Summary:
The copper-nickel-platinum group elements sulphide deposit is magmatic in origin and is hosted mainly by olivine cumulates and partly by fine-grained ultramafic volcanics, in the lowermost part of the Savukoski Group of the Central Lapland Greenstone Belt. The cumulates are overlain in the west by a multi-textured carbonate breccia and a finegrained mafic unit. Metasediments occur mainly below the cumulates. Mineralisation in the Sakatti main deposit can be divided into three main types: disseminated, stockwork and massive sulphide bodies.
The Sakatti deposit is in the north of the Sodankylä schist area. Because of the lack of exposed rock in this area of Finland the geological maps rely heavily on geophysical data and so cannot be treated as wholly reliable. The current geological map shows the location of the Sakatti deposit as being a mafic sill within the Sodankylä quartzites, however this is not compatible with the lithologies seen at Sakatti.
The majority of the stratigraphy of the Sodankylä area is the Lapponi supergroup which sits upon Archaean basement, exposed in small inliers. There is also a large layered mafic intrusion, the Koitelainen intrusion, a suite of intruded mafic-ultramafic dykes and sills and the Kevitsa ultramafic-mafic intrusion (Hanski and Huhma, 2005).
The Sakatti deposit itself is located within a zone of greenschist metamorphismthat is one of the lowest grade metamorphic areas in the Central Lapland Greenstone Belt. Rocks in this zone, termed zone IV by Hölttä et al. (2007), have preserved magmatic textures and the ultramafic rocks present also have preserved magmatic mineralogy (as seen at Sakatti).
The Sakatti deposit is located within the Palaeoproterozoic CLGB, which extends across Lapland from northern Norway to the Finnish-Russian border. It is a complex succession containing sedimentary rocks as well as volcanic rocks ranging from komatiitic to rhyolitic in composition. The evolution of the CLGB spans around 600 Ma, starting with andesitic volcanism at ca. 2.5 Ga and ending with deposition of molasse-type coarse-clastic sediments at <1.9 Ga (Hanski and Huhma, 2005). The belt is also host to several mafic-ultramafic intrusions, having a range of ages which at the one end includes the 2439±3 Ma Koitelainen layered intrusion and at the other end includes the 2058±4 Ma Kevitsa intrusion (Mutanen and Huhma, 2001).
Based on current understanding, the Sakatti deposit consists of three spatially distinct mineralised bodies of olivine cumulate named 'main body', 'north-east body' and 'south-west body'. Mineralisation in all three is hosted within or at a basal contact of the olivine cumulate. In hand specimen, there are no discernible petrological differences between these three bodies.
The major host and wall rock units of the deposit comprise the Olivine Cumulate or Peridotite Unit, Aphanitic Unit, Mafic Suite, Breccia Unit and Volcanosedimentary or Volcaniclastic Unit.
Olivine Cumulate or Peridotite Unit
The Olivine Cumulate Unit is the principal constituent of the main cumulate body, which itself can be over 400 m thick. This is an olivine cumulate with variable oikocrystic pyroxene content and rarely minor interstitial plagioclase. Most of the unit is pervasively serpentinised, and should technically be termed a serpentinite. However, the cumulate texture is preserved therefore the rock can be considered in terms of its protolith. Textures within the main body range from adcumulate to orthocumulate with the groundmass typically also composed of serpentine with minor talc.
Plagioclase-rich Picrite or Aphanitic Unit
The Aphanitic Unit, so named because of its grain size and likely volcanic origin, forms the hanging wall, footwall and sidewall to much of the main cumulate body, notably along the southern edge and the far western side. Referring to a unit as aphanitic is clearly problematic as it is not an adequate rock name. This name has endured because the unit has proved difficult to classify. The mineralogical and textural features of this unit all point towards a volcanic origin.
Where it is within 50 m of the contact with the Olivine Cumulate Unit, the Aphanitic Unit exhibits an unusual texture containing injections of the Olivine Cumulate Unit. Further from the contact this texture is not present and the rocks merely show serpentine veining.
Mafic Suite
In addition to the Aphanitic Unit, the hanging wall of the Sakatti deposit contains several other lithological units, including the Mafic Suite. It is present in the south-western part of the deposit where it occurs between the Olivine Cumulate or Aphanitic Units and the Breccia Unit. It
comprises three separate logging units: the Mafic Volcanic Rock, the Scapolite-Mica Rock and the Hanging-Wall Gabbro. The Mafic Volcanic Rock is a strongly chlorite-amphibole-altered lithology that, when in close proximity to the Breccia Unit, has undergone in-situ brecciation and precipitation of matrix and vein calcite. The Scapolite-Mica Rock is a strongly foliated, almost schistose rock with a biotite matrix hosting scapolite porphyroblasts. The Hanging-Wall Gabbro comprises a series of gabbroic sills that intrude the mafic volcanic and scapolite-mica rocks but do not intrude the main cumulate body. At the contact between the scapolite-mica rock and the Aphanitic Unit, a cryptocrystalline serpentine unit is frequently present.
Breccia Unit
The Breccia Unit is a 100-300 m thick hematite-dolomite-albite-talc-altered, and exceptionally heterogeneous, polymict breccia package that lies stratigraphically above the main cumulate body. Various zones can be differentiated within the Breccia Unit, including those with predominantly albite or carbonate (calcite/dolomite) alteration, as well as polymict zones where rounded to angular clasts of talc, chlorite and quartz typically occur in a calcite matrix.
Volcano-sedimentary or Volcaniclastic Unit
The Volcano-sedimentary Unit is the stratigraphically uppermost unit in the hanging wall of the Sakatti deposit. It is a phyllite with biotite porphyroblasts and a crenulation cleavage throughout. It is interpreted as a metamorphosed volcano-sedimentary package. The thickness of this unit is at least 600 m.
Footwall units
In the eastern part of the deposit, where the Aphanitic Unit forms the primary footwall below the Olivine Cumulate Unit, a clay-rich zone, interpreted as a fault structure, is present beneath the Aphanitic Unit. In the west, this fault structure occurs directly at the base of the Olivine Cumulate Unit and has often necessitated the cessation of drilling. Beneath this fault, a strongly laminated carbonate-rich metasediment is present. No sulphides have been observed in this metasediment.