INTRODUCTION

Precambrian diamondiferous magmatism is locally known in various world regions, in particular, Brazil, Venezuela, northeastern Canada, French Guinea, the Republic of South Africa, Western Australia, and India. This topic was recently reviewed in [1]. The relatively limited amount of these occurrences is due to the fact that the ancient primary diamond sources (kimberlite and lamproite bodies) have often eroded over long geological evolution or have been overlapped by younger sedimentary rocks. Any new evidence that indicates a possible find of Precambrian diamondiferous rocks within ancient cratons is therefore important for both the solution of fundamental geological problems and the forecasting of diamond sources of territories.

The possible presence of Precambrian diamondiferous rocks within the Siberian Platform was first suggested by Metelkina et al. [2] and further developed by Afanas’ev and Pokhilenko [3]. In this work, the authors support their hypothesis using analysis of placer diamonds from the north of the platform and an assumption on the Riphean (1268 ± 12 Ma, Rb–Sr isochron for bulk rocks [4]) age of lamproites of the Ingash field (the south of the Siberian Platform). The reviewed placers, however, are Paleozoic and Mesozoic [3], whereas the lamproites are younger than 300 Ma [5]. We thus can state that no direct evidence for a Precambrian stage of Siberian diamondiferous magmatism has been found yet.

This situation dramatically changed after diamonds in Late Precambrian terrigenous rocks of the southern part of the Siberian Platform, in particular, in rocks of the Khuzhir Formation of the Ediacaran Moti Group, were found. Information on this find was published in an open resource concerning objects of mineral management of the Respekt-invest Investment Company. As follows from information on the sale of the Khuzhir deposit of gold-bearing conglomerates, placer diamonds were found in these coarse-clastic sediments. The diamonds of the Khuzhir deposits were later described by researchers of the Irkutsk Research Institute for Noble and Rare Metals and Diamonds (Irgiredmet).

This deposit, which is unique in nature, is a single representative of a nontraditional geological–economic type of gold deposits. The most famous genetic analog of the Khuzhir deposit includes the Witwatersrand deposit in South Africa, which, however, differs in the significantly older (Archean) age of host sequences, the grandiose scale, and the global role in world gold extraction.

To determine the age of diamondiferous gold-bearing terrigenous sediments of the Khuzhir Formation of the Moti Group in the southern Siberian Platform, we identified the U–Pb LA-ICP-MS age of detrital zircons extracted from these sedimentary rocks. Our results and their interpretation are presented in this paper.

The diamondiferous terrigenous rocks studied occur within the Bulun Block of the Sharyzhalgai Uplift of the basement of the Siberian Platform. This region hosts rocks of the TTG series with ages of 3.30 and 3.25 Ga, which are the most ancient Archean rocks currently known within the Siberian Platform [6]. The Archean rocks of the region include migmatites, granite–gneisses, amphibolites, garnet amphibolites, and biotite and garnet–biotite gneisses, which are overlapped by weakly altered Neoproterozoic–Ediacaran sedimentary rocks of the Moti Group with a sharp angular unconformity. This group hosts three formations (from bottom to top): terrigenous Khuzhir with diamondiferous gold-bearing conglomerates, carbonate-terrigenous Shaman, and mostly carbonate Irkut [7]. The rocks of the Khuzhir Formation were the main object of studies; the section of the formation within the eponymous deposit starts from cherry–gray small-pebble gold-bearing conglomerates and polymictic sandstones with interlayers and lenses of siltstones. The thickness of the formation is 90–180 m. Its rocks within adjacent territories are conformably overlapped by carbonate-terrigenous rocks of the Shaman Formation.

Until recently, the accumulation of rocks of the Moti Group in the Ediacaran was accepted on the basis of inter-regional stratigraphic correlations and biostratigraphic studies [8, 9]. The age of the Shaman and Irkut formations of the Moti Group, however, was significantly reconsidered in recent years. For example, it was found that the age of the youngest zircon population in sandstone of the Shaman Formation is 538 Ma [10], whereas the results of comprehensive studies of rocks of the Irkut Formation reliably substantiate their Early Cambrian sedimentation [11].

Taking into account these new data, we should note that the Precambrian (Ediacaran) age of the Khuzhir Formation is indicated by both its stratigraphic occurrence below the Shaman Formation and the results of the most recent detailed studied of Sovetov [9].

To determine the maximum age of the accumulation of rocks of the Khuzhir Formation, two sandstone samples (21 163 and 21 171) were taken from its section directly in the area of the Khuzhir deposit for the U–Pb LA-ICP-MS geochronological studies of detrital zircons.

The U–Pb ages were analyzed in the Center for Geodynamics and Geochronology, Institute of the Earth’s Crust, Siberian Branch, Russian Academy of Sciences (IEC SB RAS, Irkutsk), using laser ablation inductively-coupled mass spectrometry (LA-ICP-MS) on an Agilent 7900 spectrometer equipped with an Analyte Excite excimer laser and a HelEx II double volume cell. The diameter of the laser beam was 35 µm. The experiment run included 20 s for background, 40 s for signal accumulation, and 60 s for purging before the next cycle. The analytical signals were reduced using the Iolite 4.0 program [12]. Further calculations and plotting diagrams were conducted in the Dezirteer program (http://dezirteer.com/) [13]. The “best age” was chosen, and the discordance was estimated in the regime “from lesser error.” The values with discordance of >5% were filtered. The age values with a discordance of <5% were plotted on a diagram of the probability age distribution with estimation of the maxima and their percent contribution to the total distribution. The maximum possible age was calculated as the weighed average of no less than three youngest age values (if there are less than three values, then the program moves up along the timescale until it finds three or more values within the measurement error). Zircon 91 500 with an age of 1065.4 ± 0.3 Ma was used as the primary standard [14]. The quality of measurements of zircons of unknown age was checked by the measurement of secondary zircon standards R33 and Plešovice with the measured 206Pb–238U ages of 421.8 ± 4.1 and 342.7 ± 1.8 Ma, respectively (uncertainties at the level of the 95% confidence interval). The measured values by the R33 and Plešovice standards occur within a ~1% analytical error with a recommended value (419.26 ± 0.39 Ma [15]) and are higher by ~1.6% than the recommended value (337.13 ± 0.37 Ma [16]).

