Oxygen isotopes in the Cretaceous-Paleogene granites of Primorye and some problems of their genesis

The Cretaceous-Paleogene granites of the Eastern Sikhote Alin volcanic belt (ESAVB) and Late Cretaceous granitoids of the Tatibin Series (Central Sikhote Alin) are subdivided into three groups according to their oxygen isotope composition: group I with δ¹⁸O from +5.5 to +6.5‰, group II with δ¹⁸O from +7.6 to +10.2‰, and group III with less than +4.5‰. Group I rocks are similar in oxygen isotope composition to that of oceanic basalts and can be derived by melting of basaltic crust. Group II (rocks of the Tatibin Series) have higher δ¹⁸O, which suggests that their parental melts were contaminated by sedimentary material. The low ¹⁸O composition of group III rocks can be explained by their derivation from ¹⁸O-depleted rocks or by subsolidus isotopic exchange with low-¹⁸O fluid or meteoric waters. The relatively low δ¹⁸O and ⁸⁷Sr/⁸⁶Sr in the granitoids of Primorye suggest their derivation from rocks with a short-lived crustal history and can result from the following: (1) melting of sedimentary rocks enriched in young volcanic material that was accumulated in the trench along the transform continental margin (granites of the Tatibin Series) and (2) melting of a mixture of abyssal sediments, ocean floor basalts, and upper mantle in the lithospheric plate that subsided beneath the continent in the subduction zone (granites of the ESAVB).     

The supergene thorium and rare-earth element deposit at Morro do Ferro, Poços de Caldas, Minas Gerais, Brazil

The thorium and rare-earth element (Th-REE) deposit at Morro do Ferro formed under supergene lateritic weathering conditions. The ore body consists of shallow NW-SE elongated argillaceous lenses that extend from the top of the hill downwards along its south-eastern slope. The deposit is capped by a network of magnetite layers which protected the underlying highly weathered, argillaceous host rock from excessive erosion. The surrounding country rocks comprise a sequence of subvolcanic phonolite intrusions that have been strongly altered by hydrothermal and supergene processes.From petrological, mineralogical and geochemical studies, and mass balance calculations, it is inferred that the highly weathered host rock was originally carbonatitic in composition, initially enriched in Th and REEs compared to the surrounding silicate rocks. The intrusion of the carbonatite caused fenitic alteration in the surrounding phonolites, consisting of early potassic alteration followed by a vein-type Th-REE mineralization with associated fluorite, carbonate, pyrite and zircon. Subsequent weathering has completely decomposed the carbonatite forming a residual supergene enrichment of Th and REEs.Initial weathering of the carbonatite has created a chemical environment that might have been conductive to carbonate and phosphate complexing of the REEs in groundwaters. This may have appreciably restricted the dissolution of primary REE phases. Strongly oxidic weathering has resulted in a fractionation between Ce and the other light rare earth elements (LREEs). Ce³⁺ is oxidized to Ce⁴⁺ and retained together with Th by secondary mineral formation (cerianite, thorianite), and by adsorption on poorly crystalline iron- and aluminium-hydroxides. In contrast, the trivalent LREEs are retained to a lesser degree and are thus more available for secondary mineral formation (Nd-lanthanite) and adsorption at greater depths down the weathering column. Seasonally controlled fluctuations of recharge waters into the weathering column may help to explain the observed repetition of Th-Ce enriched zones underlain by trivalent LREE enriched zones.     

Polygenic nature of granite clastites: Communication 2. Secondary and supergene reworking of granitic clastites

