From their webpage, under West Nickel project, it appears to be a gravitational gradiation map:

The West Nickel claim blocks cover the majority of a significant gravity anomaly located immediately west central of the “Ring of Fire” magnetic trend. The gravity anomaly is of sufficiently large size and amplitude to suggest that it was caused by mafic/ultramafic bodies that could be the central intrusive source for the massive nickel/copper mineralization in the area. The major nickel camps of the world such as Sudbury, Thompson, Raglan and Norilsk have coincident or adjacent regional gravity anomalies as well as magnetic signatures. The presence of mafic ultramatic rocks and gravity anomalies present a promising exploration environment.

Further down: