Redistribution of petrogenic elements under speleolithogenesis in Okhotnichiya cave

(Baikal region, Irkutsk oblast)

Bazarova E.P. ¹, Markova Yu.N. ²

¹ Institute of the Earth Crust SB RAS; ² Institute of Geochemistry SB RAS, Irkutsk, Russia

[e-mail]

Mineralogy and geochemistry of the caves are interconnected and relatively insufficiently known, new and recently developed scientific branch. The caves are represents the accessible for a man cavities in the upper part of the Earth crust, which are having unlighted parts and the length (the depth) bigger than two other dimensions. Cave mineral is recognizing as homogeneous solid substance with a definite chemical composition and well-ordered atomic structure normally founded in a cave. Cave minerals are often forms a different speleothems which are the secondary mineral formations, originated in the cave environment from the primary mineral as the result of physicochemical reactions. Well-known stalactites, stalagmites, draperies, cave pearls, moondmilch (lunar milk), flowstone cores and many others are relating to such formations. Cryomineral formations of the caves are also should be taking for the cave minerals, which are originated as a result of chemical equilibrium change of aqueous solution due to its supercooling which is giving rise to crystallization of earlier dissolved components. Cave minerals are allowed to judge about rock composition, which are forming the karsting massif and geochemical researches of secondary mineral formations provides the information about a little known for the present time processes of chemical elements loss and redistribution in natural underground cavities.

Discovered at 2006, Okhotnichiya cave is the third by the length cave of Baikal region at present time [4]. The cave located at oncolyt and stromatolite limestones and dolomites of uluntuiskaya strata of Late Proterozoic. Right away the entrance is a large grotto location of 15 – 20 m width and the height of 5 – 10 m in which for the moment of discovery there were long-term frazil with the area of 4 m².There is no the frazil at present time. The cave extension is 5700 m with the amplitude of 77 m [4]. The cave is representing the galleries system of a height nearly 25 m with the average of 8-10m and width of 1-15 m with the average of nearly 3 m. The secondary formations of a different genesis are widely spread in the cave, among them: residual, collapsed, aqueous mechanical, aqueous chemogenic, cave snow and ice (seasonal formations) and organogenic, the bones of ancient animals are referred to it [2]. Aqueous chemogenic formations of the cave are formed by calcite, aragonite, monohydrocalcite and gypsum. In the area of expansion of seasonal glaciations there were found the cryomineral formations, formed by the rare mineral ikaite (CaCO₃·6H₂O) [1].

It is known, that the composition of secondary minerals of the cave is controlling by the composition of bringing solution, which is, in it’s turn, depends on the composition of enclosing rocks. Moreover, the mechanism of mineral aggregate formation is of a high significance, and in a certain cases is of a decisive importance. The authors were used the classification of Stepanov-Maltsev [3, 5] based on the method of active solution feeding and operating with such the notions as cores, for the classification of aqueous chemogenic formations of Okhotnichiya cave. According to the classification, there are aggregates of gravitational, subaquatic, corallite and antolite cores are divided, and the mineral composition is given in [1].

The authors investigated the redistribution of petrogenic oxides under the secondary mineral formation in the cave. It was determined that under the formation of aggregates of gravitational cores occurs the loss of SiO₂, TiO₂, Al₂O₃, Fe₂O₃, MgO, Na₂O, K₂O and accumulation of SO₃, small accumulation of P₂O₅, and СаО and СО₂ are inactive. During the formation of aggregates of corallite’ cores occurs insignificant redistribution of SiO₂, TiO₂, Al₂O₃, Fe₂O₃, MgO and P₂O₃ and large accumulation of SO₃. The concentration of СаО and СО₂ is practically permanent, the concentration of К₂O in common is also corresponds to the enclosing rocks, and Na₂O is insignificantly caring out.

Thus, the aggregates, formed under more active inflow of solution are the most “pure” in a chemical term. The greatest number of impurities is observed in calcite of corallite cores, which formed under the less active feeding of solution and thin films motion of water under the influence of capillary forces. The presence of SiO₂, TiO₂, Al₂O₃, Fe₂O₃, MgO and P₂O₃ in corallites is also can be connected with clay admixture, trapped by the mineral aggregates during the growth process. The concentration of MgO in corallite composition is relatively increased in comparison with the enclosing rocks, which is confirm the author’s supposition about monohydrocalcite formation due to magnesium ions appearance in karst water.

