The ancient basins are of a wide interest to geoscientists as they archive the early development of plate tectonics, life, and ore-bearing systems. Several basins, especially those involving volcanic-derived materials, contain ore and mineral deposits. The formation of these ores is often related to hydrothermal activity and the migration of mineralising fluids, notably derived from volcanogenic massive sulphide deposits. As such, the Tampere basin (1.92-1.88 Ga) hosts several mineralised deposits: the Haveri deposit, mined in 1942-1962, and the Kutemajärvi deposit, mined in 1994-2003. The basin is associated with a convergent margin of an island arc type. Geodynamically, an evolution from a back-arc to an intra-arc settings is implied, based on the systematically changing geochemical signatures. The closure of the basin resulted in the formation of a large-scale, tight to isoclinal syncline with steeply dipping fold axial planes. Despite the evidently large amount of shortening, the primary sedimentary and volcanic structures are generally remarkably well preserved. The Tampere Basin, therefore, offers an excellent opportunity to examine the volcano-sedimentary evolution of an ancient convergent marginal basin, and the mechanics of and strain distribution during its subsequent closure.
In order to investigate the deposition, the tectonic deformation and the fluid migration, geological mapping of a key area of the basin has been performed. In addition, georadar reflection profiles have been acquired to confirm the position of important intrusions and faults. The field measures have also been backed up with microstructural thin-section analysis and petrography. At a larger scale, public airborne geophysics data have been compared to the geology in order to extend geological contacts when covered.
This study highlighted several sulphide-rich horizon within the basal succession of the basin infill. The sulphides are either in relation with tectonic structures or with genuinely clay-rich deposits. The studied succession shows a progressive deepening of the basin through time with facies evolving from fluvial-coastal associated with intensive volcanic activity toward some turbidite lobe deposits. Stratigraphic continuity is difficult to assess but the change of environment seems to have been sudden (continental break-up?). It is between the turbiditic lobes that clay-(sulphides-)rich deposits occur. However most of the sulphides seems to be associated with an important tectonic structure. The structural study suggests that these sulphide-bearing horizons originally localised within relatively low-angle thrusts that later rotated into a subvertical orientation.
The suggested model implies that early gently dipping thrusts acted as both channels and traps for the mineralising fluids that possibly sourced from relatively shallow depths from the base of the basin infill. The origin of the fluids might therefore be either in relation with early metamorphosis of the sedimentary pile and/or to intense intrusion of igneous material. The continued compression caused a subsequent rotation of the thrusts into their present subvertical position. Some steeply dipping, mineralised fault zones might, therefore, in fact source from relatively shallow depths rather than from deep basement sources.