2011

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Cis-Anabarian Lamproite igneous rocks: still not realized perspectives

Vishnevsky S.A.

Institute of Geology & Mineralogy SB RAS, Novosibirsk, RUSSIA; svish@igm.nsc.ru

Review on the Cis-Anabarian lamproite investigations and on the perspective of the diamond potential of these rocks.

Rich diamond placers on the East off the Anabar Shield are known for more than 40 years, but their source was not found for a long time (Epifanov, Rodin, 1991; Koptil', 1994; Grakhanov, Mitukhin, 2003).Large volume of quest efforts directed to idea of kimberlite sources was failed to give an answer on this question: these rocks in both the Anabar and Kuonamka placer areas are barren of or very poor of diamond potential. It become clear that new non-traditional native sources should be taken into account; moreover, the local placer diamonds have rather specific typomorphism which is not similar to that of the kimberlites (Grakhanov et al., 2007; Afanasiev et al., 2011).

Meanwhile, several complexes of basic and ultrabasic alkaline rocks are known here (Shpunt et al., 1981), and their presence can serve as indication of native diamond potential for a given area (Porshnev et al., 1986*; Epifanov, Rodin, 1991). Besides, lamproites and lamproite-like rocks were found here (Filatov et al., 1986; Vishnevsky et al., 1986). Later on, the series of explosive lamproite tuff and tuffizite bodies was found in 1989–1995 on the Anabar area by the combined efforts of the SNIIGGIMS and «ALROSA» Company. Initially, much of the bodies, changed by erosion and weathering, were considered to be the «banks of karst+erosion bauxites», but later their tuff-lamproite origin was supposed (Epifanov, 1993) and soon was confirmed (Prokop'ev et al., 1996*; 2001*, 2008). Our studies (Vishnevsky, 2006) also show the broad extent of lamproites on the Kuonamka area, but earlier these rocks were considered to be the peralkaline potassium trachytes (Vaganov, Konstantinovsky, 1978). Lamproites are also reported by (Vladykin, Torbeeva, 2005) in the Tomtor massif of alkaline ultra-basites and carbonatites, although (Bagdasarov, 2009) does not support the conclusion.

The story of studying the native diamond potential of the areas under consideration is highly instructive. First diamonds from the lamproite-derived «bauxites» (Gribun, 1977*; Kirillin, 1988*) were not perceived seriously (at that time the «kimberlitic» idea of quest dominated) and were explained by the contamination from other rocks. When it was shown that the «bauxites» and other country «allites» are the weathering crusts derived from the lamproite tuffs (Tolstov, 1991*; Epifanov, 1993*), and that these tuffs are diamond-bearing (Prokop'ev et al., 1996*; 2001*; 2008; Spetsius et al., 2006), the native diamond source in the areas under consideration was determined; nevertheless, the quest of the bodies with industrial diamond potential is still an actual problem, but a number of subjective causes are present here. One among the causes is an inadequate attention to non-kimberlitic models of diamond origin (Epifanov, 2006). It should be noted that the «kimberlitic» school of geologists is still assuming a pose of ignoring as far as the findings mentioned above are concerned; the students of the school consider that the native source of the diamonds is still unknown (Zinchuk, Koptil', 2003; Afanasiev et al., 2011). Nevertheless, even these authors allow yet the possibility of the lamproitic source. There are also some other controversies on the results obtained, including the age of the native sources among them. Following to (Afanasiev et al., 2011), the lamproitic (?) source is Precambrian one; following to (Prokop'ev et al., 2001*; 2008; Grakhanov et al., 2010; Spetsius et al., 2006), the local lamproite tuffs as the native source of the diamond placers were originated in Middle and Later Triassic time. Some other controversies are also present as far as geologic history and origin of the river net of the area are concerned; this current state of the knowledge does not stimulate the right strategy of the future studies on the problem, but the strategy is very actual one in scope of crisis state of the home geology. In the meantime, the new data presented by the Popigai astrobleme are still not taken into account by the local geological organizations. Following to these data, an imposing cover of Mesozoic–Cenozoic marine and coal-bearing sediments of up to 150 m in thickness was present within the area of the Popigai astrobleme, when it was originated 35.7 Ma ago (Vishnevsky et al., 1995). This fact calls in the question whether the Mesozoic erosion of the Anabar shield really took place here. It is also supposed (Vishnevsky, 1994), that the origin of the Popigai astrobleme beheaded the Paleo-Popigai R.; damming of the river valley led to origin of large water reservoir with subsequent gap on the East, when the sub-latitudinal part of Big Kyonamka R. valley was created. The Mesozoic sediments in the dead-end siding of the Paleo-Popigai R. valley (Staraya river basin) show its old age.

