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Grong - Stekenjokk District - Joma, Skorovass, Gjersvik, Godejord, Stekenjokk

Norway

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The Grong - Stekenjokk copper-zinc-pyrite deposits are located on the Norwegian-Swedish border, ~150 km northeast of Trondheim and include the Joma, Skorovass, Gjersvik and Godejord deposits/prospects in Norway and Stekenjokk to the east in Sweden.

The Grong - Stekenjokk copper-zinc-pyrite deposits are ~150 to 160 km NE of the similar Røros (see the Røros record), and Lokken orefields (see the Lokken record) south and SE of Trondheim.

The deposits of the Grong - Stekenjokk district are hosted within a Cambrian to Silurian succession of the Scandinavian Caledonides, a belt of late Neoproterozoic to early Palaeozoic rocks emplaced as a series of thrust nappes (Roberts and Gee 1985; Hacker and Gans 2005), that overlie the Palaeoproterozoic rocks of the Fennoscandian Shield and constitute the northernmost section of the composite Caledonian-Appalachian belt in Scandinavia and eastern North America (Grenne et al., 1999).

The nappes forming the Caledonides have been grouped into four main allochthons, the Lower, Middle, Upper and Uppermost Allochthons (Roberts and Gee 1985). The Grong - Stekenjokk district deposits are located in part of the Upper Allochthon, which comprises the Seve and overlying Köli Nappe systems, respectively consisting of high-grade metamorphic continental rocks thought to represent the tectonically shortened outermost margin of Baltica, and lower metamorphic grade exotic terranes comprising ophiolitic volcano-sedimentary rocks from oceanic environments outboard of Baltica, interpreted to represent remnant oceanic lithosphere of the former Iapetus ocean (Stephens, 1988; Hacker and Gans 2005). The Grong - Stekenjokk deposits lie within the Leipikvattnet nappe (Zachrisson, 1969; Olsen, 1980), a local division of the Rantser nappe within the Köli Nappe system (Kollung, 1979; Reinsbakken and Stephens, 1986).

The deposits are hosted by the possibly early Ordovician Røyrvik Group which comprises an alternating assemblage of tholeiitic to alkaline pillow lavas, pillow breccias and hyaloclastites interbedded with commonly graphitic phyllites and ribbon cherts. The deposit occurs near the structural top of the overturned Røyrvik Group within a pillowed greenstone unit.

The host rocks underwent lower greenschist facies (biotite zone) metamorphism during D2, giving rise to the principal schistosity and mineral lineation. Similar metamorphic conditions were experienced in D3, since some components of the D2 schistosity recrystallised and grew within the D3 crenulation foliation. By D4, metamorphism had declined to subgreenschist facies.

The exposed Joma deposit occurs as two arcuate massive sulphide layers which merge below the surface. Metamorphosed pillowed and massive basalts, and thinly bedded basic tuffs are the immediate host rocks, which together with quartz-rich and graphitic phyllites, make up the Leipikvattnet nappe in the district. Basalt and pillow lavas intercalated with quartzitic and graphitic phyllites make up three major greenstone units, named (from NE to SW), the outer, middle and inner greenstones. The mineralisation is hosted within the middle greenstone unit. The metamorphic grade is upper greenschist facies.
    The ore lens is internally complex comprising intercalated chalcopyrite-pyrrhotite and various pyritic ore types with thin silicate layers that have been disrupted by shear zones during tight to isoclinal folding, with clasts enclosed within a sulphide matrix to produce a durchbewegt (durchbewegung) texture (Vokes,1 969; Marshall and Gilligan, 1987, 1989).
    The Joma ore and hosts have been subjected to four periods of deformation (Reinsbakken and Stephens,1986; Marshall et al., 1987; Odling,1988). The progressive D
1 - D2 event produced regional and mine-scale isoclinal folding, while the D3 resulted in the regional Joma synform and numerous mine- and meso-scale folds with a NE trending, NW dipping crenulation foliation, as well as the arcuate surface trace of the exposed orebody. D4 resulted in small normal faults and shallowly dipping kink bands, with a negligible macro- or meso-scale influence on the orebody.
    The Joma ores are massive pyrite and chalcopyrite-pyrrhotite, concordantly (structurally) overlain by chlorite schist and a fragmental albite-rich rock.   The fragments of the albite rich rock are separated by irregular quartz-pyrite veins and veinlets and pass up into pyrite veined greenstone.

