2011

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Ðóäíûé ïîòåíöèàë ùåëî÷íîãî, êèìáåðëèòîâîãî

 è êàðáîíàòèòîâîãî ìàãìàòèçìà

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Lamproites of the Kostomuksha ore area, West Karelia

(mineralogy and 3D-method of investigation)

Rudashevsky V.N. *,  Gorkovetz V.Ya. **, Rudashevsky N.S. *, Popov M.G. **, Raevskaya M.B. **

*ÐÑ+ LTD, St. Petersburg, Russia, ** Geological Institute of Karelian Scientific Center of Russian Academy of Sciences, Petrozavodsk, Russia

vlad.rudashevsky@gmail.com

New compositionally alkaline and sub-alkaline intrusive rocks, including lamproites and kimberlites (Gorkovetz et al., 1981; 2007; et other) were recently discovered in the Kostomuksha area (West Karelia). More than 110 lamproite dikes ranging 0.5-4 m were discovered. These rocks are located in the rock packs of gimolsky series (2.8 billion years in age - Scherbak et al., 1986; magnetite quartzites, quartz-feldspar-biotite and C-containing fillite-like shales with pyrrhotite mineralization) and among komatiite peridotites which are crossing rocks of gimolsky series. The absolute age of lamproites (developing along phlogopite) is 1230 mln years (Gorkovetrz et al., 1981).

Two drill core samples (~2.5 kg each) of lamproites from two dikes ranging 1.6 to 2.1 m in thick were studied by means of newly developed “3-DIMENTIONAL” mineral processing technology (Rudashevsky et al., 2002; Rudashevsky, Rudashevsky, 2007; et other) followed by electron microscopy and microprobe analysis of polished sections of primary rocks, monolayer polished sections of the heavy mineral HS concentrates and grains after their hand picking from HS concentrates, in order to perform reconstruction of mineral paragenesises of above mentioned lamproites: 1) sample 597, located among rocks of gimolsky packs  and 2) sample 572, located among komatiites.

These rocks relate to the series of high-Mg (MgO 19.9-23.6 wt %), alkaline-K (K₂O 4.4-5.9 %, K₂O/Na₂O >30), Ca-poor and Al-poor (CaO 4.5-4.7 wt %; Al₂O₃  4.2–7.75 wt %), medium concentrations of SiO₂ (41.7-42.2 wt %) rocks, thus corresponding to classical lamproites (Bogatikov et al., 1991). Ka = (Na₂O+K₂O) /Al₂O₃ for sample 597 is 0.8, whereas for sample 572 it corresponds to 1.1.

These lamproites have a characteristic porphyry-like medium-grained texture (grain sizes 0.03-0.5 mm). The rocks are extensively replaced by secondary minerals (sample 597 – saponite and calcite, sample 572 – serpentine and calcite) in the mica matrix. Mica and calcite-saponite (serpentine) aggregates contain fine (<50 mm) inclusions of different accessory minerals. No other rock-forming minerals were found in polished thin sections of both samples. Heavy mineral HS concentrates are formed by different accessory minerals and, in less amount, by relicts of primary minerals of lamproites.

The reconstruction of the primary mineral paragenesises allows to determine that sample 597 is extensively replaced phlogopite-amphibole-sanidine basic lamproite and the sample 572 is extensively replaced phlogopite-diopside-olivine ultra-basic lamproite (according to nomenclature of A.O. Bogatikov et al., 1991).

The primary micas (phlogopite and tetra-ferriphlogopite) are high-Mg (MgO_avg. 22.3 wt %), Ti-rich (TiO_2avg. 4.9 wt %), Al-poor (Al₂O_3avg. 6.4 wt %), containing admixtures of BaO (up to 1.3 wt %). Amphiboles correspond the edenite-ferro-edenite isomorphic series and grunerite. Diopside is high–Mg, poor in Al₂O₃ and Al^IV. K-feldspar (sanidine?) contains Fe₂O₃ admixture. Olivine has high-Mg composition: mg = Mg/(Mg+Fe) = 0.92-0.94.

