Sampling and methods Results

siltstones and siderite concretions. These beds of the lower part of the Neruen Formation were deposited in a transgressive, high energy marine sequence, most likely tidal flat to distal sublitoral, near the provenance Semikhatov and Sere- brykov, 1983. These beds were enriched in heavy minerals, especially zircon. The upper terrigenous unit of the Neruen For- mation in the central Maya sections Fig. 3, sec- tion 78 contains varigated shales with minor laminae of siltstones, rare sandstones, siderite lenses, and stromatolitic limestones Semikhatov and Serebrykov, 1983. The mudstones are inter- preted to have formed in low-energy, partly anoxic, outer shoal environments Semikhatov and Serebrykov, 1983. The source rock of clay material was probably, as with the lower unit, the adjacent Aldan Shield. The Belaya River section is the one that was studied in the Uchur – Maya trough. It is located in the north-east part of the region, along the Belaya River on the eastern flank of the Gornos- takh Anticline Figs. 1 and 4. The section sec- tion 52 is composed of mostly platform carbonate rocks up to 82 in the Neruen and 95 – 97 in the Ignikan Formations, and minor shales and siltstones, and sandstones Semikhatov and Serebrykov, 1983; Podkovyrov and Vino- graadov, 1996; Semikhatov et al., 1998. The se- quence is divided into seven transgressive – regressive carbonate – shale units. The limestones and dolomites were formed in a marine shoal in subtidal and tidal environments. Black thinly lam- inated shales in lower units represent predomi- nantly low-energy subtidal, distal ramp and open marine environments, whereas, multi-colored shales with thin horizontal and low-angle cross- bedding, small-scale flazer lamination and small symmetrical ripple marks in upper units were deposited in low-energy tidal, lagoonal and, prob- ably, supratidal settings.

3. Sampling and methods

Representative samples 200 – 400 g were ob- tained from the Maya River and Belaya River at reference sections 77,78, 51,52; Figs. 3 and 4. The samples were collected at 5 – 15 meter inter- vals in thin-bedded and more heterogeneous se- quences and at 30 – 35 meter intervals in thick-bedded and more homogenous sequences. Each sample was cut in half. One half was used for the preparation of thin sections or epoxy- based polished sections for microprobe analysis, and the other half was used for other chemical analyses. A total of twenty samples were ana- lyzed. The major elements were analyzed by X-ray fluorescence and partly by standard wet-chemical methods in the Central Chemical Laboratory, NW Geological Centre, St Petersburg, Russia along with USGS standard rocks. The total Fe content is reported as FeO. The precision of SiO 2 and Al 2 O 3 are better than 9 5, and that of FeO total, TiO 2 , MgO, CaO, Na 2 O are better than 9 8 – 10; P 2 O 5 and MnO are better than 12 – 15. The concentrations of V, Cr, Co, Ni, Rb, Sr, Zr, Ba and Pb were analyzed by X-ray fluores- cence in IGGD RAS, St Petersburg using USGS standards. The precision of most trace elements are better than 8, but that of Co, Ba, Rb and Pb are 10 – 12. The elements Fe, Na, La, Ce, Nd, Sm, Eu, Tb, Yb, Lu, Co, Sc, Hf, Th, Ba and Rb were analyzed by neutron activation at Kansas University after Gordon et al., 1968; Jacobs et al., 1977. The precision of all trace elements but Yb and Lu are better than 5, and the precision of the Yb and Lu are better than 7. The values of standard rocks are periodically analyzed and compared with average values e.g. Cullers et al., 1985, 1987.

