Chapter 9 Svecofennian mafic-ultramafic intrusions

Cover page: Orbicular peridotite from the Kylm~koski Ni-Cu deposit. Photo: Jari V~t~inen.

Peltonen, P., 2005. Svecofennian mafic-ultramafic intrusions. In: Lehtinen, M., Nurmi, RA., R/ira6, O.T. (Eds.), Precambrian Geology of Finland - Key to the Evolution of the Fennoscandian Shield. Elsevier B.V., Amsterdam, pp. 407-442. 9 2005 Elsevier B.V. All rights reserved.

Three types ofmafic-ultramafic intrusions (Groups I, II, and III) were emplaced during the Svecofennian orogeny at ~ 1.89-1.87 Ga. Altogether, this magmatism represents a significant fraction of the Paleoproterozoic crustal growth of the Fennoscandian Shield and it also had a major influence on its metamorphic evolution. Most of the Group ! intrusions were emplaced close to the peak of the Svecofennian orogeny (~ 1.89 Ga) and were derived from hydrous arc-type basalts. They bear striking geochemical, mineralogical, and structural similarities to the mafic-ultramafic complexes exposed in younger deeply eroded oceanic and continental arcs (e.g., the Aleutians and Andes). The Group I intrusions were emplaced over a protracted period during the amalgamation of the Svecofennian arc collage; some of them represent conduits of arc basalts or were emplaced within an accretionary wedge, others were emplaced during the Svecofennian arc-Archean craton collision along transtensional shear zones. The Group I intrusions show evidence for syncrystallization deformation and assimilation of country rocks. They have a high potential for magmatic Ni-Cu sulfide deposits and have been the main source of Ni in Finland. The Group II intrusions are large synvolcanic layered gabbro complexes located in the Southern Finland arc complex. They represent low-pressure crystallization products of relatively juvenile subalkalic tholeiitic basalts within an oceanic arc and are not spatially associated with the Group I bodies. This suggests that the southern Finland oceanic arc terrain was amalgamated to the Western Finland arc complex only after the emplacement of the Group I and Group II intrusions. The latter have low potential for magmatic sulfide and oxide deposits. The Group III intrusions are Ti-Fe-P-rich gabbros within the Central Finland granitoid complex region. They share the geochemical similarities with anorogenic gabbros and probably do not all have a common origin. Several of these intrusions are genetically related to K-rich granitoid plutons and form, together with the granites, a bimodal magmatic suite that was generated by magmatic underplating at the postkinematic stage of the Svecofennian orogeny. However, a few of the Ti-Fe-P gabbros yield synorogenic crystallization ages and may actually represent evolved Group I magmas. Some of the Group III intrusions host important Ti ore reserves.

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I. Introduction Mafic-ultramafic plutonic rocks emplaced during Precambrian orogenic episodes are a poorly defined category of intrusions. This is not only because of the long tectonometamorphic history of such terrains and scarcity of young analogies (deeply eroded oceanic or continental arcs), but also because orogenic emplacement of magmas is often accompanied by subsequent breakup or boudinage of the intrusions, syncrystallization deformation, metamorphism, assimilation of country rock material, and prolonged thermal re-equilibration with the country rocks. These processes cause lithologic and textural diversity, which hampers their characterization and exact timing of emplacement relative to the major stages of the orogeny. Proper characterization of orogenic mafic plutonism is of prime importance for a number of reasons. First, their parental melts have ultimately been generated in the upper mantle, and these rocks thus provide information on the nature of the mantle source beneath convergent plate margins. Often, however, the composition of the parental melts becomes strongly modified by assimilation of crustal material at lower crust-upper mantle boundary region or during magma ascent towards higher crustal levels (Hildreth and Moorbath, 1988). Second, orogenic mafic-ultramafic plutonic rocks may constitute a significant part of the new crust generated at convergent tectonic settings (Robins and Gardner, 1974; Boyd and Mathieson, 1979; Snoke et al., 1982; Thompson, 1984; Burns, 1985; Butler, 1989; DeBari and Coleman, 1989; Grissom et al., 1991; Kepezhinskas et al., 1993; Skirrow and Sims, 1999; Schersten, 2001). Third, mafic plutonism provides a mechanism to transport heat upwards within the crust and thus has a strong impact on the metamorphic evolution of the lower and middle crust (Komatsu et al., 1994). As any other mafic-ultramafic magmas, orogenic melts can also lead to accumulation 410

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of valuable magmatic Ni-Cu-PGE sulfides, Ti-Fe-V oxides or apatite. Clearly, proper understanding of both the magmatic and subsolidus history of orogenic mafic-ultramafic intrusions is a prerequisite for the modeling of crustal evolution and development of successful exploration strategies. This chapter provides a review of the mafic-ultramafic intrusions of the Svecofennian orogen (Figure 9.1 ). Distribution of various intrusion types in the Svecofennian domain will be outlined and several case studies will be described in detail. In most cases, the cumulus terminology of Irvine (1982) is followed unless only conventional rock names were used in the original descriptions and corresponding cumulus names cannot be deduced. The mutual relationship of the intrusion types, timing of their emplacement relative to the tectonic evolution, and significance for regional studies of the Svecofennides will be scrutinized. Lahtinen et al. (Chapter 11) describe in detail the geodynamic context of the Svecofennian magmatism and give necessary tectonic information that is not repeated here.

2. Classification of the intrusions Mafic-ultramafic plutonic rocks can be classified according to their isotope ages, petrology, and geochemistry or tectonic setting. According to the classification scheme of Naldrett (1989), all Svecofennian intrusions belong, in a broad sense, to Category I V - "Intrusions emplaced in an active orogenic belt".

Such bodies are characterized by syndeformational intrusion resulting in fragmentation and boudinage, partial metamorphism, and presence of primary hydrous phases that, at least in some cases, indicate origin above active subduction zone. Also typical are complex contact phenomena and emplacement at relatively deep crustal levels where high ambient temperatures result in assimilation of country rock material. For the Svecofennian intrusions, however, a more specific classification scheme MAFIC-ULTRAMAFIC

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Fig. 9. I. Generalized geological map of central and southern Finland (modified from Korsman et al., 1997, and R~m6 et al., 2001 ) show the occurrences of different types of mafic-ultramafic intrusions.The synorogenic Group I bodies are found both within the Primitive andWestern Finland arc complexes and also intrude the Archean basement gneisses (and their metasedimentary cover), but are absent from the H~me belt. Group I intrusions have variable and relatively non-depleted initial Nd isotope composition. Much of the H~me belt consists of Group II synvolcanic intrusions and associated volcanic formations with depleted Nd isotope signatures. Group II bodies are absent from the Pirkanmaa belt suggesting a major tectonic boundary between H~me and Pirkanmaa belts. Group III Ti-Fe-P gabbros are found within the peripheral zones of the Central Finland granitoid complex, some of them closely associated with postkinematic granitoid plutons. Initial ~Ndvalues after Huhma (1986; unpubl.), Patchett and Kouvo (1986), Makkonen (I 996), R~m6 et al. (2001). CHAPTER

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is desirable. U-Pb zircon ages of the Svecofennian mafic-ultramafic intrusions form a rather continuous spectrum spanning the synorogenic (~1.87-1.89 Ga) stage of the Svecofennian orogeny (Figure 9.2). Some intrusions record slightly older ages that, however, tend to be associated with large errors, probably indicative of heterogeneous zircon populations. Most clearly, this may be the case for Lapinlahti, which is a rare example of Svecofennian intrusions emplaced into the Archean crust (Figure 9.1). Whether some of the mafic-ultramafic intrusions within the Svecofennian orogen were emplaced at the early orogenic stage (~1.9 Ga) remains uncertain. Similarly, the samples yielding the youngest ages (~-1.87 Ga) have rather large errors. The Soukkio gabbro, for example, is a bimodal mafic-felsic igneous complex (Eerola et al., 2001; Huhma, 1986) in which mixing of older mafic intrusive rocks with younger granite remains a possibility. Therefore, the actual range of emplacement ages for the Svecofennian mafic-ultramafic intrusions is probably somewhat smaller than the range indicated by the minimum and maximum ages of the chronogram (Figure 9.2). Most of the mafic-ultramafic intrusions record ages between 1875 Ma and 1885 Ma, which corresponds to the peak of the synorogenic stage of the orogeny. Two small flexures on the chronogram divide this population into three subgroups, 1885, 1880, and 1875 Ma. All these contain samples from both southern and central Finland and from various geotectonic and lithologic units and correlations between age and geographic location are absent. One kind of relationship is, however, evident: all intrusions that contain significant magmatic Ni-Cu sulfide deposits belong to the 1880 Ma age group. Because age data cannot be used to divide the intrusions into meaningful lithologic groups, a geotectonic domain concept is used. Basically, the terminology for the geotectonic units follows that presented elsewhere (Kors412

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man et al., 1997). Below, a general outline of the intrusions and some detailed case studies will be provided for the following subgroups of mafic intrusions: Group I: Intrusions of the Arc complex of western Finland Group Ia: Intrusions close to the Archean craton margin Group Ib: Intrusions of the Tampere and Pirkanmaa belts Group II: Synvolcanic intrusions of the Arc complex of southern Finland Group III: Ti-Fe-P gabbros of the Central Finland granitoid complex.

