The 1580-1570 Ma old Vehmaa rapakivi granite batholith was studied with the aim to define the geological constraints for localization of natural stone deposits in the batholith. The batholith comprises four roughly concentric granite intrusions from the margin inwards: pyterlite, coarse-grained porphyritic rapakivi granite and two types of mediumgrained porphyritic rapakivi granite. Also porphyry aplite and even-grained rapakivi granite occur. The batholith has intruded as a succession of pulses of subhorizontal sheet-like intrusions conceivably through repeated cauldron subsidence. Natural stone quarries are confined only to certain intrusions within the batholith. We show that the mediumgrained porphyritc granite body in the centre of the batholith comprises two almost identical but different intrusions, with only slightly different appearances: the inner (IG) and the outer (OG) granite. The quarries are confined to the outer intrusion which has an appearance more attractive to the market than that of the inner intrusion. The localization of the natural stone deposits in the Vehmaa batholith is a result of the magmatic history of the batholith, producing intrusions of different appearances with different commercial potential.
We tested the applicability of a newly developed broadband (0–800 Hz) digital-based seismic landstreamer for open-pit mine planning in the apatite-bearing Siilinjärvi mine in central Finland. Four seismic profiles, in total approximately 2.5 km long (2–4 m source and landstreamer receiver spacing), two inside the pit and two on its margins, were acquired in combination with wireless recorders connected to 10 Hz geophones and fixed at every 10 m spacing along the seismic profiles while the streamer data were being acquired. Downhole logging and laboratory physical property measurements on core and rock samples were carried out to not only support the seismic interpretations but also to provide information about the possible geophysical signature of these unique types of deposits. In spite of a highly noisy mining environment, seismic data of high quality were acquired; however, reflection processing and interpretations were challenged by the geologic complexities of several generations of basic and carbonatite dikes. To complement the reflection data imaging and to account for the steep elevation changes and crookedness of some of the seismic profiles, 3D first-arrival traveltime tomography and 3D swath reflection imaging were also carried out. Clear refracted arrivals from the open-pit profiles suggest the possibility of low-velocity zones associated with either blasting or several shear zones intersecting the seismic profiles. In terms of reflectivity, reflections have a different appearance from short and flat to longer and steep ones. The downhole- and borehole logging data suggest that some of these reflections are associated with diabase dikes and some are likely from zones of weaknesses in the alkaline-carbonatite complex. We determine the potential of using seismic streamers for cost- and time-effective open-pit mine planning and encourage further testing in simpler geologic settings to be established.
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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