Nikolenko E.I. Lobanov S.S.
V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia
The basal horizon of Carnian stage of Upper Triassic sediments are the most ancient diamondiferous sediments of the northeastern margin of the Siberian platform[2, 3]. Diamonds of this deposit were discovered by geologists of NPO "Aerogeologia" on Taas-Ari island in 1979. Further investigations of the Triassic collector revealed that it is widely spread from lower Lena region to Tsvetkov cape, Eastern Taimir (Fig.).
In the lower Lena flow region the diamondiferous coarse-grained sediments of Carnian stage underlaid by the thick-tabular sandstones of Ladin (Mid Triassic) stage and can be easily observed in relief as a ridges. Thickness of the basal horizon differs between 0,1-1 m. The highest diamond content of this deposit established in the mouth of Bulkur river (left inflow of Lena) and reaches 10 car/10m3 . Diamonds of variety V and VII according to Orlov’s classification (so-called Ebelyakh type) are peculiarity of this sediments and amount up to 30-40 % of whole diamond population [4, 5]. Such diamonds are widely spread in quaternary placers of the northeastern margin of the Siberian platform, but a totally absent in the Paleozoic kimberlites of Yakutia. Thus, primary deposits for quaternary and Carnian placers are unknown.
The staff of IGM SB RAS in collaboration with SNIIGIMS have sampled of the basal conglomerates of Carnian stage during their fieldworks in 2010 (Fig.). Significant diamond and/or kimberlite indicator minerals content were detected in the profiles of r. Bulkur (pr. 1i/10-23), c. Tumul (pr. 3i/10-8), c. Istanah-Hocho (pr. 8i/10-6), i. Taas-Ari (pr. 11i/10-31), r. Hatistah (pr. 12i/10-1) and r. Ulahan-Aldirhai (pr. 3e/-10-17; 3e/10-18).
From the above mentioned probes 1776 garnets were analyzed. For 1141 of them we performed full X-ray spectrometry analysis on GEOL JXA 8100 microprobe, other garnets were preliminary studied in terms of their composition on TESCAN MIRA 3 LMU SEM with EDS SDD detector X-Max by OXFORD Instruments.
Fig. Exposure of the basal horizon of Carnian stage T3 on the northeastern margin of the Siberian platform and probing places; a. Mg# vs MnO for Mn-rich E-type garnets (MnO>0.5 wt.%): 1 - from Carnian sediments, 2 – inclusions in diamonds from modern alluvials of the northeastern margin of the SP , 3 - inclusions in diamonds of Vth variety , 4 – inclusions in diamonds from Finsch kimberlite (South Africa) ; b. Mn-rich E-type garnets from Carnian sediments in terms of Ca-Mg-Fe+Mn (at.%), shaded area represents the composition of E-type garnets according to [7, 10].
The pyropes of lherzolitic paragenesis prevail among analyzed garnets. The concentration of garnets of diamond association (low-Ca Cr-pyropes of harzburgite-dunite paragenesis)  in Carnian sediments is moderate and varies between 0 (pr. 8i/10-6) and 3,9% (pr. 3i/10-8). Apart from the peridotitic, variable amount of eclogitic (E-type) garnets  were also observed. Their average content is 9 wt.%, minimal concentrations are typical of 3e/10-17, 1i/10-23, 3e/10-18, 12i/10-1 probes (3,3%; 4,6%; 7,4%; 9,3% respectively) and maximum are characteristic of probes 8i/10-6, 11i/10-31, 3i/10-8 (20%; 10,8%; 14,6%). Among the analyzed garnets (1141) only 131 are of E-type  and Mg# and Ca-content differ significantly in this group (27,6-79,6% and 1,9-12,4% respectively), Cr2O3 varies in between 0 and 0,95 wt.%, TiO2 0,01-0,63 wt.%, Na2O 0-0,21 wt.%. MnO of eclogitic garnets from Carnian placers reaches 3,2 wt.% and the amount of Mn-rich garnets (MnO>0,5 wt.%, ) is as high as 29%.
