Hunting for massive sulphides between density logs

Summary

Gravity measurements are one of the more useful datasets in the exploration for high-density massive sulphide mineralisation. However, the fall-off of signal strength with depth limits the usefulness of surface data alone. Borehole and core density measurements can be used to constrain surface gravity inversions and extend both their depth of investigation and lateral resolution. Conversely, surface gravity measurements can be used to constrain, via inversion, the interpolation of downhole density measurements when building density (specific gravity) models for resource calculations. In this study, various specific gravity models act as reference models for surface gravity inversions, with appropriate unweighting away from drill holes. The resulting inverted models fulfil two roles: (1) they improve upon interpolated 3D specific gravity models for resource calculations, and (2) they provide vastly superior exploration tools over inversions of the surface data alone. The brownfields exploration mandate at the Kevitsa Ni-Cu deposit in northern Finland is to find areas of massive sulphide enrichment within a large-tonnage, low-grade orebody. For this reason, we have concentrated on the differences between inverted results and the resource density models. Any positive differences represent previously unmodelled high density zones, which are potential massive sulphide targets between drill holes.

Key words::

• gravity inversion
• constrained
• resource modelling
• massive sulphides