3. Intrusions close to the craton

mar igigig~(Group laJ The craton margin environment in central Finland shows a marked concentration ofmafic-ultramafic intrusions, several of which host small magmatic Ni-Cu sulfide occurrences or deposits (e.g., Papunen and Gorbunov, 1985). Within this domain the areal distribution of the intrusions is not restricted to any major geotectonic unit (Figure 9.1). They are found both in the older (~ 1.92 Ga) Primitive arc complex and the younger ( 1.89-1.87 Ga) Arc complex of western Finland. Some of them are found east of the (subsurface) Archean-Proterozoic boundary and are intrusive both to the Archean basement gneisses and the overlying metasediments (e.g., the Lapinlahti gabbro-anorthosite). Traditionally, the emplacement of these intrusions has been related to development of subvertical D 3 wrench lineaments (e.g., Gafil, 1972). Spatial association of shear zones and intrusions is especially evident adjacent to the suture zone in the southeast (Figure 9.1). Most of the intrusions are found within a broad belt outside these shear zones and an unambiguous genetic relationship between them has not been established. Structural analysis implies MAFIC-ULTRAMAFIC

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Fig. 9.2. Chronogram of U-Pb zircon ages of the Svecofennian mafic-ultramafic plutonic rocks. Data sources for targets discussed in the text: Soukkio (Huhma, 1986), Hitura (Isohanni et al., 1985), Laukunkangas (Huhma, 1986), Hyvink~i~i (Patchett and Kouvo, 1986;Suominen 1988), Koivusaarenneva (K~rkk~iinen, 1999b), Kotalahti (Ga~l, 1980),Vammala (H~ikli et al., 1979), Lapinlahti (Paavola, 1988), Perlimaa (R~m6 et al., 2001), Saarisenj~irvi andTyypekinlampi (Ekdahl, 1993).The remaining non-labeled data from Helovuori (I 979), Honkamo (I 988), Hopgood et al. (I 983), Marttila (I 981), Nurmi et al. (1984), Nyk~inen (I 983), Suominen (I 99 I),Vaasjoki (I 989),Vaasjoki et al. (I 988, 1996) and from the unpublished database of the Geological Survey of Finland. CHAPTER

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that the intrusions were emplaced already during early D 2 deformation and were deformed by recumbent D 2 folding that predates the D 3 lineament formation (Jokela, 1991; Koistinen, 1996). The present author favors a model in which plutonism occurred over a wide zone due to the westward subduction during the final stages of the closure of the basin between the Primitive arc complex and the Arhean craton. Synchronous or subsequent to the amalgamation, transtensional shear systems developed at the continental margin locally facilitating the ascent of melts along subvertical shear zones. This model also explains the more primitive composition and higher Ni potential of the shear zone-associated intrusions compared to intrusions elsewhere. Within shear zones, magmas are expected to rise faster and undergo less fractionation during emplacement thus also retaining higher potential to saturate nickeliferous sulfides. During the D 3 phase these zones were further reactivated, deforming and brecciating the intrusions. Mafic magmatism continued for some time after the amalgamation as evidenced by intrusions (e.g., Saarisenjfirvi, Tyypekinlampi) that yield postkinematic crystallization ages of ~1875 Ma (Figure 9.2; Ekdahl, 1993). Three representative intrusions, Laukunkangas, Kotalahti, and Lapinlahti, of the craton margin environment are described in more detail. Two of these, Laukunkangas and Kotalahti, hosted economic magmatic Ni-Cu sulfide deposits.

3. I. Laukunkangas The Laukunkangas mafic intrusion is a small mafic-ultramafic body within a zone of intense transcurrent faulting adjacent to the southwestern margin of the Archean craton (Figure 9.1). It is enclosed by high-grade Svecofennian graphite-bearing migmatites that, close to the intrusion margins, may contain garnet, cordierite, and orthopyroxene porphyroblasts. The associated magmatic Ni-Cu sulfide deposit 414

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was one of the largest in the Svecofennian intrusions and yielded 6.6 Mt ore (0.78 wt.% Ni and 0.22 wt.% Cu) in 1984-1994 (Puustinen et al., 1995). The Laukunkangas intrusion is elongated, pipe-shape& approximately 1 km long, 200 m wide, and more than 800 m deep. The mineralized eastern part of the body is well known because of extensive drilling. Breccia structures and graphite-rich gneiss xenoliths are common throughout the body, and are considered indications of tectonic disturbance during intrusion and solidification (Grundstr6m, 1980). Laukunkangas intrusion can be divided into marginal and layered series (Figure 9.3). The marginal series is heterogeneous, noritic and shows reverse fractionation. The layered series comprises two distinct zones: peridotite and norite. The peridotite zone is located in the eastern tip of the intrusion and consists of olivine, olivine-plagioclase, and olivineorthopyroxene cumulates. The peridotite zone is overlain by the norite zone that is more than 200 m thick, comprising rhythmically layered orthopyroxene-plagioc|ase and plagioclaseorthopyroxene cumulates. On the basis of the Ni content of silicates and sulfides the norite zone can be divided into three subzones (Figure 9.3). Generally, the norite subzone 1 overlays the peridotite zone but is locally missing, whereas subzone 2 lies directly on the peridotite zone. Subzone 3 is above subzone 2 and consists of evolved plagioclase-rich cumulates. Subzone 1 has the highest Ni content, subzone 2 is intermediate, and subzone 3 is most depleted (Pertti Lamberg, pers. comm., 2001 ). Clinopyroxene, plagioclase, magmatic amphibole, phlogopite, and Ni-Cu sulfides dominate as intercumulus minerals. The Ni-Cu mineralization is associated with the peridotite zone close to intrusion margin. The mineralization includes both disseminated, massive, and breccia-textured ore types. Sulfide breccias and sulfide veins, which are confined to the contact zone between the intrusion and country rocks, consist of massive sulfides and MAFIC-ULTRAMAFIC

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Laukunkangas intrusion i

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Peridotite zone

Norite zone Norite subzone 3 Ni(opx) <50 ppm

Norite subzone 2 Ni(opx) 50-250 ppm

Norite subzone I Ni(opx) >250 ppm

Fig. 9.3. Subdivision of the Laukunkangas intrusion (Pertti Lamberg, pers. comm., 2001). Opx-orthopyroxene.

contain abundant country rock fragments. Pyrrhotite, pentlandite, and chalcopyrite are the major ore-forming minerals and sphalerite, gersdorffite, violarite, ilmenite, magnetite, rutile, graphite, and molybdenite are found as minor constituents (Grundstr6m, 1980). Fractional crystallization and sulfide saturation modeling suggest that crustal contamination of a relatively primitive parental magma resulted both in a shift of the melt composition from the olivine field to the orthopyroxene field and sulfide saturation (Pertti Lamberg, pers. comm., 2001). 3.2. Kotalahti

The Kotalahti intrusion is one of several mafic-ultramafic intrusions within the NWrunning tectonic shear zone, the Kotalahti Ni belt (Gafil, 1972). It hosted the largest Svecofennian magmatic Ni-Cu sulfide deposit with total production of 12.3 Mt of ore (0.66 wt.% Ni and 0.26 wt.% Cu) in 1957-1987 (Puustinen et al., 1995). According to Gafil (1980) the magma was emplaced along subvertical axial plane of a NNW-trending synform in the Archean bedrock. Next deformation phase CHAPTER

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was associated with high strain, refoliation, and generation of the NW-trending Kotalahti Ni belt shear zone. The Kotalahti intrusion is a subvertical plate that is ~1300 m long and 200 m wide at maximum. The nothern part of the body is steeply dipping and shows normal order of fractionation form footwall peridotites towards hanging-wall gabbros (Figure 9.4). The central part of the plate is characterized by "upside-down" structure with pyroxenitic and peridotitic cumulates in the upper parts of the body. Some structures indicate that the emplacement of the ultramafic rocks postdates that of the gabbros. The southern part of the Kotalahti intrusion consists of a mineralized ultramafic pipe that does not display internal fractionation. Heterogeneous gabbros are abundant in the lower parts and at the marginal zones of the intrusion. They include olivine gabbros, olivine norites, norites, gabbros and hornblende gabbros. The most fractionated rock types are diorites and quartz diorites at the bottom of the complex (Papunen and Koskinen, 1985). Breccia textures are common between the peridotitic, pyroxenitic, and gabbroic units and indicate polyphase intrusion. The sulfides can be classified as disseminated (interstitial), breccia, and massive ore veins (Papunen, 1970). They are associated with the ultramafic cumulate units and a separate breccia-textured ore body (Jussi Ore) within the graphitic gneisses outside the ultramafic intrusion proper. Such offset ores, also present in most of the other Svecofennian Ni-Cu deposits, are of high-grade and form the economic backbone of these otherwise rather low-grade deposits. The mineralogical composition of the Kotalahti ore is simple: the main minerals are monoclinic and hexagonal pyrrhotite, rare troilite, pentlandite, and chalcopyrite. The Jussi Ore also contains pyrite, millerite, and bornite (Papunen, 1970).

