Associations of Ore Minerals in the Deposits of the Seinäjoki District and the Discussion on the Ore Formation

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 discussion 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 investigations. There are two ore associations: quartz-antimony and pyrrhotite-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 intimately with the antimony. The ores of the Seinäjoki deposits are characterized 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 antimony, 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 as pääk- the group of very rare minerals specific to the könenite, seinäjokite, Sb—westerveldite, Seinäjoki ore district (seinäjokite, Sb-west- altaite and others were formed almost simul- erveldite, pääkkönenite and others) was taneously with native antimony or developed found. metasomatically, as is revealed, for example, by the antimony relicts in idiomorphic crystals of Sb-westerveldite (Mozgovaet al. 1976 The quartz-antimony association and 1977). The youngest ore mineral of this

The principal ore mineral of this association, native antimony, occurs as irregular grains and nests in the intersticies between quartz grains and as short veinlets in quartz. The antimony may occasionally distinctly re- place the quartz. Native antimony normally contains small idiomorphic inclusions of arsenopyrite and löllingite and other more rare minerals. Idio- morphism of arsenic minerals grains, and the presence of antimony veinlets earring the crystals of arsenopyrite and löllingite suggest 500 if that these minerals were formed earlier than Fig. 2. Native antimony (S) intergrown with pääk- the native antimony (Pääkkönen 1966). How- könenite (PÄ) and arsenopyrite (AP) in quartz ever, because they are regularly arranged in (black). 8 Yu. S. Borodaev, N. S. Bortnikov, N. N. Mozgova,.. .

Fig. 3. Crystal of löllingite (LÖ) replaced by arsenopyrite (AP) along the boundaries, a) backscattered electron image; b) SKa, X-ray image.

association is stibnite, which forms rims pregnations are quite large. This is probably around and occurs as veinlets cutting the due to the accumulative recrystallization of grains of native antimony. primary pyrrhotite under the influence of hydrothermal solutions. Pyrrhotite was partially redeposited, thereby cementing the Pyrrhotite-antimony association fragments of metamorphic rocks in the zones of brecciation (Fig. 4). The principal mineral of this association is Pyrrhotite is often replaced by marcasite. pyrrhotite, which also occurs on a regional Pyrrhotite grains are frequently surrounded scale. According to Pääkkönen (1966), this by gudmundite rims (Figs. 4 and 5) that in mineral is normally found, albeit in small some places exhibit submicroscopic and amounts, in mica gneiss. In some places it is coarse graphic textures of gudmundite and so abundant that it causes magnetic anoma- pyrrhotite intergrowth (Fig. 6). Pääkkönen lies. In accordance with Lepp (1957), Pääkkö- (1966) believed that the development of gud- nen (1966) was of the opinion that metamor- mundite was due to the reaction of pyrrho- phism produces iron sulphides from melni- tite and native antimony: kovite and other black unstable compounds FeS + Sb -> FeSbS. of ferrous sulphide known to accumulate on The reactional origin of the gudmundite rims the floor of sea basins and lakes together with around pyrrhotite is confirmed by the pres- organic material subjected later to graphitiza- ence of small pyrrhotite inclusions whose tion. Some chalcopyrite, sphalerite and hem- optic orientation is the same as that of the atite seem to be of similar origin. main pyrrhotite grain. In the ore zones of the Seinäjoki deposits The pyrrhotite and the gudmundite rims the pyrrhotite seems to be of an older genera- are replaced by berthierite, which sometimes tion, and in some places the pyrrhotite im- contains non-replaced relicts of these miner- Associations of ore minerals in the deposits of the Seinäjoki district... 9

1000 Fig. 4. Fragments of the enclosing metamorphic Fig. 5. Pyrrhotite (PO) surrounded by gudmundite rock cemented by gudmundite (G) and marcasite rim (G). (M), which replaces pyrrhotite.

als and their graphic intergrowths (Fig. 7). reaction between antimony-bearing solutions The three minerals are in turn replaced by and pyrrhotite. stibnite that forms reaction rims around their Contact relationships between pyrrhotite grains and cutting veinlets inside them. It has and native antimony - the main minerals of been observed that stibnite veinlets in pyr- the various associations - have not been ob- rhotite crosscut chalcopyrite veinlets formed served. In the comparatively rare cases when earlier. Stibnite is most intensely developed the two minerals are found in the same ore after pyrrhotite and is partially replaced by aggregate, gudmundite and berthierite occur laminar marcasite. Complete stibnite-marca- between them: The absence of crosscutting site pseudomorphs after pyrrhotite are often relationships suggests that, as pointed out encountered (Fig. 8). The relationships be- earlier by Pääkkönen (1966), the two associa- tween pyrrhotite and antimony minerals de- tions were formed from the same solution but monstrates that the latter resulted from the in different physical and chemical environ-

Fig. 6. Micrographic texture of pyrrhotite in Fig. 7. Intergrowth of gudmundite (G) and gudmundite (G) close to the contact with pyrrhotite pyrrhotite (PO) in berthierite (B). (PO). 10 Yu. S. Borodaev, N. S. Bortnikov, N. N. Mozgova,.. .

cow State University. The conditions of the microprobe analyses were as follows: accel- erating voltage 25 kV, sample current 0.5— 2.0 • 10-8A, X-ray take-off angle 40°, probe diameter — In. Analytical Kai lines were em- ployed for S, Fe, Co, Ni, Cu, Zn; Lj for Sb and Te; and Ma for Au and Pb. As was measured

on K«x or Lax, depending on the composition of the mineral phase. Pure metals (Fe, Co, Ni, Cu, Zn), minerals - galena (PbS), chalcostib-

ite (CuSbS2), arsenopyrite (FeAsS), stibnite

(Sb2S3) and artificial compounds (FeS and GaAs) were used as standards. Corrections Fig. 8. Complete replacement of pyrrhotite by marcasite and stibnite. were done according to a computer program modified from one by Springer (1967). ments. The rock were probably subjected to Pyrrhotite (Fei-xS) the most intense reworking in the sections of quartz-antimony mineral association, which Pyrrhotite occurs as a fine-grained (up to resulted in the complete removal of pyrr- 2 mm) impregnation that normally extends hotite. Antimony-sulphide association was along the schistosity. The grain size of this formed in the enclosing rocks, which were mineral occasionally reaches 0.5 cm. As was less intensely altered. The order of crystalliza- first established by Pääkkönen (1966) using tion of the main ore minerals is shown by the the X-ray method, both monoclinic and following idealized scheme: hexagonal pyrrhotite modifications are pre- sentin the ores. The presence of the two mod- Quartz-antimony association: ifications corroborated by applying magnet- löllingite > arsenopyrite > native ic suspension. The microprobe analyses re- antimony vealed that the composition of the magnetic

monoclinic pyrrhotite was Fe0.86-o.87S and Antimony-sulphide association: that of the nonmagnetic hexagonal pyrrhotite pyrrhotite > gudmundite Fe087_092S. The pyrrhotite exhibits great berthierite compositional variations, even within a single grain (from Feo.86S to Fe0 92S). Alter- stibnite (+ mar- nating zones of magnetic and non-magnetic casite) modifications of pyrrhotite were often ob- marcasite (+ pyrite) served. None of the pyrrhotite grains anal- ysed contained Ni.

Ore minerals of the Seinäjoki Deposits Marcasite (FeS2) The present section deals with specific features of the chemical composition of the min- This mineral often replaces pyrrhotite with- erals, which were studied using the micro- in the mineralized zones. Three types of re- probe JXA-5 at the Laboratory of Ore Mi- placement can be observed: 1. partial or com- croscopy of the Geological Faculty of Mos- plete replacement by laminar marcasite crys- Associations of ore minerals in the deposits of the Seinäjoki district... 11

Fig. 9. Replacement of pyrrhotite by laminar Fig. 10. Loop-and-ring replacement texture of marcasite. Stibnite rims can be observed at the pyrrhotite (PO) by marcasite (M). boundaries of the grains.

tals (Figs. 9, 8 and 4); 2. formation of loop- »intermediate product» (Fig. 12). Microprobe shaped and ring-shaped replacement tex- analyses of the latter revealed that it was tures. The marcasite rings frame the fresh close to the stoichiometric FeS2 in composi- pyrrhotite nuclei concentrically (Fig. 10) or tion but that it was characterized by the pres- are developed within it to form lenses and ence of antimony (from 0.9 to 3.5 %). Antimo- oval inclusions (Fig. 11); 3. development of a ny is also present in the marcasite of septa in cellular substitution texture. In this texture approximately the same amounts. the pyrrhotite grain is broken into individual blocks separated by septa of microcrystalline Native antimony (Sb) and distinctly anisotropic marcasite, marcasite itself being partially or completely re- Native antimony normally occurs as a fine- placed by a fine-grained anisotropic sub- grained impregnation. The grains are irregu- stance described by Ramdohr (1975) as an lar in shape and up to 2-3 mm in size. Rare

Fig. 11. Ring-shaped marcasite (M) aggregates in Fig. 12. Cellular texture of pyrrhotite replaced by pyrrhotite (PO). Sb-bearing marcasite (lighter, relief) and »intermediate product» (light grey, poorly polished). 12 Yu. S. Borodaev, N. S. Bortnikov, N. N. Mozgova,.. .

Fig. 13. Zonal crystal of Sb-bearing arsenopyrite. a) backscattered electron image; b) Sb Lai X-rays image.

larger blebs (up to 10 cm in diameter) may as 5 mm. It usually forms distinct short-prism also be countered. A specific feature of the or elongated sharpedged crystals that are native antimony is its arsenic content. Rou- idiomorphic in respect of other minerals such tine chemical analyses indicated that the con- as pyrrhotite, native antimony and gudmun- tent of As in a large hand specimen of anti- dite. Arsenopyrite inclusions are regularly mony from the Törnävä deposit was 3.4 % present in native antimony, even in the small (Pääkkönen 1966) whereas in the Syrjämö de- grains. posit it was 3.26 %. Our investigations re- The arsenopyrite crystals in the ores of the vealed that this mineral has a higher content Kalliosalo deposits exhibited some zonality, of arsenic (up to 7-11 %). Arsenic is distrib- which is caused by variations in the antimony uted uniformly in native antimony and its content and in the Me/S ratio (Fig. 13, Table content is practically the same at the contact 3). The central and intermediate zones have with the inclusions of arsenic minerals (arse- different Me/S ratios, whereas the external nopyrite and löllingite). A lower As content zone is composed of a brush of small pris- (0.3 %>) has been recorded only in native anti- matic crystals of antimony-bearing arseno- mony whose grains contain inclusions of sei- pyrite that has grown on the surface of the näjokite and Sb-westerveldite (Mozgova et main crystal. The rims of antimony-bearing al. 1977). arsenopyrite have also developed around gudmundite crystals (Fig. 14, Table 3). Arse- Arsenopyrite (FeAsS) nopyrite with a high content of Sb (11 %) has been found in the ores of the Urultan deposit Arsenopyrite is widespread and occurs as a in the northeastern USSR (Gamyanin et al. diffused impregnation with grain sizes of up 1981). In the report by Fleischer (1955), it to 1 mm. Individual crystals may be as long was shown that the Sb content in arsenopy- Associations of ore minerals in the deposits of the Seinäjoki district... 13

Table 3. Chemical composition of Sb-arsenopyrite from the Kalliosalo deposit, wt.% (according to the microprobe data).

Place of Fe As Sb Total Formula analysis

Zonal crystal of Sb-arsenopyrite (see Fig. 13)

Central part 34.0 48.3 1.8 17.6 101.7 Fei.ocKAsi.oeSbo .03)1.09^0.91 Intermediate part 31.3 49.6 1.9 16.4 99.2 Feo.96(As! i4Sb0 03)17Si0.88 External zone 33.5 37.3 12.0 18.6 101.4 r I e1.0l(AS0.84Sb0 17)! 01So.98 Margin of Sb-arsenopyrite around gudmundite (see Fig. 14)

Inner zone 33.8 38.9 6.1 18.0 96.8 Fej 05(As0.90Sb0 09)0.9980.97 External zone 33.8 46.1 4.8 17.1 101.8 Fe! 01(As! 03Sb0 07)i.i0S0 89

rite does not exceed 0.2 %. The data obtained lingite from different deposits in the district by us demonstrate beyond doubt that rather exhibits marked compositional variations. extensive solid solution exists in nature be- The löllingite found at Sikakangas and re- tween arsenopyrite and gudmundite (up to 17 placed by arsenopyrite (Fig. 3) reveals the at. % As). highest content of sulphur (~ 2 %) and an absence of Ni, whereas the mineral at Kalliosalo contains a minimal amount of sulphur (~ Löllingite (FeAsz) 0.2 %) but a maximal amount of nickel (~9 %), Löllingite occurs as small short-prismatic much greater in fact than that encountered and elongated crystals up to 0.5 mm in size. earlier (Holmes 1947; Busek 1963). The Kal- The löllingite from the Seinäjoki ores con- liosalo löllingite contains antimony, too, tains nickel, sulphur and antimony (Table 4), which has not been found in any other de- which are the most typical elements of this posit in the district. mineral (Holmes 1947; Mikheev 1952). Löl-

Gudmundite (FeSbS)

Gudmundite was found in the form of small grains up to 1 mm in size, mainly rim- ming the pyrrhotite grains beyond the silicification zone. The chemical composition of the gudmundite is in good agreement with the theoretical one (Table 5). Note that, unlike its isostructural arsenic analogue, arsenopyrite (in which arsenic is largely replaced by antimony, as shown above), the gudmundite, even when intimately associated with arsenopyrite, lacks arsenic. This may suggest an in- Fig. 14. Gudmundite (G) with rims of finely complete solid solution between these min- crystalline Sb-bearing arsenopyrite (AP). erals. 14 Yu. S. Borodaev, N. S. Bortnikov, N. N. Mozgova,.. .

Table 4. Chemical composition of löllingite, wt..% (according to the microprobe data).

Deposit Fe Ni As Sb S Total Formula

s Kalliosalo 18.5 9.0 70.7 3.4 0.2 101.8 (Feo.68Ni0 32)i.oo(A- i.93Sbo.o6S(i .01)2.00 Routakallio 25.2 2.4 73.5 - 0.6 101.7 (Fe0.91Ni0.08) o.99(A-S!.9780.04)2.01 Routa kallio 25.2 2.3 70.9 - 1.1 99.5 (Feo.92Nio.o8)i.oo(As1.93So.o7)2.oo Sikakangas* 27.2 - 71.2 - 2.0 101.4 Feo.97(As [ 9oSfl 13)2.03

* Replaced by arsenopyrite (see Fig. 3) with composition Fe - 33.6 %; As - 49.1 %; S - 17.4 %; total - 100.1 %, as indicated by microprobe data.

Berthierite (FeSb-iSj phides (pyrrhotite, gudmundite and berthierite) and native antimony. Stibnite vein- Berthierite is much less abundant than lets have also been observed to cut these min- gudmundite, being encountered only as small erals. In composition stibnite is almost stoi- isolated grains in the enclosing rocks. It is chiometric (Table 5). often intergrown with pyrrhotite and gudmundite and forms reaction rims around Sb-westerveldite [Fe(As, Sb)] them. In the Kalliosalo ores, berthierite may occasionally be observed in association with Westerveldite occurs as small (up to 0.1 and replacing native antimony. The largest mm) idiomorphic grains in native antimony grains of this mineral attain 1.6 mm in dia- (Mozgova et al. 1976 and 1977). Unlike west- meter. The composition of berthierite is con- erveldite from other deposits, the mineral en- sistent with the stoichiometric one (Table 5). countered in the Seinäjoki district is the only variety of antimony with the empirical formula Fe(Aso.9 Sbo.o5So.o4)i.o3 (Table 6). The Stibnite (Sb S-J 4 2 mineral studied is distinguished by the reg- Stibnite occurs as thin veinlets in the en- ular absence of Co and Ni, although these closing rocks (mainly parallel to their schisto- elements have always been present in all sity) and accumulations (up to 2 mm in dia- westerveldites described earlier (Oen et al. meter) of small acicular crystals. The mineral 1971 and 1977; Sizgoric and Duesing 1973; has very often grown along the boundaries of Karup-Moller 1978). Sb-westerveldite has a individual grains and aggregates of other sul- comparatively high content of S (~ 1 %).

Table 5. Chemical composition of gudmundite, berthierite and stibnite, wt.% (according to microprobe data).

Mineral Deposit Fe Sb S Total Formula

Gudmundite Routakallio 27.8 58.9 14.7 101.4 Fej ^Sbj ,oiS0 95 Gudmundite Kalliosalo 27.9 57.1 15.1 100.1 Fe1.o4Sb0.98S0 98 e Berthierite Kalliosalo 12.6 58.0 30.3 100.9 F 0.96Sb2 02S4 02 Berthierite Kalliosalo 12.4 56.5 29.4 98.3 Fe0.97Sb2.03S4 oo S S Stibnite Routakallio - 74.0 28.5 102.5 k>2.00 3.00 Stibnite Kalliosalo - 71.2 28.1 99.3 sb2.00s3.00 Associations of ore minerals in the deposits of the Seinäjoki district... 15

Table 6. Chemical composition of westerveldite from various deposits (wt.%).

Deposit Fe Co Ni Cu As Sb S Total

Seinäjoki 42.4 _ 53.7 4.7 1.0 101.8 (Finland) La Gallega 28.1 0.7 13.4 - 55.7 - - 98.9 (Spain) - » - 26.6 0.65 17.4 - 55.55 - - 100.2 Igdlunguaq 35.9 <0.1 6.6 <0.05 57.2 0.15 <0.05 99.85 (South Greenland) - » - 36.5 <0.1 5.65 <0.05 57.2 0.15 <0.05 99.5 Illimaussaq 42.23 0.38 0.12 - 55.48 - 0.35 98.56 (South Greenland) Birchtree 30.1 0.5 13.9 - 55.1 0.5 100.1 (Canada)

Seinäjokite Fe(S, As)2 Stibarsen (AsSb)

Seinäjokite was first discovered in the ores Stibarsen (allemontite) forms rows of small of the Routakal'lio deposit (Mozgova et al. (up to 0.06 mm) crystals and grains of irreg- 1976), where it was associated with Sb-west- ular shape in native antimony (Fig. 15). The erveldite grains of native antimony. Seinäjo- chemical composition of this mineral corre- kite has since been found in only one locality sponds precisely the stoichiometric one in Ilimaussaq (Karup-Moller, 1978). The min- (Mozgova et al. 1976). The presence of sti- eral of the type occurrence has higher con- barsen with native antimony may indicate tents of Co, Ni and AS (Table 7). that the complete solid solution between an-

Fig. 15. Stibarsen (SA) aggregates in native antimony (S). a) backscattered electron image; b) As Ka, X-ray image. 16 Yu. S. Borodaev, N. S. Bortnikov, N. N. Mozgova,.. .

Table 7. Chemical composition of seinäjokite from various deposits, wt.% (according to data of microprobe analysis).

No. Deposit

1 Routakallio 15.4 0.6 4.0 6.9 73.5 1.2 _ 101.6 (Seinäjoki)

2 Ilimaussaq 17.41 0.33 0.28 1.75 81.45 - 0.08 101.3 (South Greenland)

1 - (Feo.78Nial9Coo.o3)(Sbi.71Aso.26Ten.o3), average of the analysis of 6 grains (Mozgova et al., 1976, 1977) s 2 - (Fe0.92Co0 02Ni0.0i)0.95( bi.97As0.07S0'0i)i.05> average of the analysis of 10 grains (Karup-Moller, 1978).

timony and arsenic established at high tem- Pääkkönenite (Sb2AsS2) peratures (Hansen, Anderko, 1957) does not Pääkkönenite was first discovered in the exist under natural conditions. ores of the Kalliosalo deposit and called after the late Veikko Pääkkönen, a Finnish geolo- Aurostibite (AuSb^j gist who made a great contribution to the Aurostibite, like stibarsen, was initially dis- study of the deposits in this district (Boro- covered in the Seinäjoki ores by Mozgova daev et al. 1981). Pääkkönenite occurs as et al. (1976). The mineral occurs as individual laminated and rounded grains (up to 0.4 mm isometric inclusions (up to 0.15 mm in size) in in size) intergrown with native antimony (Fig. native antimony (Fig. 16). The chemical com- 17), and with arsenopyrite and nickel-bearing position of aurostibite is in full agreement löllingite. The grains of pääkkönenite are with the theoretical one. sometimes twinned. A recalculation of the

Fig. 16. Aurostibite (AuS) grain in native antimony (S) at the contact with quartz (black), a) backscattered electron image; b) Au Ma X-ray image. Associations of ore minerals in the deposits of the Seinäjoki district... 17

Table 8. Chemical composition of pääkkönenite and artificial solid solutions, vt.% (according to the data of microprobe analysis).

Deposit _ Kalliosalo 65.8 17.8 15.5 99.1 Sb2.14As0 g4Si g2 (Seinäjoki) - » - 65.3 18.9 - 15.1 99.3 Sb2.l3ASi .00S1.86 - » - 65.9 18.7 - 15.4 100.0 Sb2.13As0 98Si 89 - » - 69.5 17.0 - 16.2 102.7 Sb2.19AS0.87S! g4 - » - 68.0 17.9 - 15.5 101.4 Sb2.l8As0 93S1 89 Average 66.9 18.6 - 15.5 101.0 Sb2.14AS0.97S1 89 »New phase» in equilibrium 65.9 17.4 - 16.7 100.0* Sb2.10AS0.90S2.00 with Sb2S3 and Sb (Craig et al., 1974) Phase X 67.7 15.3 0.2 16.1 99.3 Sb2.20AS08lS1.99 (Luce et al., 1977) - » - 62.8 19.9 0.1 17.2 100.0 Sbi gfiASi ooS2.04 Theoretical 63.7 19.6 - 16.7 100.0 Sb2AsS2 composition

* in wt.% the analysis is recalculated from mol.% given by the authors.

microprobe analysis (the average of five anal- al. (1974) and Luce et al. (1977) in associa- yses, Table 8) gives us the following formula: tion with native antimony, it is characterized Sb2.14Aso.97S 1.89- Note that the composition of by an antimony content in excess of stoichio-

pääkkönenite exhibits insignificant varia- metric Sb2AsS2. The X-ray powder diffrac- tions and, like the artificial solid solution ob- tion pattern of pääkkönenite is indexed to

tained in the system Sb—As—S by Craig et have a monoclinic unit cell with a0 5.372; bQ

Fig. 17. Pääkkönenite (PÄ) intergrown with native antimony (S) in quartz. a) without analyser; b) with analyser.

22 18 Yu. S. Borodaev, N. S. Bortnikov, N. N. Mozgova,.. .

Table 9. Chemical composition of minerals Y and Z, zinkenite, fahlore, chalcostibite and sphalerite, wt.% (according to microprobe data).

Deposit Minerals Pb Zn Cu Fe Mn Sb As S Total Formulas

— Mineral Y Kalliosalo 10.0 _ _ _ 59.7 15.4 14.8 .99.9 (Pb0.99Sbl0.07As4 03S9 42 Mineral Z Kalliosalo 17.0 - - - - 56.9 11.9 15.0 101.4 Pk»! 08Sb5.94As2.0305.96 34.8 - - - 45.8 - 22.1 103.6 Zinkenite Kalliosalo 0.9 (Pb0.94CU0.08)l.02Sb2.llS3.85 - - - - Fahlore Kalliosalo 38.6 6.9 29.6 25.0 100.1 Cu106Fe2.o4Sb4 03S12.88 - » - Kalliosalo - 2.0 38.8 5.2 - 30.3 - 25.9 102.2 Cu9 88(Fei.52Zno.5o)2.02Sb4 03S13 06 - » - Satamo - 1.7 39.1 5.5 - 31.3 - 25.0 102.2 Cu1o.o5(Fei.61Zno.43)o.o4Sb4.19So.72 Chalco- stibite Kalliosalo - - 25.9 - - 48.1 - 25.3 99.3 CUl.O38bg.99S1 98

Sphalerite Kalliosalo - 56.8 - 8.9 1.0 - - 33.3 100.0 (Zno.83Feo.i5Mno.o2)S - » - Kalliosalo - 56.0 - 9.2 1.7 - - 33.5 100.1 (Zn0.81Fe0.i6Mn0.03)S

3.975; c0 11.41 Å; ß 89.71°, which is consistent Sphalerite (Zn, Fe, Mn)S with the lattice constants - a0 5.40, b0 3.98; Sphalerite occurs in pyrrhotite as small in- c0 11.40 Å; ß 90° suggested by Craig et al. clusions (up to 0.5 mm) of irregular shape. (1974) for the synthetic phase Sb2AsS2. Microprobe analysis of sphalerite in the Kalliosalo deposit shows that it contains iron Mineral Y (Pb^bioAsgSis) and manganese (Table 9).

Mineral Y was also discovered for the first Chalcopyrite (CuFeS2) time in the ores of the Kalliosalo deposit. This mineral occurs as grains of irregular shape up Chalcopyrite was found as exsolution to 0.1 mm in size in the contact with pääkkö- bodies and veinlets in sphalerite and as small nenite. No compound of similar composition inclusions and veinlets in pyrrhotite. has ever been described either from nature or a synthetic system. A microprobe analysis of Pyrite (FeS?) mineral Y has the recalculated idealized for- According to Pääkkönen (1966), pyrite oc- mula Pb2Sb2oAs8S19 (Table 9). curs in appreciable amounts in graphitic phyllite and in the zones of sericitic quartzite, but infrequently in association with antimo- Mineral Z (PbSbäAs2S6) ny minerals. Pyrite and marcasite both re- Mineral Z is another phase that has not place pyrrhotite. been encountered either in nature or as a synthetic variety. It was observed in associa- Fahlore Cu10 (Fe, Zn) Sb^^ tion with zinkenite in the same polished section of ore from Kalliosalo deposit as mineral Fahlore has been found as a subordinate Y. It is about 0.1 mm in grain size. The re- mineral as small irregular grains up to 0.5 mm sults of the microprobe analysis of mineral in size in the ores of three deposits: Routa- Z are consistent with the idealized formula kallio, Kalliosalo and Syrjämö. The mineral PbSb6As2S6 (Table 9). is usually intimately associated with pyrrho- Associations of ore minerals in the deposits of the Seinäjoki district... 19 tite, sphalerite and chalcopyrite. Fahlore Discussion on the physical and chemical from the Seinäjoki deposits is pure tetrahed- conditions of formation of ores in the rite with no arsenic (Table 9). A specific Seinäjoki district feature of this fahlore is the preponderance of iron over zinc. In some places, the pure The fresh data given above on the mine- iron-bearing variety, koppite, has also been ralogy, paragenesis and chemical composi- noted as have variations in Me/S content. tion of minerals based on microprobe analyses and the results of experimental and thermodynamic studies on the systems Chalcostibite (CuSbS2) Fe-As-S, Fe-Sb-S and As-Sb-S contri- Chalcostibite has only been encountered in bute to a better understanding of the chemi- one polished section from the ores of the cal and physical conditions of the antimony Kalliosalo deposit in an intimate in inter- ore formation in the Seinäjoki district. growth with tetrahedrite and stibnite en- The temperature of formation of the quartz- closed within a gudmundite crystal. The antimony mineralization can be estimated maximum size of the chalcostibite grains is using: 0.05 mm. The composition of the mineral a) solubility of sulphur in löllingite, is close to the theoretical one (Table 9). b) composition of arsenopyrite coexisting with löllingite and

Zinkenite (Pb1~nSb4-nS7 c) the content of arsenic in native antimony associated with phase X(Sb2AsS2) - a Zinkenite occurs as small grains (up to 0.4 synthetic analogue of pääkkönenite. mm in size) in the Kalliosalo deposit in close association with native antimony and mineral The experimental data of Clark (1960) show Z. The chemical composition of the zinkenite that the solubility of sulphur in löllingite from the deposit (Table 9) is in the composi- is 3.4 wt-% at 702°C and 3 wt.% at 650°C. tional range of the mineral established by The position of the solvus curve, which Mozgova and Bortnikov (1980). determines the limit of sulphur solubility in löllingite in equilibrium with arsenopyrite, has not been determined accurately enough Altaite (PbTe) at lower temperatures. Nevertheless a rough Altaite forms graphic intergrowths with extrapolation indicates that the increased native antimony (Mozgova et al. 1976 and content of sulphur in löllingite (~ 2 %) from 1977). It has been found in association with the Sikakangas deposit suggests that the other tellurides in the ores of the Routa- temperature of formation was about 500°C' kallio deposit only. The tellurides are closely Similar results are obtained when the tem- associated with seinäjokite, which contains perature is determined using the experi- only traces of Te. mental data of Kretschmar and Scott (1976) The main feature of the ores studied is on the composition of arsenopyrite in as- the extensive occurrence of native metal, and sociation with löllingite and pyrrhotite. the presence of intermetallic compounds and According to their data in this association, sulphoarsenides (pääkkönenite, minerals Y arsenopyrite with 36.6 wt-% As forms at and Z) characterized by a sulphur content approximately 500°C. Lower temperatures lower than that required by the valence of are obtained when the content of As in the cations. Arsenic appears to play the role of native antimony coexisting with pääkkönen- anion in them, as it does in löllingite. nite is used as a geothermometer. Luce and 20 Yu. S. Borodaev, N. S. Bortnikov, N. N. Mozgova,.. . his co-authors (1977) have established that The fugacity of sulphur during the forma- the minimal As content in antimony in tion of the quartz-antimony association can association with the Sb2AsS2 phase, which is be estimated from the data on the stability a synthetic analogue of pääkkönenite, is re- of the association of löllingite, the FeSb2 duced from 14.8 wt.% at 500°C to 12.1 wt.% phase - the end member of the seinäjokite at 425°C. Consequently, the content of 11 % solid solution (Fe,Ni) (Sb,As)2, and the FeAs As in native antimony must indicate a lower phase - the end member of the westervel- temperature of formation (below 425°C) of the dite solid solution (Fe,Ni,Co) (As,Sb). Accord- quartz-antimony association. ing to Barton (1969 and 1971), the stability The authors believe that in this case, the fields of the FeAs and FeSb2 phases at 400°C most reliable results are obtained by esti- (a maximal temperature for the ore associa- 17 mating the upper thermal limit from the tion in Seinäjoki) lies between fs2 = 10 and 15 composition of the coexisting pääkkönenite fs2 = 10 atm. These values are much lower and native antimony, since the relationship than the sulphur fugacity at which, according of these minerals observed in polished sec- to Barton, the mineral associations of sulp- tions suggests that they were formed simul- hides of the »main line» (Fig. 18) were de- taneously. The other minerals pair, löllingite posited from hydrothermal solutions. The and arsenopyrite, however, is out of equili- lower limit of sulphur fugacity during the brium owing to the distinct replacement of formation of sulphide-antimony association löllingite by arsenopyrite. We may therefore is restricted by the stability of gudmundite. reasonably assume that the temperature of The minimal fs2 that allows the formation formation of the quartz-antimony mineral of this mineral at 280°C (thermal limit of its 20 association was within 380°-400°C. stability) must be below 10 atm (calculated from Barton's equation Ag = -79400 + 38.8 T The sulphide-antimony association form- for reaction 2Fe + 2Sb + S2 = 2FeSbS). The ed at temperatures lower than the quartz- maximal sulphur fugacity during the depo- antimony assemblage. The upper thermal sition of these ores at the same temperature limit can be estimated using the temperature of 280° can be estimated from the stability of gudmundite stability. According to Clark data on the association of löllingite with (1966), this temperature is 280° ± 10°C, arsenopyrite and pyrrhotite whose stability which is confirmed by the presence in the range is restricted at this temperature by a ores of monoclinic pyrrhotite, a mineral that 15 value of sulphur fugacity of 10" atm. is stable below 250°C (Scott & Kissin 1974). It is interesting to note the difference in the temperature of formation between the Thus it is possible that the sulphur fugacity ores of the Seinäjoki district and of stibnite varied during the formation of the Seinäjoki deposits, which are the principal source of minerals from 1017 to 1015 atm at 400°C and antimony. from 1015 to 10-20 atm at 280°C. These values