Among 125 grains analyzed, 104 zircon grains from the sandstone sample 21 163 showed values with a discordance level of <5% within the age of 605–3225 Ma and a maximally possible age of sedimentation of 622 ± 6 Ma (Fig. 2a). Ten maxima are distinguished on the diagram of relative probability: 625 (15%), 777 (25%), 985 (4%), 1089 (3%), 1419 (3%), 1859 (21%), 2575 (10%), 2743 (13%), 3003 (3%), and 3225 (1%) Ma.

Fig. 1.
figure 1

Scheme of the geological structure of the southern Siberian Platform. (1) Phanerozoic sedimentary cover of the Siberian Platform, (2) Central Asian Orogenic Belt, (3) sedimentary rocks of the Moti Group, (4) Late Precambrian sedimentary rocks of the southern margin of the Siberian Platform, (5) Neoproterozoic Belaya Zima complex: ultramafic and alkaline rocks, (6) Early Proterozoic postcollision granitoids, (7, 8) Early Precambrian basement inliers: (7) Sharyzhalgai, (8) Biryusa, (9) Early Proterozoic Urik- Iya graben, (10) area of works.

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Fig. 2.
figure 2

U–Pb age of detrital zircon grains from samples (a) 21 163 and (b) 21 171. The size of ellipses on diagrams with concordia (left) correspond to an uncertainty of 2σ. Only values with a discordance of <5% are shown. The histograms and diagrams of the relative age probability (right) indicate the ages of statistically significant maxima. The maximum age of sedimentation was calculated as the weighed average from no less than the three youngest ages, which are overlapped by their errors.

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Among 117 zircon grains analyzed in the sandstone sample 21171, 97 grains showed values with a discordance level of <5% within the age range of 604–3230 Ma with the maximum possible sedimentation age of 607 ± 6 Ma (Fig. 2b). Ten maxima are also distinguished on the diagram of the relative age probability: 615 (9%), 781 (41%), 997 (2%), 1153 (3%), 1567 (2%), 1855 (16%), 2297 (3%), 2541 (17%), 2835 (5%), and 3231 (2%) Ma.

The comparison of ages shows that the distribution of the zircon ages from both samples is similar, which is also supported by the Kolmogorov–Smirnov test (Fig. 3). The combined sampling exhibits nine maxima: 621 (14%), 781 (35%), 1113 (2%), 1421 (2%), 1857 (20%), 2301 (3%), 2553 (13%), 2761 (10%), and 3229 (2%). The maximum age of sedimentation is 605 ± 6 Ma corresponding to the Ediacaran according to the International Stratigraphic Chart.

Fig. 3.
figure 3

Cumulative curves of distribution of ages for samples 21 163 (lower curve) and 21 171 (upper curve).

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On the basis of the minimum ages of zircon in sandstone of the Khuzhir Formation and the present-day stratigraphic correlations [9], it thus can be stated that the age of the Khuzhir Formation is Late Precambrian and therefore its diamonds were contributed to the sedimentary basin from Precambrian diamond-bearing rocks.

In the first approximation, the ages of the youngest zircon grains are close to the period of formation of carbonatite and alkali-silicate magmatism of the region [17]. The second maximum (780–1000 Ma) is the dominant period for rock associations of the Tuva–Mongolian composite terrane of the Central Asian Orogenic Belt (CAOB) [18], whereas the four most ancient maxima correspond to the age of mafic magmatic and metamorphic processes in the southern part of the basement of the Siberian Platform [19].

By the genesis and the age of formation, the potentially Precambrian diamond-bearing sources could be associated with Late Precambrian mantle igneous complexes, which are known in the southern Siberian Platform, including the rocks of the Zhidoi (632 ± 2 Ma) and Belaya Zima (Beloziminskii) (643 ± 3 Ma) plutons [17] (Fig. 1). The presence of similar complexes allows us to suggest that the geological conditions in this area of the platform basement were favorable for possible coeval (or older), but in any case, Precambrian diamond-bearing magmatism, which is related to the alkaline rocks.

In considering the possible contribution of the CAOB diamonds, note that no Archean geological complexes (except for a small fragment of the Tuva–Mongolian composite terrane, namely, the Gargan Block) are known in the CAOB areas adjacent to the sedimentation basins of the Moti Group. Judging from this, according to Clifford’s rule [20], the CAOB complexes can hardly be potential host rocks for the Precambrian diamond-bearing kimberlites and/or lamproites; thus, the CAOB rock associations could not be considered the possible diamond provenances for sediments of the Ediacaran Khuzhir Formation.

The prospects of the entire Moti Group and its possible analogs in the southern Siberian Platform for finding diamonds are most likely negative, because no diamonds or placer gold were found in these rocks (the rocks of the Ust-Tagul Formation of the Sayan region and the Ushakovka Formation of the Baikal region). This circumstance may indicate the local enrichment of rocks of this age in both gold and erosion products of the Precambrian diamond-bearing rocks, which occurred only in rocks of the Khuzhir Formation in the area reviewed or close to it.

The prospects of finding of a Precambrian primary diamond source in the reviewed territory (as well as in the southern Siberian Platform) is relatively low, because the diamond-bearing rocks could likely have eroded over the long and intense geological evolution of the territory.