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Fig. 9.4. Two representative cross-sections from the northern part of the Kotalahti intrusion (after Papunen and Koskinen, 1985).

3.3. Lapinlahti gabbro-anorthosite The Lapinlahti gabbro-anorthosite was emplaced into the Archean crust close to the craton margin (Figure 9.1). A coarse-grained gabbro dike from the central part of the body yielded a zircon age of 1895 + 15 Ma (Paavola, 1988) implying that also this gabbro belongs

to the major phase of Svecofennian mafic plutonism (Figure 9.2). Lapinlahti intrusion is a subrounded body with a narrow 8-km-long "tail" protruding towards southwest from the main body, and has a total areal coverage of 44 k m 2 (Figure 9.5). The intrusion is completely enclosed by Archean banded tonalite-trondhjemite migmatites and has a concentric struc-

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ture with steeply (60-90 ~ dipping layering and a strike parallel to the intrusion margins. Magmatic layering proceeds from olivine gabbronorites and gabbronorites within the outer rim of the body towards more evolved leucogabbros, anorthosites, and hornblende gabbros in the central parts (Figure 9.6A, B). Minor ultramafic rocks include olivine websterites, websterites, and their hornblendebearing varieties. Crystallization started with plagioclase and olivine and was followed by orthopyroxene and clinopyroxene. Apparently, the parental melt had high volatile content as not only calcic amphibole but also biotite is present as large poikilitic intercumulus grains throughout the crystallization sequence. In the more evolved rock types hornblende and apatite appear as cumulus minerals. Kerkkonen (1985) argued that the parental melt of the Lapinlahti was probably high-A1 basalt. The internal structure of the Lapinlahti gabbro-anorthosite is well displayed on aeromagnetic and ground gravity survey anomaly maps. The Bouguer anomaly (Figure 9.5B) is tightly restricted within the exposed area of the gabbro showing maxima in the center of the body. These features are consistent with the intrusion being an almost vertical funnel-shaped pipe (Kukkonen, 1981). The second vertical derivate of the gravity data visualizes the concentric structure of the body (Figure 9.5C). Ultramafic cumulates, olivine gabbro-norites, and hornblende gabbros outcome as dense outer and inner layers while the anorthosite-dominated middle layers appear as a gravity low. Gray-tone and obliquely illuminated low-altitude aeromagnetic maps bring out more subtle features of magmatic layering (Figure 9.5D, E).

4. Intrusions of the Tampere and Pirkanmaa belts (GmuR The supracrustal belt between the Central Finland granitoid complex and the HS.me belt CHAPTER

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consists of two distinct lithological domains, the high-grade Pirkanmaa belt and the medium-grade Tampere belt (Figure 9.1). The Pirkanmaa belt consists of high-grade and polydeformed tonalitic migmatites derived from psammitic protoliths (Koistinen, 1996). The Tampere belt is a narrow volcano-sedimentary sequence of basaltic to rhyolitic mature arc-type rocks and turbiditic graywackes (K~ihk6nen, 1989). The boundary between the Pirkanmaa and the Tampere belts is located within a major shear zone that is not a major terrain boundary but a thrust or reverse fault (Nironen, 1989). These belts are believed to represent the upper and middle crustal expressions, respectively, of the same volcanic arc-accretionary wedge terrain. The mafic-ultramafic intrusions of the Pirkanmaa belt are, in general, more mafic compared to those of the Tampere belt and have high potential for magmatic Ni-Cu sulfide deposits (Lamberg, 1990; Papunen and Gorbunov, 1995; Peltonen, 1995a). Within these belts a broad correlation exists between the nature of the Group Ib intrusions and the metamorphic grade of their country rocks. Intrusions within high-grade domains tend to be more metamorphosed and deformed, smaller, and more mafic (and Ni-ore potential) than their counterparts in the lower-grade crustal domains. This is a typical feature of synorogenic intrusions and suggests that the depth of their emplacement corresponds to the pressure determined from the metamorphic mineral assemblage of the enclosing supracrustal rocks (Peltonen, 1995a). A complete layered series is not preserved in any of the intrusions but a cumulate pseudostratigraphy- obtained by combining petrographic data from over 50 bodies- illustrates some salient features of their fractionation. Figure 9.7 shows an idealized layered series that would result from closed-system crystallization of the parental melt, as well as the extent of layered series in some example intrusions. The layered series and country rocks are sepa-

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Fig. 9.5. Geological (A), Bouguer gravity anomaly (B), second vertical derivative of Bouguer anomaly (C),gray-tone low-altitude aeromagnetic (D),and obliquely-illuminated aeromagnetic anomaly (E) maps of the Lapinlahti gabbro-anorthosite. Geology modified after Kerkkonen (1985) and Paavola (1988). Geophysical data from the Geological Survey of Finland (processed by Seppo EIo). 418

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rated by a hybrid zone of magma-sediment mingling and by a more extensive marginal zone. The marginal zone shows a reverse trend of differentiation dominated by two-pyroxene cumulates. At the top, direction of differentiation changes to normal and marginal contact zone gives way to the cumulates of the layered series. Peridotite zone is distinguished by the presence of cumulus olivine. Earliest cumulates are composed of cotectic proportions of olivine and chromite and intercumulus material (locally including abundant magmatic sulfides). Olivine-chromite cumulates are followed by olivine-two-pyroxene cumulates until the cumulus termination of olivine marks the top of the peridotite zone. Pyroxenite zone is dominated by two-pyroxene cumulates with plagioclase as a common intercumulus phase; the amount ofplagioclase increases with stratigraphic height. In some bodies of the Pirkanmaa belt plagioclase is absent and magmatic amphibole is the major intercumulus phase. Beginning of the gabbro zone is marked by the appearance of cumulus plagioclase. Lower gabbro zone is dominated by clinopyroxeneplagioclase and clinopyroxene-orthopyroxene-plagioclase cumulates. The most evolved rock types are plagioc|ase-orthopyroxene rocks and plagioclase cumulates; these may contain abundant euhedral apatite, ilmenite, and magnetite. Upper contact zones are only sporadically exposed and are composed of cognate gabbro xenoliths in a matrix of hybrid gabbro-metapelite rocks.

4. I. Ultramafic intrusions of the Vammala Ni province More than 50 small (100-1000 rn long) ultramafic cumulate bodies are concentrated within a roundish crustal block ~ 15 km in diameter near Vammala (Figure 9.8). This a r e a - the Vammala Ni province- is associated with a moderate gravity anomaly maximum that is not explicable by the small and extensively serpentinized cumulates exposed in the area, CHAPTER

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but rather indicates the presence of voluminous mafic-ultramafic massifs at subsurface levels (Elo, 1992). Most of the ultramafic bodies can be depicted as boudins or lenses that "float" in polydeformed, medium- to high-grade paragneisses. The intrusions consist mainly of olivine-chromite and olivineclinopyroxene cumulates with clinopyroxene, orthopyroxene, magmatic amphibole, and phlogopite as intercumulus phases (Figure 9.9A). The entry of plagioclase was delayed, probably as a consequence of crystallization at relatively high confining pressure; moderate crustal pressures are also indicated by relatively aluminous pyroxenes and chromian spinels (Peltonen, 1995a). Metamorphism, deformation, subsolidus equilibration, and geochronology suggest that the emplacement of these cumulatetextured bodies coincided with the peak of regional metamorphism and deformation (Figure 9.6C; Jokela, 1991; Peltonen, 1995a; Marshall et al., 1995). Internal structures of the cumulate lenses and absence of strong penetrative tectonic fabric (in spite of their deformed large-scale morphology) may indicate that the cumulate-textured sills were boudinaged while not completely solidified. Partial recrystallization of cumulate bodies is also consistent with their synkinematic intrusion. Lack of prograde reactions in cumulate cores indicates that, between igneous crystallization and regional metamorphism, the cumulate bodies became hydrated only close to their margins. The metamorphic conditions reached upper amphibolite-lower granulite facies, i.e., 600-700 ~ and 5-6 kbar (Peltonen, 1990). Folding of the host migmatites probably resulted in flexing and melt-facilitated fracturing of the ultramafic bodies and their veining by neosome material (Marshall et al., 1995)- a feature common also in the Hitura and Kotalahti intrusions (Papunen, 1970). The slow cooling of cumulates from peak conditions is evidenced by extensive subsolidus reequilibration of olivine and chromian spinel ULTRAMAFIC

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(Peltonen, 1995c) and redistribution of Ca between pyroxenes. Slow cooling rates and the absence of significant contact aureoles may reflect a low temperature gradient between the cooling intrusions and their metapelite surroundings undergoing migmatization. Later, Fe-Mg silicates became extensive replaced by pseudomorphic lizardite. Although ultramafic in bulk composition, these cumulates were not derived from ultramafic melt as suggested by the relatively low and uniform forsterite content of olivine (Fo = 77.0-82.4 mol.%). Instead, they either crystallized in an open system or represent fragments of much larger intrusions (Figure 9.8). However, because only minor amounts of more evolved cumulates are found in the region, the bodies cannot represent the basal units of fragmented layered intrusions, neither do they represent ophiolitic cumulates or the products of ultramafic magmas. They have been interpreted as remnants of synorogenic conduits for tholeiitic arc-type magmas that became choked by cumulus crystals and were boudinaged into small lenses and fragments by concomitant tectonic movements (Peltonen, 1995a, b). Several of the Vammala Ni province intrusions are mineralized (Figure 9.6D). The largest magmatic Ni-Cu sulfide deposit was hosted by the Vammala intrusion (HLkli et al., 1979)- it yielded 7.6 Mt of ore (0.68 wt.% Ni, 0.42 wt.% Cu) in 1974-1994 (Puustinen et al., 1995). Most of the sulfides are interstitial,

