| Abstrakt: |
Petrophysical, rock magnetic and anisotropy of magnetic susceptibility (AMS) measurements have been carried out on a porphyry type Cu-Au deposit in Kedonojankulma, on an orogenic gold deposit in Uunimäki and on the exploration target of Mäyrä in the Palaeoproterozoic Häme belt of Southern Finland. The mineralizations occur in strongly altered and sheared zones. The main aim of the laboratory studies has been to identify and characterize ore-related alteration processes that are reflected in the physical properties of rocks. The studies have focused on magnetic properties and the identification of magnetic minerals using rock magnetic tests. Magnetic mineralogy was verified by SEM studies. In the Kedonojankulma quartz-plagioclase porphyrite occurrence, the induced and remanent magnetizations are slightly higher in the altered auriferous shear zone than in the less altered host rock. In the altered zone, the only magnetic mineral is monoclinic pyrrhotite with a high Curie temperature, while in the host rock the main magnetic mineral is ilmenite with minor magnetite. In the Uunimäki gabbroic occurrence, the highest magnetization values correlate with strong IP anomalies, the magnetization being carried by monoclinic pyrrhotite with a high Curie temperature. In the more weakly magnetized rocks, the magnetization predominantly resides in ilmenite. In the strongest IP anomaly area, which is also regarded as the most gold bearing, the remanent magnetization also has strong intensity, and a Svecofennian age remanence could be isolated. The result thus suggests that provided that gold and pyrrhotite are contemporaneous at the site, the gold is post-tectonic. Based on AMS data, the magnetic foliation planes follow the general foliation structures in both formations, although the degree of anisotropy varies considerably. In the Mäyrä occurrence, the magnetization of the shear zone has decreased due to hydrothermal alteration. The surrounding gabbros, whether coarse-grained dark gabbro or fine-grained lighter coloured gabbro, contain magnetite as the main magnetic mineral, while the shear zone also contains pyrrhotite in addition to magnetite. [ABSTRACT FROM AUTHOR] |
