ASSOCIATIONS OF ORE MINERALS IN THE DEPOSITS OF THE SEINÄJOKI DISTRICT AND THE DISCUSSION ON THE ORE FORMATION YU. S. BORODAEV; N. S. BORTNIKOV; N. N. MOZGOVA; N. A. OZEROVA; P. OIVANEN and V. YLETYINEN BORODAEV, YU. S.; BORTNIKOV, N. S.; MOZGOVA, N. N.; OZE- ROVA N. A.; P. OIVANEN and V. YLETYINEN, 1983: Associations of ore minerals in the deposits of the Seinäjoki district and the dis- cussion on the ore formation. Bull. Geol. Soc. Finland 55, 1, 3-23. The mineral associations of the antimony deposits in the Seinä- joki region (Finland) are described on the basis of microprobe inves- tigations. There are two ore associations: quartz-antimony and pyr- rhotite-antimony. The latter is characterized by the presence of the new minerals seinäjokite and pääkkönenite first discovered in these deposits. The data obtained suggest that minerals were formed in the deposits under specific conditions: at relatively high temperatures for antimony deposition, and at extremely low sulphur fugacity for the hydrothermal process. Key words: Antimony, seinäjokite, pääkkönenite, sulphur fugacity Seinäjoki. Yu. S. Borodaev, N. S. Bortnikov, N. N. Mozgova and N. A. Ozerova: IGEM, Staromonetnyi 35, Moscow 109017, USSR. P. Oivanen and V. Yletyinen: Geological Survey of Finland, SF-02150 Espoo 15, Finland. Introduction ny ores in detail and to advance some sug- gestions on their genesis (Pääkkönen 1966). The Seinäjoki ore district, situated in west- During the last few years exploration has ern central Finland, is unique for its deposits concentrated primarily on two deposits, Kal- of native antimony. The content of native lio salo and Tervasmäki. antimony amounts to 80-90 per cent in some The results presented here are based on a of the occurrences. For some years the Seinä- study of the ore samples collected by N. N. joki district has been subjected to geological, Mozgova and N. A. Ozerova from the deposits geophysical and explorational investigations of Syrjämö, Routakallio, Tervasmäki, Kallio- conducted by the Geological Survey of Fin- salo and Sikakangas during an excursion or- land. The work was headed by V. Pääkkönen, ganized by the Geological Survey of Finland who was the first to study thoroughly the ge- in the Seinäjoki ore district in 1978, on sam- ology of the deposits, to describe the antimo- ples collected by P. Oivanen and V. Yletyi- 4 Yu. S. Borodaev, N. S. Bortnikov, N. N. Mozgova,.. nen, and on minerals kindly placed at our dis- The intrusive magmatic rocks found in the posal by the late V. Pääkkönen. The ores were district constitute younger complex of syn- investigated at the Geological Faculty of the orogenic granitoid rocks: quartz diorites, Moscow State University and at the Institute granodiorites, granites and the pegmatites as- of Geology of Ore Deposits, Petrography, sociated with them. The rocks were formed Mineralogy and Geochemistry (IGEM) of the during the main phase of metamorphism and USSR Academy of Sciences. The unique deformations that occurred 1800-1900 mil- antimony ores from Seinäjoki were studied lion years ago (Simonen 1980). The antimony employing electron microprobe analysis. mineralization took place during the second This allowed us to add considerably to V. phase of metamorphism associated with oro- Pääkkönen's data on the chemical composi- genesis (Pääkkönen 1966). tion of ore minerals, to reveal some specific The Seinäjoki ore district is an anticlinal features of paragenetic correlations, to dis- structure about 20 km wide. It consists of a cover some minerals previously unknown in series of isoclinal folds with a steep north- this ore district and to find some mineral eastern plunge. The folded structure is com- species which had not been found anywhere plicated by faults of several systems in which in nature before. Our knowledge of the spe- faults striking north west predominate. The cific features of the geochemistry and miner- ore zone, which is about 8 km long and at alogy and of the conditions of formation of least 1 km wide, is located within the faults. the natural antimony deposits of the Seinä- It includes nine deposits, and a tenth one is joki district has been thereby greatly en- situated somewhat to the south-west of the hanced. Detailed investigations of native zone (Fig. 1). metals and intermetallic compounds have Various rocks of the schist series are miner- been carried out recently on the deposits in alized: quartz-sericitic and graphite schists, which the inclusions of these minerals occur mica gneisses, acid igneous rocks and por- as separate grains of very small and even mic- phyrites, often along the contact of volcanic roscopic size (Karup-Moller 1978; Novgoro- rocks differing in composition. The majority dova 1980 et al.). The Seinäjoki deposits are of the