Clastogenic rocks spatially associated with granite massifs have been reported in the geological literature from different regions: Caucasus (Leonov, 1974, 1991), Urals (Puchkov, 1968), Kazakhstan (Svarichevskaya and Skublova, 1973), Transbaikal region (Leonov, 2008; Lobanov et al., 1991), Tien Shan (Leonov et al., 2008), North America (Beroush, 1991; Lukin, 1989, 2007; Pippin, 1973). In some places, they represent crushed rocks of indigenous massifs. In other places, they make up accumulations and aprons of clastic products of the granitic composition both on the surface and beneath the sedimentary cover. In the first communication (Leonov et al., 2014) devoted to the origin of granite clastites, we examined specific features of the structure and evolution of granite bodies at the posthumous development stage, i.e., after cooling and introduction into the consolidated layer of the Earth’s crust. It was shown that such rocks are formed at least due to two main processes: supergene1 (chemical and physical weathering) and tectonic (prototectonics and posthumous disintegration). Although the rocks are highly similar in composition, structure, and bedding conditions, they are marked by several specific features described in the first communication that provide insight into their genetic nature. However, the problem of morphostructural characteristics and genetic interpretation of granite clastites cannot be closed here. Reconstruction of the “primary” origin of clastic granitic bodies in some, far from single, cases is complicated by the following fact: the exhumed massifs of tectonically disintegrated granitoids undergo supergene transformations, while sediments in the weathering crust are involved in tectonic reworking. Thus, clastites can be formed in several stages with different successions of events: supergene processes (formation of the weathering crust) can precede the tectonic reworking of rocks or succeed the formation of tectonomixtites. Determination of diagnostic properties of genetically different clastic rocks and stages of their lithostructural alteration is important for solving the issues of regional geology, development of methods for the study of genetically complex sequences, as well as paleogeographic and paleotectonic reconstructions. This problem acquires a specific importance because of two circumstances: first, its solution is at the intersection of two geological disciplines (lithology and tectonics); second, granitic clastite bodies often represent commercial hydrocarbon reservoirs (Areshev et al., 1997; Gavrilov, 2000; Izotov et al., 2003; Lobanov et al., 1991; Lobusev et al., 2002; Lukin, 2007; Martynova, 2002; Pippin, 1973; Sitdikova and Izotov, 2002). Let us discuss two scenarios of the succession of events: scenario 1—“tectonic mixtite” → “supergene reworking”; scenario 2—“weathering crust” → “tectonic reworking”. All other versions are combinations of these two types.     

Chai Mat Kaolin–Bauxite Deposit (South Vietnam): Typomorphic Features of Kaolinite and Formation Mechanism of the Zonal Profile of the Bauxite-Bearing Weathering Crust of Granites

Analysis of the evolution of bauxite-bearing weathering crust in the geological history of Asia, as well as detailed mineralogical–geochemical and hydrogeochemical studies, has made it possible to consider supergene infiltration metasomatism as a single mechanism for the formation of the eluvial zonal profile, using the weathering crust of southern Vietnam weathering crust as an example. It is established that all weathering crust zones develop simultaneously throughout the existing fracturing as rocks interact with the solutions percolating through them. All supergene minerals are formed directly from original parent rock components rather than by staged development at each other’s expense. The resulting paragenetic assemblages of newly formed minerals result in zoning.

     

Timing of supergene enrichment of low-grade sedimentary manganese ores in the Kalahari Manganese Field,South Africa

Low-grade carbonate-rich manganese ore of sedimentary origin in the giant Kalahari Manganese Field, South Africa, is upgraded to high-grade todorokite–manganomelane manganese ore by supergene alteration below the unconformity at the base of the Cenozoic Kalahari Formation. Incremental laser-heating ⁴⁰Ar/³⁹Ar dating of samples from the supergene altered manganese ore suggest that chemical weathering processes below the Kalahari unconformity peaked at around 27.8 Ma, 10.1 Ma and 5.2 Ma ago. Older ages are dominant in the upper part of the weathering profile, while younger ages are characteristic of the deeper part of the profile. Younger ages partially overprint older ages in the upper part of the weathering profile and demonstrate the downward progression of the weathering front by as little as 10 cm per million years. The oldest age obtained in the weathering profile, namely 42 Ma, is considered a minimum estimate for the onset of the post African I cycle of weathering and erosion that followed the break up of Gondwanaland and formation of the Cretaceous to early Cenozoic African land surface. The youngest ages, recorded at around 5 Ma, in turn, correspond well to the Pliocene transition from humid to arid climatic conditions in Southern Africa.     

The structure of a small complexly‐deformed area within Ordovician rocks near Mansfield,Victoria

Two distinct groups of granitoids occur on the eastern side of the Kosciusko Batholith. Those considered to be derivatives of sedimentary source rocks (S‐types) are usually foliated and either contain cordierite or white‐mica secondary after cordierite. The granitoids produced from igneous source material (I‐types) are generally massive and frequently contain hornblende. Geochemical parameters provide the best discriminant between the two groups, I‐types have higher Ca, Al, Na₂O/K₂O, and Fe₂O₃/FeO, and lower Fe, Mg, Sc, V, Cr, Co, Ni, Cu, Zn, Ba, Rb, Th, La, Ce, and Y than S‐types of comparable SiO₂ values.

