Manganoan Ilmenite from the Säviä Ore Deposit, Pielavesi, Central Finland

MANGANOAN ILMENITE FROM THE SÄVIÄ ORE DEPOSIT, PIELAVESI, CENTRAL FINLAND LEA AHO AHO, LEA 1977: Manganoan ilmenite from the Säviä ore deposit, Pielavesi, Central Finland. Bull. Geol. Soc. Finland 49: 25—31. Electron microprobe analyses of nine ilmenites from a sulphide ore deposit at Säviä, Pielavesi are given. For comparison, two ilmenite analyses, one from an ore erratic and one from Säviä schists, are included. The MnO content of the ilmenites from the sulphide ore is as much as 16.6 wt. per cent. The manganoan ilmenite forms a myrmekitic intergrowth with sulphide minerals. Associated minerals are magnetite (0.3 wt. per cent TiO»), cassiterite, ferroan gahnite (54.3 mole per cent ZnAlgOi) and garnet (30.6 mole per cent spessartine). The enrichment of Mn in ilmenite is assumed to be caused by hydrothermal activity during the formation of the Säviä ore. Lea Aho, Geological Survey of Finland, SF-02150 Espoo 15, Finland Introduction tent and the peculiar, myrmekitic texture of the ilmenite intrigued the author into making The discovery of two chalcopyrite-bearing a more detailed study of this mineral. ore erratics in the Pielavesi area led the Ex- Previous investigations of manganese oc- ploration Department of the Geological Sur- currences in Finland have not shown manga- vey of Finland to investigate the area in 1960 nese-rich ilmenite or pyrophanite. Laitakari —1964. Minor ilmenite and garnet rich in (1967) has mentioned one pyrophanite occur- manganese were observed besides the main rence from Joensuu. Re-investigation of the ore minerals chalcopyrite, pyrrhotite, pyrite original material using an electron microana- and sphalerite. lyser revealed that the mineral is common ilmenite with a low manganese content from A copper-zinc ore body in the village of 1 to 2 per cent MnO. Säviä (lat. 63°12'30"N, long. 26°40'20"E), Pie- lavesi, which has not been submitted to mi- ning activities, was discovered by drilling in General geology 1966—1968. The same mineral assemblage with manganese-bearing minerals was found The schist belt at Säviä is part of the in the ore body as had been found in the crystalline schist suite of the Pielavesi dis- erratics. The unusually high manganese con- trict described by Wilkman (1938). The schists 26 Lea Aho are conspicuously banded and veined migma- sign of the compositional zoning that is com- titic rocks, mainly of supracrustal origin. mon in metamorphic garnet. The MnO con- In an unpublished report, Laitakari (1968) tent of garnet generally decreases with in- has given a comprehensive description of the creasing metamorphic grade. Since Mn+2 ions Säviä schists, largely based on material collec- are the most appropriate in size for entering ted during exploration. The schists consist the crystal structure of Ca-poor garnet, only mainly of cordierite-anthophyllite gneiss, garnet rich in Mn+2 can be formed at very hornblende-cummigtonite gneiss and amphi- low temperatures (Miyashiro 1953). bolite with considerable abundances of alu- Spinel is green in colour and varies in mina-rich minerals, e.g. garnet and sillima- grain size from 0.02 mm to 2 mm. The spinel nite. The ore body lies within a fracture zone grains contain small zonally arranged exsolu- which is roughly concordant with the bed- tion bodies of magnetite and numerous inclu- ding, but also clearly cuts across the schists. sions of pyrrhotite, chalcopyrite, galena and The beds strike approximately from N to S, ilmenite. Microprobe analysis proved the spi- and dip from 40° to 70°E. nel to be a ferroan gahnite (Table 2). Table 2. A microprobe analysis for ferroan gahnite Mineral description from Säviä ore. Anal. T. Paasivirta The oxide ore minerals in the Säviä ore Wt. per Number of ions Mol. per cent body are magnetite, ilmenite and a minute cent based on 24 (O) end-members amount of cassiterite. They occur either as A1203 57.8 Al 16.04 ZnAlä04 54.3 rounded discrete grains or as dense irregular FeO 14.0 Fe2+ 2.75 ) MgAlä04 11.6 MgO 3.3 Mg 1.15 <7.93 aggregates or nodules with garnet and spinel. FeAl204 33.8 ZnO 25.3 Zn 4.03 ) (AI2O3 0.3) The associated accessories are biotite, chlo- 100.4 rite, anthophyllite, cordierite, sillimanite and sulphides. The aggregates vary in size between 2 to 5 cm and can clearly be detected Magnetite in the quartz and sulphide ore matrix. Electron microprobe analysis (Table 1.) re- The main oxide ore mineral in the Säviä vealed the garnet to be an almandine-spessar- ore is magnetite, which occurs as discrete tine. The scanning image of Mn shows no grains or as rims around the sulphide grains. Table 1. A microprobe analysis for garnet from Säviä ore. Anal. T. Paasivirta Wt. per Number of ions Mol. per cent cent basec on 24 (O) end-members Si02 37.6 Si 6.08 TiOo 0.1 Ti 0.01 Almandine 49.3 AI2O3 20.1 Al 3.82 (3.90 Spessartine 30.6 a 3 FeO 22.4 Fe + 0.07 Pyrope 19.3 2 MnO 13.4 Fe + 2.96 Andradite 0.8 MgO 4.8 Mn 1.83 CaO 0.3 Mg 1.15 98.7 Ca 0.04 a Total iron expressed as FeO Manganoan ilmenite from the Säviä ore deposit, Pielavesi, Central Finland 27 A few minute exsolution blebs of spinel and to occur independently of magnetite, but may some thin ilmenite lamellae (0.02 mm) are form aggregates with it. This ilmenite con- found within the magnetite grains. The thin tains minute lamellae of exsolved hematite ilmenite lamellae remaining within magnetite orientated parallel to (0001) of the host ilme- are considered to reflect later stage oxidation nite. Small corundum grains are found as at very low temperatures (Vincent 1960). exsolution products within the ilmenite. Ram- dohr (1969) concluded that some ilmenite ori- Table 3. Average of seven microprobe analyses ginating from ore differentiates very rich in and calculated normative components for magne- A1203 may contain corundum grains. tite grains from Säviä ore. Anal. T. Paasivirta The secondary ilmenite, the »exsolved il- Wt. per Normative components menite», forms with magnetite four texturally cent per cent different types of intergrowth: 1) coarse sub- hedral plates (grain size up to 0.3 to 1 mm) TiOg 0.3 Mn2Ti04 0.1 AI2O3 0.3 FegTiOi 0.3 which contain minute lamellae of exsolved a FeO 30.5 Fe304 97.5 hematite (Fig. 1), 2) internal granular grains Fe203 67.3 Total R3O4 98.4 MnO 0.1 (Fig. 2), 3) thin magnetite lamellae and 4) MgO 0.2 Fe2Ti04b myrmekitic intergrowth. The lamellar and -= 0.3 98.7 Fe2Ti04 + Fe304 myrmekitic intergrowths are clearly seen in Fig. 1. The textural types 2 to 4) of the ilme- a Fe0/Fe203 assumed ideal for magnetite, b Ratio expressed as lOOx mole ratio of compo- nite-magnetite intergrowth do not contain nents. (Anderson 1968) hematite as exsolution blebs. Removal by solution of the magnetite component from textural type 3) can produce The microprobe analyses (Table 3) indicated a low titanium content for the magnetites (0.2 to 0.5 wt. per cent Ti02). A low titanium content (0.3 wt. per cent Ti02) was also noted for magnetite from the Säviä schists in one analysis. The Ti02 content of magnetite varies with the metamorphic grade of the rock, and may be used as a general temperature indicator (Buddington et al. 1955). Magnetite formed at low grade metamorphic temperatures and pressures is nearly pure Fe304 (Ab- dullah and Atherton 1964). Ilmenite Two texturally different types of ilmenite, primary and secondary, can be distinguished in the Säviä ore. Fig. 1 Lamellar and myrmekitic intergrowth be- The primary ilmenite occurs as discrete tween ilmenite (dark grey) and magnetite (grey), grains, grain size 0.3 to 0.5 mm, and intersti- and pyrrhotite (white). In the middle of the pho- tomicrograph ilmenite (dark grey) with exsolu- tial to the silicates (mostly quartz). It appears tion blebs of hematite (grey). 28 Lea Aho Fig. 2 Granular intergrowth between magnetite Fig. 4 Ilmenite (dark grey) partly replaced by (light grey) and ilmenite (grey). Cassiterite (black) chalcopyrite (white). Slightly harder pyrrhotite myrmekitic intergrowth with ilmenite. White grains (white) within the chalcopyrite. grains are pyrite. rite, cubanite, pyrite, galena and sphalerite. This sulphide texture within an ilmenite is caused by the replacement of the magnetite component of the ilmenite-magnetite intergrowth by sulphides (Figs. 1 and 3). They commonly replace magnetite more readily than ilmenite, and they may also partly replace ilmenite, although this mineral is very resistant (Fig. 4). Cassiterite forms a myrmekitic intergrowth with the secondary ilmenite (Fig. 2). Exsolved cassiterite in magnetite or ilmenite has been described by Desborough and Sainsbury (1970). It is possible that also in the Säviä ore, tin originally occurred as solid solution in magnetite and was remobilized during late- stage alteration, accompanied by the intro- Fig. 3 Lamellar pyrrhotite and chalcopyrite (light duction of sulphide minerals. grey) within ilmenite (dark grey). Investigation of ilmenite by electron skeletal ilmenite grains. The magnetite re- microprobe moved is replaced by silicate minerals. A peculiar lamellar and myrmekitic inter- The data were obtained using a Geoscan growth occurs between secondary ilmenite, electron probe microanalyser with an accele- and sulphides such as pyrrhotite, chalcopy- rating potential of 20 kV and 0.1 «A specimen Manganoan ilmenite from the Säviä ore deposit, Pielavesi, Central Finland 29 current. Chemically analysed ilmenite served Discussion as standard, and manganese was measured using pure element. Raw data were corrected If coexisting magnetite and ilmenite crys- applying the method described by Bence and tallize in equilibrium, both the oxygen fuga- Albee (1968).

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