The Gjersvik depositt is hosted by metabasalts of the Gjersvik nappe which is structurally above the Leipikvattnet nappe.

The Skorovass deposit occurs within a sequence of greenstone of volcanic and sedimentary origin, intruded by a gabbro-trondhjemite suite. The greenstones of the Ordovician Stø Group dip SW while the gabbro is mainly restricted to two large differentiated masses in the SW part of the area, the closest being 2 km from the ore zone. The greenstones are dominantly semi-massive, with lesser schistose varieties that are generally ~1m thick and have associated thin carbonate layering. The ore is located within, and near the middle of the semi-massive greenstones, which are interpreted to represent basaltic lava flows, and several hundred metres from any rocks of possible sedimentary origin.
    Ore occurs as a few large lenses and a number of closely spaced satellites. The long axis of the main ore lens strikes north-south and is 800 m long, with a maximum width of 300 m, with maximum thicknesses that rarely exceed 30 m. The ore is apparently conformable with the schistosity, which is also conformable with stratification. In detail the massive sulphides are slightly discordant, climbing up the succession from north to south, with sharp but irregular contacts. The ore minerals are pyrite, chalcopyrite and sphalerite in a gangue of quartz, chlorite and carbonate. Much of the ore consists of massive pyrite, with >50% pyrite. In the northern section of the deposit, the massive sulphide from the footwall give way to disseminated ore with coarser grained pyrite wit 20 to 40% sulphur, almost devoid of Cu and Zn. To the south, first chalcopyrite and then sphalerite become more abundant (Gjelsvik, 1968).

The Godejord stratabound polymetallic sulphide deposit is located in the metamorphic Palaeozoic rocks of the Grong district, just to the north of the east-west belt of basement Palaeoproterozoic gneisses known as the Grong culmination which is generally transverse to the axial direction of the Caledonides, and separates the Grong district metavolcanic and metasedimentary rocks from similar rocks of the Trondheim district. The deposit is hosted by similar rocks to those described for Joma and Skorovass.
    The sulphide mineralisation exposed in trenches along the outcrop at Godejord prospect shows a distinct banding parallel to the ore walls, mainly defined by variable sulphide-gangue ratios and to a less extent by variations in the relative proportions of the sulphide minerals themselves. The ore mineral assemblage includes pyrite, sphalerite, chalcopyrite, galena, tennantite, bornite, idaite, covelline, Cu-Ag-S minerals, an Ag-Cu-Te sulphide and an Sn-Cu sulphide, in a gangue of predominantly quartz, calcite, tremolite and mica. A composite trench sample assayed 1.84% Cu, 15.34% Zn, 0.20% Pb, 4400 g/t Ag. The ore shows a typical metamorphic recrystallised fabric, with subhedral to anhedral grains of 0.5 mm pyrite, embedded within softer "matrix" sulphides and gangue minerals in varying proportions. The copper-silver sulphide minerals occur in the matrix to the pyrite grains as irregular anhedral masses, either as a filling between these grains, or in a polygonal mosaic along with the other matrix minerals.

Between 1972 to 1998, the Joma mine produced a total of 11.5 Mt @ 1.49% Cu, 1.45% Zn (Grong Gruber/Geol. Survey of Norway, NGU).
The Gjersvik deposit produced 0.5 Mt @ 2.15% Cu, 0.6% Zn from 1994 to 1998 (Grong Gruber/Geol. Survey of Norway, NGU).
Skorovass contained ~7 Mt @ an unspecified grade.
Stekenjokk in Sweden contained ~20 Mt @ an unspecified grade

For more detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1994.     Record last updated: 14/11/2012
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Giles A D, Marshall B  1994 - Fluid inclusion studies on a multiply deformed, metamorphosed volcanic-associated massive sulfide deposit, Joma Mine, Norway: in    Econ. Geol.   v 89 pp 803-819
Gjelsvik T,  1968 - Distribution of Major Elements in the Wall Rocks and the Silicate Fraction of the Skorovass Pyrite Deposit, Grong Area, Norway: in    Econ. Geol.   v.63 pp. 217-231
Marshall B,  1990 - Pseudostratigraphy and thrusting in relation to the structural evolution of the Joma ore-body, North Trondelag, Norway : in    Ore Geology Reviews   v5 pp 175-210
Stephens M B, Swinden H S, Slack J F  1984 - Correlation of massive Sulfide deposits in the Appalachian-Caledonian orogen on the basis of Paleotectonic setting: in    Econ. Geol.   v79 pp 1442-1478


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