Accessory minerals of the sample 597 are the following: anatase (Nb₂O_5avg. 1.2 wt %), rutile (Nb₂O_5avg. 1.3 wt %), apatite (SrO_avg. 3.2 wt %; CeO_2avg. 0.7 %), calcite-Sr (SrO_avg. 8.2 %), monazite-(Ce,La,Nd,Pr), chromespinels, barite, henrymeerite (Ba,K)(Ti,Fe²⁺)₈O₁₆, priderite(?) (K,Ba)_1+x(Ti,Fe²⁺)₈O₁₀, baotite (Ba,K)₄(Ti,Nb)₈Si₄O₂₈Cl, alstonite/ baritocalcite BaCa(CO₃)₂, strontianite-(Ca,Ba,Ce), zirconium Na-Ca- è Ca-K-silicates, magnetite («bubbles»), sulphides (pyrite, chalcopyrite, pentlandite, galena, sphalerite, millerite), native iron (“microdroplets” and irregular-shaped grains), (Fe,Cr,Ni) and (Cu,Ni) alloys, wüstite, native copper and tin; the sample 572 – chromespinels, ilmenite-(Mg), geikielite, ilmenite-(Mn), apatite (SrO_avg. 2.0 wt  %; CeO_2avg. 0.7 wt  %), magnetite, baddeleyite, zircon, barite, anatase (Nb₂O₅ 0.6 wt  %),  henrymeerite, hyalophan (K,Ba)(Si,Al)Si₂O₈, perovskite-(Ce-Nb-Sr-Nd), unnamed CuLa₂O₄, sulphides, arsenides and arsenide-sulphides (pyrrhotite, pentlandite, chalcopyrite, galena, sphalerite, niccolite, maucherite, cobaltite, and gersdorffite; «microdroplets»  and irregular-shaped grains), native cupper and tin.

The primary rock-forming silicates (micas, diopside, olivine and K-feldspar) and main accessory minerals (chromespinels, apatite, anatase, rutile) are whose mineralogical characteristics match those of the Western Australia diamond fields, of classical lamproites from USA, Spain, Greenland, Africa and Antarctic. In other words barium, strontium, zirconium, and rare-earth accessory minerals, as well as rare-metal admixtures in apatite, anatase, rutile, perovskite, and calcite are very characteristic signatures of the rare-metal–rare-earth specialty of the studied samples corresponding to all lamproites (Jaques et al., 1986; Bogatikov et al., 1991).

By hand picking from heavy mineral HS concentrates of both lamproite samples the following xenocrystals derived from different deep earth rocks were collected: à) chromium-diopside (kosmochlor 5-7 mol. %, jadeite 6-10 mol. %) with admixture of K₂O_avg. 0.33 wt % (sample 597); b) pyrope (sample 597 - mg = 0.87, Cr₂O₃ 7.3 wt %, CaO 4.5 wt %); d) chromespinels, including high-Cr (up to 66.5 wt % Cr₂O₃), e) ilmenite-(Mg) and geikielite (sample 572); f) almandine; g) corundum (sample 597). Obviously the majority of these have the same source of mantle xenoliths, and almandine is captured from metamorphic rocks of the crystalline foundation (Sobolev, 1974). These xenoliths prove mantle source of magmatic melt for studied lamproites and possible finding there of diamond-bearing xenoliths.

