T.Yu.Cherkashina Institute of the Earth's Crust, SB RAS, Irkutsk, Russia

1. T.Yu.CherkashinaInstitute of the Earth's Crust, SB RAS,Irkutsk, Russia Geochemical Features of the Phosphate Deposit Sedimentogenesis of the Bokson-Khubsugul Basin (Russia-Mongolia)

2. Bokson-Khubsugul Basin A number of large deposits of phosphorites are included in Bokson-Khubsugil basin in Russia (Khara-Nurskoye) as well as in Mongolia (Uhagolskoye, Tsagannurskoye, Khubsugulskoye, Burenkhanskoye, etc.). The reconstruction of the depositional environments is the basis problem to geological development history understanding of the region under investigation. Geochemical features of the phosphorite sediments of large deposits Khara-Nurskoye (Bokson series), Khubsugulskoye, Burenkhanskoye (Khubsugul series) are considered.

3. For carrying out the geochemical investigation of the hposphate sediments of Bokson and Khubsugul series the following analytical methods were used: X-ray fluorescence (XRF). Atomic-emission spectrometry (AES). Mass-spectrometry with inductively coupled plasma (ICP MS). Used analytical methods

4. Some aspects of X-ray fluorescence technique The measurement conditions are chosen for each determined element.The NbKα-, ZrKα-, YKα-, and SrKα- lines were chosen for the technique. The possibility of quite precise determination of Y, Zr, Nb contents is restricted by corresponding RbKα, SrKα, YKα spectral line overlaps. For the correction of matrix effects the background standard method was used. When using this technique the scattered X-radiation at the background position IB has been used to obtain the more precise information about a chemical composition of a sample. The intensity at this background angular position must be free from overlaps of spectral element lines presented in the sample.

5. X-ray spectrum in YKβ1 — PbLβ1 wavelength range For choice of this position the spectrum of CRM SG-1A has been registered in the range from 19.6˚ to 29.9˚. The figures shows that the background position at angle 2θ = 29° is the most convenient for YKα-,SrKα-, and RbKα- analytical lines.

6. Certified Reference Materials (CRMs) For the investigation the CRMs of different compositions were chosen. GPOS301 and GPOS302 (dolomitic limestone), OOKO204 (carbonate-silicate loose deposition), OOKO303 (carbonaceous background mud) were received from the Research Institute of Applied Physics at ISU and Vinogradov Institute of Geochemistry SB RAS in Russia. BF (Bronnitsky phosphorite), AK (apatite concentrate), AP (apatite ore) CRMs were received from Bronnitskaya geological-geochemical expedition at the Geological Ministry of USSR. HF (phosphorite) was received from the Central Geological Laboratory of Mongolia. SCo-1 (shale) CRM was received from the Geological Survey of USA.

7. The ranges of determined contents of the studied elements in the phosphorites The main chemical compositions of analyzed samples vary within the wide range (in %): 2.2-40.7 for P2O5, 5.1-52.3 for CaO, 1.2-86.0 for SiO2, 0.01-0.65 for TiO2, 0.06-10.5 for Al2O3, 0.11-5.9 for Fe2O3(total), 0.01-0.07 for MnO, 0.01-15.8 for MgO, 0.03-4.09 for Na2O+K2O. The ranges of determined contents of sdudied elements in the phosphorites are the following (in %): 0.0083-0.13 for Sr, 0.0003-0.0082 for Y, 0.0004-0.011 for Zr, 0.0004-0.0015 for Nb, 0.0006-0.018 for V, 0.0007-0.051 for Cr, 0.0004-0.0063 for Co, 0.0004-0.0027 for Sc, 0.014-0.15 for Ba.

8. Continuation To obtain the most complete pattern of depositional environment the phosphate ores were divided into cherty (SiO2 content is from 26.0% to 86.0%) and carbonaceous (SiO2 content is from 1.7% to 26.0%) in composition. In addition, they were divided into groups of poor (1.0 - 8%), medium (up to 16%), and rich (more than 16%) ores according to their phosphate content. The average composition of marine phosphorite was used for normalizing in studying the distribution of trace element contents. The main chemical compositions of the marine phosphorite are the following: CaO — 31.4%, SiO2 - 5.6%, P2O5 — 13.8%, Fe2O3(total) — 0.77%, Al2O3 - 0.91%, TiO2 — 0.064%, MnO — 0.12%, MgO — 0.18%, K2O+Na2O - 0.87%.