According to many investigators, the ma- are much lower than those of fS2, according jority of the antimony minerals in the stib- to Barton (op.cit.) are characteristic of the nite deposits formed at 130°-180°C. In the process of hydrothermal sulphide mineral gold stibnite deposits only an early quartz- formation at temperatures of 280°-400°C. It pyrite-arsenopyrite association was depo- should be stressed that the antimony ores sited at higher temperatures (280°-380°C), in the stibnite deposits crystallized at higher whereas antimony ores (quartz-stibnite- sulphur fugacity because the pyrite-stibnite- berthierite association) formed at 130°- 180°C. berthierite association observed in these de- Associations of ore minerals in the deposits of the Seinäjoki district. 21

-6

-8

-10

-\k

-16

-18

-2D

-22

io3/T ,°K

Fig. 18. Diagram of sulphur activity - temperature for sulphurization reactions in systems Fe-As-S and Fe-Sb-S, from data provided by Barton (1969, 1971). Reactions gud = po + ant and gud = po + FeSb2 are shown schematically. Hatched part is the region of sulphur temperatures and activities characteristic of sulphide ore formation - »main line», according to Barton (1970). The dotted part is the region of sulp hur temperatures and activities during the formation of the deposits of the Seinäjoki district. Abbreviations: ant - antimony, ars - arsenic, asp - arsenopyrite, ber - berthierite, gud - gudmundite, ir - iron, A - liquid, po - pyrrhotite, lö - löllingite, py - pyrite, st - stibnite, sn - seinäjokite, wes - westerveldite. The crossings of equilibrium lines in the system Fe-Sb-As-S are not nonvariable points. 22 Yu. S. Borodaev, N. S. Bortnikov, N. N. Mozgova,.. .

u 12 posits is stable at fs2 of about 10 -10 the existence of temperature gradient (Kor- atm at 130°-180°C. shinskii 1955). However, that the content of The present study helps us to understand the components of the mineral solid solu- the specific features of the behaviour of the tions (native antimony, arsenopyrite-gud- main ore-forming elements: antimony, arse- mundite solid solution, tetrahedrite, seinä- nic and sulphur during the process of mineral jokite, löllingite) fluctuate and that zoning in formation and suggests a model of meta- these minerals can be accounted for by the somatic formation for the ores in the depo- non-equilibrium conditions of ore deposi- sits of the Seinäjoki district. A complete tion. The paragenetic sequence observed in extraction of pyrrhotite from the enclosing the mineral formation reflects the process of rocks of intensely altered metamorphic series sulphurization as the concentration of sul- indicates undersaturation of the hydro- phur in the minerals formed increases pro- thermal solutions in iron and sulphur, which gressively. Such a sequence in mineral-for- resulted in the dissolving of pyrrhotite and mation can be attributed either to enhanced the remobilization of Fe and S (Korzhinskii sulphur concentration due, for example, to 1955). The progressive saturation of the in- a dissolution of sulphides (pyrrhotite) in the tergranular solutions with iron gave rise to rear zone of the column or to a lowering of the formation of the next metasomatic zone temperature. made up of the minerals of sulphide anti- The data obtained therefore suggest that mony association due to the interaction of mineral formation in the deposits of the antimony-bearing solutions with pyrrhotite. Seinäjoki district occurred under specific This suggests inert behaviour of iron in this conditions. The minerals were formed at zone. The formation in the same zone of a temperatures relatively high for antimony number of reaction minerals of a similar deposition, and at sulphur fugacity extremely qualitative composition but differing in the low for hydrothermal processes. These two quantitative ratio of their constituent com- factors resulted in the formation of the uni- ponents (for example, löllingite, Sb-wester- que antimony ores made up of virtually mo- veldite, gudmundite-berthierite) is a result of nomineral native antimony and unusual asso- a change in the activity of the components, ciations of native metals, intermetallic com- that is, of antimony, arsenic and sulphur, pounds and minerals incompletely saturated in the mineral-forming solution together with with sulphur.

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Pääkkönen V., et al., 1976. Seinäjokit (Fe0.gNio,2) (Sb1.7Aso.32) i sarmjanistii westerveldit Fe- Manuscript received. May 19, 1982