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

Ermolina O.S.

Institute of Mineralogy ub ras, Miass, Russia

A_lira@rambler.ru

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

Mn-rich eclogitic garnets from Carnian stage (Upper Triassic) sedimentary deposits of the northeastern margin of the Siberian platform

Nikolenko E.I. Lobanov S.S.