The differences between the two groups are not the result of differences in the melt‐forming process but reflect differences in the nature of the source material. Thus the geochemical features of the S‐type granitoids are indicative of their source rocks having been through a process of chemical weathering in a sedimentary cycle. Conversely, the I‐type granitoids were derived from fractionated rocks that had not been involved in weathering processes.     

非洲锰矿床成因类型、地质特征及表生富集作用

[研究目的]非洲锰矿资源丰富,储量3.1亿t、资源量6.6亿t排名在世界上均列第一,加强非洲锰矿床的研究和认识对推动锰矿找矿工作具有重要的指导意义.[研究方法]通过对重要成矿带典型矿床的解剖总结了非洲锰矿床的成因类型、地质特征.[研究结果]非洲锰矿成因类型主要有前寒武系条带状铁建造(BIF)型、海相沉积型、陆相(三角洲...     

Source Diversity of Ore Sulfur from Mesozoic-Cenozoic Mineral Deposits in the Korean Peninsula Region

Abstract: Sulfides from the Daebo Jurassic granitoids and some ore deposits from Korean Peninsula and Sikhote Alin occurring in different basement settings were analyzed for δ³⁴S values. Highly positive values were obtained from Jurassic Mo skarn deposit at Geumseong of the Ogcheon belt (average +13. 0%), Au‐quartz vein deposits at Unsan, North Korea (+6. 7%), and late Paleozoic Sn‐F deposit at Votnesenka (+8. 2%), Khanka massif, Russia. Together with published data of that region, regional variation of δ³⁴S values is shown across Korean Peninsula. Sulfur isotopic data published are compiled on 88 ore deposits, whose mineralization epochs belong to Cretaceous (58 deposits), Jurassic (25 deposits) and Precambrian (4 deposits) in South Korea. Average sulfur isotopic values vary across South Korea as follows: Cretaceous deposits in the Gyeongsang basin, +4. 8% ranging +1.2 ? +12.7‰ (n=28); Jurassic and Cretaceous deposits in the Sobaegsan massif, +3. 5% ranging 0.0 ? +7.8‰ (n=20); those of the Ogcheon belt, +6. 4% ranging ‐0.5 ? +15.4‰(n=19); those of the Gyeonggi massif, +5. 5% ranging +2.1 ? +9.0‰(n = 21). The δ³⁴S values of South Korea tend to be concentrated around +5. 5 permil, exhibiting little, if any, a systematic variation across the geotectonic belts. This tendency is seen also in North Korea and Northeast China within the Cino‐Korean Block, and may be called as Cino‐Korean type. Sulfur of this type is derived mostly from the crystalline basement. Khanka massif of Russia seems to have features of the Cino‐Korean type. In contrast, paired positive/negative belts corresponding to magnetite‐series/ilmenite‐series granitic belts are overwhelming in the Japanese Islands, especially in Southwest Japan. The similar trend is also seen in southern Sikhote Alin and northern Okhotsk Rim, which may be called as Japanese type. Source of the sulfur in this type is likely in the subducting oceanic slab for positive value and accreted sedimentary complex for the negative value, respectively. The Daebo granitoids have an average rock δ³⁴S value of +5. 3 permil, which should have reflected that of the source rocks in the continental crust. The ore sulfur heavier than this value may have been originated in other granitoids having even higher δ³⁴S values, or the ore fluids interacted directly with sulfate sulfur of the host evaporites or carbonate rocks. Rock isotopic values of granitoids and basement rocks need to be examined in future from the above point of view in mind.     

Geochemistry and detrital modes of Proterozoic sedimentary rocks,Bayana Basin,north Delhi fold belt: implications for provenance and source-area weathering

Bayana Basin, sited along the eastern margin of the north Delhi fold belt of the Aravalli Craton, contains an ～3000?m-thick sequence comprising one volcanic and seven sedimentary formations of the Delhi Supergroup. The sedimentary units are the Nithar, Jogipura, Badalgarh, Bayana, Damdama, Kushalgarh, and Weir formations in order of decreasing age. Petrographic study of the sandstones as well as major and trace elements (including rare earth elements) and bulk-rock analyses of the shales and sandstones allow the determination of their provenance, source-rock weathering, and basinal tectonic setting. The sandstones are quartz rich and were derived mainly from exhumed granitoids typical of a craton interior. Geochemical patterns of the sandstones and shales are similar. However, trace element abundances are low in sandstones, probably due to quartz dilution. The coarser clastic Damdama and Weir sandstones, which occur at higher stratigraphic levels, have strikingly low trace element concentrations compared with the underlying Bayana and Badalgarh sandstones. All samples show uniform LREE-enriched patterns with negative Eu-anomalies (Eu/Eu*?=?0.16–0.23) and are similar to those of post-Archaean Australian shales (PAAS). However, the (La/Yb) _n ratios (averages 11–18) of all the sedimentary units are higher than those of PAAS, except for the Bayana Sandstone, which has low values (average 6.77). The chemical index of alteration (70–78) and the plagioclase index of alteration (87–97) values and the A–CN–K diagram suggest moderate to intense weathering of the source area.

