Bulletin of the Geological Society of Finland (Dec 2011)
Rock magnetic investigations constraining relative timing for gold deposits in Finland
Abstract
Palaeomagnetic and anisotropy of magnetic susceptibility (AMS) studies were carried out on a orogenic gold deposit in Jokisivu, located in the western part of the Pirkanmaa Belt in the Svecofennian domain of southern Finland. These results are compared with previous studies obtained from Satulinmäki, belonging to the Forssa Group in the western part of the Häme Belt, southern Finland and also results from the Central Lapland Greenstone Belt in northern Finland. The main aim of the studies was to test the capability of palaeomagnetic and AMS methods to provide relative age constraints about the structurally controlled gold formation processes. Palaeomagnetic data were used to obtain timing for the emplacement of hydrothermal fluids relative to geological structures. AMS was used to delineate the magnetic fabric. Petrophysical measurements and rock magnetic tests were carried out to define the magnetic minerals and their magnetic domain states as they have importance in preservation of the ancient remanent magnetization. Both the magnetic carriers and the remanence directions of the gold deposits in southern Finland deviate from those previously reported from the Central Lapland Greenstone Belt. The main magnetic mineral in the southern Finland deposits is coarse- to fine-grained monoclinic pyrrhotite whereas in the Central Lapland Greenstone Belt the magnetization is carried by fine-grained magnetite/titanomagnetite. The remanence directions in the southern Finland deposits are rotated and deflected so that no Svecofennian directions have been preserved. This, coupled with correlations between the orientation and orientation distribution of the magnetic and the rock fabric elements imply that the hydrothermal fluids were injected pre/syntectonically during the late stages of Svecofennian orogeny. This is contrasting to the Central Lapland Greenstone Belt, where previous works inferred that the well-preserved 1.88–1.84 Ga Svecofennian palaeomagnetic directions indicate that the gold-bearing hydrothermal fluids were emplaced in existing fractures during the late- or post-deformational stage of the orogeny.
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