9. Geochemical features of trace elements distribution in the phosphorites of Bokson and Khubsugul series Bokson series Khubsugul series The figure shows that more higher contents of V, Cr, Sc, and Zr are typical for the ores of Bokson series in comparison with Khubsugul series. The highest content of Zr (up to 110 ppm) have been noted in cherty ores, that it's not typical for this ones. The distribution of Zr have smooth character, that it is supposed the stability of sediment sourcelands. Cr and Co are elements of ultrabasic specialization and took part in the forming of the phosphate rock terrigenous part. Chromium accumulate is typical for a shallow zone with active hydrodynamical regime. Chromium content in the poor ores of Bokson series is twice as higher than in the marine phosphorite. Considering the smooth character of the distribution of Zr the accumulation of the poor phosphate ores enriched by chromium is assumed to take place in shallow settings of the shelf zone.

10. Continuation Inverse correlation in the accumulation of Co and Cr denote the separate ways of the elements transportation. V and Sc are elements of basic specialization. In the phosphate ores of Khubsugul series Sc content is lower than the detection limit (CL = 3 ppm). In the rich and the poor ores of Bokson series V-Sc geochemical specialization is appeared. These elements were fallen into the sediment as a result of their sorption by phosphatic and organic material. This material available in shallow shelf sediments of Tuva-Mongolian Microcontinent in a big quantity. Sr is an element, which almost completely include in the composition of the calcic minerals. The maximum contents of Sr (up to 1300 ppm) available in carbonaceous rich ores of Bokson and Khubsugul series. It denotes the enrichment of the marine water by this element in the sedimentation moment of the phosphorites.

11. Continuation Nb is most immobile element. It have low ability to the precipitation. For the carbonaceous phosphorites the high concentrations of Nb are not typical due to its low migratory ability and high specific weight of its minerals-carriers. The Y content is more than 5 times as lower in comparison to the Y content in the marine phosphorite. It is connected with the including this element to ocean water as a result of the hydrothermal activity [Gurvich, 1998], and the accumulation of the phosphorites of studied deposits was taken place in the shelf zone in a passive tectonic setting [Letnikova et al., 2005].

12. The distribution of Ba in the carbonaceous phosphate ores of Khubsugul series The accumulation of Ba connects with the precipitation of phosphorus. So, more enriched sediments have the high Ba contents (up to 1500 ppm). The existence of geochemical traps is assumed, which are opportune for the precipitation of Ba during the sediment accumulation.

13. Geochemical features of REE distribution in the phosphorites of Bokson series REE contents in the phosphate ores of Hara-Nur deposit vary within from 3 ppm to 48 ppm, and the ones of Khubsugul deposit vary from 0.03 ppm to 36 ppm. The figure shows that REE contents in the ores are lower than in the marine phosphorite. The sample of the marine phosphorite have the explicitnegative Ce-anomaly (Ce/Ce* = 0.43), and the positive Eu-anomaly (Eu/Eu* = 1.16). The cherty ores have the gently sloping REE distribution trend (La/Yb = 5.8-6.7). For their negative Eu-anomaly (Eu/Eu* = 0.56-0.76) and not explicit negative Ce-anomaly (Ce/Ce* = 0.83-0.9) were noted. In carbonaceous phosphate ores the explicitnegative Ce-anomaly (Ce/Ce* = 0.24-0.44) was revealed. The Eu-anomalywas not found (Eu/Eu* = 0.92-0.99).

14. The figure shows the distribution of REE in carbonaceous phosphate ores of Khubsugul (а) and Burenkhan (б) deposits. For this ores the gently sloping REE distribution trend (La/Yb = 5.83-8.68) is typical. This type of the ores is characterized by the explicitCe-minimum: Ce/Ce* = 0.24-0.88 for Khubsugul deposit (a), and Ce/Ce* = 0.24-0.59 for Burenkhan deposit (b). The figure (a) shows the almost complete absence of the negative Eu-anomaly (Eu/Eu* = 0.54-1.0) in the ores of Khubsugul deposit. In the figure (b) the positive Eu-anomaly (Eu/Eu* = 1.14-1.47) is seen very well. The distribution of REE in carbonaceous phosphate ores of Khubsugul and Burenkhan deposits

15. The distribution of REE in cherty phosphate ores of Burenkhan deposit This figure shows the explicit negative Ce-anomaly (Ce/Ce* = 0.19-0.29). The Eu-anomaly is poorly evident (Eu/Eu* = 0.79-0.96). It will be noted the single occurrence of the Eu-maximum (Eu/Eu* = 3.2) in the sample MM5-17P2.

