Epigenetic REE-U-Th-anomalous Fe oxide mineralisation in the Narken area, NE Sweden
|Location||International Geological Congress,oslo 2008|
|Holding Date||08 September 2008|
The subeconomic Fe oxide mineralisations in the Narken area (c. 7445/1804, Swedish National Grid, RT90) were first noted in the late 1960s (see Frietsch, SGU C 672, 1972). The mineralisations occur closely related to a major set of mainly N-S-trending shear zones and lineaments, which in this area mostly outline the Kalix river valley. The zones comprise part of a very large system of polyphase tectonic structures that have been characterised as part of the Baltic-Bothnian megashear or on a regional scale, the Pajala shear zone. This Fe oxide mineralisation type is presently known from at least 3 discrete locations in the Narken area; Vattuvaara, Myllyniemi-Kuusisaari and Kartovaara, all featuring slightly different appearances and host rock relations. The most well-exposed locality (overburden recently removed by a private prospector), Vattuvaara, consists of an Fe oxide breccia, with at least two generations of originally magnetite-dominated assemblages cutting, and incorporating fragments of, Karelian to Svecofennian metasupracrustal rocks. Most of the magnetite at Vattuvaara has been oxidised to hematite ("martite"), and only strongly corroded remnants of the original (often euhedral to subhedral, now pseudomorphic) magnetite phase remains. Sulphides occur very sparingly.
At Kartovaara, Fe oxide mineralisation is spatially clearly related to brittle deformation and related quartz veining as well as epidote-chlorite alteration, but seems to be intimately tied to the present suite of (comagmatic) granitic to gabbroic intrusives. Oxidation of magnetite is more restricted here, and sulphides (primarily pyrite) occurs abundantly. Elevated contents of REE (in particular Ce) together with high Th and U, as well as slightly elevated contents of Cu and Au are characteristic of the studied deposits. The host for REE, Th and U is suggested to be epidote-allanite phases, which occur abundantly, both in the mineralisation and in the altered host rocks.
The latter is particularly evident in the case of host rock fragments occurring within the mineralisation. Tendencies towards local to semi-regional epidote-chlorite alteration can also be observed in the area, and particularly within, and in association with, brittle deformation zones. Overall, the Narken Fe oxide mineralisations exhibit a number of features suggestive of IOCG-affinity.