The provenance analyses indicate that basin sedimentation was discontinuous. It received input from a terrain comprising granitoids, mafic rocks, sedimentary sequences, and tonalite-trondhjemite-granodiorite (TTG) suites. The Nithar and Badalgarh sandstones received input from a source consisting predominantly of granitoids. The succeeding Damdama and Weir sandstones received debris from granitoids and TTG in different proportions. The Kushalgarh shale was possibly derived from a source consisting granites and mafic rocks with a TTG component. The pre-existing sedimentary formations also contributed intermittently during the different phases of sedimentation.

Bulk-rock geochemical data suggest Mesoarchaean gneisses and late Archaean granites of BGC/BGGC (Banded Gneissic Complex/Bundelkhand Granitic Gneiss Complex) basement as possible source terrains. These data indicate deposition in a continental rift setting. The coeval formation of many rift-related Proterozoic sedimentary basins in the BGC/BGGC terrain suggests that the North Indian Craton underwent major intracratonic extension during Proterozoic time, probably triggering the break up of Earth's first supercontinent.     

Heterogeneous magnesium isotopic composition of the upper continental crust

High-precision Mg isotopic data are reported for ∼100 well-characterized samples (granites, loess, shales and upper crustal composites) that were previously used to estimate the upper continental crust composition. Magnesium isotopic compositions display limited variation in eight I-type granites from southeastern Australia (δ²⁶Mg = −0.25 to −0.15) and in 15 granitoid composites from eastern China (δ²⁶Mg = −0.35 to −0.16) and do not correlate with SiO₂ contents, indicating the absence of significant Mg isotope fractionation during differentiation of granitic magma. Similarly, the two S-type granites, which represent the two end-members of the S-type granite spectrum from southeastern Australia, have Mg isotopic composition (δ²⁶Mg = −0.23 and −0.14) within the range of their potential source rocks (δ²⁶Mg = −0.20 and +0.15) and I-type granites, suggesting that Mg isotope fractionation during crustal anatexis is also insignificant. By contrast, δ²⁶Mg varies significantly in 19 A-type granites from northeastern China (−0.28 to +0.34) and may reflect source heterogeneity.Compared to I-type and S-type granites, sedimentary rocks have highly heterogeneous and, in most cases, heavier Mg isotopic compositions, with δ²⁶Mg ranging from −0.32 to +0.05 in nine loess from New Zealand and the USA, from −0.27 to +0.49 in 20 post-Archean Australian shales (PAAS), and from −0.52 to +0.92 in 20 sedimentary composites from eastern China. With increasing chemical weathering, as measured by the chemical index of alternation (CIA), δ²⁶Mg values show a larger dispersion in shales than loess. Furthermore, δ²⁶Mg correlates negatively with δ⁷Li in loess. These characteristics suggest that chemical weathering significantly fractionates Mg isotopes and plays an important role in producing the highly variable Mg isotopic composition of sedimentary rocks.Based on the estimated proportions of major rock units within the upper continental crust and their average MgO contents, a weighted average δ²⁶Mg value of −0.22 is derived for the average upper continental crust. Our studies indicate that Mg isotopic composition of the upper crust is, on average, mantle-like but highly heterogeneous, with δ²⁶Mg ranging from −0.52 to +0.92. Such large isotopic variation mainly results from chemical weathering, during which light Mg isotopes are lost to the hydrosphere, leaving weathered products (e.g., sedimentary rocks) with heavy Mg isotopes.     

