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The Tjrrojkka area in northern Sweden hosts a number of Fe-oxide Cu-Au (IOCG) occurrences.
The major Cu-prospect in the area has been studied by magnetic and electron microprobe analyses of four selected rock samples. The samples are from an exploration well that cuts the main Cu-mineralised body. Magnetic susceptibility thermal treatment of samples indicates that magnetite (and maghemite) are the dominant magnetic minerals, while haematite and other Fe-oxides and Fe-hydroxides are present in minor amounts. Fe-sulphides are not detected in the selected samples. The electron microprobe observations confirm the presence of magnetite and further indicate that haematite is present as an alteration product of magnetite at its rims.
The formation of the highly magnetic minerals is possibly contemporaneous with deposition of the Tjrrojkka Fe-prospect, which is a "Kiruna type" iron deposit. However, our study shows that highly oxidised Fe-minerals (i.e., haematite, goethite and/or lepidocrocite) occur in spatial association with the mineralised rocks of the main Cu prospect. The relatively low magnetic susceptibility of these minerals, compared to the high susceptibility of magnetite and maghemite, and their spatial relationship with the Cu-mineralised rock, may be used as a tool for targeting Cu-deposits. Low magnetic anomalies within areas showing generally strongly magnetised rocks may indicate oxidised areas associated with Cu deposits.
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