Very interesting, re geo tech had to go up to 2000- 2500 ft. vs 1000 ft. before they could zero their instruments. The first thing I thought of was Texas Gulf Sulphurs' huge VMS discovery 1964 in Timmins Ont. 150Mt. The Kidd Creek Mine. For years before the discovery, planes would fly over the area and their instruments would go hay wire for no apparent reason. huh!