What you're seeing is a colour-coded image of the magnetic intensity of these rocks. The bright colours—the reds and purples—are relatively magnetic rocks and the blue colours are non-magnetic rocks. This particular map is from the previous generation of geophysical surveys that were flown in the 1970s in analog mode, so they came out as paper maps with contours on them. About 25 years ago we went through the process of digitizing all that and making it colour coded like this. So there were digital images available. However, the resolution of these was still very coarse.
On October 19th, 2011. See this statement in context.