Age and composition of Antarctic bedrock reflected by detrital zircons, erratics, and recycled microfossils in the Prydz Bay–Wilkes Land–Ross Sea–Marie Byrd Land sector (70°–240°E)

The age and composition of the 14 × 10⁶ km² of Antarctica's surface obscured by ice is unknown except for some dates on detrital minerals. In remedy, we bring together proxies of Antarctic bedrock in the form of (1) detrital zircons analysed for U–Pb age, T_DM^C, εHf, and rock type, including five new analyses of Neogene turbidites, (2) erratics that reflect age, composition, and metamorphism, and (3) recycled microfossils that reflect age, facies, and metamorphism. Each sample is located in its ice-drainage basin for backtracking to the potential provenance. Gaps in age between sample and upslope exposure are specifically attributable to the provenance. This work indicates that the central Antarctic provenance about a core of the Gamburtsev Subglacial Mountains (GSM) and Vostok Subglacial Highlands (VSH) contains a basement that includes igneous (mafic granitoids) and metamorphic rocks with peak U–Pb ages of 0.5–0.7, 0.9–1.3, 1.4–1.7, 1.9–2.1, 2.2–2.3, 2.6–2.8, and 3.15–3.35 Ga, T_DM^C 1.3–3.6 Ga, and εHf + 12 to − 40. Other modelled cratons with similar ages are set in a matrix of foldbelts of 0.5–0.7 Ga age. The basement in the core is surmounted by Permian red beds, at the periphery by Permian and Triassic sedimentary rocks unaffected by igneous heating or load metamorphism, and west of the Transantarctic Mountains (TAM) in the Wilkes Basin arguably by Late Cretaceous through Pliocene marine sediments. Erratics of undated red sandstone along the coast of Wilkes Land and George V Land indicate a red-bed provenance in the interior. The Prince Charles Mountains (PCM) provide an exposed example of a crust of Precambrian igneous and metamorphic rocks and Permian and Triassic sedimentary rocks.     

Sr isotopic characteristic of neoproterozoic carbonate sediments from the southern Yenisei Ridge

This paper presents the first Sr isotopic data for the Late Precambrian carbonate rocks of the southern Yenisei Ridge. Their geochemical study allowed estimation of the degree of secondary alterations and gave the possibility to reveal rocks with a less disturbed Rb-Sr isotopic system. The Sr isotopic data indicated Neoproterozoic sedimentation of the rocks about 1070–750 Ma ago. Sr and C isotopic data showed that carbonate rocks of the Sukhoi Pit, Tungusik, and Shirokino groups are Late Riphean and could be comparable with sedimentary sequences of three Precambrian key sections of the Northern Eurasia: the subsequent Derevnino, Burovaya, and Shorikha formations from the Turukhansk Uplift, the Lakhanda Group from the Uchur-Maya region, and the Karatav Group from the South Urals. All studied carbonate rocks are older than 750 Ma and, according to the International Stratigraphic Chart, accumulated prior to global glaciations in the Cryogenian. This is evident from sedimentological study indicating the absence of tillite horizons in the studied sections. δ¹³C values in the sections vary from +0.4 up to +5.3‰, which testifies to the absence of periods of great cold.     

Geochemical behavior under tropical weathering of the Barama–Mazaruni greenstone belt at Omai gold mine,Guiana Shield

Mineralogical, petrographical, and geochemical studies of the weathering profile have been carried out at Omai Au mine, Guyana. The area is underlain by felsic to mafic volcanic and sedimentary rocks of the Barama-Mazaruni Supergroup, part of the Paleoproterozoic greenstone belts of the Guiana Shield. Tropical rainy climate has favoured extensive lateritization processes and formation of a deeply weathered regolith. The top of the weathering profile consists of lateritic gravel or is masked by the Pleistocene continental-deltaic Berbice Formation. Mineralogical composition of regolith consists mainly of kaolinite, goethite and quartz, and subordinately sericite, feldspar, hematite, pyrite, smectite, heavy minerals, and uncommon mineral phases (nacrite, ephesite, corrensite, guyanaite). A specific feature of the weathering profile at Omai is the preservation of fresh hydrothermal pyrite in the saprolith horizon. Chemical changes during the weathering processes depend on various physicochemical and structural parameters. Consequently, the depth should not be the principal criterion for comparison purposes of the geochemical behavior within the weathering profile, but rather an index that measures the degree of supergene alteration that has affected each analyzed sample, independently of the depth of sampling. Thus, the mineralogical index of alteration (MIA) can provide more accurate information about the behavior of major and trace elements in regolith as opposed to unweathered bedrock. It can also aid in establishing a quantitative relationship between intensity of weathering and mobility (leaching or accumulation) of each element in each analyzed sample. At Omai, some major and trace elements that are commonly considered as immobile (ex: TiO₂, Zr, etc.) during weathering could become mobile in several rock types and cannot be used to calculate the mass and volume balance. In addition, due to higher “immobile element” ratios, the weathered felsic volcanic rocks plotted in identification diagrams are shifted towards more mafic rock types and a negative adjustment of ∼20 units is necessary for correct classification. In contrast, these elements could aid in defining the material source in sedimentary rocks affected by weathering. Generally, the rare-earth element (REE) patterns of the bedrock are preserved in the saprolith horizon. This can represent a potentially useful tool for geochemical exploration in tropical terrains. Strong negative Ce and Tb anomalies are displayed by weathered pillowed andesites, which are explained by the influence of the water/rock ratio.     