Fig. 9.7. An idealized magmatic cumulus stratigraphy for a Group I intrusion crystallized from a single pulse of magma.The column was reconstructed from several bodies of the Pirkanmaa belt; complete stratigraphy is not preserved in any of the intrusions.The extent of cumulus sequence observed in some well-studied intrusions is outlined on the right (Lamberg, 1990; Peltonen, 1995a; Peltonen and EIo, 1999). Mineral abbreviations for the cumulate (C) names after Irvine (1982): o-olivine, b-bronzite, a-augite, p-plagioclase, s-spinel.

indicative of early formation of immiscible sulfide liquid in the magma (Figure 9.9B). Minor remobilization of the interstitial ore occurred during metamorphism and deformation resulting in formation of thin massive sulfide

Fig. 9.6. (facing page) (A) Layered leucogabbro, Lapinlahti gabbro-anorthosite. (B)"Mottled" anorthosite, Lapinlahti. (C) Pyroxenitic dike in polydeformed graywacke-slate migmatite in the proximity of the Piim~sj~rvi intrusion,Vammala Ni province. Such features can be applied to constrain the timing of the magmatism relative to the regional deformation. (D) Orbicular peridotite from the Kylm~koski Ni-Cu deposit. Several origins have proposed for such a texture, e.g., the orbicules could represent rounded cumulate fragments (autoliths) that settled to the base of the magma chamber together with the immiscible sulfide liquid or be products of rapid olivine crystallization due to supercooling. (E) Pothole structure in the layered series of the Hyvink~ intrusion. (F) Layered ultramafic cumulates, Hyvinkli~. (G) Rhytmically layered gabbronorite cumulate layers, Hyvink~. (H) Fragmental unit with gabbro autoliths embedded in fine-grained gabbm, Hyvink~. Photos: Petri Peltonen (A, B, G), Markku Tiainen (C), Jari v~t~inen (D) and Riku Raitala (E, F, H). CHAPTER

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veins. The dominating sulfide assemblage is monoclinic pyrrhotite + pentlandite + chalcopyrite + cubanite + mackinawite + valleriite. Less common sulfides include gersdorffite, niccoline, various tellurides, sphalerite, galena, molybdenite, gold, silver, and PGM (Peltonen, 1995b). The petrogenesis of these magmatic sulfide ores will be discussed in Section 7.

Porrasniemi layered gabbro is a typical example of mafic-ultramafic intrusions within the high-grade Pirkanmaa belt. The internal structure of Porrasniemi is well-preserved and enables detailed study of magmatic evolution and primary structures. Porrasniemi exhibits an extensive and complete cumulate sequence and has some special interest because of its apparent potential for magmatic sulfide deposits at depth. The Porrasniemi layered gabbro comprises three tectonic blocks separated from each other by migmatites (Figure 9.10). Originally, the gabbro was probably emplaced as a continuous 2-km-long and 400-m-thick stratiform sheet but became boudinaged by tectonic movements. In the subsequent deformation, the three blocks were rotated relative to each other and the magmatic layering was tilted close to vertical (Lamberg, 1990). Each block can be subdivided into layered series and marginal series. Marginal series is located between the layered series and the footwall contact of the intrusion and shows a reverse trend of fractionation: olivine content increases and plagioclase content decreases upwards in the sequence towards the peridotite zone. The marginal series is an up to 75-m-thick heterogeneous unit with abundant country rock xenoliths and poorly developed cumulus textures. Lamberg (1990) distinguished a country rock xenolith-rich "fragmental unit" within the marginal series of the KS.rki block. Transition from the marginal series to the layered series

is relatively abrupt and is marked by a change from reverse to normal fractionation. The layered series can be divided into peridotite, pyroxenite, and gabbro zones. The peridotite zone is relatively thin (---50 m) and consists of peridotites and olivine websterites at the base and wehrlites at the top. The overlaying pyroxenite zone is approximately 250 m thick and consists of two-pyroxene cumulates (Figure 9.9C). The modal amount of intercumulus plagioclase gradually increases upwards in the pyroxenite zone. The peridotite and pyroxenite zones consist of ~ 40-cm-thick layers of uniform composition. At the base of the peridotite zone modal rhytmic layering is present: 15-cm-thick augite-bronzite-olivine cumulate layers are frequently separated by 1-2-cm-thick olivine-rich laminae. In the gabbro zone, melano- and leucocratic laminae alternate and, at the highest stratigraphic levels, 1-3-cm-thick pyroxene laminae alternate with plagioclase-rich layers. This type of layering resembles the schlieren-lamination of Irvine (1982). The ~ 100-m-thick gabbro zone is characterized by cumulus plagioclase gabbros and norites. The contacts between peridotite, pyroxenite, and gabbro zones are phase boundaries: beginning of the pyroxenite zone is marked by disappearance of cumulus olivine, and the start of the gabbro zone by appearance of cumulus plagioclase. Smooth geochemical and mineral chemical trends suggest that Porrasniemi crystallized from a single pulse of magma. Because chilled margins are not exposed and the rocks are cumulates, the composition of parental magma is difficult to estimate. However, back-calculation from cumulus mineral compositions implies that the parental magma was close to tholeiitic basalt. Ubiquitous hydrous intercumulus minerals suggests that it had relatively high volatile content. Mass balance calculations have shown that the cumulus sequence is incomplete also in Porrasniemi. The most primitive olivinecumulates are missing from the intrusion, and

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4_2_Porrasniemi layered gabbro

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Fig. 9.8. Simplified geology of the Vammala Ni province.The small size, deformed morphology, and internal textures of ultramafic bodies suggest that the bodies represent originally much larger ultramafic sills or dikes that became boudinaged by deformation that was synchronous with their emplacement (after Peltonen, 1995a).

olivine may have saturated already within the feeder system. Also, the most evolved plagioclase-titanian magnetite cumulates, predicted to occur at the top, have not been found; they may remain unexposed above the norites or the most evolved melt may have escaped the magma chamber (Lamberg, 1990).

4.3. Kaipola layered intrusion The Kaipola layered intrusion (Figure 9.11) is located close to the easternmost tip of the Tampere belt close to the boundary of the Central Finland granitoid complex (Sandholm, 1970). The intrusion is situated between two NW-trending faults about 20 km to the north of the shear zone separating lower grade rocks CHAPTER

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of the Tampere belt (upper crustal milieu) from the strongly deformed and metamorphosed rocks of the Pirkanmaa belt (middle crustal milieu). The intrusion is a 4.6-km-long and 2.2-km-wide oval gabbro-diorite body, which is beautifully displayed in the aeromagnetic map (Figure 9.11A). Its residual gravity anomaly is about 13 mGal and, according to gravity modeling, the body dips to the north-northwest with an average depth extent of about 1.4 km. The associated volcanic rocks give rise to a magnetic maximum on the northwest side of the intrusion (Peltonen and Elo, 1999). Unlike most other Svecofennian mafic-ultramafic intrusions, Kaipola is not enclosed by metasedimentary rocks but by syn- and postkinematic granitoids. Gabbro-granite ULTRAMAFIC

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Fig. 9.9. (A) Olivine-clinopyroxene-orthopyroxene cumulate with magmatic amphibole occupying the intercumulus space. Note the resorbed outlines of the cumulus clinopyroxe crystals indicative of the peritectic reaction clinopyroxene + intercumulus melt = amphibole. Vammala Ni province. (B) Olivine + chromite cumulate with interstitial Ni-Cu-Fe sulfides consisting of pentlandite (light yellow), chalcopyrite (yellow), and pyrrhotite (light brown).Vammala Ni-Cu deposit. (C) Clinopyroxene-orthopyroxene cumulate with intercumulus plagioclase. Porrasniemi layered gabbro. (D) Clinopyroxene-orthopyroxene-plagioclase cumulate with brown poikilitic intercumulus amphibole (pargasite). Note the resorbed outlines of the cumulus crystals due to their peritectic reaction with hydrous intercumulus melt. Kaipola layered intrusion.Width of the images corresponds to 2.5 mm.

relationships are well exposed at a road cut close to the western tip of the intrusion where the contact zone is characterized by gabbro enclaves in the granite and small mica-rich clots in the gabbro- both features implying immiscibility of two melts. Two distinct types of granitoid dikes intrude the gabbro" older fine-grained, 5-50-cm-wide, and deformed dikes with smooth and irregular boundaries and younger coarse-grained pegmatite dikes that sharply cut both the gabbro and the finegrained granite dikes. The relationship of the gabbro with the older dikes gives an impres-

sion of immiscibility and co-existence of felsic and mafic magmas, whereas the younger dikes clearly postdate the solidification of the gabbro. The older dikes are interpreted to be coeval with the synkinematic granitoids enclosing the gabbro and the younger dikes are related to the emplacement of the somewhat younger, postkinematic, Kaipola granitoid pluton nearby (cf. Nironen et al., 2000). The Kaipola layered intrusion is characterized by a well-preserved layered series showing distinctive large-scale repetitive layering with at least seven zones (Figure 9.11B). Thin