References:

1. Bazarova E.P., Gutareva O.C., Kononov A.M., Uschapovskaya Z.F., Nartova N.V., Osintsev A.V. Minerals of the Okhotnichiya cave (Baikal region, Irkutsk oblast)// Speleology and karstology, 2011. №7. p. 5 – 14.

2. Klementev A.M., Korshunov E.O., Osintsev A.V. Okhotnichya cave - a new location of fossil fauna at Primorskiy range (Western PreBaikal’e) // Proceedings of the laboratory of ancient technologies. Irkutsk: ISTU Publishing. 2007. Ed. 5. p.146-153.

3. Maltsev V.A. Minerals of karst system of Kup-Kutan caves (south-east of Turkmenistan)//World of Stones, 1993, №2 <vl-maltsev.narod.ru> accessed 2011 May 31.

4. Osintsev A.V. Large caves of Baikalskiy region – newest researches// Speleology and spelestology: the development and interaction of the sciences. Abstract book of international scientific-practical conference. – Nabereznie Chelny:NGPI - 2010. p. 99 - 101.

5. Stepanov V.I. The periodicity of crystallization processes in karst caves// Fersman mineralogical museum proceeding/ - Ed. 20- Мoscow, 1971. p. 161 - 171

Gold-bearing ore mineral association of Piilola occurrence (Kuhmo greenstone belt, eastern Finland)

Ermolina O.S.

Institute of Mineralogy UB RAS, Miass, Russia

[e-mail]

Archean greenstone belts, which can be found both within the Finnish and Russian sectors of the Fennoscandian Shield, interesting because of their gold potential that connected with mesothermal (orogenic) type of deposit.

The object of study was Piilola ore occurrence, located in the central part of the Archaean greenstone belt Kuhmo (Eastern Finland). The material was collected at core depot Geological Survey of Finland. The aim is to characterize ore mineralization of the central part of Kuhmo greenstone belt. Objectives are to characterize host rocks, to define mineral association, to define the morphology of gold, to study chemical composition of gold and associated minerals.

Archaean greenstone belt Kuhmo, whose age is estimated at 2800-2750 Ma, is characterized by the meridional strike. Rocks of greenstone belt are presented by volcanic rocks that have been metamorphosed to epidote-amphibolite facies of regional metamorphism (from upper greenschist to amphibolite). Frame rocks are granite-gneiss complex with relics of metamorphosed Fe-tholeiitic basalt that modified to banded amphibolite migmatite [3].

There are 15 ore occurrences within Kuhmo greenstone belt [5]. The most interesting occurrence is Piilola occurrence. In general, the site has monoclinal structure with eastern dip. In the central part of the area (boreholes R429, R423, R427, R425, R428) the structure is more difficult because of numerous veins of granite, zones of granitization and mylonitization.

The section presents chlorite-tremolite-biotite, biotite, mica shists, amphibolites, complex granitoids bodies [2]. Productive gold zones are associated with mica, mostly biotite, shists with various dip angle and dip azimuth, located between big granitic veins. Gold content that exceed 1 g / t are found out in most drill holes, which including exocontact of granitic "core" with shists.

Increased gold grades are associated with sulphide mineralization. Depending on the prevailing sulphide minerals are divided into two associations 1) mainly pyrrhotite association and 2) pyrrhotite-arsenopyrite association.

Fig. Shapes of Native gold aggregates. SEM REMMA-202M (analyst V. A. Kotlyarov)

The dominant mineral of first association is pyrrhotite, which forms euhedral and anhedral crystals. The size of crystals changes from 0,001 to 2-2.5 mm. Pyrrhotite is presented as disseminations in rocks and cement for silicate minerals. Big aggregates show a porous structure and fractures. Pyrrhotite is characterized by beige color with distinct pleochroism from beige color to brownish one. Anisotropy is different, depends on the orientation of the units and ranges from unnoticeable to evident in the greenish-gray. As a result of pyritization of pyrrhotite some pyrrhotite crystals have "bird's eye" structure. The second common mineral is chalcopyrite. Bright yellow aggregates of chalcopyrite are presented both as inclusions in pyrrhotite and intergrowths with pyrrhotite, and as a separate units in the shists. The size of aggregates does not exceed 0.5 mm. Pentlandite forms elongated aggregates that oriented according to the cleavage of pyrrhotite. The size of units to lengthen no more than 0.05 mm. The color is straw-yellow similar to pyrite. Sphalerite presented as single inclusions in pyrrhotite and as intergrowth with pyrrhotite. The size of sphalerite aggregates is less than 0.01 mm.

