The Paleoproterozoic Kristineberg mining area, northern Sweden: Results from integrated 3D geophysical and geologic modeling, and implications for targeting ore deposits

Abstract

The Kristineberg mining area in the western part of the Paleoproterozoic Skellefte Ore District, northern Sweden, is well known for its base-metal and recent gold discoveries. A pilot 3D geologic model has been constructed on a crustal scale, covering an area of 30ϫ 30 km to depths of 10 km. Constrained 3D inverse and forward gravity modeling have been performed to confirm and refine previous modeling along seismic profiles using mainly 2.5D techniques. The 3D inverse gravity modeling was geared to generating isodensity surfaces that enclose regions within the model of anomalous density contrast. The 3D forward gravity modeling was conducted to include faulting and folding systems that are difficult to include in the inversion. The 3D geologic model supports many previous interpretations but also reveals new features of the regional geology that are important for future targeting of base-metal and gold deposits. The margins of a thick granite in the south dip steeply inward, suggesting the possibility of room to accommodate another large base-metal deposit if the granitic rocks are juxtaposed with volcanic rocks at depth. Gravity modeling also suggests the observed Bouguer gravity high within the western metasediments can be explained by a large mafic intrusion that has dioritic to tonalitic composition and no significant magnetic signature. Because mafic-ultramafic intrusions within metasediments can indicate gold, this interpretation suggests the western metasediments have a high gold potential.

3D interpretation by integrating seismic and potential field data in the vicinity of the proposed COSC-1 drill site, central Swedish Caledonides

“…While there are several methods for separating the regional field from the gravity data (e.g. Li & Oldenburg 1998b;Nabighian et al 2005;Malehmir et al 2009), we chose a simple second-order polynomial approach to estimate the regional field (Fig. 3d).…”

Unravelling the internal architecture of the Alnö alkaline and carbonatite complex (central Sweden) using 3D models of gravity and magnetic data

“…A second-order polynomial field was also tested but was not optimal as it removed the long wavelength components of the target anomaly. The main Alnö gravity anomaly is successfully separated from the background field after removing the regional trend, this implies that a first-order regional field is suitable for this purpose (Li and Oldenburg, 1998b;Malehmir et al, 2009;Hedin et al, 2013).…”

New zircon data supporting models of short-lived igneous activity at 1.89 Ga in the western Skellefte District, central Fennoscandian Shield

“…Other suggestions for the basement are 2.0-1.9 Ga granitoids south of the Skellefte district (Billström and Weihed, 1996) and the Bothnian Basin rocks beneath a north-dipping crustal-scale reflector in the Kristineberg area (Malehmir et al, 2007). The structural evolution of the Skellefte District is controlled by a complex fault pattern developed during early crustal extension, later followed by fault inversion (Allen et al, 1996;Bauer et al, 2011) during the main compressional deformation at around 1.87-1.82 Ga (Bilström and Weihed, 1996;Rutland et al, 2001b;Weihed et al, 2002).…”

“…The Viterliden intrusion and the Kristineberg VMS deposit occur in the core of the Kristineberg antiform, where the greatest strains were localised into sub-vertical, curviplanar faults which led to large variations in strain and structural geometry both across and along the regional antiformal structure (Malehmir et al, 2007(Malehmir et al, , 2009Skyttä et al, 2009;Dehghannejad, 2010). Contacts between the different intrusive phases, as well as contacts between the intrusion and the bounding volcanic rocks are generally sheared.…”