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Fig,. 9. I O. Geological map of the Porrasniemi layered gabbro (modified after Lamberg, 1990).The originally sill-type intrusion that crystallized from a single pulse of magma is interpreted to have boudinaged into three roundish fragments rotated relative to each other during regional deformation.

olivine-bearing (augite-bronzite-olivine cumulates, aboC) and pyroxenitic layers (augite or augite-bronzite cumulates, aC/abC) represent the most primitive fractionation products and have been observed at the base of some of the zones. Most of the cumulus sequence is composed of more evolved leucocratic-mesocratic plagioclase-dominated orthocumulates (paC/pabC/pbC/pC) with apatite and titanian magnetite as minor cumulus phases. A striking feature of the intrusion are large postcumulus oikocrysts of green and brown amphibole, implying that the parental melt of the intruCHAPTER

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sion was relatively hydrous (arc-type basalt?). Some of the poikilitic amphibole was formed by interstitial crystallization, but most of it was produced by peritectic replacement of cumulus pyroxenes and plagioclase (Figure 9.9D). Other intercumulus minerals include ilmenite, apatite, phlogopite, quartz, zircon, and plagioclase. Sulfides are uncommon which, together with low PGE contents, implies that Kaipola layered intrusion has low potential for magmatic sulfide deposits.

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5. Synvolcanic intrusions of the Arc complex of southern Finland

5. I. Forssa gabbro The Forssa gabbro consists of medium-grained amphibole and pyroxene gabbros, diorites, and quartz diorites. Ultramafic and anorthositic varieties are uncommon. Aeromagnetic low-altitude map (not shown) brings out the concentric structure of the gabbro with gabbroic rocks in the core and diorite in the margins. In many places the diorite brecciates the gabbro. This, together with the lack of fine-scaled magmatic layering, is indicative of a dynamic environment of crystallization. In the northwest, the plutonic rocks grade into a plagioclase-phyric and weakly ophitic hypabyssal rock type that gradually grades into hornblende-plagioclase porphyry (Neuvonen, 1956). All these features, also supported by the U-Pb zircon ages (Figure 9.2), are indicative of a synvolcanic nature of the gabbros and a comagmatic origin of the spatially associated volcanic formations.

The H~ime belt is a major volcanic-dominated terrain in southern Finland bounded by the Pirkanmaa belt in the north and high-grade metamorphic gneiss complexes in the south (Figure 9.1). Hakkarainen (1994) divided the supracrustal formations into the Forssa Group and stratigraphically younger H~ime Group. The volcanic rocks of the Forssa Group are mainly medium-K, calc-alkaline pyroclastic andesites probably related to mature arc stratovolcanoes with intervening sedimentary basins. The H~ime Group volcanic rocks are medium-K tholeiitic basalts and basaltic andesites believed to represent fissure eruptions during late intra-arc rifting. These volcanic formations are spatially associated with mafic intrusions, the most extensive of these are the Forssa gabbro and the Hyvink~i layered intrusion (Figure 9.1). Age data from the H/ime belt and associated mafic intrusions are few. An andesitic lava close to the western margin of the Forssa gabbro yielded an age of 1888 + 11 Ma (Vaasjoki, 1994). This felsic lava unit represents the uppermost units of the H~ime belt (Hakkarainen, 1994) and thus provides a minimum age for the Forssa gabbro. Patchett and Kouvo (1986) reported an age of 1880 + 5 Ma for a gabbro pegmatoid of the Hyvink~ layered intrusion. Plagioclase porphyrite close to the eastern margin of the intrusion yielded an age of 1880 i 3 Ma and is considered to be indicative of the cogenetic origin of the gabbro and volcanic rocks (Suominen, 1988). A somewhat younger age, 1870 + 8 Ma, was yielded by a hornblende gabbro from the Soukkio complex ~20 km east of Hyvink~i~i (Huhma, 1986). In the light of these data the possibility remains that the mafic magmatism gets younger from west to east: i.e., Forssa gabbro (>1888 Ma) > Hyvink~i~i layered intrusion (~1880 Ma) > Soukkio complex (~1870 Ma).

The synvolcanic Hyvink~i~i layered intrusion in the H~ime belt (Figure 9.12) has an areal extent of---120 km 2 and is one of the largest Svecofennian layered gabbro complexes. It is an oval lopolithic body consisting of layered peridotites, pyroxenites, olivine gabbros, gabbronorites, non-layered isotropic gabbros, and granophyre. According to Raitala (1997), the body has been slightly tilted from its primary position so that the western part exposes deeper levels of the igneous stratigraphy and thus more primitive cumulates than those exposed in the east. In the east, the gabbros are rich in hornblende, biotite, and magnetite which, together with some quartz and alkali feldspar, imply proximity of roof. The layered series of the Hyvink~i~i intrusion has been studied in detail by Raitala (1997). The outermost shell is a hybrid zone between the country rocks and the first marginal series cumulates. The hybrid zone may reach

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5.2. Hyvink~ layered intrusion

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Fig. 9. I I. (A) Low-altitude aeromagnetic anomaly map of the Kaipola layered intrusion and surroundings.The intrusion is located between two parallel NW-trending faults.The lower part of the image illustrates the sharp boundary between the high-grade Pirkanmaa belt and lower-grade Tampere belt (after Peltonen and EIo, 1999; data from the Geological Survey of Finland). (B) Geology of the Kaipola layered intrusion. Mineral abbreviations for the cumulate names as in Figure 9.7. CHAPTER

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600 m in thickness but locally may be only a few meters. It consists of a variety of rock types that formed through mixing between the magma and country rocks. Frequently, the hybrid rock types bear evidence for strong interaction with late magmatic fluids. The presence of some disseminated sulfides and tellurides, including some Pt group minerals, has raised economic interest for the hybrid zone (Raitala, 2000). The marginal series is believed to have formed from cumulus crystals that nucleated and grew higher in the magma chamber but which, due to density contrast or convective currents, settled at the sidewalls of the magma chamber. The rock types of the marginal series are cumulates with clinopyroxene, orthopyroxene, and olivine as cumulus phases and plagioclase and clinopyroxene as the most common intercumulus minerals. The cumulus paragenesis alternates in a random manner and is distinct for example from the marginal series of the Porrasniemi intrusion (above) in which the order of crystallization was reverse to that of the layered series. The layers are frequently graded and 1 to 5 m thick. Magmatic erosion, slumping, gliding as well as textures implying filter pressing are common (Figure 9.6E). Erosional discordance separates the marginal series from the overlying layered series. Most of the layered series consists of rhytmically layered gabbro to gabbronorite cumulates (Figure 9.6E G). The layers are generally 0.5-30 cm thick. A characteristic feature are country rock xenoliths and autolithic fragments (Figure 9.6H). These have settled parallel to the magmatic layering. A late magmatic dunite pipe, which consists of cumulus olivine and chromite and intercumulus clinopyroxene, intrudes the layered series (Figure 9.12). Later, the Hyvink/i/i layered intrusion was intruded by K-rich granite and diabase (Raitala, 1997).

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6.Ti-Fe-Pgabbros of the Central

Finlandgranitoid complex

Several gabbroic intrusions, some of them hosting important magmatic oxide deposits, are found within the Central Finland granitoid complex (Figure 9.1). Most of them are located in the peripheral areas of the complex and are confined to two clusters: the Kauhaj/irvi and Koivusaarenneva gabbro provinces. Although intrusions within these two provinces share a number of features, distinct origins have been proposed. R/im6 et al. (2001) argued that the Kauhaj/irvi gabbros are genetically related to the ~1.87 Ga postkinematic Lauhanvuori granite and form a bimodal magmatic suite. In contrast, according to K/irkk/iinen (1999a), the Koivusaarenneva gabbros are Group I intrusions emplaced during the synorogenic stage of the orogeny.

6.I.Kauhaj~ryig_abbrgprovjnce The Kauhaj/irvi gabbro province (Figure 9.1) consists of five, 2-10-km-long and 1-3-kmwide gabbro intrusions. These gabbros are located between the postkinematic (1867 4- 6 Ma) metaluminous to peraluminous Lauhanvuori granite in the west and a foliated synkinematic (1886 + 11 Ma) granodiorite in the east (R/ira6 et al., 2001). Field observations imply that gabbros are younger than the synkinematic granitoids but are intruded by the Lauhanvuori granite that postdates the main stage of the Svecofennian orogeny (R/im6, 1986). The Per/imaa gabbro has yielded a similar crystallization age (1874 + 14 Ma; R/im6 et al., 2001) and has been related to the same geotectonic event as the Lauhanvuori granite. The intimate association of the Ti-Fe-P gabbros and latekinematic granites imply an mature, postorogenic type setting for the magmatism. Nironen et al. (2000) interpreted the MAFIC-ULTRAMAFIC

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Fig. 9.12. Geology of the Hyvink~i~ layered intrusion after Raitala (1997) and Vuori (1999). contact features of the postkinematic granites and spatially associated gabbros as a result of mixing and mingling of coeval felsic and mafic magmas. They disregarded the possibility of a single parental magma and presented a two-stage model for the coexistence of the felsic and mafic magmatic suites. According to this model, compositionally variable and anhydrous lower crust was first produced as a result of extraction of synkinematic magmas. This granulitic residue was then melted due to CHAPTER

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the mafic underplating. Both anatectic silicic melts and underplating mafic magmas contributed to the bimodal postkinematic magmatism that took place in response to extensional or transtensional events that modified the tectonically thickened Svecofennian crust.