The second association includes not only pyrrhotite and arsenopyrite but also chalcopyrite, pyrite, sphalerite, native gold and native bismuth. Number of pyrrhotite varies from 3 to 15%. Pyrrhotite forms isometric and irregular aggregates. The size changes from 0.01 to 1.5 mm. Also pyrrhotite develops along the edges of arsenopyrite aggregates and as inclusions in arsenopyrite. Pyrrhotite is characterized by brownish color with a distinct pleochroism, by fractures, by anisotropy in the brownish-gray color. Arsenopyrite forms aggregates, ranging in size from 0,005 to 2-2.5 mm. Euhedral crystals predominate. There are star-shaped twinnings. Chalcopyrite forms inclusions in pyrrhotite, arsenopyrite as well as intergrowths with pyrrhotite and separate units. The size of aggregates change from 0.01 to 0.5 mm. Isometric forms of units is mostly predominated. Sphalerite is presented as individual inclusions in pyrrhotite.

Native gold forms angular aggregates, isometric and elongated shape. Some grains show crystal form (Fig.). Native gold is presented in the silicate matrix and as inclusions in arsenopyrite crystals. Intergrowth native gold with native bismuth was described. The size of units varies from 0.09 to 0.30 mm. Microprobe analysis results show that native gold contains silver and copper as impurities.

For the first time in Piilola ore manifestation native bismuth and maldonite were described. Native bismuth occurs as inclusions in arsenopyrite crystals. The aggregates are characterized by angular shape. The size of units up to 50 microns. Microprobe studies showed that the impurities in the bismuth are absent. Native bismuth forms intergrowths with native gold. In reflected light native bismuth is characterized by pale-gray color with a pinkish tint, by isotropy.

Maldonite was found jointly with native bismuth. Maldonite is presented as inclusions in arsenopyrite. Aggregates have an angular shape. The size is about 10 µm. Silver and copper are determined as impurities at the crystallographic position of gold.

Thus, native gold associated with arsenopyrite, pyrrhotite, chalcopyrite, pyrite, sphalerite, native bismuth and maldonite. Angular anhedral form of native gold and native bismuth aggregates perhaps due to crystallization of the relatively low temperature melt among the thermally more stable silicate phases [1,4]. For the first time in Piilola ore manifestation native bismuth and maldonite were described.

The author is thankful to the Mineral Exploration Network (Finland) Ltd. for help.

References:

1. E. V. Belogub, V. P. Moloshag, K. A. Novoselov, V. A. Kotlyarov. Native bismuth, tsumoite and lead-bearing variety of tsumoite from Tarnierskoye copper-zinc massive sulfide deposit (The North Urals). Zapiski RMO, N 6, 2010. P. 108 – 119.

2. Ermolina O. S., Novosyelov K. A. Petrography of the rocks from the western part of the Ilomantsi greenstone belt, Finland // Metallogeny of ancient and modern oceans – 2010. Ore potential of spreading and island arc structures. Scientific edition. Miass: IMin UB RAS, 2010. P. 271 - 274.

3. Luukkonen E., Halkoaho T., Hartikainen A. Ita-Suomen arkeeiset alueet – hankken (12201 ja 210 5000) toiminta vuosina 1992-2001 (Suomussalmen, Hyrynsalmen, Kuhmon, Nurmeksen, Rautavaaran, Valtimon, Lieksan, Ilomantsin, Kiihtelysvaaran, Enon, Kontiolahden, Tohmojarven ja Tupovaaran alueela. 2002. 265 p.

4. Tomkins A. G., Pattison D. R. M. and Frost B. R. On the Initiation of Metamorphic Sulfide Anatexis //Journal of Petrology Advance Access originally published online on December 6, 2006

5. http://en.gtk.fi/index.html

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