the initial Sr isotopic ratios of the Quaternary and Middle Pliocene dacite lavas from the same territory. Considered together with petrogeochemical and geological data, this suggests that all young rocks in Southern Georgia were produced in similar tectonic and geodynamic environments.

K-Ar Age and Sr-Nd Characteristics of Subalkali Basalts in the Central Georgian Neovolcanic Region (Greater Caucasus)

Late Cenozoic magmatism in the Greater Caucasus during the last 8.5 Ma was manifested in the Elbrus, Kazbek, and Central Georgian neovolcanic regions. In our work, we present K‐Ar dates obtained for young rocks of Central Georgia, which, in turn, allowed us to determine the age of volcanism in this region and specify its role in the Neogene‐Quaternary magmatic history of the Greater Caucasus. In addition, based on new Sr‐Nd isotopic‐geochemical and petrochemical data, we consider the petrogenesis of volcanics in this region and the problem of material source for young igneous rocks in the Greater Caucasus as a whole. The present work continues our recent geochronological investigations in this region [1‐5]. The Central Georgian neovolcanic region is located

Lead isotopic composition from data of high-precession MC-ICP-MS and sources of matter in the large-scale Sukhoi Log noble metal deposit, Russia

The lead isotopic composition of 33 sulfide samples from orebodies of the Sukhoi Log deposit was studied by high-precession MC-ICP-MS with a precision of ±0.02% (±2SD from 120 analyses of the SRM 981 standard sample). The deposit is located in the Bodaibo gold mining district in the northern Baikal-Patom Highland. Gold mineralization is hosted in Neoproterosoic black slates. Variations of lead isotope ratios of the Sukhoi Log sulfides are generally typical of Phanerozoic deposits and ore fields. They are significant for 206Pb/204Pb (17.903–18.674), moderate for 208Pb/204Pb (37.822–38.457), and relatively narrow for 207Pb/204Pb (15.555–15.679). In the Pb-Pb isotope diagrams, the data points of pyrite and galena constitute a linear trend. The points corresponding to pyrite from metasomatic ore occupy the left lower part of the trend. Galena from late gold-quartz veins shows more radiogenic Pb, and corresponding data points are located in the upper part of the trend. According to the Stacey-Kramers model, the end points of the trend, which is regarded as a mixing line, have μ2 = 9.6 and μ2 = 13.2 and model Pb-Pb ages 455 and 130 Ma, respectively. The isotope characteristics of ore lead, their relationships in pyrite and galena, and the mixing trend of Pb isotopic compositions are clearly tied to two Paleozoic stages in the formation of the Sukhoi Log deposit (447 ± 6 and 321 ± 14Ma) and testify to the leading role of crustal sources, which are suggested as being the Neoproterozoic black-shale terrigenous-carbonate rocks.

Age of Granodiorite Porphyry and Beresite from the Darasun Gold Field, Eastern Transbaikal Region, Russia

The Darasun ore field situated in the southern West Stanovoi Terrane near the Mongolia-Okhotsk Suture comprises the Darasun (>100 t Au), Talatui (∼38.2 t Au), and Teremki (3 t Au) lode gold deposits. In the opinion of many researchers, the Darasun deposit is spatially and paragenetically linked to granodiorite porphyry of the Amudzhikan Complex and related metasomatic rocks (beresites). Whole-rock samples of granodiorite porphyry, monomineralic fractions of plagioclase, K-feldspar, and biotite, as well as sericite from beresite (26 samples in total), were analyzed by the Rb-Sr method. Eight biotite and sericite samples were analyzed by the K-Ar method. The Rb-Sr mineral isochrons obtained for individual granodiorite porphyry samples yielded initial 87Sr/86Sr ratios varying from 0.70560 to 0.70591. The consistent results of both methods allowed us to accept the ages of granodiorite porphyry and beresite as 160.5 ± 0.4 and 159.6 ± 1.5 Ma, respectively. The age of granodiorite porphyry of the Amudzhikan Complex of 160.5 ± 0.4 Ma corresponds to the boundary between the Early and Middle Jurassic and marks the completion of collision between the East Siberian and Mongolia-China continents and related orogeny. Since that time, the eastern Transbaikal region has been involved in the postorogenic (within-plate) stage of evolution, characterized by the formation of large gold, uranium, and other ore deposits.

Geochronology and genesis of the young (Pliocene) granitoids of the Greater Caucasus: Dzhimara multiphase Massif of the Kazbek neovolcanic area

