Hessjø Copper Mines, Holtålen, Trøndelag, Norwayi

Regional Level Types
Hessjø Copper Mines	Mine
Holtålen	Municipality
Trøndelag	County
Norway	Country

From the NGU's mineral database:

"The Hersjø ore deposit is situated above the tree-line c.1020 m.a.s.l. on the westside of the Kjølidalen/Kjurrudalen valley. The mineralization is outcropping in two main areas, where some small-scale mining have been done. The northern part of the deposit, known as the old Hersjø mine, was claimed by Henning Irgens as early as c. 1670. The mine was handed over to Røros Kobberverk in 1687, which produced c.4000 t of copper ore until 1693. The production was not profitable and the mine was handed over to Irgens again. Since copper production was not economic, Irgens started producing copper vitriole and later ochre. The mine was closed when Irgens died in 1699. Some sporadic mining of pyrite was done in the late 1800s, after investigations by Røros Kobberverk in 1873 and 1881. The southernmost part of the Hersjø deposit, about 1.5 km to the south of the old Hersjø mine, is known as Nygruva. It was claimed and mined by Røros Kobberverk around 1830, but only c. 70 t of copper ore was extracted from a small excavation into the hillside and two small waterfilled sinks. According to reports the deepest sink is about 13 m deep. Geophysical investigations by Geofysisk Malmleting in 1948 using TURAM, showed strong and extensive electromagnetic conductors in the Hersjø field. These zones got the designations A, B, C, D, E and F. The A-conductor which was the strongest, was not known before, but turned out to be caused by the largest ore lens when it was drilled later. The B- conductor is due to the mineralization present in the old Hersjø mine, while the C-conductor is represented by two minor showings between the old Hersjø mine and Nygruva. The E-conductor represents the mineralization in Nygruva. The conductors D and F, as the A-conductor, were not known before. Diamond drilling commenced in 1970 by Røros Kobberverk AS, and was continued in 1971. Drilling was followed up by AS Sydvaranger in 1974-1975 and by Killingdal Grubeselskap AS in 1976-77. Altogether 12139.9 m were drilled, distributed on 68 holes. Together with ground geophysics and geophysical measurements in the drillholes, about 3 mill. tons of ore was identified containing 1.7 % Cu and 1.4 % Zn. The A lens was intersected by 24 drillholes distributed in 6 profiles to a vertical depth of 415 m (i.e. 600 m along the ore axis). Altogether 2.04 mill. t. of identified, probable and possible ore with 2.01% Cu and 1.56% Zn is present in the A lens. The B lens was intersected by 14 drillholes distributed in 4 profiles to a vertical depth of 320 m (i.e. 450 m along the ore axis). The drilling resulted in a calculated ore resource of 0.67 mill. t. of probable and possible ore with 1.17% Cu and 0.86% Zn. The C lens is calculated to contain 0.28 mill. t of probable ore with 0.74% Cu and 1.54% Zn. This is based on 7 drillholes to a vertical depth of 250 m (i.e. 300 m along the ore axis). Geology and mine workings: The Hersjø ore deposit is situated in the Fundsjø Group (aka. Hersjø Formation) in the eastern Trondheim Region Caledonides. The dominating lithology is massive greenstone, composed of mainly dark green hornblende and feldspar with subordinate chlorite, epidote and calcite. Around the ore lenses this rock has been altered to a dark chlorite schist, often with lenses and layers enriched in quartz ± feldspar. Subordinate lithologies in the area includes felsic metavolcanics (quartz keratophyre), metagabbro and pillow metabasalt. The workings in the old Hersjø mine field consist of two waterfilled shafts and several small diggings in an area about 105 m along strike and about 30 m wide. Mining has been done on several parallel ore lenses with a common strike of 190-210o and dipping 50-59o W. The trend of the ore lenses is controlled by folding, shown by a strong lineation with trend 250o WSW and plunge 46o. Three lenses of ore are present, varying from 0.7 m to 2 m in width. The dominating ore type is a massive pyrrhotite ore with subordinate, but varying amounts of chalcopyrite and scattered cubes of pyrite. Locally the ore contains considerable amounts of sphalerite. Five samples of ore from the dumps of the Hersjø mine contained between 1.58% and 6.11% Cu and between 0.12% and 1.37% Zn. Contents of Pb, Ag and Au in these samples were extremely low (Pb < 3 ppm, Ag <0.3-6 ppm, Au <0.02 ppm). The Kjøllufjellet prospect is situated about 300 m to the south of the old mine. The small showing contains massive and semi-massive pyrrhotite ore, with variable amounts of chalcopyrite and sphalerite, partly in stripes. Two samples contained 1.06 and 1.34% Cu and 2.77 and 4.09% Zn, respectively. Another small showing is situated c. 500 m to the south of the old mine. Here a semimassive to massive 1 m wide mineralized zone is present with mainly finegrained pyrrhotite and chalcopyrite. A sample from the dump contained 2.63% Cu and 0.16% Zn. The Nygruva mine in the south consists of an excavation in a small hillside, and two minor shafts. The excavation is 3-4 m across strike towards the west, before in turns to the north and follows a 1 meter thick zone of massive, fine-grained diffusely banded pyrite ore with subordinate sphalerite. The ore zone wedges out in both directions along strike, but may continue downwards along the strongly defined lineation/fold axis. In the footwall are numerous bands and lenses of disseminated to semimassive pyrite in quartz. One sample from the dump contained 1.71% Cu and 3.01% Zn. The closest shaft is about 70 m to the north. Here are two thin zones (20 and 80 cm wide) of massive, banded pyrite mineralization. Two samples contained 1.32 and 1.43% Cu and 0.99 and 7.94% Zn, respectively. The second shaft is another 70 m to the north. Here a sample of massive pyrrhotite ore with subordinate chalcopyrite and pyrite contained 1.76% Cu and 0.81% Zn. In all the samples from the Nygruva mine, the contents of Pb, Ag and Au were extremely low (highest values were 24 ppm Pb, 4 ppm Ag and 0.1 ppm Au)."

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