Seismic forward modelling constraints for seismic ore exploration at the Kylylahti Cu-Co-Zn-Ni-Ag-Au sulfide deposit

The Kylylahti Cu-Co-Zn-Ni-Ag-Au sulfide deposit is located within the Paleoproterozoic North Karelia Schist Belt in eastern Finland about 25 km northeast from Outokumpu. The Kylylahti deposit is a part of the deformed and discontinuous chain of occurrences of Outokumpu assemblage in the Outokumpu area. Outokumpu assemblage refers to a lens-shaped serpentinized peridotite body enveloped by carbonate-skarn-quartz rocks. In the entire Outokumpu area the Cu-Co-Zn-Ni-Ag-Au massive to semi-massive sulfide deposits are hosted in the thin carbonate-silica alteration zones around the serpentinite bodies and are surrounded by black schists. The Kylylahti serpentinites are fine-grained and massive antigorite serpentinites. Tremolite skarns are abundant within the alteration zones of the Kylylahti massif. Previous seismic reflection data from the Outokumpu area includes OKU1, OKU2 and OKU3 survey lines acquired in connection with the Finnish Reflection Experiment FIRE in 2001–2005 and V1, V2, V3, V7, V8 and E1 survey lines in connection with the High Resolution Reflection Seismics for Ore Exploration HIRE in 2007–2010. The goals of this work were, 1) to estimate the seismic reflectivity characteristics of the Outokumpu-type ores and various rock types in the Outokumpu area, 2) to build a geological 3D model of the Kylylahti massif, 3) to use this 3D model and the estimated seismic velocity and density values for creating synthetic seismic sections over the Kylylahti massif in order to test for optimal 2D reflection seismic survey geometries and 4) to compare the synthetic sections with real reflection seismic sections already available from the Kylylahti area. For seismic forward modelling, a geological 3D model and densities and seismic velocities of different rock types are needed. Density values for each rock type were chosen from and compared with previous density measurements done in the entire Outokumpu area. Several rock types show a trend of increasing density towards Kylylahti. Seismic velocity values were either chosen from previous measurements or theoretically calculated from the density values. The geological 3D model of the Kylylahti massif was built with GOCAD® 3D modelling program and is based on drill holes and geological cross-sections of Kylylahti. The seismic forward modelling was done in GOCAD® Mining Suite with the BMOD3D-program which calculates synthetic seismograms using Born approximation theory. The modelled reflection seismic sections were compared with the real E1, V1 and V8 reflection seismic sections that cross the Kylylahti area. The forward modelling suggests that the geological 3D model used for the seismic forward modelling cannot fully explain the reflections seen on E1 and V1 sections. The lithological contacts of the 3D model are much too vertical to create as wide and continuous reflections as seen on E1. The Kylylahti massif may continue further southeast than the geological 3D model suggests as reflections indicate V1 section may crosscut talc-carbonate-skarn–serpentinite and black schist–talc-carbonate-skarn contacts. The less clear reflections on V8 section indicate that the parts of the massif that V8 crosses may be the less reflective mica schist–black schist contacts or the contacts within the Kylylahti massif are vertical at the location of V8. The most reflective parts of the Kylylahti massif are the black schist–talc-carbonate-skarn and talc-carbonate-skarn–serpentinite contacts. The ore has been modelled as an ore–black schist contact. If the contact is associated with density values and especially with theoretical seismic velocity values similar to those used for the modelling, the ore will produce a detectable signal, even if not very strong. A potential 2D seismic survey geometry in Kylylahti could consist of several seismic reflection lines along and perpendicular to the Kylylahti massif, taking into account that in the northern part the massive deviates about 20Ëš towards east from the north–south axis.