Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Mineralogy of diamond bearing lamproite diatreme in the Kostomuksha region (Karelia, Russia)

Rudashevsky V.N.*, Gorkovetz V.Ya.**, Rudashevsky N.S.*, Popov M.G.**, Antonov A.V.***

*PC+ Co.LTD, St.Petersburg, Russia; **Geological Institute of Karelian Research Centre RAS, Petrozavodsk, Russia; ***A.P. Karpinsky Russian Geological Research Institute (VSEGEI), St.Petersburg,  nrudash@list.ru

Lamproites (Raevskaya, Gorkovetz, 1978; Proskuriakov e. a., 1989; etc) and kimberlites (Gorkovetz e. a., 2009; etc) were determined in the Kostomuksha ore region (West Karelia) based on study of its petrography and petrochemistry. There have been more than 120 dike lamproite bodies (0.5-4 m wide) discovered already, the majority of which are located within Kostomuksha iron ore deposit. The age of lamproites (by Rb-Sr method – Beliatsky e.a., 1997) is 1230 mln. years. The mineralogy of two dike bodies of lamproites was previously studied in detail (Rudashevsky e.a., 2012).

One diatreme of lamproites having diameter of 200-250 m and cross section square of 3 hectares was discovered within the limits of Kostomuksha iron ore deposit in 2005. The tube is breaking through Archaean metamorphic rocks of Gimolskaya series (2.8 bill. years – Scherbak e.a., 1986). This diatreme is localized in quartz-feldspar-biotite shales, associated with magnetite quartzites. This presentation represents results of the 3D-mineralogical investigation of lamproites from this diatreme.

The rock sample of >3 kg was studied. Polished sections were prepared from the studied sample. Also, special sample preparation for 3D-mineralogical investigation (Rudashevsky e.a., 2002) was performed: stage-by-stage grinding (exposition of 10 seс. followed by dry sieving through 0.5 mm after each grind), wet sieving in 500–100 microns interval, hydroseparation (using CNT HS-11 – Rudashevsky, Rudashevsky, 2007) and preparation of monolayer polished sections from the heavy mineral concentrates after hydroseparation. Also, from the same heavy mineral concentrates under binocular microscope the selections of different indicator minerals were hand picked by color, shine, shape of grains and crystals. The polished sections of the rocks, heavy mineral concentrates and selections of indictor minerals were studied by electron microscopy (Camscan-4DV, Link AN-10000, UK).

The chemical composition of the diatreme rocks (average of 4 samples) is the following (wt. %): SiO₂ 42.2 (40-51)[1], TiO₂ 2.55 (1-5), Al₂O₃ 4.75 (3-9), FeO_total. 8.17 (5-9), MnO 0.12, MgO 23.34 (12-28), CaO 5.67 (4-13), Na₂O 0.11 (<2), K₂O 2.34 (3-9), H₂O+CO₂ 10.12 (2-8, without СО₂), P₂O₅ 1.16 (1-3). The following parameters of geochemical criteria of the studied rocks are characteristic for olivine lamproites (Mitchell, 1995)[2]: K₂O/Na₂O 21.3 (>3), K₂O/Al₂O₃ 0.5 (>0.8), Mg# = MgO/(MgO+FeO_total) = 0.74 (0.45-0.85), k = K₂O/(K₂O+Na₂O) = 0.96 (>0.7), FeO_total 8.17 wt. % (<10 wt. %), CaO 5.67 wt. % (<10 wt. %).

The diatreme rock has breccia-like texture (pieces of xenoliths ranging from 0.1 mm to 1 cm). Xenoliths are rounded or have irregular shape. They consist of the fine grained aggregates – intergrowths of talc and serpentine.  Marginal parts of xenoliths at the contact with matrix rock are serpentine-rich. The matrix rock cementing xenoliths is formed by average grain size and coarse grained aggregates of reddish-brown mica ranging between 10 and 1000 microns (phlogopite and tetra-ferriphlogopite), and fine grained accumulations of secondary minerals (serpentine, talc, calcite, dolomite and quartz). Rare relicts of of K-feldspar and (Ti-K)-richterite were identified as well. Xenoliths and their cementing matrix rock are crossed by serpentine veins.

The following minerals are accumulating in the heavy mineral concentrates: first of all, sulphides (pyrrhotite, pyrite, pentlandite, chalcopyrite, galena, sphalerite), as well as other accessories – apatite (including Sr-apatite), barite, and more rare minerals, such as garnets (pyrope and almandine), chromium diopside, ilmenite, monazite-(Ce), rutile, zircon, Zr-priderite, and several others.

Chemical compositions of the studied micas (56 microprobe analyses of various phenocrysts, wt. %) – TiO₂ 1.5-10.4, average - 4.7; Al₂O₃ 1.5-10.4, average - 6.3; MgO 19.9-26.3, average - 23.0 – show variation trends which are very characteristic for lamproite micas (Mitchell, 1995).

