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Address : 1Luleه University of Technology, Sweden; 2Hifab-Envipro, Sweden;
Celebration date : 4 Saturday October 2008
Author name : Martinsson, Olof1; Allan, إsa2; Niiranen, Tero3; Wanhainen, Christina1; Eilu, Pasi4; Ojala, Juhani4; Nykنnen, Vesa4; Weihed, Pنr1

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  • Article title : Iron oxide-Cu-Au deposits in the northern part of the Fennoscandian Shield
    Article type : Mineral processing
    Location : International Geological Congress,oslo 2008

    Fulltext :

    The northern part of the Fennoscandian Shield, which formed during repeated extensional and compressional events at 3.1-1.8 Ga, is an ore province characterized by regionally developed albite and scapolite alteration and the occurrence of world class Fe-oxide (Kiirunavaara) and Cu-Au deposits (Aitik). It host several styles of Fe-oxide-Cu-Au deposits, including skarn and apatite-iron style deposits, many of them with features that also warrant classification as iron oxide-copper-gold (IOCG) deposits.
    The apatite-iron ores are economically most important with a total production of c. 1900 Mt from 10 mines during the last 100 years and with a total pre-mining resource of c. 4100 Mt. In these deposits, the Fe and P content vary between 30-70 % and 0.05-5 %, respectively. The ore minerals magnetite and hematite occur in lenses or as breccia infill. The ores are usually enriched in LREE, sulphides are rare but subeconomic amounts of Cu may occur.
    Skarn-like iron occurrences consisting of magnetite and Mg and Ca-Mg silicates have been less important with c. 20 Mt mined from 6 deposits and a pre-mining resource of 760 Mt. Most of them occur as conformable lenses with a banded internal structure. Pyrite, pyrrhotite and minor chalcopyrite are commonly present disseminated or as veinlets. Typical grades are 30-55% Fe, 0.2-3.5 % S, 0.05-0.3% Cu, 0.005-1g/t Au and 0.02-0.2% P. A few of the deposits are also enriched in LREE.
    Epigenetic Cu±Au occurrences include the porphyry-style giant Aitik deposit with a pre-mining resource of 2000 Mt at 0.3% Cu and 0.2 g/t Au and a total production of 465 Mt. Other deposits vary in style from disseminated to breccia infill or veins. Chalcopyrite is the most important ore mineral but bornite, pyrite, pyrrhotite, magnetite, molybdenite and native gold may occur in varying amounts.
    The skarn-like ores occur in 2.1Ga Karelian greenstones in association to carbonate rocks, BIF and graphite schist. The apatite iron ores are hosted by 1.9 Ga Svecofennian intermediate to felsic porphyries. The epigenetic Cu±Au deposits occur in both Karelian and Svecofennian volcanic and sedimentary rocks and 1.9 Ga intrusive rocks. The two last type of deposits show similar alteration styles including albite, K-feldspar, biotite, scapolite, carbonate, amphibole and tourmaline, whereas the skarn-like deposits are associated with diopside, amphibole, scapolite and biotite alteration.
    Deposit studies and geochronological data reveal a multiphase origin of the Fe oxide and Cu±Au occurrences with multiple sources of the ore fluids and peaks of mineralization at c. 1.88 and 1.77 Ga. These events are temporally related to major orogenic stages in the evolution of the Fennoscandian Shield. This implies that mineralization formed in different tectonic settings, and with different magmatic associations. Thus, the IOCG deposits are not uniform in origin, which may well explain their diverse features and also makes a simple genetic model for them dubious.

     
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