3D Seismic Data Are an Asset for Exploration and Mine Planning: Kevitsa Ni-Cu-PGE Deposit, Northern Finland

Kevitsa is a disseminated Ni-Cu-PGE (platinum group elements) ore body in northern Finland, hosted by an extremely high-velocity (6500-8500 m/s) ultramafic intrusion. It is currently being mined at a depth of approximately 100 m using open-pit mining method. The life of mine is expected to be nearly 20 years, with the final pit depth reaching around 550-600 m. Based on a series of 2D seismic surveys and given the expected life of mine, a high-resolution 3D seismic survey was justified and acquired in winter 2010 prior to the start of the mining operation. Various researchers and teams have exploited these data since the acquisition because of the unique nature of the host rock and the data being challenging to interpret however rich in reflectivity. Here, we first present the earlier 3D reflection data processing results and then complement them by 3D first arrival traveltime tomography that was recently conducted. The combined results help to provide constraints on the nature of some of the reflectors within the intrusion. It for example shows how the tomography results can be correlated with rock quality data and for further planning of the pit. In particular, we observe a major fracture system, resolved by the tomography results and running in the northern parts of the planned pit, with the reflection data providing better information on its depth extent, estimated to be at least 600 m with a lateral extent of 1000 m. The fracture system appears to spatially limit the lateral extent of the economic mineralzsation and partitioned mainly within the intrusion. It can be related to the paleostress regime forming similar features in the study area and will likely be important for mining at deeper levels. Using the Kevitsa 3D seismic data set, we argue that 3D seismic data should routinely be acquired prior to the start of mining activities to not only maximize exploration efficiency at depth, but also to optimize mining as it continues towards depth. 3D seismic data are valuable and can be revisited for various purposes but difficult to impossible to be acquired with high quality when mining commences.