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Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Ultrapotassium volcanites and native oxide mineralization in them (South Timan)

Udoratina O.V.*, Varlamov D.A.**, Kulikova K.V.*, Savelyev V.P.***

*Institute of Geology Komi SC UB RAS, Syktyvkar, Russia

**Institute of Experimental mineralogy RAS, Chernogolovka, Russia

***OJSC Ukhtageoservis, Ukhta, Russia

udoratina@geo.komisc.ru

The thin white layers with volcanogenic genesis have been found at South Timan (within Yarega deposit) in core material of boreholes drilled in Dzherskaya (D₃dz) formation. The formation is composed of two strata: volcanogenic sedimentary yaregskaya stratum (basalts, tuffs and tuffites, layers and members of argillites, sandstones, aleurolites), thickness 0-120 m, and terrigenous one (sandstones, argillites, layers and lenses of conglomerate breccias) with thickness 45 m. Thin (generally the first centimeters at maximum 10-15 cm) white layers are contrasting among grey argillites and aleurolites, but the enclosing rocks are not subject to any visible changes. The analysis of the material from various boreholes confirms that thin layers are completely altered, however the thicker volcanogenic layers contain these white varieties in the lower part.

The thin light interlayers (homogeneous volcanogenic injections), with distinct boundaries with enclosing rocks, often show amygdaloid  texture. The amygdales compose about 8% of rocks and have roundish or irregular ameboid shape. Their size varies 0.8-3 mm. The amygdales have a complex filling; carbonate of several generations and amorphous substance (analog of opal with sulphide impurity) are observed; in many amygdales the marginal inner part is filled with muscovite. Microscopically the rocks are characterized by porphyric, glomeroporphyric structure and trachyte basic texture.  The porphyric, often glomeroporphyric inclusions (aggregates of two-three crystals) are represented by crystals of K-feldspar (KFS) of tablet form with size not more than 0.2-0.6 mm. A part of the inclusions is substituted with carbonate. The basic tissue is composed of long microlites (lathes) of KFS, interstitions are filled with glassy mass.

The amygdaloid texture, porphyric structure, glass relics – all these clearly testify to that the primary rocks were volcanic – basalt?. However the presence of KFS and muscovite/sericite suggests that the protholite was subject to powerful process of K-metasomatosis. Considering the metasomatic version, it is hard to explain why K-metasomatosis “avoids” the amygdales, they never show KFS, however all the microlithic basic mass and inclusions are composed of them.

The microprobe studies confirmed the petrographic characteristic of the rocks and thin dispersed specific ore mineralization was determined, that was similar to previously characterized one for alkali basic volcanites (Varlamov et al., 2010).

The amygdales are filled with zonal ankerite (with varying Ca:Mg:Fe  ratios) with inclusions of high magnesium ankerite. K-spar, both in the inclusions and in microlathes, corresponds to KFS (?) and is stable by the composition. It does not contain considerable Na, Ca, Ba impurities. The composition of KFS is extremely clear K₂O to 16 wt. %, sometimes there are compositions with Na₂O – 0.04-0.4 and CaO – 0.11-0.17. Glasses are alumosilicate (Al:Si ≈0.9), without other components (sometimes up to 5-6 wt.% TiO₂), possibly with large quantity of water (up to 15 wt.%).

In the rocks the presence of several types of ore mineralization of varios compositions was determined – sulphides, intermetallides, oxides. The nain phases of sulphides – rather large khalkopyrite and pyrite, rarer galenite. The intermetallides and oxides are represented by small, individual (to 5-10 per polished section) micrograins – copper gold (to 3 mcm, to 20at% Cu), wustite (to 10 mcm as balls, sometimes with faceting elements, molibdenite, intermetallide Fe, W, Zn and Sn (or possible their carbides – this requires additional studies). Generally the sizes of microstructures do no exceed first micrometers, they are localized at grain contacts, boundaries of interlayers, amygdales, in carbonates of filling amygdales. They were not observed as general solid phase inclusions in minerals. The shape of the micrograins - roundish isometric, rare elongated or hackly. The mineral associations testify to highly reductive character of mineralogenetical environment. Together with confinedness of intermetallides to deformational structures of the rocks (cuts, fractures, voids, intergrainular boundaries and others) it suggests their genesis in certainly non-equilibrium conditions at the latest stages of formation of the volcanic bodies or later – in the process of their latest transformations. Contrary to the previously described similar micromineral associations they contain sulphides that seem to decrease the number of intermetallide-oxide phases in the rocks.