4. Results

4 . 1 . Mineralogy The B 1 mm fractions were separated from seven samples 51 – 38, 52 – 9, 52 – 117, 77 – 3, 77 – 6, 77 – 12 and Lh-13 and analyzed by X-ray diffrac- tion T.L. Turchenko, analyst in order to corre- late the mineralogy and chemistry. The samples contain mostly illite – muscovite, and remnants of mixed layer illite – smectite, chlorite, kaolinite, py- rophyllite, quartz, calcite, and feldspars. In the Belaya River section of the Yudoma – Maya trough, sample 52 – 117 consists mainly of 2M 1 muscovite-type minerals 95 – 97 with minor chlorite, quartz and pyrophyllite, and they reflect the high degree of diagenetic alteration among the Lakhanda shales. Samples 52 – 9 and 51 – 38 con- tain progressively decreasing amounts of minerals Fig. 4. Stratigraphic section along the Belaya River with the main lithologies identified. R .L . Cullers , V .N . Podko 6 yro 6 Precambrian Research 104 2000 77 – 93 83 Table 1 Mudstones of the Belaya River and Maya River regions Mudstones from the Belaya River Area — near source 52–11 52–31 52–34 52–44 52–46 52–59 52–80 52–85 52–103 Element 52–1 51–38 52–9 62.1 65.2 SiO 2 57.2 56.0 61.5 57.3 60.2 62.8 59.6 49.3 54.1 57.0 21.3 21.3 18.7 20.4 23.4 18.5 22.3 22.3 20.3 Al 2 O 3 22.6 23.4 18.9 1.27 1.29 1.19 1.3 1.11 1.27 1.45 1.28 1.18 1.14 1.18 1.25 TiO 2 10.5 9.64 6.59 3.86 14.0 3.66 4.94 5.73 2.21 9.27 Total FeO 7.67 14.4 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 MnO 0.01 0.07 1.31 1.95 0.56 0.64 1.72 2.2 1.68 2.05 1.48 1.63 1.9 1.59 MgO 0.13 0.75 0.38 0.22 0.31 0.54 0.4 1.11 0.28 0.6 0.2 0.13 CaO 0.28 0.37 0.28 1.13 0.4 0.18 0.38 0.87 Na 2 O 0.44 0.38 1.02 1.11 4.10 3.23 2.17 2.61 2.21 5.54 4.29 6.21 4.28 5.22 K 2 O 4.99 3.54 0.12 0.08 0.02 0.10 0.08 0.07 0.02 0.09 0.12 P 2 O 5 0.02 0.12 0.11 4.26 3.46 4.92 3.87 3.96 3.15 5.59 4.22 4.07 3.79 5.04 5.03 LOI 99.55 99.48 99.62 99.46 99.48 99.5 99.54 99.56 99.57 99.5 99.4 99.8 Sum 155 162 97 184 134 204 220 249 Rb 165 146 223 166 57 70 56 18 74 16 Sr 165 34 25 67 91 78 350 450 242 553 233 443 428 249 352 379 Ba 508 472 254 200 269 300 249 283 370 265 295 195 182 223 Zr 137 60 115 100 93 132 101 86 119 60 192 144 V 16.2 6.5 38.1 11.8 15.7 4.7 20.7 8.2 Co 16.5 9 33 14.2 25 15 Ni 36 20 27 38 17 31 21 29 37 30 28.2 31.0 16.8 16.8 20.5 16.6 28.5 27.4 25.8 23.6 Th 17.3 21.9 9.5 7.4 9.8 15.1 6.8 6.7 8.5 6.5 8.4 8.8 7.5 8.0 Hf 2.5 2.9 1.6 1.9 2.1 1.8 Ta 1.9 2 2.1 2.3 2.2 2.6 127 140 83 72 100 95 153 148 64 Cr 124 140 91 27.5 16.8 18.3 26.2 16.8 14.7 21.8 18.6 31.7 24.7 32.7 27.3 Sc 66.4 56.9 60.2 73.4 42.2 46.7 58.6 32.5 79.2 61.1 84.6 73.4 La 118 151 105 82.8 123 65.7 125 170 156 135 Ce 118 120 53.8 40.6 48.4 – 34.8 29.4 50 24.1 57.6 54 64.6 54.6 Nd 10.1 6.61 10.2 18.7 6.59 4.57 11.0 3.9 8.73 11.0 13.0 11.5 Sm 1.97 2.95 1.37 0.75 1.73 0.59 1.82 0.86 Eu 2.03 2.27 1.98 1.04 1.41 0.96 1.72 2.6 0.92 0.61 1.42 0.53 0.99 1.41 1.56 1.33 Tb 6.66 8.32 4.06 3.47 4.79 2.57 6.73 5.47 4.62 Yb 6.00 6.39 5.98 0.95 0.68 0.9 1.21 0.59 0.52 0.69 0.39 0.81 0.88 1.00 0.88 Lu 0.588 0.491 0.587 0.518 0.659 0.53 0.52 0.50 0.35 0.6 0.59 0.61 EuEu 3.29 2.80 2.51 3.18 2.69 