Kauhaj~rvi gabbro The Kauhaj~irvi gabbro consists of two chemically distinct zones: relatively thin (~50 m), poorly layered basal zone and thicker (>400

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m) well-layered main zone (Figure 9.13). The basal zone consists of gabbro, gabbronorite, and olivine gabbronorite and layering is only weakly developed. Ilmenite and apatite are strongly concentrated in the uppermost part of the basal zone. The main zone is texturally and modally layered and extends from peridotite to anorthosite. Layers are 0.2-20 m thick. The chemical compositions of the basal zone and main zone are distinct - the main zone is enriched in Fe, Ti, and P and depleted in Mg, Cr, Si, and A1 relative to the basal zone. Ilmenite was saturated early in the main zone and was concentrated into the most primitive Mg-rich cumulates. The main zone contains ~1500 ppm F, which is believed to reflect interaction of basaltic parental melt with coeval granitic magma. The compositional variation of both the basal zone and main zone can be explained by a closed-system fractional crystallization of a single pulse of tholeiitic magma. However, redox conditions during their crystallization were variable. The basal zone crystallized under low fO z, which resulted in strong enrichment ofTi, Fe, and P at the top of the zone. The main zone crystallized under relatively high oxygen fugacity resulting in co-precipitation of ilmenite and apatite throughout the layered series, thus preventing formation of massive oxide ore layers (KS.rkk/iinen, 1999a).

R/ira6 (1986) described rhytmic layering where individual 10-15-cm-thick layers consist of thin seam of magnetite in the bottom followed first by gabbro and then leucogabbro on the top. As was in the case of Kauhaj~rvi intrusion, also in Pefiimaa the Fe, Ti, and P abundances are highest in the melanocratic cumulates. The Perfimaa intrusion shears many features in common with anorogenic intrusions; this reflects its emplacement into stabilized crust during the latest stages of the Svecofennian orogeny (Nironen et al., 2000). Before the final emplacement the primary melt for the Perfimaa gabbro evolved under low fO~ at almost closed system, which resulted in a Ti-Fe-P-rich parental magma for the intrusion (Rfim6, 1986). During crystallization, however, fO, increased as evidenced by the gravitative accumulation of oxides at the base of individual cumulate layers. Importantly, the H~O content of the melt was low. This is in marked contrast to the synorogenic intrusions such as Kaipola that crystallized from hydrous (arc-type) magma.

6.2. Koivusaarenneva layered intrusion

The PerS.maa intrusion is found in the same tectonic setting as the Kauhajfirvi gabbro (Figure 9.1). Intermediate differentiates dominate but ultramafic-gabbroic rocks make approximately one third of the total volume of the intrusion (Figure 9.14). This mafic part of the body consists of rhytmically layered or massive cumulus-textured peridotites, olivine gabbronorites, gabbronorites, and gabbros, derived from a tholeiitic parental magma (R/ira6, 1986). Plagioclase (An32_5~), olivine (Fo35_70), titanian magnetite, ilmenite, apatite, and clinopyroxene are cumulus phases, plagioclase and orthopyroxene are intercumulus.

The Koivusaarenneva layered intrusion- the host for a major magmatic ilmenite deposit - is located ~170 km north-northeast of the Perfimaa gabbro (Figure 9.1). It is an elongate& 0.5-1-kin-wide and 3-km-long sill-like intrusion belonging to a suite of several similar intrusions that are found adjacent to the intersection of SW- and SE-trending fault zones. The intrusion itself bears some similarities with the intrusions of the Kauhaj/irvi gabbro province- it is intrusive to the Central Finland granitoid complex and shares some compositional features of anorogenic mafic plutonism. However, the Koivusaarenneva intrusion is not associated with postkinematic granites and has yielded an older zircon age of 1881 i 6 Ma (K/irkk~iinen, 1999b). K/irkk/iinen (1999a) interpreted it to belong to the synorogenic group and to share common origin with other

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Fig. 9.13. Geology of the Kauhaj~rvi gabbro (after K~rkk~iinen, 1999a). craton margin intrusions. On the basis of modal mineral variations, K/irkkfiinen (1999a) divided the Koivusaarenneva intrusion into lower, middle, and upper zones characterized by titanian magnetite, ilmenite, and apatite, respectively (Figure 9.15). The 150-500-m-thick lower zone consists of layered gabbro and gabbronorite (plagioclase-pyroxene cumulates). Thin (0.2-1 m) pyroxenitic layers are spatially associated with oxide-rich layers. The lower zone hosts disseminated (8-18 vol.% ilmenite) to semimassive titanian magnetite-ilmenite ore that CHAPTER

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is characterized by low TiO2/Fe203.The lower zone is, however, of only minor economic importance. The beginning of the middle zone is marked by an abrupt increase in ilmenite and magnetite and normative pyroxenes coupled with decrease in plagioclase. The middle zone hosts a 5-20-m-thick layer of massive, magnetite-poor, ilmenite ore (18-48 vol.% ilmenite) overlain by up to 40 m of disseminated ilmenite. The massive ore layers are associated with both pyroxenitic and gabbro layers, the remainder of the middle zone consists of gabbro and gabbronorite. The middle zone is the

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Fig. 9.14. Geological (A), Bouguer gravity anomaly (B), second vertical derivative of Bouguer anomaly (C),gray-tone low-altitude aeromagnetic (D), and obliquely illuminated aeromagnetic (E) anomaly maps of the Per~imaagabbro.Ti-rich oxide layers give rise to pronounced aeromagnetic maxima in the central part of the body. Geology after R~im6 (1986). Geophysical data from the Geological Survey of Finland, processed by Seppo EIo.

The chemical compositions of mafic-ultramafic intrusions are determined by initial composition of the parental magma, assimilation of country rock material, possible segregation of immiscible sulfide liquid, and by cumulus processes. As all Svecofennian intrusions are composed of various types of cumulates and fresh chilled margins are not well-preserve& the composition of the parental magma has to

be determined indirectly. Because of their potential for Ni-Cu + PGE deposits, several attempts have been made to determine the parental magma composition of the Group I intrusions (MS.kinen, 1987; Lamberg, 1990; Peltonen, 1995a; Makkonen, 1996). The Mg content of the primary magma was close to 12 wt.% MgO as backcalculated from the forsterite content of the most magnesian cumulus olivine (Lamberg, 1990; Makkonen, 1996). This implies that the ultramafic cumulates were not derived from ultramafic magma but represent cumulates from more evolved melts. Peltonen (1995a) argued that the incompatible trace element composition of the most primitive olivine cumulates of the Vammala Ni province closely approximates that of the parental magma. This approach - based on the fact that olivine has

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economically most important ore horizon. The 300-800-m-thick upper zone is only weakly layered and is dominated by fluorapatite-rich gabbro with minor ilmenite.

7. Chemical and isotope composition of the mafic-ultramafic intrusions

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Fig. 9.15. Igneous stratigraphy of the Koivusaarenneva layered intrusion (after K~irkk~iinen, 1999a). extremely low mineral/melt partition coefficients for incompatible trace elements and that trapped melt thus determines the trace element composition of such cumulates- implied that the incompatible trace element composition of the magma was similar to that of the enclosing metaturbidites. This, together with lower than depleted mantle ENd (at 1.9 Ga) values (Fig. 7.1; Huhma, 1986, unpubl.; Makkonen, 1996), low Se/S of the sulfides (Peltonen, 1995b), and presence of graphite in the ores (Peltonen et al., 1995) shows that the trace element composition of parental magma for the Group I intrusions was strongly modified during emplacement through the Svecofennian crust. Thus it bears no unequivocal information of its mantle source. Primary magmatic intercumulus amphiboles are particularly common in the Group I bodies (Lamberg, 1990; Makkonen, 1996; CHAPTER

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Peltonen and Elo, 1999) indicating high water content of the parental magma. This is characteristic for arc cumulates of Phanerozoic terrains (Snoke et al., 1982; Regan, 1985; Butler, 1989; DeBari and Coleman, 1989; Kepezhinskas et al., 1993; Skirrow and Sims, 1999) and led Peltonen (1995a) to suggest that the Group I bodies are cumulates of Svecofennian arc basalts. Arc-type volcanic rocks are, however, uncommon in the vicinity of the Pirkanmaa belt intrusions. The belt is characterized by metaturbidites and fragments of primitive oceanic crust and this led Lahtinen (1994) to favor an accretionary wedge setting for the intrusions of the Pirkanmaa belt. Major element ternary plots (Figure 9.16) are used to illustrate fractional crystallization of the intrusions. The crystallization started with olivine in Vammala, Porrasniemi, Laukunkangas, and Kotalahti. In the ultramafic cu-

