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Pahtavaara |
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Finland |
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Au
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Super Porphyry Cu and Au
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The Pahtavaara gold deposit is located 25 km NNW of Sodankylä, 250 km NNE of Kemi and 850 km north of Helsinki in Finland. The mine has been in production between 1996 and 2000, and from 2003 to the present (#Location: 67° 37' 49"N, 26° 24' 52"E).
The Pahtavaara deposit is hosted within an altered komatiitic sequence of the 2.5 to 1.89 Ga Central Lapland Greenstone Belt that covers large parts of Finland north of the Arctic Circle and comprises mainly komatiitic lava, volcaniclastic sedimentary and mafic metavolcanic rocks. The deposit is in an east-west oriented, 40 x 5 km belt containing a several kilometres thick sequence of Geluk-type komatiitic lavas and pyroclastic rocks and komatiite-related mafic metavolcanic rocks, the Sattasvaara Formation. The deposit is located near the northern margin of the sequence, near the contact zone between a komatiitic pyroclastic and a komatiitic lava unit (Hulkki, 1990; Korkiakoski, 1992; Korkiakoski et al., 1989; Lehtonen et al., 1998; Lanne, 1995; Eiluet al., 2007).
The host rocks have been hydrothermally altered and metamorphosed in three different stages: i). pre-ore albitisation and some carbonate alteration; ii). a biotite±tourmaline and pyrite phase with the concurrent formation of talc-carbonate veins and gold mineralisation, involving CO2-H2O fluid, that was slightly reducing, Au-, K- and S-bearing (S content relatively low; Korkiakoski, 1992) and iii). later amphibole (tremolite) overgrowing the earlier assemblage during progressive regional metamorphism, which peaked during the crystallisation of tremolite and metamorphic recrystallisation of the assemblage that was produced by the early alteration (i.e., biotite, carbonates and talc). This stage involved CO2-undersaturated, silica-saturated, Ba-rich?, slightly oxidising fluids (Korkiakoski, 1992). Peak metamorphism accompanied D2, while thrusting during D3 was at least partly post-peak, late metamorphic (Hulkki, 1990; Niiranen, 1998; Korkiakoski, 1992; Eilu,1997; Hölttä, 2007). Gold mineralisation probably occurred in conjunction with the Svecokarelian orogenesis between 1.9 and 1.8 Ga (Lapland Goldminers AB, 2011).
The combination of these three stage has produced a proximal alteration/metamorphic overprint assemblage of unfoliated, biotite-talc-dolomite/ankerite-tremolite/actinolite-quartz-pyrite-rutile±albite, richterite, barite, magnetite, tourmaline assemblage, as well as coarse-grained, nearly mono-minerallic amphibole rock with locally abundant quartz and barite and minor amounts of dolomite or ankerite, talc and albite over an area of ~600 x 30 to 120 m. These assemblages are enclosed within an intermediate talc-carbonate-chlorite-biotite-rutile-tremolite-quartz±albite, magnetite, tourmaline envelope to a larger distal alteration halo that occurs discontinuously over an east-west oriented area of 8 km by 100 to 300 m, characterised by the mineral assemblage chlorite-calcite-talc-tremolite±albite (Hulkki, 1990; Niiranen, 1998; Korkiakoski, 1992; Eilu,1997).
The proximal alteration is characterised by enrichment in Au, Ag, B, Ba, CO2, Fe(?), K, Mn, Na (locally), S, Sr, while anomalous Au, Ba, Ca, CO2, K, Na, P, Si, Sr, Te and W in is found in the intermediate zone. In addition, LREE is enriched in most carbonate-rich zones (Hulkki, 1990; Korkiakoski, 1992; Korkiakoski and Kilpelä, 1997).
The major gangue opaques are magnetite and pyrite, while the minor opaques are chalcopyrite, rutile, chromite, haematite, pentlandite, pyrrhotite, violarite, millerite, cubanite, gold, clausthalite, merenskyite (Hulkki, 1990; Korkiakoski, 1992; Korkiakoski et al., 1989; Karvinen, 1999; Eilu,1997).
A Pb-Pb whole rock age of 1814±32 Ma has been obtained for the host rocks, and 1811±87 Ma for pyrite and magnetite. If these ages represent the second stage of mineralisation, the age of ~1820 Ma can be considered as the minimum age for the main stage of gold mineralisation (Mänttäri, 1995). Mineralisation took place at ~1870 to 1840 Ma (Mänttäri, 1995).
The deposit comprises a swarm of subparallel, structurally controlled mineralised veins which have an east-west strike, and dip at 70 to 80° to the north. The underground resource persists to a depth of from 80 to 330 m below the surface (Scan Mining 2003. Press release). These lodes, which are individually 5 to 10 m wide, occur in a range of lithologies, including biotite-tremolite skarn, talc-biotite schist, quartz-barite veins, talc-barite-dolomite veins, and are sometimes accompanied by magnetite, pyrite and chalcopyrite. The lodes have an anomalous barite-gold association and very high fineness (>99.5 % Au). The gold generally occurs as free grains with only minor associated sulphide minerals.
Pahtavaara has been interpreted to represent a metamorphosed sea-floor alteration system with ore lenses as either carbonate- and barite-bearing cherts or quartz-carbonate-barite veins. The gold may have been introduced late, although its grain size, occurrence as free native gold and association with silicates rather than sulphides, as well as its high fineness suggest a pre-peak metamorphic timing.
The total in situ estimated original size is 15.2 t of gold from 3.5 Mt of ore @ 4.3 g/t Au (production + resource, in February 2006; Scan Mining Oy, 2003, 2005).
Published mineral resources and ore reserves include:
Mineral resources, 2005 - 2.975 Mt @ 3.2 ppm Au (Scan Mining Oy, 2005. Press release 03/05/2005);
Measured + Indicated mineral resources, 1 January, 2012 - 0.962 Mt @ 1.95 ppm Au (Lapland Goldminers, 2012), plus:
Inferred mineral resources, 1 January, 2012 - 0.061 Mt @ 2.94 ppm Au (Lapland Goldminers, 2012; 0.5 g/t Au cut-off), plus:
Proved + probable ore reserves, 1 January, 2012 - 1.078 Mt @ 1.60 ppm Au (Lapland Goldminers, 2012; 0.8 g/t Au cut-off).
The most recent source geological information used to prepare this decription was dated: 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.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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