The Mustavaara V-Fe-Ti deposit is part of the 2.45 billion-year-old Koillismaa Intrusion. The positioning of this sheet-like intrusion has been attributed to the periodic replenishing of magma between the Kuusamo Schist Belt (2.45-2.0 billion years old) and the Archaean Basement Complex (>2.5 billion years old). The layer of magnetite gabbro, which is the host rock for the Mustavaara Deposit, formed during the last phases of the magma chamber crystallization. Due to later tectonic events, the intrusion has been uplifted and broken into a number of blocks that can be found from Kaukuanjärvi in Posio to the Pintamo district in the Pudasjärvi municipality. The Mustavaara Deposit is located in one of the biggest and central blocks of the intrusion, Porttivaara Block (figure 1).
Figure 1. Simplified geological map of the Koillismaa area.
The magnetite gabbro is clearly reflected in magnetic survey data (Figure 2). In the Porttivaara Block area, the magnetite gabbro is about 19 km in length in total, nearly east-west striking and generally dipping 40° north. Gravimetric data suggests that the depth of the deposit goes down to around 2000 m.
Figure 2. 3D-modelled airborne magnetic survey data of the Porttivaara Block area (block dimensions shown as black line in upper image). The upper image illustrates magnetic intensity shown as topography, and the lower image shows the modelled dip of the magnetite gabbro.