This paper reports an integrated petrological, geochronological, and isotopic geochemical study of the Pliocene Dzhimara granitoid massif (Greater Caucasus) located in the immediate vicinity of Quaternary Kazbek Volcano. Based on the obtained results, it was suggested that the massif has a multiphase origin, and temporal variations in the chemical composition of its granitoids and their possible sources were determined. Two petrographic types of granitoids, biotite-amphibole and amphibole, were distinguished among the studied rocks of the Dzhimara Massif belonging to the calc-alkaline and K-Na subalkaline petrochemical series. The latter are granodiorites, and the biotite-amphibole granitoids are represented by calc-alkaline granodiorites and quartz diorites and subalkaline quartz diorites. Geochemically, the granitoids of the Dzhimara Massif are of a “mixed” type, showing signatures of S-, I-, A-, and even M-type rocks. Their chemical characteristics suggest a mantle-crustal origin, which is explained by the formation of their parental magmas in a complex geodynamic environment of continental collision associated with a mantle “hot field” regime.The granitoids of the Dzhimara Massif show wide variations in Sr and Nd isotopic compositions. In the Sr-Nd isotope diagram, their compositions are approximated by a line approaching the mixing curve between the “Common” depleted mantle, which is considered as a potential source of intra-plate basalts, and crustal reservoirs. It was suggested that the mantle source (referred here as “Caucasus”) that contributed to the petrogenesis of the granitoids of the Dzhimara Massif and most other youngest magmatic complexes of the region showed the following isotopic characteristics: 87Sr/86Sr − 0.7041 ± 0.0001 and + 4.1 ± 0.1 at 147Sm/144Nd = 0.105–0.114.The Middle-Late Pliocene K-Ar ages (3.3–1.9 Ma) obtained for the Dzhimara Massif are close to the ages of granitoids from other Pliocene “neointrusions” of the Greater Caucasus. Based on the geochronological and petrological data, the Dzhimara Massif is formed during four intrusive phases: (1) amphibole granodiorites (3.75–3.65 Ma), (2) Middle Pliocene amphibole-biotite granodiorites and quartz diorites (∼3.35 Ma), (3) Late Pliocene amphibole-biotite granodiorites and quartz diorites (∼2.5 Ma), and (4) K-Na subalkaline biotite-amphibole quartz diorites (∼2.0 Ma).The close spatial association of the Pliocene multiphase Dzhimara Massif and the Quaternary Kazbek volcanic center suggests the existence of a long-lived magmatic system developing in two stages: intrusive and volcanic. Approximately 1.5 Ma after the formation of the Dzhimara Massif (at ca. 400–500 ka), the activity of a deep magma chamber in this area of the Greater Caucasus resumed (possibly with some shift to the east).

Isotope Geochemistry (O, C, S, Sr) and Rb-Sr Age of Carbonatites in Central Tuva

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 S isotopic compositions were established: δ18Ocarb = +(8.8−14.7)‰, δ13Ccarb = −(3.6−4.9)‰, δ18Oquartz = +(11.6−13.7)‰, δ34Spyrite = +(0.3−1.1)‰, and (87Sr/86Sr)i =0.7042−0.7048 for ankerite-calcite carbonatite and δ18Osid = +(9.2−12.4)‰, δ13Csid = −(3.9−5.9)‰, δ18Oquartz = +(11.2−11.4)‰, δ34Spyrite = −(4.4–1.8)‰, δ34Ssulfate = +(8.6−14.5)‰, and (87Sr/86Sr)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.

Lead isotope ore provinces of eastern Transbaikalia and their relationships to regional structures: Results of high-precision MC-ICP-MS study of Pb isotopes

Lead isotopic composition was studied at 12 deposits of eastern Transbaikalia, which differ in type and scale of mineralization. The high-precision Pb-Pb data obtained using multicollector inductively coupled plasma mass-spectrometry allowed us to outline two large lead isotope provinces spatially coinciding with the West Stanovoi and Argun tectonic blocks. The difference in Pb isotopic composition of deposits in these provinces indicates that regional ore sources contrasting in geochemistry took part in ore formation. In the deposits at the southern margin of the West Stanovoi Block with predominance of Au and Mo mineralization, the lead role is played by a mixed mantle-type source, whereas the source of lead in the deposits of the Argun Block has U-Th-Pb isotopic characteristics inherent to a continental crust of orogenic type.

Oxygen isotopic composition of quartz veins and host rocks at the Sukhoi Log deposit, Russia

The relationships between the δ18O of quartz veins and veinlets pertaining to the main stage of gold mineralization at the Sukhoi Log deposit and metasomatically altered host slates are estimated. The oxygen isotopic composition of veined quartz and host slates is not uniform. The δ18O of quartz veins from the Western, Central, and Sukhoi Log areas of the deposit vary from +16 to + 18 ‰. The δ18O range of metasomatically altered slates in the Western and Sukhoi Log areas attains 6 ‰. The δ18O of quartz veins are always higher than those of host slates by 3–7‰. The regular difference in the δ18O between quartz veins and host slates indicates that the oxygen isotopic composition of the ore-bearing fluid forming the system of quartz veins and veinlets at the Sukhoi Log deposit could have formed as a result of interaction with silicate rocks, for instance, terrigenous slates enriched in δ18O. Such interaction, however, took place at deeper levels of the Sukhoi Log deposit. It is suggested that the fluid phase participating in the formation of the vein and veinlet system had initially high δ18O(>+10‰) due to interaction with the rocks enriched in δ18O at a low fluid/rock ratio. The oxygen isotope data indicate that the fluid participating in the formation of gold mineralization at the Sukhoi Log deposit was not in equilibrium with igneous rocks at high temperatures.