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

Epigenetic gold mineralisation in
central and SW Finnish Lapland
More than 60 drilling-indicated, epigenetic gold
occurrences have been discovered in the Palaeoproterozoic
greenstone belts in the central and SW
Finnish Lapland. Suurikuusikko (Kittilä Mine, Table
1), being the largest deposit so far discovered,
is a classic example of a gold-only orogenic deposit
hosted by a N-trending shear zone in lower-greenschist
facies greenstones (Eilu & Pankka 2009).
Nearly all occurrences in Central Lapland
probably belong to the orogenic category in the
sense the deposit class is defined by Groves et al.
(1998) and Goldfarb et al. (2001). For example,
more than 30 drilling-indicated deposits and occurrences
are in the Sirkka Shear Zone and subsidiary
faults branching from this crustal-scale, >100 km
long, structural break within the Central Lapland
greenstone belt in Finland (Eilu et al. 2007). Locally,
the two most significant controls to mineralisation
are structure and rock type: the ore bodies typically
are hosted by the local dilatational sites and by the
locally most competent lithological units. For many
Pahtavaara Gold Mine
Nicole L. Patison
Agnico-Eagle Finland, Kittilä, Finland
V. Juhani Ojala
Store Norske Gull AS, Longyearbyen, Norway
Pasi Eilu
Geological Survey of Finland, Espoo, Finland
Introduction
Pahtavaara is an active gold mine with a total in situ
size estimate of 12.5 t gold (production + resource,
as of January 2011; Finnish Ministry of Employment
and the Economy official statistics, Lappland
Goldminers 2011. Initial production took place during1996–2000
and the mine was reopened in 2003
(Eilu & Pankka 2009). The deposit is hosted by an
altered komatiitic sequence at the eastern part of the
Central Lapland greenstone belt (Fig. 2 in Introduction
and Fig. 4 below). It comprises of 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 barite-gold
lodes, part of the local control is intersection of two
faults or a fault along boundary between lithological
units with contrasting competence (Sorjonen-Ward
et al. 2003, Holma & Keinänen 2007, Patison 2007,
Saalmann & Niiranen 2010). Fluid compositions
(Billström et al. in press) suggest variable, mixed,
origins for volatiles and metals with no obvious indications
of a local source. These features are present
for both the gold-only and the anomalous metal
association (typically Au-Cu) subtypes. Obvious
IOCG-type deposits have been detected only in the
westernmost Finnish Lapland, in the western margin
of the Central Lapland greenstone belt. The IOCG
deposits are covered by another field excursion of
the IAGS congress and, hence, not discussed here.
A possible exception to the orogenic type
of gold mineralisation within the Central Lapland
greenstone belt is represented by the Pahtavaara
gold deposit. Pahtavaara has an anomalous baritegold
association and a very high fineness (>99.5%
Au) of the gold. Furthermore, the geometry of
high-grade quartz-barite lenses and amphibole
rock bodies relative to biotite-rich alteration zones
is anomalous to an orogenic or an IOCG deposit.
association and a very high fineness (>99.5 % Au)
of gold (Kojonen & Johanson 1988, Korkiakoski
1992). The geometry of high-grade quartz-barite
lenses and amphibole rock bodies relative to biotiterich
alteration zones is also anomalous, as is the δ 13 C
of alteration carbonate minerals. 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
(David Groves, pers. comm. 2006). The gold may
have been introduced later, but its grain size, textural
position (nearly all is free, native, and occurs
with silicates, not sulphides) and high fineness point
to a pre-peak metamorphic timing which is highly
anomalous for orogenic gold.
Geology and hydrothermal alteration
The following description is extracted from
Korkiakoski (1992) unless otherwise is indicated.
Pahtavaara gold mine is hosted by the predominantly
pyroclastic Sattasvaara komatiite complex within
the Sattasvaara Formation of the Central Lapland
greenstone belt. There is no reliable radiometric age
data of the volcanic rocks of the Sattasvaara Formation
in Finland, but one of its branches continues far
in northern Norway where Krill et al. (1985) have
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