I suspect it is not a magnetic survey but a gravity Geophysics....

I suspect it is not a magnetic survey but a gravity Geophysics. The reddish or yellowish color means the rocks in area has low density( might be caused by fault structure whereas fault is the space for gold to be mineralized). I think the Geophysical anomalies(brighter color) is created by contrast the survey reading. the reddish and yellowish color means that there is larger differences in reading than surrounding area. Blue color probably means that rocks in that area are relatively uniform, therefore, no structure no mineralization. I dont know where this survey map comes from, I might have better answers if I can have the full copy. probably better with geological map in same scale.