Hiekkapohja hydrothermal system – ore mineral, lithogeochemical and paleomagnetic evidence from the Paleoproterozoic Central Finland Granitoid Complex
The Paleoproterozoic Svecofennian Central Finland Granitoid Complex (CFGC) has been regarded as an area of low mineralisation potential. The Hiekkapohja area, 20km north-east of the town of Jyväskylä, host a concentration of variable metalliferous showings. Samples from mineralised boulders and o...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Geological Society of Finland
2023-01-01
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| Series: | Bulletin of the Geological Society of Finland |
| Subjects: | |
| Online Access: | https://www.geologinenseura.fi/sites/geologinenseura.fi/files/heilimo_et_al_bulletin_vol_94_part_2_145_164.pdf |
| Summary: | The Paleoproterozoic Svecofennian Central Finland Granitoid Complex (CFGC) has been
regarded as an area of low mineralisation potential. The Hiekkapohja area, 20km north-east
of the town of Jyväskylä, host a concentration of variable metalliferous showings. Samples
from mineralised boulders and outcrops display variable combinations of anomalously high
concentrations of Cu, Mo, Zn, Pb, W, Pb, Ag, As, and Au. The area is composed mainly of
peraluminous and ferroan granitoids. The dominant porphyritic Hiekkapohja granodiorite
(~1.88 Ga) is cross-cut by the equigranular Soimavuori granite of similar age. The
porphyritic Lehesvuori granite on the western side of the study area represents marginally
older (~1.89 Ga) magmatism. The paragenetic sequence of the ore minerals shows that
the Hiekkapohja area has been affected by at least two separate stages of hydrothermal
activity. The first mineralisation stage was widespread, crystallising typically chalcopyrite,
pyrrhotite, sphalerite, galena, arsenopyrite, magnetite and Ag-bearing minerals. After the
first stage, a low temperature oxidising phase formed hematite and marcasite. The second
mineralisation stage enclosed low temperature minerals, such as marcasite and native
Ag and Ag-minerals, as inclusions inside chalcopyrite, pyrite, pyrrhotite, sphalerite, and
arsenopyrite. The mineralised samples typically display signs of K-metasomatism and
less commonly signs of propylitic alteration. During the second mineralisation stage the
fluid flow was controlled by the dominant 120°–135° trending shear zones. Both the
hydrothermal activity and the regional geology indicate that porphyry type ore forming
processes have occurred in the Hiekkapohja area. Paleoproterozoic resetting of the
remanent magnetisation is further evidence for the role of the hydrothermal system. |
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| ISSN: | 0367-5211 1799-4632 |