However, it is not possible to ignore such mineralogical peculiarities of these rocks, which are not characteristic of classical lamproites such as: in the sample 597 - 1) the presence of edenite and grunerite in the matrix instead of typical for lamproites such as high-Mg diopside and Ti-K-richterite (Jaques et al., 1986; Bogatikov et al., 1991); 2) the availability of magnetite «bubbles»; 3) the presence of native iron (including «microdroplets»), grains of (Fe,Cr,Ni) and (Cu,Ni) alloys; in the sample 572 - 1) presence of sulphide «microdroplets» (pyrrhotite+pentlandite+galena; chalcosine+Cu); and 2) presence of Ni-arsenides (niccolite and maucherite, including their «microdroplets») and Ni-Co-sulphide-arsenides (cobaltite, and gersdorffite). The change of composition of the lamproite matrix of the sample 597 may be a result of an increase in the activity of silica and growth of iron content of a primary melt of the parent magmas and contamination by xenoliths from host rocks (iron quartzites and quartz-feldspar-biotite shales with carbon and sulphide mineralization). The incorporation of carbonaceous shale by lamproite magma formed a local reducing fluid and crystallization of native metals, first of all native iron. Lamproites of the sample 572, obviously was getting enriched by Ni, Co and As as the result of contamination of fragments of hosting komastiites. High temperature nature of interaction between lamproite magma and hosting rocks is documented by occurrence of microglobules («microdroplets» of melt) of magnetite, native iron, sulphides and niccolite.

The differences in chemical and mineralogical compositions of the studied lamproite samples, obviously can be explained as result of combined influence of deep earth differentiation of primary melt (Mitchel, 1988), and contamination of side rocks hosting lamproites by matrix magma.

The use of newly developed “3-DIMENTIONAL” mineral processing technology was able to characterize 38-40 different minerals, including those considered most characteristic for lamproites even though these rocks were completely replaced by secondary silicates.  In contrast, only 16-18 minerals were found in polished sections of the primary sample.  Such complex mineralogical results, preserving unique ontogenetic evidence of minerals, allows for reconstruction of the mineral paragenesises for all stages of formation as well as providing important data on the potential diamond-bearing nature of these lamproites.  Additionally, we have found two unusual minerals [CuLa₂O₄ and K(Ti,Fe)₆O₁₂] that may be new species, currently under study.

The studied lamproites, as well as kimberlites of Fenno-Karelian province (Gorkovetz et al., 2007) could represent new potential source of Russian diamonds.

References:

Bogatikov Î.À., Ryabchikov I.D., Kononova V.A. et al.  Lamproites. 1991.Ì. Nauka. 302 p. (In Russian).

Gorkovetz V.Ya., Rayevskaya M.B., Belousov E.F. et al. Geology and metallogenesis  of the Kostomuksha iron-ore deposit region. Petrozavodsk, «Karelia», 1981. 143 p. (In Russian).

Gorkovetz V.Ya., RayevskayaM.B., Popov M.G. et al. Prognosis of diamond-bearing nature of rocks of Karelian region of Fennoskandian shield // Geodainamics, magmatism, sedimentaiom-genesis and mineral-genesis of North-West Russia. Geological Institute of Karelian Scientific Center of Russian Academy of Sciences, Petrozavodsk. 2007. Petrozavodsk. P.110-113. (In Russian).

Jaques A.L., Lewis J.D., Smith C.B. The kimberlites and lamproites of Western Australia // GSWA Bulletin 132 / Translation, edited by N.V. Sobolev. 1989. Moscow. Mir. 430 p. (In Russian).

Mitchell R.H. Lamproites – family of alkali rocks // Proceeding of the Russian Mineralogical Society. 1988. Part 117. No 5. P. 575-586. (In Russian).

Rudashevsky N.S., Rudashevsky V.N. Device for separation of solid particles // Russian Patent (utility model) ¹69418. Russian Federation. 2007. (In Russian).

Sobolev N.V. Deep-earth inclusions in kimberlites and problem of the composition of upper matle. Novosibirsk: Nauka. 1974. 264 p. (In Russian).

Scherbak N.P., Gorkovetz V.Ya., Dodatko A.D. et al. Scheme of correlation of strartigraphy sections of iron-silica formations of pre-cambrian  rocks of the European part of USSR // Geological Journal. 1986. Vol.46. No 2. P. 5-17. (In Russian).

Rudashevsky N.S., Garuti G., Andersen J.C.Ø. et al. Separation of accessory minerals from rocks and ores by hydroseparation (HS) technology: method and application to CHR-2 chromitite, Niquelândia, Brazil // Trans. Inst. Min. Metall. (Section B: Appld. Earth Sci.). 2002. Vol. 111. P. B87-B94.