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mulate bodies ofVammala Ni province (black circles) the extent of fractional crystallization is restricted to olivine and olivine-pyroxene cumulates. In all bodies olivine is followed by pyroxenes in the cumulate sequence. In this respect, intrusions of the craton margin environment and Pirkanmaa belt show contrasting behavior. Generally, clinopyroxene saturated after olivine in the Pirkanmaa belt intrusions (such as Porrasniemi), shifting the cumulate compositions towards the CaO-apex in the CMA-ternary (Figure 9.16). Figure 9.16 also shows that the most primitive olivinebearing cumulates are absent in Porrasniemi. This implies early segregation of significant amounts of olivine (+ chromite) during ascent or intermediate magma storage- a process that significantly decreased the potential of the Porrasniemi intrusion to host magmatic sulfide deposits. Orthopyroxene dominates in the craton margin intrusions resulting in Laukunkangas-type trends with low and ~constant CaO abundances (Figure 9.16). This feature was recognized by Mfikinen (1987), who divided intrusions into Vammala-type (cpx-dominated) and Kotalahti-type (opx-dominated) and interpreted the division to reflect higher degree of mantle melting for Kotalahti-type magmas. However, crustal contamination provides an alternative explanation: extensive assimilation of country-rock sediments would increase the silica content of the melt resulting in early crystallization of orthopyroxene instead of clinopyroxene (Haughton et al., 1974). This is supported by the Nd isotope composition of the intrusions. In the Juva area (Figure 9.1), where Kotalahti- and Vammala-type bodies coexist in the same region, two Kotalahti-type intrusions yield an average ENd(at 1.9 Ga) of +0.7, whereas three Vammala-type bodies yield an average eNa(at 1.9 Ga) of +1.7. This suggests higher amount of crustal material in the Kotalahti-type magmas (Makkonen, 1996). The Laukunkangas intrusion, which is a typical Kotalahti-type intrusion and hosts a 434

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major magmatic Ni-Cu sulfide deposit, has also a relatively low ENd(at 1.9 Ga) value of +0.2 + 0.5 (Huhma, 1986). It seems reasonable to conclude that the crustal environment of emplacement had a profound effect on the chemical evolution of the melts and thus on the mineralogy of the forming cumulates. Intrusions that were emplaced through the thick sialic Archean crust or the Primitive arc complex were more likely to become contaminated by SiO 2 and crystallize orthopyroxene. Intrusions in western Finland became contaminated as well, but the main contaminant was carbonaceous and calcareous sulfidic black schist material resulting in early sulfide and clinopyroxene saturation in the melt. The Group II synvolcanic intrusions of the arc complex of southern Finland have somewhat different chemical and isotope characteristics compared to Group I intrusions (Huhma, 1986; Patchett and Kouvo, 1986; Vuori, 1999). The HyvinkfiS. layered intrusion, for example, crystallized from a subalkalic tholeiitic magma (Vuori, 1999). Low Nb and Zr, together with slight LREE enrichment, high Sr and Ba/Rb suggest an island arc setting. In the AFM ternary diagram the Hyvink/i/i layered intrusion follows a typical tholeiitic trend with local production of Ti-Fe-V-rich residual melts (Figure 9.16). The compositional trend in the CMA ternary implies that the cumulate compositions are largely related to orthopyroxene fractionation - this is consistent with the noritic bulk composition of HyvinkS./i. Vuori (1999) concluded that, although crustal xenoliths are locally common in the cumulate sequence of the Hyvinkfi/i layered intrusion, the effect of contamination on the melt composition was relatively small. This is consistent with the high ENd(at 1.9 Ga) value of +2.7 (Patchett and Kouvo, 1986). The Group Ill PerS.maa intrusion records e~a(at 1875 Ma) of-0.1 (average of five) and is less juvenile compared to the intrusions of the Arc complex of southern Finland but is similar MAFIC-ULTRAMAFIC

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Fig. 9. I 6. Major element CMA and AFM ternary plots illustrating the trends of the fractional crystallization for selected Group I, II, and III mafic intrusions. Boundary between the calc-alkaline and tholeiitic fields in the AFM diagram is after Irvine and Baragar (1971). to other mafic-ultramafic intrusions outside Central Finland granitoid complex (R~im6 et al., 2001). The time-corrected Pb isotope ratios for Per~imaa gabbro and diorite average at 2~176 = 15.64 and 2~176 = 15.28 (R/im6 et al., 2001); these are close to those of the adjacent postkinematic granitoid plutons. They both plot close to the composition of average crustal Pb. R/im6 et al. (2001 ) favored these values to reflect an enriched subcontinental lithospheric mantle source. However, they could also indicate pervasive interaction CHAPTER

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of depleted mantle derived magmas and crust during the genesis of postkinematic granite plutons and associated mafic intrusions.

8. Economic aspects and petrogenesis of the ores Svecofennian mafic-ultramafic intrusions show high potential for both magmatic NiCu + PGE sulfide deposits (Papunen and Gorbunov, 1985) and ilmenitic (FeTiO3) Ti

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ores (K/irkk/iinen et al., 1997). Magmatic Ni sulfide deposits are restricted to the orogenic Group Ia and Ib bodies. The potential of the synvolcanic Group II intrusions has also been extensively evaluated, but the results have been discouraging (H/ikli, 1970; Raitala, 1997; Vuori, 1999). Since the start up the production of the Makola deposit in 1941, altogether nine deposit have been exploited from Group I bodies. The total production has been 41 Mt ore with an average (weighted) grade of 0.67 wt.% Ni and 0.28 wt.% Cu. Most of the deposits have low abundances of Pt group elements. Rare examples of PGE mineralized Group I intrusions are Ekojoki (Peltonen et al., 1995) and Uudiskorhola (Papunen, 1989). Group I intrusions have been the most important source rock for Ni in Finland and their production has far exceeded that from other type of formations (e.g., Archean komatiites in eastern Finland ). Sulfide mineralogy and ore textures indicate that the Ni-Cu deposits originated as concentrations of immiscible sulfide liquid (Figures 9.6D, 9.9B). The mineralized zones are frequently located within the most primitive cumulates at the stratigraphic base of the intrusions (Papunen and Gorbunov, 1985; M/ikinen, 1987; Peltonen, 1995a; Makkonen, 1996). In addition, deformation has resulted in remobilization of primary sulfides and formation of economically important high-grade offsets in several intrusions (Kotalahti, Laukunkangas, Telkk/il/i). Chemical, isotope, and mineral composition of the ores require that assimilation of sedimentary rocks by the magma, combined with decreasing temperature, was the ultimate cause for the sulfide saturation and formation of immiscible nickeliferous sulfide liquid. For the deposits of the Vammala Ni province (Group Ib bodies), Peltonen (1995b) suggested that in the metamorphic environment, H2S-bearing C-O-H-S fluids were continuously produced in the surrounding schists through the conversion of pyrite to pyrrhotite in the presence of graphite. The 436

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circulation of such fluids enabled selective transfer of large quantities of S and Zn from black schists into the cooling magma. In the craton margin intrusions (Group Ia) the ore genesis was basically similar, but black schist sulfides were less important contaminants. In this case, the magmas gained e x c e s s S i O 2 from their country rocks, which promoted orthopyroxene crystallization and sulfide saturation (Makkonen, 1996). The Group III intrusions host important ilmenite resources. The Koivusaarenneva layered intrusion is estimated to contain 44 Mt ore down to 150 m with 15% ilmenite and 6% vanadiferous magnetite (K/irkk/iinen et al., 1997). The Koivusaarenneva intrusion is currently under feasibility study. The whole Koivusaarenneva gabbro province (Figure 9.1) has exploration potential as also several other intrusions host Ti deposits. K/irkk/iinen (1999a) proposed a two-stage model for the Koivusaarenneva intrusion and the oxide mineralizations. At the first stage, a primary tholeiitic arc basalt magma underwent fractional crystallization in a deep magma chamber. This took place at very low fO 2 to prevent early saturation of Ti-rich oxides. In the second stage, the modified residual magma - enriched in TiO 2 up to 3.3 wt.% - was emplaced at higher crustal levels to form the intrusion. The average chemical compositions of the lower, middle, and upper zones are not consistent with the origin from a single batch of magma, but requires successive magma pulses from the same source. The lower zone crystallized from a single magma pulse and fractional crystallization of ilmenite and ferroan titanian spinel led to the stratification. The middle zone and the ilmenite ore were probably formed through open-system fractional crystallization in a dynamic system as the amount ofilmenite ore exceeds what might be expected to saturate from the volume of the magma represented by the middle zone. The upper zone crystallized from a more fractionated and P-rich pulse of the parental magma. MAFIC-ULTRAMAFIC