Mesozoic sedimentation on the Anabar shield and on its northern vicinity, followed by the lamproite magmatism, shows that Late Triassic + Jurassic–Cretaceous sediments were the primary placer collectors of diamond (Grakhanov, 2006; Grakhanov et al., 2007). Not quite at all, the Ebelyakh R. placers are followed by Cretaceous sediments and Mesozoic weathering crusts, whose remnants are still present in this river valley. Let us note in this connection that the first findings of non-impact bort-class diamonds in the Popigai crater were made near the outcrops of the Cretaceous sands (Kirjushina, 1955*) in klippen+megabreccia zone on the west side of the crater. Taking into account that there is large province of alkaline potassium-rich igneous rocks (including lamproites) with possible diamond potential on the Northern part of the Siberian platform, the Mesozoic diamond placers could have a large areal distribution. Possibly, the main part of the placers was buried within the Leno-Khatangsky marginal trough, being exposed and affected to Cenozoic re-washing on the northern (the Laptevs Sea Cost) and southern (Anabar-Olenek zone) flanks of the trough.

It's highly-instructive also, that the similar story with the quest of native sources takes place for the diamond placers of Uralian-Timanian region; at this, the story is being prolonged for about two hundred years (Sychkin, 2006). The local kimberlites are either barren of, or have a very poor diamond potential (Mal'kov, 2006; Makeev et al., 2008), but attempts «to bring» the diamonds here from the Russian platform are unconvincing (Alexeevsky, 2005). At the same time, the new type of native sources – diamond-bearing lamproites and tuffizites – including those with the industrial diamond potential (Rybal'chenko et al., 1997; Makeev et al., 1999; Alexeevsky, 2005; Rybal'chenko, 2006) are still ignored by the traditional geology (Vaganov et al., 2001). Like in the case of the Anabar-Kuonamka area, this scepticism hampers the solving of the problem, and one can only be sorry about the past times when the possibilities of the home geology were much more than they are now.

The problem of quest of native diamond sources (and not only on the areas mentioned above) is complicated both by the «kimberlite» imprint of some our geologic schools, and by the discussions on the multiple Р–Т variants of the natural diamond formation also. The pioneer works by Kaminsky (1984), Letnikov (1993) and others should be noted here; in particular, one of the generalizations of so kind (Shumilova, 2003) shows that the endogenic diamonds can originate at a very broad range of Р–Т parameters of the process, with pressures varying from deep vacuum to 100 GPa, and with room to 4000–5000 ^oC temperatures. The discussions still continue, but one can note unfortunately that the «high-pressure» mantle way, being only the one among the cases, is still the most preferable among the geologists today. Alkaline- and fluid-stimulating models of origin for the metasomatic fluidized melts after (Kogarko, 2006) and for origin of their diamonds as well (including the hydrocarbons participation in the diamond growth (after Epifanov, Rodin, 1991; Vaganov, 1993)) are of interest for the current discussion. The role of fluids in diamond transportation (Rybal'chenko et al., 1997; Silaev et al., 2004; Rybal'chenko 2006; Oleinikov, Barashkov, 2005) is also very important. At this, the source of the diamonds themselves can be both the upper-mantle and lower-crustal one.