Sulfur Isotopic Variation of Yanshanian Magmatic-hydrothermal Deposits in Southern China

Abstract: Sulfur isotope data (δ³⁴S) of sulfides of more than 6700 samples from 157 ore deposits associated with Early and Late Yanshanian granitic and volcanic activities in South China are reviewed and summarized. Averaged δ³⁴S values of individual deposits vary from ‐9. 3 to +20. 6%, and show a normal distribution pattern with the average of +2%. About 88 % of the ore deposits have values within the range, ?2.5 ? +13.6‰, of associated Yanshanian granitoids. There is a temporal‐spatial variation of δ³⁴S values of the ore deposits. However, no clear zonal distribution parallel to geotectonic NNE lineaments was observed. Spatial distribution of ore sulfide δ³⁴S values in most of the NE part of the whole studied area coincides with that of Yanshanian granitoids and volcanic rocks. A downward tendency of the average values in time is: +3. 0% (n=7, J₁) → +1. 6% (n=29, J₂) → +1. 7% (n=68, J₃) → +1. 8% (n=37, K₁) → ?1. 5% (n=16, K₂). There is an “island” of high and variable δ³⁴S values (0? +16.5‰) occurring within a generally low trough zone (?8 ? 0%) of N‐S about 800 km and E‐W 100 to 300 km, bounded by 110°E ? 116°E longitudes and 22°N ? 31°N latitudes. The island occurs at the junction of three tectonic units and a NE‐trending crustal matching line implying a variety of magmatism occurred at the junction. The low trough zone coincides with a low ferric/ferrous ratio zone of Early Yanshanian granitoids, indicating their genetic relationship. Different genetic types of ore deposits show different histogram patterns suggesting different relationships to magmatic rocks and host strata. Granite/greisen/pegmatite type deposits are most closely associated with granitoids, with average ore sul‐fide δ³⁴S values for individual ore deposits ranging between ‐2. 0 and +4. 1%, and an average of +0. 5% (n = 15) close to type meteoric value of 0%. Porphyry‐type deposits have also narrow range of ?2.2 ? + 4.9‰, with an average value of +1. 1% (n = 18). Skarn‐type dominated ore deposits have a nearly normal distribution pattern with an average of +1. 6% (n = 62), ranging from ‐5. 3 to +11. 5%. Volcano‐subvolcanic ore deposits range between ‐3. 1 and +5. 9% with an average of +2. 3% (n = 19). Other types of hydrothermal ore deposits have averaged δ³⁴S values of individual ones from ‐9. 3 to +20. 6%, with average value of +1. 3% (n=43). Vertical and horizontal zonations of δ³⁴S values of ore deposits around their associated granitoid plutons are observed in several localities. Such zonations may be caused by interaction between magma and/or magmatic fluids and host sedimentary rocks, as well as the evolution of physico‐chemical conditions of ore‐forming fluids. Spatial distribution of ore sulfur isotope compositions is also clearly controlled by tectonics and deep faults. Ore sulfur isotope composition is sometimes strongly affected by host sedimentary rocks, especially by evaporite sulfur with much higher δ³⁴S value and partly by biogenic sulfur with low δ³⁴S value. The δ³⁴S values of Yanshanian granitoids are from ‐2. 5 to +13. 6% for both rock samples and pyrite/pyrrhotite separates from granitic rocks, with similar spatial distribution pattern to those of associated ore deposits. The ore deposits associated with ilmenite‐series granitoids have δ³⁴S values ranging between ‐7. 5 and +10. 4% with an average of +1. 0%, while the ore deposits associated with magnetite‐series granitoids ranging between ?8.0 ? +11.5‰ with an average of +1. 1%. δ³⁴S values of ore deposits tend to converge to +3% as the Fe₂O₃/FeO ratio of associated granitoids increases from 0. 45 to 8. 7.     

