Ultrapotassic basic volcanites of the Enganepe Uplift (the Polar Urals) and noble-metal mineralization in these rocks

Onishchenko S.A.¹, Soboleva A.A.², Varlamov D.A.³

¹ Gold Minerals, Syktyvkar, Russia; ² Institute of Geology, Russian Academy of Sciences Komi Sc. Centre, Ural Branch, Syktyvkar, Russia; ³ Institute of Experimental mineralogy, Russian Academy of Sciences, Chernogolovka, Russia

dima@iem.ac.ru

 

The volcanic flow of ultrapotassic basic volcanites is studied in the southern part of the Enganepe Uplift in the basement of the Uralides rock sequence. Rocks with so high concentrations of potassium (K₂O up to 9 wt. %) are interesting not only because of uniqueness for this region but also owing to detected non-sulfide noble metal mineralization presented by tiny segregations of gold and platinum in association with copper, mercury, silver, thallium and antimony.

The investigated rocks are exposed near the interflow of the Right Iz’yavozh and Left Iz’yavozh rivers on a right bank of the Right Iz’yavozh. The volcanic flow occurs conformable with general bedding of enclosed rocks and is traced on nearby 60 m. The thickness of the flow changes from 3 m in a southwest part of the exposure (near the water line) to 0.6 m in a northeastern direction (in the brow of a slope). Volcanites lie 6-7 m above the surface of sharp unconformity of Uralides and Pre-Uralides. Basal conglomerates of the Late Cambrian-Early Ordovician Manitanyrd formation overlap the Late Neoproterozoic (Vendian) metasediments of the Enganepe formation. Volcanites are located among lilac-grey fine-grained sandstones and siltstones of the Manitanyrd formation. V.A.Dushin (Dushin, 1986) has defined these rocks as epileucitic tephrites and has refered them to Iz’yavozh alkaline-basaltic complex. Ya.E.Yudovich (Yudovich et al., 2002) considers that originally these rocks were close to alkaline picrites and contained leucite and then have undergone deep-seated potassic and carbonic metasomatism that has resulted in replacement of earlier crystallized minerals by orthoclase, hydromica and carbonates.

Usually rocks are amygdaloidal, less often - massive. Amygdules compose 5-20 vol. % of rock and consist of dolomite; calcite, sericite, quartz and chlorite are less abundant. They also contain epidote, hematite, barite, and thread veinlets of anhydrite in calcite. The rocks contain phenocrysts (5-10 vol. %) of feldspar completely replaced by sericite and carbonate. Euhedral phenocrysts of mafic minerals are also present. Possibly it was clinopyroxene replaced by chlorite. The matrix of the rocks consist of feldspar laths replaced by orthoclase, sericite and albite, and also euhedral  and subeuhedral grains of mafic minerals pseudomorphically replaced by chlorite, sericite, carbonate and quartz. The rocks have relict intersertal and poikilophitic texture (if the laths were primary plagioclasic), or trachytoid texture (if they initially consisted of soda-potash feldspar). Pseudomorphs of mafic minerals make up to 10 vol. % of the rocks. In matrix there are also recrystallized glass transformed into feldspar and quartz.

Table 1. Chemical composition of the ultrapotassic basic volcanic rocks, wt. %

 

 

Remark: massive texture rocks from the middle of the volcanic flow (2116-03, 2116-04, 2116-05); amygdaloidal texture rocks from the upper part of the volcanic flow (2116-09, 380, 317). 380, 317- Yudovich et al., 2002; Nikulova et al., 2007

Now rocks contain (vol. %) potash feldspar (35-40), sericite (5-15), carbonate (5-20), albite (5-20), chlorite (5-10), Fe-Ti oxides (5-8), quartz (3-5). We can only assume their primary mineral composition. A chemical compound of rocks (Table 1), results of its recalculation into normative mineral composition, and also petrographic observations allow to suppose that primary rocks has been composed by mainly soda-potash feldspar with the subordinated quantity of plagioclase. Both feldspars were presented by lath grains. Pyroxene made about 10 vol % of the rock. It is impossible to exclude also presence of primary olivine. The leucite content could reach up to 10 vol.%.

