Developing Geophysical Signatures To Constrain Geologic Mapping

The Greens Creek ore deposit is one of the largest producing volcanogenic massive sulfide<br>(VMS) deposits in southeast Alaska, and is found on north-central Admiralty Island in southeastern<br>Alaska. This deposit and all other known VMS prospects in the region are found only in Triassic Hyd<br>Group volcanic rocks. Future success of mineral exploration in the region is thus highly dependent on<br>accurate geologic maps that accurately portray where these Hyd Group rocks are located in a region<br>that is largely covered by dense vegetation, water, and tidal mud-flats. Because of this extensive cover,<br>we have used ground and airborne geophysical information to constrain the geologic mapping. A large<br>airborne geophysical survey, incorporating both electromagnetic (EM) and magnetic sensors was flown,<br>supplemented by ground magnetic and VLF-EM resistivity profiles over key areas where geology<br>could be unequivocally mapped. From these, we developed a matrix of geophysical signatures of key<br>rock units that was then used in predictive geophysical models (essentially pseudo-geology maps)<br>derived from the airborne data. Geologic ground control was then used to convert these models into a<br>substantially revised geologic map of the region that now shows more than a 30% increase in Triassic<br>Hyd Group rocks.