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Lokken, Hoydal, Dragset, Amot

Norway

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The Løkken ophiolite hosted massive sulphide copper deposit is located 10 km south of Trondheim in north-western Norway.   It is believed to be one of the largest deposits of this type yet found.

Løkken is ~110 km NW of the similar Røros orefield (see the Røros record), and 100 km north of the Folldal orefield (see the Folldal District record).

The deposits of the Løkken district 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 Løkken 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 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). Köli sequence rocks are recognized over a distance of more than 500 km north of Trondheim. The Løkken district, which is in the west of the Trondheim region, falls within the Støren Nappe, in the inverted limb of a recumbent nappe structure that was refolded to produce a major east-west trending synform.

The deposit is hosted by the middle of three units that make up a 1 to 2 km thick sequence of basaltic volcanic rocks believed to represent a fragment of early Palaeozoic (Ordovician) oceanic crust now within the upper allochthon of the Caledonian nappe pile.

The central unit of this sequence marks a period of intense sea floor faulting activity with abundant talus breccias and of more vigorous volcanic activity with voluminous eruptions of fluid magmas that formed thick sheet basalts and subordinate intermediate flows.   These volcanics were derived from shallow magma chambers represented by the now exposed intrusive gabbros.

The ore deposit is composed of a conformable massive sulphide body overlying a fissure related stockwork-stringer zone in the footwall.   This fissure feeder has resulted in the ore deposit being strongly elongated.   The massive sulphide body is thickest over the fissure, thinning away from it, with maximum dimensions of 400 m in width and 60 m in thickness, but persisting over a length of 4 km. Post-depositional deformation has fragmented the original deposit into one large and several smaller bodies.

The massive ore is composed predominantly of pyrite with chalcopyrite and lesser sphalerite, and averages 2.3% Cu, 1.8% Zn, 0.02% Pb, 16 g/t Ag.   Minor galena, magnetite, hematite and bornite are also present as are accessory tetrahedrite-tennantite.   The ore is mainly massive and compact with lesser banded sulphides.   Sulphides are fine and contain porous and colloform pyrite aggregates with base metal sulphides or quartz filling the interstices.

The underlying feeder zone follows the controlling fissure for the entire 4 km length of the deposit and is well defined to a depth of 300 m below the massive sulphides.   Mineralisation occurs as "breccia ore" (the term for intense stockwork development).   The central 40 to 50 m of the stockwork has a network of sulphide veins which are a few mm to 10 cm thick, separated by intense disseminations, with a total of around 15 to 20% sulphides.   The intensity of mineralisaiton decreases outwards and downwards, with the zone being generally around 100 m wide.   Conversely the density and thickness of veins increases upwards and inwards with the contact between the stringer and massive sulphides being gradational.

Stratabound beds of jasper (hematitic chert), composed essentially of SiO2 (69 to 95 wt %) and Fe2O3 (3 to 25 wt.%), can be traced several kilometres along strike in the Ordovician Løkken ophiolite, Norway. These siliceous beds are closely associated with the volcanogenic massive sulfide (VMS) deposits and are interpreted as sea-floor gels that were deposited by fallout from hydrothermal plumes in silica-rich seawater, in which plume-derived Fe oxyhydroxide particles promoted flocculation and rapid settling of large (~200 µm) colloidal particles of silica-iron oxyhydroxide.

The Løkken deposit was discovered in 1654 and was almost continuously mined until 1987, along with the nearby but much smaller Hoydal, Dragset, & Amot deposits.   Hoydal has produced around 100 000 tonnes of ore and is 2 km from Løkken.

The Løkken mine produce 24 Mt @ 2.1% Cu, 1.8% Zn, 0.02% Pb, 19 g/t Ag, 0.2 g/t Au between 1654 and 1987 (NGU, Norway).

For detail see the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1989.     Record last updated: 20/11/2012
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:
Grenne T and Slack J F,  2005 - Geochemistry of Jasper Beds from the Ordovician Løkken Ophiolite, Norway: Origin of Proximal and Distal Siliceous Exhalites: in    Econ. Geol.   v100 pp 1511-1527
Grenne T,  1989 - The feeder zone to the Lokken Ophiolite-hosted massive Sulfide deposit and related mineralizations in the central Norwegian Caledonides: in    Econ. Geol.   v.84 pp. 2173-2195
Grenne T, Slack J F  2003 - Bedded jaspers of the Ordovician Lokken ophiolite, Norway: seafloor deposition and diagenetic maturation of hydrothermal plume-derived silica-iron gels: in    Mineralium Deposita   v38 pp 625-639
Grenne T, Vokes F M  1990 - Sea-floor Sulfides at the Hoydal volcanogenic deposit, central Norwegian Caledonides: in    Econ. Geol.   v85 pp 344-359


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