铀的地球化学性质与成矿——以华南铀成矿省为例

铀是强不相容元素,随着岩浆演化而不断富集,在岩浆演化末期受结构氧增加影响进入独居石、磷钇矿等副矿物中。岩浆演化通常无法直接形成达到工业品位的铀矿床。铀是对氧逸度敏感的变价元素。在表生风化过程中岩体(层)中的铀被氧化为UO_2~(2+)而极易溶解进入水体中,并可在还原环境沉淀而富集成矿,氧化还原界面是找矿的理想选区。大气水可通过断裂构造系统进入一定深度,并受热源作用形成高氧逸度的热液而萃取出岩体(层)中的铀在还原位置沉淀富集形成矿床。新元古代氧化事件以及Marinoan冰期结束使得表生风化过程中更多的U进入水体;而寒武纪生命大爆发,易在沉积盆地底部形成还原环境,有利于U的沉淀富集。受上述三方面因素控制,在华南形成了广泛分布的富铀黑色页岩层,并被之后的沉积物覆盖,成为华南各型铀矿床的铀源层。印支期构造运动使部分富铀黑色页岩层发生部分熔融形成了富铀的S型花岗岩,该类岩石亦是之后铀成矿作用的铀源岩。燕山运动后期华南发生伸展构造背景下的岩浆热事件为以大气水为主的高氧逸度热液的形成并作用于铀源岩(层)提供了有利条件,促使华南各类型铀矿床开始在白垩纪集中形成。     

Rare earth elements in supergene phosphorites

An analysis of rare earth elements in various types of supergene phosphorites established the following sequence of increasing average total contents (ppm): phosphorite from Christmas Island in the Indian Ocean, 3.89; spelean coprolitic phosphorite, 21.98; phosphorite from the weathering zone of sedimentary rocks, 27.41; phosphorite from the weathering zone of endogenous rocks, 372.32; and lacustrine coprolitic phosphorite, 461.59. Supergene phosphorites, especially the most common among them from the weathering zone of sedimentary rocks, are significantly depleted relative to marine phosphorites both in average and maximum REE contents. The REE contents of supergene phosphorites are controlled by several factors, including the REE contents in the primary rocks affected by weathering, the physicochemical conditions of phosphorite formation, the presence of a biogenic component in the phosphatogenetic system, and the structural type of the phosphorites. There is a strong positive correlation within the group of light and, in part, middle REEs (La, Ce, Nd, Sm, and Eu) and between the heavy REEs Yb and Lu, whereas the correlation between these two groups is weaker or insignificant. Gd and Tb are well correlated with the elements of both groups.     

蒙古国巴彦洪格尔铜金矿集区地质特征及成因

巴彦洪格尔地区位于蒙古国中部 ,由古生代俯冲体系构成。该体系包括前寒纪微板块 (白得拉格和伯得高尔构造带 )、逆冲蛇绿岩带、加积型沉积岩 (巴彦洪格尔和得扎格构造带 )和弧前沉积岩 (可汗盖构造带 )。巴彦洪格尔地区岛弧岩浆作用主要以早古生代钛铁矿系列和晚古生代磁铁矿系列花岗岩类为特征。许多诸如斑岩型、矽卡岩型及脉岩型等各种类型的热液矿床与这些花岗岩类伴生。该区四大矿床的 K Ar同位素测年显示 :南方 Cu Au斑岩型矿床、呼布金洪迪矽卡岩型 Cu Au矿床、罕乌尔含金剪切带和塔磁高尔伟晶岩 W Au矿床的形成年龄值分别为 2 40± 5 Ma、2 5 2± 5 Ma、2 83± 6 Ma、32 9± 7Ma。由此可见 ,前三个地区的矿床与二叠纪—三叠纪初期磁铁矿系列花岗岩类具亲缘关系 ,而塔磁高尔W— Au伟晶岩则与早石炭世钛铁矿系列花岗岩类具亲缘关系。斑岩型和矽卡岩型 Cu— Au矿化作用发生在二叠纪末期—三叠纪初期 ,此时紧随着安第斯型岛弧岩浆作用 ,发生了白得拉格和塔巴嘎泰微板块的碰撞。     

Comparison of petrology and isotope geochemistry of the Bulka peridotite–gabbro massif and granitoids of the Kyzykchadra complex (West Sayan)

