Active and ongoing gold exploration and mining in ... - IAGS 2011

Active and ongoing gold exploration
and mining in Northern Finland
Pasi Eilu1 and Vesa Nykänen2 1 Geological Survey of Finland, P.O. Box 96, 02151 Espoo, Finland,
e-mail pasi.eilu[at]gtk.fi
2 Geological Survey of Finland, P.O. Box 77, 96101 Rovaniemi, Finland
Abstract
This excursion includes visits to the Pahtavaara and Kittilä gold mines, Mustajärvi
and Hanhimaa gold occurrences, and Rompas Au-U occurrences. In addition, the
field trip provides an introduction to the Palaeoproterozoic metallogenic evolution
of the region.
Pahtavaara is an active gold mine (in production 1996–2000, 2003–), with
a total in situ size estimate of 13 t gold at the average grade of 2.7 g/t (production +
resource, December 2010), in a komatiitic sequence at the eastern part of the Central
Lapland greenstone belt. It comprises a swarm of subparallel lodes; nearly all gold
is free native. It has many of the alteration characteristics of amphibolite-facies orogenic
gold deposits and an obvious structural control, but has an anomalous baritegold
association and a very high fineness (>99.5 % Au) of gold. Pahtavaara is best
interpreted as a metamorphosed seafloor alteration system with ore lenses as either
carbonate- and barite-bearing cherts or quartz-carbonate-barite veins. The gold may
have been introduced later, but its grain size, textural position (occurs with silicates,
not sulphides) and high fineness point to a pre-peak metamorphic timing.
Kittilä Mine, also known by the name Suurikuusikko, is the largest gold
deposit in northern Europe. It has a current in situ resource of 194 t gold, at the average
grade of 3.6 g/t. Production started in 2008. The deposit is a Palaeoproterozoic
orogenic gold deposit hosted by albitised, mafic to intermediate, volcanic rock and
tuffite. It comprises a number of ore bodies in a 4-km long section of the subvertical,
compressional, Suurikuusikko shear zone. This NNE-trending shear zone, which has
a dextral component, is known to be gold-enriched for its entire length of >20 km.
The deposit is open at the depth of >1.4 km and along strike. The gold is refractory
gold: 71 % of gold in the lattice of, and as tiny inclusions in, arsenopyrite and 22 %
in pyrite, in both thin veins and altered host rock.
Mustajärvi is a Palaeoproterozoic orogenic gold occurrence with no resource
estimate available. It is characterised by carbonate- and tourmaline-quartz veins in
albitised schists. The occurrence is controlled by a NE-trending shear zone possibly
branching from the WNW-trending Sirkka shear zone. Native gold is present
in quartz veins and their alteration haloes. Saprolitic part of the deposit is presently
exploited in a small scale.
Three orogenic gold occurrences are known from the N-trending Hanhimaa
shear zone which is parallel to the Suurikuusikko shear zone 10 km to the east.
The area has seen only minor exploration, including trenching, drilling, and tillgeochemical
and ground-geophysical surveys. Hosts to mineralisation include mafic
volcanic rocks. In addition to gold, some of the Hanhimaa occurrences are, partially,
also enriched in Ag, Cu, Pb and Zn.
Rompas is a new gold-uranium discovery in Palaeoproterozoic Peräpohja
Schist Belt in SW Lapland. Bonanza-grade Au and U mineralisation occur at surface
over an area 6 km long and 200 m wide. Only surface sampling has been performed
in the area: weighted average of all 80 channel samples from the 2010 program is
0.59 m @ 203.7 g/t Au, 0.73 % U. Mineralisation appears to be hydrothermal in
nature and fracture-controlled in metavolcanic host rocks. The occurrence may be
related to a buried intrusive that may be an apophyse or down-dip extension of a
granitoid complex a few kilometres to the north of the property.
Keywords: gold, mining, exploration, Palaeoproterozoic, Finland