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Folldal District - Hovedgruva, Sondre Geitryggen, Grimsdalen, Nordre Geitryggen, Nygruva

Norway

Main commodities: Cu Zn Ag
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The Folldal group of volcanic hosted massive sulphide copper-zinc deposits is located 110 km south of the ophiolite hosted Lokken copper deposits in the Trondheim district of north-western Norway.

Folldal is ~110 km SW of the similar Røros orefield (see the Røros record), and 100 km south of the Lokken orefield (see the Lokken record).

The deposits at Folldal are hosted within a Cambrian to Silurian succession of the Scandinavian Caledonides, a belt of late Proterozoic 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 Folldal district is located in part of the Upper Allochthon, which comprises the Seve and overlying Köli Nappe systems, consisting of continental rocks thought to represent the outermost margin of Baltica, and ophiolitic rocks interpreted to represent remnant oceanic lithosphere of the former Iapetus ocean (Hacker and Gans 2005).

The Trondheim Nappe Complex of the Southern Trondheim region, has been subdivided, from west to east, into the Støren, Gula and Meråker Nappes and contains Köli sequence rocks (Roberts and Wolff 1981; Gee and Sturt 1985). The Folldal district, which is in the southeast of the Southern Trondheim region, falls within the Meråker Nappe.

These deposits are hosted at three different stratigraphic levels within the Fundsjø Group volcanics which overlie the psammitic schists, quartzites and partly graphitic semipelites of the Gula Group.   The Fundsjø Group comprises volcanics, volcaniclastics and minor sediments. Basalts (and a large 488 Ma sub-volcanic trondjhemite intrusive) are found in the lower part of the group, while volcaniclastics or tuffites with minor quartzites, greywackes and graphitic schists are found in the upper parts. The tuffites include extensive intercalated rhyodacitic felsic volcanics.

The Hovedgruva deposit at the lower of the three levels is found in the basalts, while the Søndre Geitryggen and Grimsdalen ores of the middle level are within rhyodacites and volcaniclastics and the Nordre Geitryggen and Nygruva of the upper level is hosted by volcaniclastics and tuffites.   Metal ratios vary from copper predominating over zinc at the lowest level to more zinc rich ores at the higher level.

All of the deposits have massive sulphide ores underlain by disseminated mineralisation.   In contrast to the ores in volcanic rocks, the tuffite hosted deposits are flanked by thin, laterally extensive massive sulphides and have less well developed associated disseminated sulphides.

The Søndre Geitryggen deposit occurs within the upper part of a 20 to 50 m thick, extensive band of rhyodacite and is a small north trending and 60° plunging lense of massive Zn-Cu ore with a 195 m strike length and 25 m maximum thickness which narrows downward.

The Folldal district is recorded to having yielded 4.45 Mt of ore between 1748 and 1970.

The average metal contents of the massive sulphide ores vary as follows:
Hovedgruva - 1.94% Cu, 1.21% Zn, 21 g/t Ag, 0.19 g/t Au
Søndre Geitryggen - 0.67% Cu, 2.72% Zn, 27 g/t Ag, 0.35 g/t Au
Grimsdalen - 0.37% Cu, 2.62% Zn, 19 g/t Ag, 0.25 g/t Au
Nordre Geitryggen - 4.51% Cu, 2.72% Zn, 31 g/t Ag, 0.27 g/t Au
Nygruva - 1.63% Cu, 5.01% Zn, 48 g/t Ag, 0.35 g/t Au

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1996.     Record last updated: 2/11/2012
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:
Bjerkgard T, Bjorlykke A  1996 - Sulfide deposits in Folldal, southern Trondheim region Caledonides, Norway: source of metals and wall-rock alterations related to host rocks: in    Econ. Geol.   v91 pp 676-696


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