1.75 2.41 2.50 LaSc 2.69 2.59 2.47 3.39 3.72 11.29 1.11 3.96 3.73 6.91 LaCo 9.66 4.01 1.85 9.40 4.45 3.21 0.47 0.52 0.51 0.65 0.59 0.34 0.43 0.54 0.60 0.59 LaCr 0.89 0.67 2.21 2.85 2.41 4.89 1.17 1.73 1.54 1.91 2.55 2.91 2.92 1.98 LaNi 1.04 1.03 1.54 1.18 1.00 1.14 0.94 0.89 0.86 0.89 0.79 0.86 ThSc 1.74 4.77 0.44 1.42 1.31 3.53 1.38 3.34 ThCo 1.43 2.87 0.66 1.22 R .L . Cullers , V .N . Podko 6 yro 6 Precambrian Research 104 2000 77 – 93 Table 1 Continued Mudstones from the Belaya River Area — near source 52–11 52–31 52–34 52–44 52–46 52–59 52–80 52–85 52–103 52–1 Element 52–9 51–38 0.19 0.19 0.22 0.22 0.20 0.23 0.21 0.17 0.19 ThCr 0.24 0.27 0.18 1.13 2.07 0.47 0.62 0.54 0.98 0.95 0.88 ThNi 0.64 0.89 1.04 0.87 0.95 0.95 0.95 0.88 0.94 CIW 0.89 0.86 0.92 0.87 0.96 0.96 0.96 0.86 0.85 0.85 0.70 0.80 0.67 0.84 0.77 0.83 0.81 CIA 0.70 0.70 0.72 1.08 0.52 0.46 1.02 0.91 0.67 1.07 0.60 1.02 0.91 0.81 ICV 0.11 K 2 OAl 2 O 3 0.13 0.28 0.13 0.29 0.20 0.36 0.21 0.29 0.16 0.20 0.16 Mudstones from the Maya River area — platform mudstones Lh-8 77–12 77–10 77–8 52–119 77–7 78–3 77–6 77–3 Element 78–2 78–1 Lh-13 56.8 60.8 57.0 44.3 41.2 54.9 55.1 57.9 63.7 SiO 2 52.6 43.7 58.7 27.3 20.9 23.4 22.3 25.6 22.2 20.6 26.3 26.0 24.4 19.1 23.7 Al 2 O 3 1.48 1.38 1.40 1.46 1.74 1.50 1.34 1.89 1.50 1.81 1.33 1.62 TiO 2 4.55 2.21 2.23 19.3 23.2 2.36 2.58 2.79 Total FeO 9.67 20.1 2.77 2.08 0.04 0.01 0.02 0.02 0.04 0.02 MnO 0.02 0.01 0.01 0.18 0.03 0.01 0.99 0.92 0.47 0.42 0.41 0.55 0.72 0.89 1.15 0.72 MgO 1.75 0.59 0.21 0.19 0.21 0.28 0.41 0.41 0.28 0.28 0.14 0.34 0.38 0.3 CaO 0.14 0.28 0.11 0.1 0.08 0.08 0.09 0.55 0.08 0.23 0.41 0.17 Na 2 O 4.75 4.67 2.21 2.01 2.00 2.69 2.98 4.08 K 2 O 2.92 3.00 3.10 5.54 P 2 O 5 0.09 0.10 0.07 0.04 0.02 0.04 0.03 0.06 0.05 0.02 0.06 0.07 7.13 7.19 9.36 10.3 10.4 10.7 8.91 5.70 10.2 7.72 LOI 3.66 7.54 99.5 99.59 99.5 100 99.2 101 99.6 100 99.2 99.54 99.6 99.5 Sum 151 172 193 172 110 85 105 119 163 141 128 136 Rb 167 123 91 47 45 83 110 144 Sr 52 42 53 64 245 324 225 161 144 207 Ba 370 492 439 324 352 329 300 306 388 260 236 409 300 308 200 200 Zr 310 300 100 95 80 143 202 162 142 217 182 150 100 100 V 8.8 9.4 10.9 13.4 5.2 21.1 25.9 3.6 8.1 18.4 32.9 25.0 Co 20 32 29 17 4 24 15 40 Ni 15 10 15 42 19.9 21.6 24.4 22.3 21.9 16.2 21.4 21.1 20.4 25.8 17.0 24.1 Th 7.8 7.5 9.9 6.4 6.8 10.1 7.5 8.5 8.8 Hf 10.1 7.2 12.4 2.1 1.6 1.6 2.2 2.3 2.1 1.8 2.5 2.1 2.2 1.7 1.9 Ta 114 85 121 106 115 87 95 121 103 103 76 96 Cr 25.4 22.6 22.9 22.2 21.7 21.7 22.2 26.1 Sc 26.4 21.8 25.4 22.9 49.4 52.6 67.8 62.1 62.4 42.6 46.1 57.5 53.6 63.5 45.2 67.6 La 161 121 123 81.9 89.1 114 96.4 108 138 Ce 114 127 92.5 64.7 49.7 50.7 31.9 35.3 42.9 Nd 45.6 40.8 45.6 38.7 47.3 38.0 12.8 9.19 8.71 5.78 6.48 7.39 7.68 8.79 Sm 8.39 7.39 8.00 6.48 R .L . Cullers , V .N . Podko 6 yro 6 Precambrian Research 104 2000 77 – 93 85 Table 1 Continued Mudstones from the Maya River area — platform mudstones 78–1 Lh-13 Lh-8 77–12 77–10 77–8 77–7 77–6 77–3 78–3 Element 78–2 52–119 1.66 1.14 2.48 1.55 1.50 1.06 1.22 1.35 1.47 Eu 1.35 0.65 1.79 1.64 1.21 1.28 0.92 1.07 1.19 0.83 1.14 Tb 1.53 1.61 1.49 