Fig. (a, b) highlights the compositional features of Mn-rich E-type garnets from Carnian sediments of the northeastern margin of the Siberian platform and those recovered from diamonds of different localities. For much of the Mn-rich eclogitic garnets Mg# are between 35-54, Cr2O3 0-0,36 wt.%, TiO2 0,03-0,63 wt.%. Na2O content reaches 0,15 wt.%, but usually lies in between 0,02-0,07 wt.%. Nearly 10% of such diamonds contain Na2O in amounts exceeding 0,09 wt.% and could be assigned to the paragenesis of diamondiferous eclogites .
The eclogitic garnets with elevated MnO content (>0,5 wt.%) are quite rare and known currently only as inclusions in diamonds from quaternary placers of Anabar province (also northeastern margin of the Siberian platform) [6, 7, 11], in alluvial diamonds from Guaniamo, Venezuela , in E-type diamonds from Finsch kimberlite, South Africa  and also in some kimberlites of Yakutia .
Hence earlier publications mentioned above refer only to Mn-rich E-type garnets found as inclusions in diamonds; we are the first to report such garnets in diamondiferous sediments. The amount of such garnets reaches 2,6 % among the hole population of garnets in heavy fraction, which is acknowledged by means of the schlich probing of basal horizon conglomerates of Carnian stage. Thus, for the northeastern margin of the Siberian platform, Mn-rich E-type garnets, along with Cr-rich pyropes, could be employed as an indicator for diamondiferous primary rocks as recommended by [9, 14]
1. Appleyard C.M., Viljoen K.S., Dobbe R. (2004) A study of eclogitic diamonds and their inclusions from the Finsch kimberlite pipe, South Africa. derived garnets in kimberlite: a tool for investigating the mantle and exploring for diamonds. Lithos, v.77, pp. 317-322.
2. Grakhanov S.A., Zarukin A.O., Bogush I.N., Yadrenkin A.V. (2009) Discovery of Upper Trassic placers of diamonds in the bay waters Olenek of Laptev sea (in Russian). Otechestvennaya geologiya, v.1, pp. 53-61.
3. Grakhanov S.A., Smelov A.P., Egorov K.N., Golubev Yu. K. (2010) Volcanic-sedimentary composition of the Carnian stage base, the Source of diamonds in the northeastern Siberian craton (in Russian). Otechestvennaya geologiya, v.5, pp. 3-12.
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7. Sobolev N.V., Yefimova E.S., Koptil V.I. (1999) Mineral inclusions in diamonds in the northeast of the Yakutian diamondiferous province. Proceedings of the VII International Kimberlite Conference, Cape Town, Red Roof Design, v. 2. pp. 816-822.
8. Sobolev N.V., Yefimova E.S., Channer D.M. Der., Anderson P.F.N., Barron K.M. (1998) Unusual upper mantle beneath Guaniamo, Guyana shield, Venezuela: Evidence from diamond inclusions. Geology, v.26, pp. 971-974.
9. Sobolev N.V., Lavrent’ev Y.G., Pokhilenko N.P., Usova L.V. (1973) Chrome-rich garnets from the kimberlites of Yakutia and their parageneses. Cont. Miner. Petrol., v.40, p. 39-52.
10. Sobolev N.V. (1974) Deep-SeatedInclusionsin Kimberlite and the Problem of the Composition of theUpper Mantle (in Russian). Novosibirsk, Nauka Press, 264 p.
11. Sobolev N.V, Yefimova E.S., Koptil V.I., Lavrent'ev Yu.G., Sobolev V.S. (1976) Inclusions of coesite, garnet and omphacite in Yakutian diamond: the first discovery of coesite paragenesis (in Russian). Doklady Akademii Nauk, v.230. №6. pp.1442-1444.
12. Sobolev N.V., Lavrent'ev Yu.G. (1971) Isomorphic sodium admixture in garnets formed at high pressure. Contrib. Mineral. Petrol. v.31, pp. 1-12.
13. Sobolev N.V. (1971)Onmineralogicalcriteria of a diamond potential of kimberlites. Geologiya i Geofizika, v.3, pp. 71-80.
14. Sobolev N.V., Logvinova A.M. Efimova E.S. Manganese enriched garnets of eclogitic paragenesis, in kimberlites and alluvial diamonds as a new mineralogical criterion. (in print).
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