AbstractsEngineering

In the last decades Fe-Oxide Cu-Au deposits have been discovered and developed in Australia, South America and Canada. This class of mineral deposits is also present in northern Sweden and thus represents a potential target for improving the mineral production of the country. The project aims at defining a geophysical signature of IOCG deposits from regional scale to local scale by using available geophysical data and newly measured petrophysical data. The Kiruna district was selected for regional investigations, while the Tjårrojåkka prospect was selected as testing site for the local study. The Tjårrojåkka prospect is located around 50 km west of Kiruna and is constituted by an apatite-iron ore (Kiruna type) and an associated Cu(- Au) deposit. The host rock is an altered andesite of Palaeoproterozoic age. The working hypothesis to be tested was that a spatial relationship exists between IOCG deposits and major tectonic lineaments and between IOCG deposits and alteration zones and that geophysical data can be used to delineate prospective areas. Gravity and magnetic data were used to map faults and associated fracture zones at regional scale, whereas airborne radiometric data were used to map potassic alteration, which appears to be related to the Cu(-Au) mineralising event. The K/Th ratio maps at both regional and local scale seem to be able to define prospective areas for IOCG deposits. Magnetic maps are also used to indicate presence of highly magnetic bodies associated to high concentrations of Fe-minerals. In the Tjårrojåkka area, Anisotropy of Magnetic Susceptibility on oriented samples was used as tectonic indicator for the geological study of the area. Three major tectonic events were identified, one of which is interpreted to be related to the main mineralising event. Density, bulk susceptibility and Natural Remanent Magnetisation were measured for oriented samples and for borehole specimens at the main Cu deposit. Thermal demagnetisation cycles were applied to the specimens to define the main magnetic mineral, which is mostly multidomain magnetite as confirmed by Königsberger ratios lower than one. Haematite is also present and is caused by oxidation of magnetite. The haematitisation of magnetite is contemporaneous to the Cu(-Au) ore deposition and to potassic alteration. At local scale, high magnetic anomalies define magnetite bodies, while intermediate magnetic anomalies mark areas that were affected by haematitisation. These areas also show high K/Th ratios and are associated to Cu-(Au) occurences. The proposed model for IOCG exploration begins with the analysis at regional scale of potential field data to define the major tectonic lineaments and fracture zones that may have acted as pathways for mineralising fluids. The second step is to identify high/intermediate magnetic anomalies caused by magnetite/haematite deposits. High values of K/Th ratio from airborne radiometric data show areas affected by potassic alteration, which often accompanies Cu-(Au) deposition.