0.70 5.89 5.34 5.90 Yb 3.88 3.85 4.47 5.56 4.71 7.5 7.18 7.17 4.09 0.87 0.81 0.91 0.6 0.68 0.84 0.65 0.71 1.14 1.08 Lu 0.59 1.12 0.52 0.356 0.66 0.56 0.55 0.56 0.58 0.56 0.56 0.511 0.67 0.60 EuEu 2.23 2.30 2.67 2.75 2.72 1.92 2.12 2.65 2.05 2.50 2.07 2.56 LaSc 6.22 4.63 12.0 2.02 1.78 16.0 5.61 6.62 LaCo 2.7 1.37 3.451 5.60 0.56 0.59 0.54 0.49 LaCr 0.49 0.62 0.48 0.52 0.617 0.59 0.7 0.43 3.39 1.94 2.15 2.51 11.5 2.4 3.29 1.34 1.25 LaNi 4.51 4.52 4.233 0.90 0.94 0.96 0.99 0.96 0.73 0.99 0.97 0.78 1.016 0.78 0.91 ThSc 2.26 2.30 2.24 1.66 4.21 0.77 0.83 5.86 2.52 1.40 0.52 0.96 ThCo 0.20 0.21 0.19 0.19 0.23 0.17 0.17 0.20 ThCr 0.25 0.22 0.25 0.25 1.22 0.70 0.76 0.95 5.35 0.88 0.51 ThNi 0.51 1.72 1.7 1.61 1.33 0.98 0.97 0.97 0.96 0.97 0.95 0.98 0.99 0.93 0.97 CIW 0.96 0.961 0.88 0.75 0.80 0.80 0.89 0.88 0.88 0.86 0.84 0.849 0.81 0.86 CIA ICV 0.57 0.69 0.51 0.32 0.88 1.06 0.37 0.42 0.41 1.24 0.62 0.34 0.22 0.23 0.09 0.10 0.11 0.11 0.17 0.12 K 2 OAl 2 O 3 0.29 0.14 0.13 0.17 Table 2 A comparison of the elemental concentrations of the Belaya and Maya Formations Belaya element or Element Maya element or ratio ratio 53.1 9 6.5 SiO 2 58.9 9 4.4 23.7 9 2.4 21.1 9 1.7 Al 2 O 3 TiO 2 1.253 9 0.095 1.54 9 0.19 8.29 9 8.0 7.27 9 4.09 Total FeO 0.035 9 0.047 MnO 0.015 9 0.017 0.76 9 0.28 1.57 9 0.49 MgO 0.40 9 0.28 CaO 0.28 9 0.09 0.18 9 0.15 0.55 9 0.35 Na 2 O 4.1 9 1.3 K 2 O 3.2 9 1.0 P 2 O 5 0.081 9 0.038 0.053 9 0.023 8.54 9 1.65 4.23 9 0.71 LOI 140 9 33 Rb 175 9 42 89 9 43 63 9 39 Sr 396 9 106 Ba 284 9 89 288 9 65 261 9 53 Zr 110 9 36 V 142 9 44 15.2 9 9.3 Co 15.7 9 10 21.3 9 10.8 28.3 9 8.5 Ni 21.1 9 2.9 Th 22.8 9 5.1 8.4 9 1.8 8.6 9 2.2 Hf 2.1 9 0.4 Ta 2.0 9 0.3 102 9 14 109 9 31 Cr 23.1 9 5.8 Sc 23.4 9 1.8 La 55.3 9 9.3 60.6 9 15 113 9 23 122 9 28 Ce 45 9 9 Nd 46 9 12 8.3 9 1.8 9.4 9 4.0 Sm 1.51 9 0.37 Eu 1.54 9 0.73 1.25 9 0.27 1.24 9 0.56 Tb 5.5 9 1.3 Yb 5.3 9 1.6 0.84 9 0.19 0.78 9 0.22 Lu 0.531 9 0.094 EuEu 0.575 9 0.048 2.36 9 0.31 2.66 9 0.44 LaSc 5.60 9 4.36 LaCo 5.30 9 3.1 0.54 9 0.07 0.57 9 0.13 LaCr 2.3 9 1.0 LaNi 3.6 9 2.7 .90 9 0.10 1.0 9 0.2 ThSc 2.0 9 1.3 ThCo 2.1 9 1.6 0.21 9 0.03 ThCr 0.21 9 0.03 1.44 9 1.31 0.89 9 0.41 ThNi contain progressively decreasing amounts of min- erals as follows: illite with 5 – 10 of expanded I – S phase, I = 0.45 – 1.5 \ kaolinite \ chlorite \ quartz \ feldspar 9 pyrophyllite, glauconite and hematite. 4 . 2 . Geochemistry The elemental concentrations and averages of the Lakhanda shales are given in Tables 1 and 2, respectively, and the average elemental concentra- tions are compared with the average of post- Archean shales Taylor and McLennan, 1985 in Fig. 5. The average of the Lakhanda shales are significantly higher in Al 2 O 3 , TiO 2 , Zr, Th, Hf, Sc, and the REE concentrations and lower in SiO 2 , MgO, CaO, Na 2 O, P 2 O 5 , Sr, Ba, and Ni concen- trations than corresponding elemental concentra- tions in the PAAS. The Lakhanda shales and the PAAS have similar concentrations of FeO total, MnO, K 2 O, LOI, Rb, V, Co, Ta, and Cr. The enrichment of immobile elements like Al 2 O 3 and depletion in mobile elements like