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9_Concludingremarks The mafic-ultramafic intrusions of the Svecofennian orogen are classified into three major groups. Their areal distribution, petrology, and geochemical and isotope characteristics provide important constraints for crustal evolution. Group I intrusions include all synorogenic bodies that were emplaced either within the Archean craton margin, the Primitive arc complex or the Arc complex of western Finland (Figure 9.1). This implies that their emplacement was coeval with or slightly postdated the amalgamation of these arc complexes. Although Group I intrusions may have diverse country rocks, shapes, and to some extent crystallization order of minerals, they share a number of common features. Their U-Pb zircon systematics invariably record ages between 1.89-1.87 Ga - t h e time of the synorogenic stage of the orogeny. Structural analysis indicates that the bodies were emplaced during early orogenic stage into short-lived extensional structures within the arc crust. Ubiquitous boudinage and fragmentation, partial metamorphism, and extensive fluid-driven contamination are all indicative ofsynkinematic intrusion. Group I intrusions have high potential for magmatic Ni-Cu sulfide deposits. All intrusions within the Arc complex of southern Finland are Group II bodies. They differ from Group I intrusions in being much larger and spatially associated with metavolcanic rocks. In contrast to the Group I intrusions- which show evidence for crystallization at high confining pressures (Peltonen, 1995 a ) - the Group II intrusions crystallized at low pressure. The boundary of the H~rne and Pirkanmaa belts is a major tectonic boundary, no Group I intrusions are found south of it and no Group II intrusions north of it. This suggests that the emplacement of both types of mafic intrusions preceded amalgamation of these two arc complexes. This is also supported by the distinct initial ENd values for CHAPTER

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mafic-ultramafic intrusions across the boundary. These imply more a depleted source or less crustal contamination for the Group II intrusions (Figure 9.1). The Group II intrusions - apparently because of their more evolved compositions and less dynamic crystallization regime (Maier et al., 2001) - have significantly lower potential for magmatic sulfide deposits than the Group I intrusions. Group III comprises evolved gabbroic intrusions in the Central Finland granitoid complex region. This is a relatively poorly characterized suite of bodies including gabbros genetically related to postkinematic (~ 1.87 Ga) granites (e.g., Per~imaa) and intrusions that share the characteristics of evolved Group I intrusions (e.g., Koivusaarenneva). It is, however, important to notice that postkinematic gabbros are not strictly restricted to the Central Finland granitoid complex. Saarisenj~irvi and Tyypekinlampi (Figure 9.1) are examples of intrusions that were formed within the Primitive arc complex and have postkinematic crystallization ages (Figure 9.2; Ekdahl, 1993).

Acknowledgments Numerous individuals are acknowledged for putting their expertise, unpublished data or photographs at the author's disposal. Thanks go to Seppo Elo, Niilo K~irkk~iinen, Markku Tiainen (GTK), Pertti Lamberg (Outokumpu Research), and Riku Raitala (University of Helsinki). Heikku Papunen (University of Turku), Hannu Makkonen (GTK), and the volume editors carefully reviewed the manuscript and suggested numerous improvements.

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Burns, L.E., 1985. The Border Ranges ultramafic and mafic complex, south-central Alaska: cumulate fractionates of island-arc volcanics. Can. J. Earth Sci. 22, 1020-1038. Butler, J.R., 1989. Review and classification of ultramafic bodies in the Piedmont of the Carolinas. Geol. Soc. Am., Spec. Pap. 231, 19-31. DeBari, S., Coleman, R.G., 1989. Examination of the deep levels of an island arc: Evidence from the Yonsina ultramafic-mafic assemblage, Tonsina, Alaska. J. Geophys. Res. 94, 4373-4391. Eerola, T., T6rnroos, R., Lallukka, H., Kdrkkdinen, N., Valli, T., Elo, S., Tiainen, M., 2001. The Svecofennian layered mafic-ultramafic intrusions in M/ints/il/i, southern Finland. In: S. Autio (Ed.), Current Research 1999-2000, Geol. Surv. Finland, Spec. Pap. 31, 17-23. Ekdahl, E., 1993. Early Proterozoic Karelian and Svecofennian formations and the evolution of the Raahe-Ladoga Ore Zone, based on the Pielavesi area, central Finland. Geol. Surv. Finland, Bull. 373, 1-137. Elo, S., 1992. Gravity anomaly maps. In: T. Koljonen (Ed.), The Geochemical Atlas of Finland. Part 2., Geol. Surv. Finland, Espoo, pp. 70-75. Ga6l, G., 1972. Tectonic control of some Ni-Cu deposits in Finland. 24 th Int. Geol. Congr., Montreal 4, 215-224. Ga6l, G., 1980. Geological setting and intrusion tectonics of the Kotalahti nickel-copper deposit, Finland. Bull. Geol. Soc. Finland 52, 101-128. Grissom, G.C., DeBari, S.M., Page, S.P., Page, R.EN., Villar, L.M., Coleman, R.G., de Ramirez, M.V., 1991. The deep crust of an early Paleozoic arc; the Sierra de Fiambala, northwestern Argentina. In: R.S. Harmon, C.W. Rapela (Eds.), Andean magmatism and its tectonic setting. Geol. Soc. Am., Spec. Pap. 265, 189-200. Grundstr6m, L., 1980. The Laukunkangas nickelcopper occurrence in southeastern Finland. Bull. Geol. Soc. Finland 52, 23-53. Hakkarainen, G., 1994. Geology and geochemistry of the H/imeenlinna-Somero volcanic belt, southwestern Finland: a Paleoproterozoic island arc. In: M. Nironen, Y. K/ihk6nen 438

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Finland. Geol. Surv. Finland, Bull. 378, 1-128 Lamberg, P., 1990. Porrasniemen intruusio, sen rakenne ja petrologia- 1.9 Ga-magmatismi ja Ni-malmit. M.Sc. Thesis, Univ. of Oulu, Finland. (in Finnish) Maio; W.D., Li, C., de Waal, S.A., 2001. Why are there no major Ni-Cu sulfide deposits in large layered mafic- ultramafic intrusions? Can. Min. 39, 547-556. M6kinen, J., 1987. Geochemical characteristics of Svecokarelidic mafic-ultramafic intrusions associated with Ni-Cu occurrences in Finland. Geol. Surv. Finland, Bull. 342, 1-109. Makkonen, H.V., 1996. 1.9 Ga tholeiitic magmatism and related Ni-Cu deposition in the Juva area, SE Finland. Geol. Surv. Finland, Bull. 386, 1-101. Marshall, B., Smith, J V., Mancini, E, 1995. Emplacement and implications of peridotitehosted leucocratic dykes, Vammala Mine, Finland. GFF 117, 199-205. Marttila, E., 1981. Explanation to the map of rocks. Geological map of Finland 1:100 000, Sheet 3323 (Kiuruvesi), Kiuruveden kartta-alueen kallioper/i. Geol. Surv. Finland, Espoo. Naldrett, A.J., 1989. Magmatic sulfide deposits. Oxford University Press. 186 p. Nem'onen, K.J, 1956. Kallioper/ikartan selitys. Summary: Explanation to the map of rocks. Geological map of Finland 1:100 000, sheet 2113 (Forssa), Geol. Surv. Finland, Helsinki. Ni~vnen, M., 1989. Emplacement and structural setting of granitoids in the early Proterozoic Tampere and Savo Schist Belts, Finland implications for contrasting crustal evolution. Geol. Surv. Finland, Bull. 346, 1-83. Nironen, M., Elliott, B.A., R6m6, O.T., 2000. 1.88-1.87 Ga post-kinematic intrusions of the Central Finland Granitoid Complex: a shift from C-type to A-type magmatism during lithospheric convergence. Lithos 53, 37-58. Nurmi, RA., F~vnt, K., Lampio, E., Nironen, M., 1984. Etel~i-Suomen svekokarjalaiset porfyyrityyppiset molybdeeni-ja kupariesiintym~it, niiden granitoidi-is/int/ikivet ja litogeokemiallinen etsint~i. Summary: Svecokarelian porphyry-type molybdenum and -

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environment. In: K. Kojonen (Ed.), The early Proterozoic Zn-Cu-Pb sulphide deposit of Rauhala in Ylivieska, western Finland. Geol. Surv. Finland, Spec. Pap. 11, 59-65. Vaasjoki, M., 1994. Valijfirven hapan vulkaniitti: minimi H/imeen liuskejakson i~ksi. Summary: Radiometric age of a meta-andesite at Valij~irvi, H~ime schist zone, southern Finland. Geologi 46, 91-92. Vaasjoki, M., Sakko, M., 1988. The evolution of the Raahe-Ladoga zone in Finland: isotopic constraints. In: K. Korsman (Ed.), Tectonometamorphic evolution of the Raahe-Ladoga zone. Geol. Surv. Finland, Bull. 343, 7-32.

Vaasjoki, M., Pietikdinen, K., Vaarma, M., 1996. U-Pb zircon determinations from the Keikyfi breccia and other sites in the Svecofennides: indications of a Svecokarelian protocrust. Bull. Geol. Soc. Finland 68, 3-10. Vuori, S., 1999. Hyvinkfi~in em~iksisen kerrosintruusion l~insiosan rakenteesta ja geokemiasta. M.Sc. Thesis, Univ. of Helsinki, Finland. 1-78. (in Finnish)

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