Characteristic components of rocks are iron-titanium minerals. They are presented mainly by ilmenite, much less often ulvospinel. Also the Zn-bearing chromespinel is found. Ore minerals have undergone high-temperature oxidation that resulted in replacement of ilmenite by a mineral phase, similar to pseudorutile. Ulvospinel has been replaced by phases of ilmenite-hemoilmenite-hematite series, and chromespinel grains have got rims with the high content of the titanium. Unusual case-like crystals of Ti-bearing hematite are widespread in these rocks, bordering crystals of the former pyroxene (replaced by dolomite and/or potash feldspar + sericite aggregate).

Accessory minerals are presented by numerous idiomorphic grains of (Ce-Nd)-monazite and fluorapatite, xenomorphic barite (which contain up to 2 wt.% SrO), also small (10-20 mk) grains of zircon and baddeleyite and  rare segregations of xenotime.

Presence of noble-metal mineralization of various compositions is established in volcanic rocks. Basic components are copper, gold, mercury and platinum, and minor are antimony, thallium, and silver. The quantity of micrograins is sometimes great enough - up to 30-40 per 1 mm², however, bulk analyses have not shown industrial concentrations of elements (the content of gold does not exceed 3 ppb). As a rule, the sizes of micro-segregations of minerals do not exceed several micrometers. They are located on joints of grains and at borders of tiny veins, amygdules, ore minerals (ilmenite and case-like Ti-hematite), in carbonate grains from amygdules and in carbonates which partially fill “cases” of ore minerals. These noble-metal segregations haven’t been found as usual solid-state inclusions in minerals. Shapes of micrograins are various - roundish or isometric less often extended or jagged. Composition of micrograins is very unstable (taking into account that their sizes are often less than zone of excitation of a microprobe), however, multiphase structure of microphases is visible sometimes. Most often there are intermetallic phases of Cu-Au nonstoichiometric composition (up to almost pure gold, occasionally with impurity of silver, average fineness - 550-700‰), segregations of Cu-Au-Hg composition, less often - almost pure mercury (globules) and platinum. Also are found out rare individual segregations of phases with different composition: Au-Hg, Pt-Hg, Cu-Au-Pt, Au-Pt-Hg etc. In some micrograins of platinum the considerable impurity of antimony and iron is established. Also the thallium impurity in some globules of mercury is revealed. We have found also two micrograins of high-tantalum phase - to 80-90 wt. % Ta (with influence of the matrix).

Compositions of the studied phases are very unusual (for example we found Cu-Au phase in one grain with platinum, phases of Au-Hg-Pt composition) and are unstable. Associations of elements are rather atypical for deposits and ore occurrences of noble metals. Along with their localization in deformation microstructures of rocks (cleavage, cracks, emptiness, intergrain borders, etc.), it allows to assume their genesis in obviously non-equilibrium conditions at the latest stages of formation of a volcanic flow or even later – in processes of latest transformations. Native character of the mineralization is caused, apparently, by the lowest activity of sulfur. Absence of sulfides in rocks also testifies to this. The considerable quantity of “volatile” ore elements (Hg, Tl) specifies possible sedimentation of metals from a gas phase in a mode of "blowing" already formed lava flow by thermal fluids at finishing stages of existence of the magmatic chamber.

 

This study was financially supported by integration project of the Uralian, Siberian and Far East branches of RAS № 09-И-05-2002 and RFBR grant № 09-05-00991

 

References

Dushin V.A. Early Paleozoic alkaline-basaltic volcanism in miogeosyncline area of Polar Ural // Transactions of Academy of Sciences USSR (Geology), 1986. 286. № 2. С. 398-401 (in Russian)

Yudovich Ya.E., Makhlaev L.V., Ketris M.P. The ultrapotassic basaltoid in basement of the Uralides in Enganepe Ridge // Geochemistry of ancient rock series of the northern part of the Ural // Syktyvkar, Geoprint, 2002. p. 158-161. (in Russian)

Nikulova N.Yu, , Yudovich Ya.E., Shvetsova I.V. Lithology and geochemistry of rocks of interformational contact zone on the Iz’ya-vozh river (the Polar Ural)  // Reports of Komi SC Ural Branch of RAS; Syktyvkar, 2007,  Vol. 491, 40 p. (in Russian)