Linnajavri
Geology
The rocks of the Linnajavri area are built up of the Caledonian Nappe sequence thrusted onto Precambrian granitic basement.
The ultramafic rocks in the area are regarded as ophiolite fragments, with serpentinite and soapstone alteration that has probably taken place at an early oceanic stage. Later the ophiolite (oceanic crust + uppermost mantle) was obducted, exposed for weathering and then strongly eroded and dismembered. A significant dismembering of the crust and mantle may also well have taken place even at an early, oceanic stage as seen at other Caledonian ophiolites, e.g. at Raudfjellet, central Norway (Nilsson et al. 2005). Finally, the fragmented remains of the ophiolite were incorporated into Caledonian nappes being thrusted onto the Precambrian granitic basement of continent Baltica.
The geology of the area is dominated by tectonic nappes and flakes of rocks separated by a net of tectonic melange zones.
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Tectonic melange zones in the Klebervann area
Photo: I. Lindahl, NGU |
In the Linnajavri area the relatively flat lying Caledonian nappes constitute the eastern part in two open synforms with weakly dipping to undulating axes towards E -SE. Granitic basement is found to the west. To the east, on the Swedish side of the border, a large area of Caledonian rocks is found. The tectonic mélanges consist of crushed wall rocks, and are preferably developed where blocks of ultramafic rocks and soapstone are involved, i.e. within the ophiolite fragments. However, they also occur in the sediments. A typical tectonic mélange, as shown above, constitutes up to 50 % of the marble unit.
The ophiolite fragments occur in two different nappes. The most complete section is found at Ridoalggicohkka (Southern area) and includes the ultramafic rocks, pillow lava, gabbro, amphibolite and conglomeratic weathering products with well-rounded pebbles from these rocks. Well-preserved pillows are found in the not so strongly influenced block of rocks between mélange zones (see picture below). Only one dyke has so far been registered intersecting the pillow lava. However, no detailed investigation for dykes within the mafic rocks have been undertaken. Within the geographically Northern area ophiolite fragments are found and here in the same nappe as at Ridoalggicohkka, but in a different lithological setting (see geological map) .
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Pillow structure in greenstone at Ridoalggicohkka
Photo: I. Lindahl, NGU |
The major ophiolite fragments in the Northern area are in different nappes and are more tectonically fragmented than it is at Ridoalggicohkka. The rock suite includes the ultramafic rocks and dark medium-grained amphibolites. At Njaskasvarri a body of leucogabbro is occurring. Ultramafic scree material and an immature ultramafic breccia-conglomerate are also found in the Northern area. These include weakly rounded pebbles of soapstone among non-talcified boulders and gravel, one of several observations constraining talc formation to a pre-obduction, or at least pre-conglomerate formation stage.
The ultramafic rocks at Ridoalggicohkka and in the Northern area are similar: dunite and peridotite. Only in one ultramafic body, the Njaskasvarri 985 body, a clear cumulate sequence is found. The dunitic bodies are normally the most fine-grained and the peridotite coarser grained. The ultramafic rocks locally contain small pods of chromite, which have survived both morphologically and mineralogically during the alteration of their host rocks to serpentinite and further to soapstone. The gabbro bodies are normally medium grained.
The suite of rocks of the ophiolite fragments have been exposed for weathering and then become the source rock for conglomerates and grit deposited. The conglomeratic rocks (se picture below) vary a lot in composition. At the southern side of Gaskavarri a conglomeratic breccia with pebbles of barely rounded soapstone is found. The conglomerate at Ridoalggicohkka has well-rounded pebbles in most cases with onionskin structure. They vary from serpentinite conglomerate, green conglomerate with large amounts of mafic material and epidote, to more gray conglomerates with micaceous matrix. The type of conglomerate varies from monomict (serpentinite conglomerate) to polymict. Mostly the conglomerates are matrix supported with transgressions to grit and coarse-grained sediment. There is repeated change in composition of the conglomerate beds with thickness of each bed down to half a meter.
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Various types of conglomerates at Ridoalggicohkka.
Photo: I. Lindahl, NGU |
Alteration of the rocks in the ophiolite fragments is extensive. In the pillow at Ridoalggicohkka solutions introduced sulphur and magnesium have flowed through vents (hot spots). The solutions caused sulphide impregnation and in the aureole (1 meter thick) around the vent growth of flaky talc. Along shear zones in the amphibolites within the Ridoalggicohkka synform it has been introduced magnesium and talc growth can bee seen as half centimeter unorientated flaky crystals. Late hydrothermal activity in the ophiolitic rocks has bleached the rocks around veinlets wherein quartz, carbonate, chlorite and tourmaline are deposited. Silification along thin veins in the conglomeratic rock is also common.
Soapstone alteration with introduction of CO2 in the process is of varying degree in the different bodies from limited to extensive, in some places up to 100 % (Klebervann deposit). Alteration took place from cracks, joints and breccia zones in the ultramafic rocks. In several places the primary magmatic textures can be seen inherited from the dunite or peridotite. At one locality (Kleberflåget deposit) the alteration process was taken a step further: during excess CO2 contribution the soapstone broke down to listvenite (see pictures below), a rock consisting of magnesite-dolomite and quartz. The formation of listvenite is among our strongest indications/proofs that the soapstone is not formed by CO2 derived from a sedimentary source (calcite or dolomite marble, black shale, etc.) at a late, compression stage during the Caledonian Orogeny, i.e. after such rocks had been juxtaposed or come close to the ultramafic rocks, but from a strong and lasting mantle source that have been active even at a very low temperature (c. 200-300ºC) below the stability temperature of talc in ultramafic rocks.
The larger part of the ultramafic rocks is altered, and only to a small extent unaltered or only little altered dunite and peridotite is found, and then often in the cores of bodies like e.g. at Kvitfjell where a tough harzburgite makes up the very erosion resistant top area. Most of the ultramafic rocks are serpentinites, some of them with a sharp green color.
Hydrothermal activity has been pervasive for the whole nappe sequence of the area. Numerous small near vertical quartz-rich pegmatitic veins intersect the sediments preferably in a N-S orientation or parallel to the schistosity.
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Various types of soapstone alteration. Upper pictures: Alteration dunite-serpentinite-soapstone, Klebergryta øvre.
Lower left: Alteration of periodotite to soapstone from joints, soapstone interfingering from Grensegangen. Lower right: Alteration of soapstone to listvenite, Kleberflåget.
Photo: L. P. Nilsson & I. Lindahl |
Discovery and recognition of deposits
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Exploitation in the Linnajavrí area