mineralizations are in veins and stock- a fortunate exception in this respect as the works, where the ore minerals form nests, grains of native antimony are fairly large, veinlets and impregnations. and a number of minerals are associated inti- mately with the antimony. The ores of the Seinäjoki deposits are char- acterized by a relatively complex mineral composition (Table 1). Note that the mineral compositions are of almost the same type in Geological position of the Seinäjoki ore all the deposits. district The principal ore minerals are native anti- mony, pyrrhotite and arsenopyrite. The ores The Seinäjoki ore district lies at the bound- contain appreciable amounts of gudmundite, ary of the Central Finland granitoid area antimonite, berthierite, löllingite and marca- within the schist zone of metamorphic rocks site. Other minerals occur in insignificant of Middle Proterozoic age. The metamorphic amounts. The group of rare minerals found rocks are represented by biotite gneisses, for the first time in the ores of the Seinäjoki migmatites, mica schists, quartzites and vol- district is of particular interest. They include canic rocks of intermediate and acid compo- the recently discovered mineral species, seinä- sition. jokite and pääkkönenite, and compounds Associations of ore minerals in the deposits of the Seinäjoki district... 5 1 2 3 U 5km • i i • i 2. ' \ i 3. u. 5 6 • Fig. 1. Scheme of geological structure of the Seinäjoki ore district. 1. Amphibolite and hornblende gneiss, 2. Plagioclase porphyrite, 3. Mica schist and mica gneiss, 4. Quartz diorite and granodiorite, 5. Pegmatite and pegmatite granite, 6. Antimony deposits (1- and 2-Törnävä, 3-Syijämö, 4-Routakalüo, 5-Lootakallio, 6-Tervasmäki, 7-Kalliosalo, 8-Marttalanniemi, 9- Sikakangas, 10-Satamo). 6 Yu. S. Borodaev, N. S. Bortnikov, N. N. Mozgova,.. Table 1. Distribution of ore minerals in the deposits of the Seinäjoki ore district. Deposits Törnävä Syrjämö Routakallio Tervasmäki Kalliosalo Sika kangas Minerals Antimony, native © © + + + + Pyrrhotite © © + + + + Arsenopyrite © © + + + + Stibnite © © + + + + ^ Gudmundite © © + + + + Berthierite O O + + + + Löllingite O O + + + + Marcasite O © + + + + in Sphalerite O O + + + + c Chalcopyrite O O + + + + § Fahlore O + + S b-arsenopy rite + Pyrite O O + Pääkkönenite + Y-mineral + Z-mineral + Zinkenite + Jamesonite O gj Seinäjokite + p2 Sb-westerveldite + Stibarsen + Breithauptite O Aurostibite + AI taite + Gold, native O O Hematite O O Scheelite O O Note: O - from data by V. Pääkkönen; © - from data by the authors and V. Pääkkönen (1966); + — from data by the authors. previously unknown in nature, which in this mally intergrown. Wherever one of the two is context have been designated »mineral Y» predominant, the other is either absent or in and »mineral Z». subordinate amounts; they are never in con- The main types of wall-rock alteration are tact with each other. Within the ore zones the silicification and sericitization. In some cases native antimony is concentrated in quartz the ore mineralization has been traced to a veins and in intensely silicified rocks. A num- depth of up to 100 m (the Kalliosalo deposit). ber of other ore minerals are associated with both the native antimony and the pyrrhotite but in widely varying proportions. Two main Associations of ore minerals mineral associations, quartz-antimony and antimony-sulphide, can thus be established A study of the ores of the Seinäjoki depos- in the ore district (Table 2). The first associa- its reveals that the two main ore minerals, tion is characterized by a pronounced sub- native antimony and pyrrhotite, are not nor- ordinate occurrence of other antimony and Associations of ore minerals in the deposits of the Seinäjoki district... 7 Table 2. Distribution of minerals according by association. Associations Main minerals Minor minerals Rare minerals Quartz-antimony Native antimony Stibnite Seinäjokite Quartz Pyrrhotite Sb-westerveldite Arsenopyrite Pääkkönenite Altaite Löllingite Sb-arsenopyrite Stibarsen Berthierite Y- and Z-minerals Gudmundite Zinkenite Antimony-sulphide Pyrrhotite Löllingite Pyrite Gudmundite Sphalerite Chalcostibite Berthierite Chalcopyrite Breithauptite Stibnite Fahlore Arsenopyrite Native antimony Marcasite Quartz antimony-bearing minerals (stibnite, ber- the peripheral parts of the host mineral, some thierite and others). The second association of the crystals of arsenopyrite and löllingite contains comparatively large amounts of sul- associated with native antimony are younger phides of antimony such as gudmundite, and are of metasomatic origin (Fig. 2). As berthierite and stibnite. It was only in the shown by the formation of arsenopyrite rims quartz-antimony association, however, that around it (Fig. 3). Rare minerals such