MgO, CaO, Na 2 O, and Sr results in fairly high chemical Fig. 5. Selected elemental concentrations of the average of all samples are compared with those of the PAAS post Archean average shale values from Taylor and McLennan, 1985. Ex- cept for the REE the ratios are plotted in increasing concen- tration relative to the PAAS. Only selected REE are plotted in order of decreasing atomic number some of the REE like Ce and Lu plot similarly to adjacent REE. Error bars are esti- mates based on one standard deviation of the Lakhanda values for each element as no standard deviation is reported for the PAAS. as follows: dioctahedral illite – muscovite with a low index of crystallinity I = 0.20 – 0.36 \ \ Mg – Fe chlorite \ quartz : feldspars. The sam- ples from the Maya River section have a lesser degree of diagenetic transformation of primary clay minerals than do those from the Belaya River. Samples 77 – 3, 77 – 6, 77 – 12 and Lh-13 Fig. 6. The SiO 2 versus Al 2 O 3 concentrations of shales from the Belaya River open triangle and Maya River closed triangle are plotted relative to the idealized composition of the observed minerals. Much of the variation in composition may be accounted for by variation in quartz and clay miner- als-muscovite. The three samples with high FeO total and low SiO 2 are skewed in the direction of hematite, opaque minerals and Fe-rich chlorite compositions. The FeO total vs. Al 2 O 3 concentrations of shales from the Belaya and Maya Rivers are plotted relative to the composition of the observed minerals same symbols as in Fig. 6a. Again much of the variation in composition may be accounted for by variation in quartz and clay minerals-muscovite. The Fe-rich samples are again skewed toward the hematite, magnetite, and Fe-rich chlorite compositions. primary and recycled clay material Cabanis and Lecolle, 1989; Cullers, 1994b, 1995. Ancient weathering profiles are observed in some parts of the Lakhanda sequence. These profiles contain mostly kaolinite with lesser hematite and chlorite Semikhatov and Serebrykov, 1983. The B 2 mm fractions of samples in this study, however, were mainly illite – muscovite with smaller amounts of kaolinite and chlorite. Elemental plots are also consistent with these phases, but the whole rock samples have variations in major element compo- sitions that suggests a significant amount of quartz and iron oxides may be present e.g. Fig. 6. Also the high concentrations of Zr, Th, REE, Hf, and Th relative to the PAAS in the Lakhanda shales could be due to the concentration of certain accessory minerals zircon, monazite, ilmenite, ru- tile. For example, the observed correlations be- tween Zr, Hf, and TiO 2 tends to support this possibility. The correlation of Sc and Cr with Al 2 O 3 and lesser correlation of Th and the REE with Al 2 O 3 suggests these elements may also be included in the clay minerals of the Lakhanda. There are poor or no trends in the elemental compositions with time. The best of these correla- tions is a slow increase in Na 2 O in both the Maya River and Belaya River samples with time.

5. Discussion