Petrochemically the studied rocks are characterized (Table 1) by SiO₂ content similar to basalt 48.50 wt.%, but K₂O is 9 at insignificant content of Na₂O, high content of TiO₂ – 1.38.

Table 1. Chemical compositions (wt %) of South Timan basalts and ultrapotassium rocks

Notes: South Timan: 2 – basalt, 03-02-02 – ultrapotassium rocks

In Yarega basalts SiO₂=48.95 wt.%, K₂O is very small, 0.21 at Na₂O content 5.11 wt.%, TiO₂ – 0.84 wt. %.

The composition of impurity elements in ultrapotassium rocks is represented in the table 2.

 Table 2. Composition of impurity elements (ppm) in the ultrapotassium rocks from the South Timan.

Notes: element composition was determined by ICP MS LA, RGRI, Saint-Petersburg)

The total concentration of REE is 91.64 ppm. The sample generally shows decreasing normalized values from light lantanoids to heavy lantanoids (La_n=27.81, Lu_n=10.25), but the enrichment is observed in the medium spectrum (Nd_n=36.33, Sm_n=36.77). Normalized by khondrite (C1, Boynton, 1984) distribution spectra have arched form, and clear Eu minimum is observed. Spider-diagram (normalized on N-MORB, Sonders, Tarni, 1987) is characterized by sharply increased Rb and K content, related to normal values, considerable Sr decrease and insignificant enrichment in Ba related to N-MORB basalts. The enrichment of potassium and affiliated rubidium is connected, by our opinion, with overimposed metasomatic transferomation on the primary rocks. This enrichments seems to be related also to the impoverishment of ultrapotassium rocks with europium.

Can these rocks be primarily magmatical, for example alkaline basalts or trachytes, or they are metasomatically reworked basalts? Similar rocks with magmatical and (or) metasomatical by genesis are known in Middle Timan for a long time, and opinions about their genesis are different (Makeev vet al., 2006, Udoratina et. al, 2010). However ore minerals and of previously studied dyke “ultrapotassium trachytes” (Udoratina et. al, 2010). Are strikingly different from the above cited. Therefore we suggest that the rocks, studied by us, are related to previously described feldspatolites with metasomatical genesis. The presence of a very unusual ore mineralization (intermetallides, native elements, wustite, sulphides), determined in the studied structures, together with other data, allows reliable confirmation of primarily volcanogenic genesis of the rocks under study, possibly overimposed by K-metasomatosis with unclear source.

References:

Varlamov D.A., Onishchenko S.A., Soboleva A.A.  Noble metal mineralization in ultrapotassium volcanites of basic composition in Enganepe Uplift (Polar Ural) // “Geomaterials for high technologies, diamonds, noble metals, semiprecious stones of Timan-Northern Ural region”” Proceedings of All-Russia mineralogical seminar. – Syktyvkar: Geoprint, 2010, pp. 121-124.

Makeev A.B., Lebedev V.A., Bryanchaninova N.I. Magmatites of Middle Timan. – Ekaterinburg, UB RAS, 2008. 348 pp.

Malkov A.B., Phillipov V.N., Shvetsova I.V. Timanite – unique high titanium ultrapotassium variety of trachyte: Middle Timan, Late Paleozoic // Vestnik of Institute of Geology Komi SC UB RAS, 2006. No.2, P.13-21

Sonders F.D., Tarni J. Geochemical characteristics of basalt volcanism in back-arc basins. In: Geology of marginal basins. Moscow. Mir. 1987, p. 102-133.

Boynton W. V. Geochemistry of Rare Elements Meteorite Studies // Rare Earth Element Ceohemistry. Amsterdam, 1984. P. 63–114.

Udoratina O.V., Burtsev I.N., Kulikova K.V., Varlamov D.A. Ultra-potassium Trachytes from Middle Timan // Geochemistry of magmatic rocks-2010: Abstracts of XXVII International Conference School “Geochemistry of Alkaline rocks”– Moscow-Koktebel, 2010. p.211–213.