16. Thus, the existence of the negative Ce-anomaly in carbonaceous phosphate ores of the studied deposits denote the more reduce character of their depositional environments with respect to the cherty ores. The process of Eu accumulation in the phosphorites is poorly studied, but it is assumed that it is connected with the supply of the abyssal waters, which are enrichment of this element. Considering the low content of the sum of REE in the carbonaceous phosphate ores the accumulation of the ores is assumed to take place in the deep-water parts of Tuva-Mongolian Microcontinent shelf. The geochemical analysis of the distribution of the rare-earth elements in the ores of the studied deposits revealed the relationship in the HREE distribution, which is connected with the increase of the phosphate part with respect to invariable LREE contents. This is an additional argument that the mechanism of the phosphate sedimentation is upwelling.

17. Conclusions The geochemical investigation gave the following preliminary results: • The accumulation of the phosphate poor cherty ores of the studied deposits took place in the shallow part of the shelf of Tuva-Mongolian Microcontinent in active water regime. The carbonaceous ores enriched by phosphorus precipitated in the deep-water parts of the shelf zone. • Thus, the accumulation of the ores of Bokson-Khubsugul Basin took place in marginal parts of the shallow Vendian-Cambrian carbonaceous cover of Tuva-Mongolian Microcontinent.

18. Conclusions • The geochemical type of the phosphorites was established and connected with the way of the phosphatic material supply to the sedimentation basin. • Therevealeddistribution patterns of the studied elements in all types of the phosphatic ores denote the evidence of upwelling type as a mechanism of the Cambrian phosphorite forming.

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20. Due to the great distance between angle positions of NbKα and ZrKα analytical lines and the background position 2θ = 29˚, the background intensity for these lines has been determined using IB values measured on the short-wave side of these analytical lines (2θ = 20.70˚ and 21.90˚, respectively). All background coefficients have been calculated using FON.EXE program by Prof. B.I. Kitov. For the correction for matrix effects the background standard method was used in combination with the linear multiple regression equation. where Ciis the concentration of an element analyzed, a0, a1, aj, bkare regression coefficients, is the ratio between the relative intensity of the analytical line of the i-th element of the sample and the relative in- tensity of analytical line of the reference sample, respectively, and are the intensities of the background radiation from the sample and the reference sample, respectively, which have been measured at a previously chosen position of the spectrum 2θ = 29˚.

21. Atomic emission spectrometry V, Cr, Co, Sc, and Ba contents in the phosphorites of studied series were determined using DFS-12 diffraction spectrograph. For the determination the internal standard method was used. As the internal standard Pd was used. As a basic in this technique for the preparing of reference materials (CRMs) the monomineralic quartz was applied. As a buffer the mixture of coal powder and quartz was used. For decreasing matrix effects and increasing futures of the excitation the CRMs and samples were diluted by the buffer mixture consist of coal powder 40%, and SiO2 60% with addition of PdCl2 0.053%. Degree of the dilution of sample by the buffer mixture is 1:1 relation. When the analyze the analytical lines of the determined elements are the following (in Å): Ba – 3071, Co – 3102, Cr – 3014, Sc – 3353 and 3273, Pd – 3460.

22. Inductively coupled plasmamassspectrometry The determination of the REE contents in the phosphate ores was carried out using VG Plasmaquad PQ-2 mass spectrometer. For the studied rocks the techniques of open acidic decomposition and lithium metaborate fusion were applied. The choice of the decomposition technique responsible for the mineral composition of the sample analyzed. As the studied samples contain from accessory, carbonaceous, and phosphate minerals, that the complete decomposition of each sample in its acidic treatment don't take place. Thus, the optimal decomposition technique is lithium metaborate fusion. For the calibration the following CRMs were used: BHVO-1 (basalt), SCo-1 (averaged shale), SGR-1 (shale), JDO-1 (dolomite), OOPE101 (terrigenous clay), OOPE401(calcareous mud).