The classification and nomenclature questions for Cis-Anabarian lamproites are also needed in working out. On the SiO₂–(K₂O+Na₂O) diagram broadly-used for classification of feldspathoids, the Kuonamka lamproites are broadly varied from the olivine leucetites, tephrites and alkaline basalts (Vishnevsky, 2006) to the trachyandesites and trachytes to which they were early referred to by (Vaganov, Konstantinovsky, 1978; Milashev, 1984). However, based upon the richness in K–feldspar, the Kuonamka rocks can be referred to that alkaline basaltoid subgroup to which the lamproites of the Leucite Hills (USA), Western Australia and Spain are included into by (Zavaritsky, 1955). Following to Niggli (Chetverikov, 1956) the rocks have to be selected as the members of lamro-sienite and jumillite-biotite types of the lamproite group. As being the K– and Mg–rich rocks they have to be included into the same group following to (Troger, 1935) classification. After (Sahama, 1974) they are the members of that part of the orendite family group which is equal to West Australian lamproites, but they have a more broad range of SiO₂ sub-saturation (Vishnevsky et al., 1986). Later on (Milashev et al., 1987) noted also the close petro-chemical similarity between the Talakhtakh rocks and the other lamproites. On the main oxides vs. MgO diagram one can see (Vishnevsky, 2006) that the Talakhtakh and other world lamproites are equal to each other in terms of K₂O, MgO, SiO₂, MnO and, partially, of P₂O₅ amounts; as for the Na₂O content, the rocks discussed are very similar to the West Australian peralkaline potassium rocks, but in general, the world-distributed leucite lamproites have a broad dispersion of Na₂O amounts. Finally, the rocks discussed are past beyond compare with the rest of the lamproites (Vishnevsky et al., 1986) and other known volcanites and magmatites (Milashev et al., 1987) in Al₂O₃ super-saturation and in low CaO amount. So, based upon the previous classification approach, when each rock from this very mixed rank was named usually in its proper name, the Talakhtakh rocks can come in for their proper name also. However, based upon the mineralogical principle of nomenclature for these rocks after (Scott Smith, Skinner, 1984) we select the Kuonamka rocks as the sanidine type lamproites.

Talakhtakh lamproites attracted attention of domestic colleagues and references on them appeared in a rank of publications (Konev et al., 1988; Shpunt, Shamshina, 1989; Sekerin et al., 1990; Rotman, 2006; and others). However, after the critical paper by (Mitchell, 1988), who declined the lamproitic nature of the rocks and named them again as peralkaline potassium trachytes, the home interest to these specific rocks, very perspective for the aims of quest prediction, was very decreased unfortunately. Let us note in this connection that: 1) not all the Mitchell’s geochemical criteria are applied blamelessly to the known world lamproites; 2) the model of ancient deep metasomatism explaining after this author the enrichment of lamproites by compatible and incompatible elements is the possible speculation only what is spoken about by himself (Mitchell et al., 1987); let us add also that lamproitic magmatism on the Siberian platform could have its own specificity; the ancient, since the accretion stage, pre-history of the development of the Siberian mantle segment (as the source of the local lamproitic melts) could be quite another than it is supposed by the model of deep metasomatism after Mitchell; 3) the Talakhtakh lamproites are selected following to the criteria after Zavaritsky, Niggli, Troger, Sahama and some other authors, mentioned above; in spite of the old age, these criteria have one very important advantage: they were summarized on a broad base of observations and comparisons and were not thrilled by the specificity of West-Australian deposits; 4) the composition field of the Talakhtakh lamproites is varying from olivine leucitites to liparites and is very broad (Vishnevsky, 2006, Fig. 2) to select them as peralkaline potassium trachytes following to Mitchell. Finishing the discussion, one can note that lamproites of Kuonamka area are undoubtedly, the result of explosive ejection of deep-seated fluid+melt mixtures enriched with K and Mg.

One has to say the next in the conclusion. The successes of Australian geologists are well-know, but the way to the finish was not a simple one. Much more complex is the way to the Russian «Argyles». The investigators of Cis-Anabarian and Uralian-Timanian diamonds come up to the success closely, but orthodoxy of some our geological schools, un-critical tracing of foreign achievements and modern crisis events in domestic geology are served as the barrier for the «finishing the last effort».

References: Due to the limited volume of the report, the published references are indicated in form of author’s names and years of issue only. The complete bibliography of the references is presented on the next address: http://www.proza.ru/2011/05/23/611. Non-published papers are indicated as *.