The study provides new petrologic and isotope geochemical data for rocks of the 465 ± 5 Ma Bulka massif (Borodina et al., 2011). The primary amphibole from granitoid stocks cutting across the layered series of the massif yielded an Ar–Ar age of 415.9 ± 3.7 Ma. The rocks of the Bulka massif have ¹⁴³Nd/¹⁴⁴Nd ratio of 0.513243 and εNd (Т) values of +12.00. The granitoids have ¹⁴³Nd/¹⁴⁴Nd ratios between 0.512919 and 0.512961 and εNd (Т) values between +8.03 and +9.25. The Nd isotope composition indicates that the rocks of the Bulka massif and granitoids were derived from a depleted mantle source. Depletion of the rocks of the massif in LILE, LREE, and HFSE over LILE is inherited from the mantle source, which has geochemical signatures of N-MORB and subduction-related components. Granitoids are metaluminous I-type granites, which were probably generated either by differentiation of intermediate to mafic mantle-derived magmas or by melting of metabasites. The rocks of the granitoid stocks are characterized by enrichment in LILE and LREE and depletion in HFSE over LILE, which suggests derivation from arc-related parental magmas.     

Metacarbonate-terrigenous complex of the Derba block (East Sayan): petrogeochemical and isotope parameters,metamorphism, and time of formation

The Derba block is one of the largest Precambrian terranes of the Sayan-Yenisei accretionary belt in the southwestern margin of the Siberian Platform. It is composed of metamorphosed terrigenous-carbonate rocks of the Sayan Group, injected by granitoids. The geochemical features of gneiss-schist associations indicate the low maturity of their sedimentary protoliths corresponding in composition mainly to graywackes and terrigenous-carbonate rocks (marls). According to the results of U-Pb (LA-ICP-MS) dating of detrital zircons from gneisses and schists, the sedimentary protolith formed in the Vendian. Neoproterozoic subduction complexes were probably the major provenance for terrigenous material, and Early Precambrian rocks made a limited contribution. The Ar-Ar and U-Pb isotope data testify to nearly coeval and multistage events of metamorphism (up to the amphibolite facies) and granitoid magmatism (~ 510-500 and 480-465 Ma) in the Derba block. These processes were reflective of the Early Caledonian orogenic processes in the structures of the Central Asian Orogenic Belt. The similarity in the composition, time of sedimentation, and provenances of metaterrigenous-carbonate complexes of the Derba block (Sayan Group), West Sangilen block of the Tuva-Mongolian massif (Erzin and Moren complexes), and the Khamar-Daban terrane (Slyudyanka Group) suggests that these structures were a single Vendian continental margin with lateral variations in depositional environments and the sources of terrigenous material.     

Structure of the precambrian sedimentary cover and upper part of the basement in the Central Russian Aulacogen and Orsha Depression (East European Platform)

The Precambrian sedimentary section and upper part of the basement of the Central Russian Aulacogen and Orsha Depression, two largest structures located beneath the Moscow Syneclise are analyzed. It has been established that the Late Riphean Central Russian Aulacogen was initiated on the Proterozoic crust of the Transcratonic belt that separates different-aged geological blocks of the East European Platform basement. The Orsha Depression is superposed both on sedimentary complexes of the aulacogen and rocks constituting structures surrounding the Transcratonic belt. Boundaries of the sedimentary cover and basement are outlined and a new structure (Toropets-Ostashkov Trough) is defined. The Precambrian section recovered by Borehole North Molokovo is proposed to serve as a reference one for the Central Russian Aulacogen. The CMP records demonstrate seismocomplexes, which allow one to trace rock members and sequences defined by drilling. Eight seismocomplexes, combination of which varies in different structures, are defined in the Upper Riphean-Vendian part of the sedimentary section. The section of the Central Russian Aulacogen includes the following sedimentary complexes: dominant gray-colored arkoses (R₃¹), variegated arkoses (R₃²), red-colored arkoses (R₃³), and volcanosedimentary rocks (V₁²). The section of the Orsha Depression consists of dominant red-colored quartz sandstones (R₃⁴), glacial and interglacial (V₁¹), and variegated volcanogenic-terrigenous sediments. The upper seismocomplex (V₂) is composed of terrigenous and terrigenous-carbonate rocks. It represents the basal unit of the Moscow Syneclise, which marks the plate stage in development of the East European Platform. The upper part of the basement corresponds to a seismocomplex (Pr₁) represented by dynamometamorphosed rocks that form a tectonic mélange. Analysis of the lateral and vertical distribution of the defined seismocomplexes made it possible to specify the structure of the Riphean-Vendian part of the sedimentary cover and to revise their formation history in some cases.