Geochemistry of Brines and Oil Occurrences in the Udachnaya Kimberlite Pipe (Siberian Platform)
Authors: Novikov D.A. , Ilin A.V. , Kashirtsev V.A. , Chernykh A.V. , Pyryaev A.N. , Dultsev F.F. , Maksimova A.A. , Zueva I.N. , Chalaya O.N. 
Published: February 2022

—Results of a geochemical study of brines and oil occurrences in the Udachnaya kimberlite pipe are presented. Like other intrusions in the Daldyn–Alakit diamondiferous region, this diamond deposit is a unique cryohydrogeologic microstructure differing from the host sedimentary rocks and other diamond pipes of the Yakutian diamond-bearing province. Two waterlogged zones distinguished in the section of orebodies at the explored depths of the deposit correspond to the upper and middle Cambrian aquifers. Predominantly acidic (average pH = 5.5) Cl–Ca and Cl–Ca–Na brines with TDS from 94.3 to 391.3 g/dm3 are widespread within the orebodies and host rocks. The brine mineralization and contents of major salt-forming components increase with depth, to the horizon at the -365 m elevation, where TDS reaches 391 g/dm3, while below, at the –650 m level with noted hydrogeochemical-field inversion, TDS is 253 g/dm3. The mineralization of Cl–Ca, Cl–Ca–Na, Cl–Ca–Mg, and Cl–Ca–Mg–Na brines in the upper Cambrian rocks varies from 102.9 to 192.9 g/dm3, and the pH values, from 4.9 to 6.2, averaging 5.6. Among the microcomponents, the highest average concentrations (mg/dm3) are found for Br1292.8 &gt; S875.7 &gt; Sr453.7 &gt; Fe79.7 &gt; Li53.4 &gt; B32.7 &gt; I13.3 &gt; Si10.8 &gt; Mn6.4 &gt; Se3.6 &gt; Rb2.3. The values of genetic coefficients vary widely: The rNa/rCl coefficient ranges from 0.18 to 0.31; rCa/rMg, from 1.03 to 3.60; Ca/Cl, from 0.2 to 0.3; and the integrated metamorphism index S (according to S.L. Shvartsev) varies from 193 to 277. The middle Cambrian rock complex, containing more saline brines, has been examined in much more detail. It hosts Cl–Ca, Cl–Ca–Na, Cl–Ca–Mg, and Cl–Na–Mg brines with TDS from 94.3 to 391.3 g/dm3 and high average concentrations (mg/dm3) of microcomponents: Br2224.9 &gt; Sr1024.9 &gt;S500.1 &gt; B202.9 &gt; Li147.1 &gt; Fe97.0 &gt; I33.2 &gt; Rb11.4 &gt; Si9.6 &gt; Se9.5 &gt; Mn3.6 &gt; Ni1.7. As compared with brines in the overlying rocks, the middle Cambrian brines show a wider variation in element ratios: rNa/rCl from 0.14 to 0.34, rCa/rMg from 0.66 to 9.71, and Ca/Cl from 0.03 to 0.45. These brines are also characterized by a significantly higher metamorphism grade, which is indicated not only by the rNa/rCl and rCa/rMg ratios but also by the S index varying from 278 to 316. The composition of stable isotopes δD and δ18O) and dissolved inorganic carbon δ13C) of the brines was investigated. The studied waters are assumed to be of sedimentary–metamorphic origin. Their isotopic composition reflects the climatic conditions existing at the time of their burial, which were probably aggravated by the contribution of the oxygen isotope exchange with water-bearing rocks. The δ13C values of carbon dioxide dissolved in water allow an inference about its biogenic origin. The biogenic carbon isotope exchange is governed by the relationship between methanogenic and SMT processes. Analysis of the 87Rb/86Sr and 87Sr/86Sr isotope ratios of the studied brines has revealed affinity between the isotopic compositions of waters in the Cambrian deposits and in ancient seawaters. The mass chromatograms of saturated-hydrocarbon (HC) fractions show at least two individual types of oils and malthas (naphthides). The third variety resulted from their mixing at different stages of migration. The fourth is from the contact zone; it changed during the explosion of kimberlites. The first, most common, type of naphthides (“postexplosive”) is similar in all geochemical parameters to oils from the Nepa–Botuobiya anteclise, in particular, to those from the Mirnyi arch. Oils of the second (pre-explosive) type are found only in the Udachnaya Formation, within the depth range 1130–1430 m.