Vassbygda Marble Occurrences - B Carlsen (2021).pdf
Brønnøy Kalk, which today operates the large Akselberg marble quarry in Velfjord, Nordland, are in the process of developing plans to ensure that future resources of calcite marble are available when the current open-pit quarry reaches the end of its life. Options include going underground or exploring new occurrences. Some of the most promising deposits are found in Vassbygda, about 15km south of the current mine. The Vassbygda area has seen little activity in terms of geological exploration. As a result, there is a lack of maps and publications to indicate the quality, origin and structure of the marble deposits.
This contribution aims to unravel the structural configuration of the marbles in the Vassbygda area and to provide a new understanding of the marble deposits. The Vassbygda and Akselberg marbles are situated within the Helgeland nappe complex, which formed during west-vergent thrusting and folding, and associated migmatisation and magmatism before the main Scandian phase of the Caledonian orogeny. This resulted in an extensive (and in place, intensive) folding of the units in Vassbygda.
A range of techniques have been used to constrain the structural framework, mineralogical properties, depositional environments and metamorphic conditions of the Vassbygda area, including remote sensing, field mapping, optical microscopy, SEM, geothermobarometry, chemical analyses, core logs, and 3D-modelling.
The Vassbygda area consists primarily of marbles (spotted and banded) and metasandstones (from metagreywacke to metaarkose), the latter in places being garnet- and sillimanite-bearing migmatites. Structurally, the area is characterized by tight to isoclinal recumbent F1 folds that are refolded by large-scale, open, moderately north-plunging F2 folds, creating a type 2 interference pattern (Ramsay & Huber, 1987). The contact with the overlying Middle Nappe rocks is a top-to-the WNW oblique reverse-sinistral shear zone that may have been developed contemporaneously with F2 folding. Geothermobarometery applied on a garnet- and sillimanite-bearing metagreywacke yield two clusters of P-T estimates, one at c. 4.5 Kbar, and c. 640°C, and the other at c. 3 Kbar and c. 540°C. Preliminary, we interpret the higher P-T estimates to constrain conditions during migmatization and F1 folding, whereas the latter may be associated with F2 folding and overthrusting of the Middle Nappe.
The results of this study will provide a more detailed understanding of the geology of the Vassbygda area, which is important for Brønnøy Kalk in their planning for their future development.
Ramsay, J. G., & Huber, M. I. (1987). Modern structural geology. Folds and Fractures, 2, 309-700.
