Rapakivi granites are the most important raw material for granitic natural stone in Finland, as almost 70% of all granite produced in Finland consists of rapakivi granite. The main production area is the Wiborg rapakivi granite batholith in southeastern Finland. The batholith is a source of mostly brown and red coarse-grained granites with the characteristic texture of large round K-feldspar crystals. The Wiborg batholith represents a unique production area in the world.
A typical feature of rapakivi granite is the distinct surface weathering of outcrops, termed “grusification”. This special type of weathering can extend so deep into the rock that the first couple of metres down from the rock surface can, in the worst case, be unusable as natural stone. The weathering also has a crucial impact on exploration for natural stone in the batholith by changing the surficial colour and soundness of the granite, thus making prospecting more challenging than in unweathered rock areas.
Major parts (75–80%) of the outcrops of wiborgite, dark wiborgite, pyterlite and porphyritic rapakivi granite are weathered with varying intensity from the surficial parts of outcropping granites to a depth of 1–2 m. Weathering can also be found along random subhorizontal and subvertical fractures deeper in the bedrock. The colour of the weathered and stained surface parts of the granite does not represent the real colour of the fresh rock. In addition, the soundness of the rock is diminished.
Revised exploration process
Because of the surface weathering of the outcrops, subsurface quality assessment methods should always be used and the dimensions of surficial weathering should always be investigated in rapakivi granite areas of the Wiborg batholith. The thickness of the upper weathered zone, as well as the colour and soundness of rapakivi granite, can be reliably confirmed by diamond core drilling, but this is the most costly method. Hence, more affordable light drilling equipment for shallow core sampling should be utilised. The depth penetration of the light core drill should be 5–10 m. Of the non-invasive geophysical methods, electrical resistivity tomography (ERT) and induced polarization (IP) successfully exposed a ca. 1-m-thick weathered surficial zone of rapakivi granites and should be applied in the exploration of the Wiborg batholith.
A revised exploration process for natural stone was compiled to be applied especially to the weathered rapakivi granites in the Wiborg batholith (Fig. 1). The process includes the individual steps of desk study, regional mapping and detailed target studies.
It was also demonstrated that the natural stone potential for future quarrying in the Wiborg batholith is good. Prospects can especially be identified in areas dominated by wiborgite and pyterlite, but the commercially less known rapakivi varieties are also prospective.
Natural stone is defined as a piece of naturally occurring rock (EN 12670, 2019). A natural stone product is a worked piece of naturally occurring rock used in building and for monuments.
Härmä, P. 2020. Natural stone exploration in the classic Wiborg rapakivi granite batholith of southeastern Finland – new insights from integration of lithological, geophysical and structural data. Geological Survey of Finland, Bulletin 411, 90 pages, 57 figures and 3 tables. Available at: https://doi.org/10.30440/bt411
Text: Paavo Härmä
Dr Paavo Härmä is a geologist in the Energy and Construction Solutions unit. He has worked at the Geological Survey of Finland (GTK) since 1991, carrying out local, national and international investigation projects on natural stone exploration, crushed rock aggregate inventories and mapping bedrock geology. His current research interests are focused on natural stone exploration in the rapakivi granite area in southeastern Finland.