The sulphides represent two generations: 1) early stage high-temperature – layered silicate-sulphide “microdroplets” (pentlandite, pyrrhotite and chalcopyrite);  2) low-temperature later stage – sulphides, synchronous with serpentine, talc, carbonates and quartz (same sulphides +sphalerite and galena). The “microdroplets” of sulphides have similar structures to re-crystallized mantle melts of monosulphide solid solution – ultramafic-type, eclogite-type or similar to inclusions in diamonds (Taylor, Lee, 2009). According to their composition (Ni-rich: pentlandite>pyrrhotite>chalcopyrite), obviously, they have ultramafic nature. Later stage generation of sulphides was formed as the result of “binding” of liberating metal admixtures during transformation of the primary minerals of ultramafic xenoliths – Ni from olivine, chromespinels and other minerals or Zn from spinel.

Two generations of chromespinels were determined. The first - chromite I and spinel I (49 microprobe analyses), forming homogeneous grains and crystals. These chromium spinels represent practically continuous series from spinel (47.1 wt. % Al₂O₃) to Cr-rich chromite (up to 64.5 wt. % Cr₂O₃); minerals are poor in TiO₂ (<0.9 wt. %) and Fe₂O₃ (average - 3.0 wt. %). Such isomorphic series correspond to chromespinels of the lherzolites and dunite-harzburgites. They are only able to be present simultaneously in the small samples when forming ulrabasic xenoliths representing kimberlites or lamproites (Sobolev, 1974; Mitchell, 1995). The second generation – chromite II and spinel II (25 microprobe analyses), was forming on the spinel I in the form of grains with fine pores (pores are filled by phlogopite). These chromespinels are abruptly poorer (as opposed to primary spinel) in Al₂O₃, but richer in TiO₂ and FeO+Fe₂O₃. These peculiarities of chromespinels composition are characteristic for the series of mantle alkaline rocks – lamproites and orangeites (Mitchell, 1995).

From the lamproite sample of 51 kg of the studied diatreme 10 diamond crystals ranging by size between 1.0 and 1.5 mm were extracted by acid leaching (contract between KRC RAS and DeBeers,  07.08.2007) – (Gorkovetz e. a., 2009).

From the heavy mineral concentrates, 84 mono-mineral grains of pyrope of reddish-violet color were hand picked. The chemical composition of these garnets correspond to dominating Cr-rich Ca-low pyrope (50 grains from all 55 analysed) – Cr₂O₃ 4.5-5.8 wt. %, average - 5.3 wt. %; CaO 2.4-2.8 wt. %, average -  2.6 wt. %; Mg# = 0.824-0.884, average - 0.873, as well as Ca-pyrope (5 grains from all 55) - Cr₂O₃ 3.1-7.8 wt. %; CaO 4.4-5.7 wt. %; Mg# = 0.833-0.844. Cr-rich and Ca-low pyrope has close composition to pyrope of the harzburgite-dunite diamond bearing association (Sobolev, 1974) and Ca-low Cr-pyrope  (G10  group – inclusions in diamonds  and intergrowths with diamonds – Dawson, Stephens, 1975) of xenoliths in kimberlites. Ca-Cr-pyrope from diatreme correlates by composition with pyrope of the lherzolite association of the mantle ultramafites (Sobolev, 1974).

From the heavy mineral concentrates over 40 grains of chromium diopside of emerald-green color were hand picked as well. The pyroxene is represented by mono-mineral grain, or as intergrowths with talc-serpentine aggregates. This diopside (29 microprobe analyses) is Mg-rich (Mg#_avg. = 0.943), Cr₂O_3avg. 2.3 wt. %, Al₂O_3avg. 2.9 wt. %, Na₂O_avg. 2.2 wt. %, poor in TiO₂ (<0.4 wt. %). It contains 7 mol. % of kosmochlor and 8 mol. % of jadeite components and correspond to monoclinic pyroxene of the lherzolite association of phlogopite-containing inclusions in dimonds (Sobolev e. a., 2009).

Compositions of most Cr-rich chromites in lamproites of the studied diatreme (62-64.5 wt. % Cr₂O₃) correlate to compositions of chromite associating with diamonds (Sobolev, 1974; Sobolev e. a., 2009).

Obviously, rare grains of picroilmenite (11.7-13.8 wt. % MgO, 3.5 wt. % Cr₂O₃) and, possible, Cr-rutile (1.1 wt. % Cr₂O₃) which were determined in the heavy mineral concentrates could be attributed to xenocrysts of the ultrabasic paragenesis in kimberlites – (Sobolev, 1974).

According to chemical composition, associations of  rock-forming, accessory and secondary minerals, and peculiarities of their chemical composition, the rocks of diatreme should be described as heavily replaced olivine lamproites (Mitchell, 1995), saturated by xenoliths of ultrabasic rocks – heavily replaced spinel (garnet) lherzolites and harzxburgite-dunites.

The detection of diamonds and wide complex of xenocrysts of minerals of ultramafic diamond-bearing paragenesis confirms deep (level of garnet lherzolites) magma source and diamond bearing nature of the studied diatreme of Kostomuksha ore region.

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