Targeting Uranium Alteration Zone with 3D Gravity and 3D Geology

Abstract:

Using the Midwest uranium deposit as an example I will compare the traditional complete Bouguer anomaly, CBA vs. Residual Free-Air Anomaly, RFAA. The Midwest uranium deposit is in the eastern Athabasca Basin, Saskatchewan, Canada, under Mink Arm Lake. The complete Bouguer anomaly at the Midwest deposit is 0.5 mGal as compared to 4.0 to 5.0 mGal RFAA. The RFAA reflects the low density alteration zone commonly associated with Athabasca Basin unconformity uranium deposits. The geology model used to calculate the CBA assumes a single uniform density from the topographic surface to the reference datum, in this case zero elevation. Additionally, the Bouguer density used for the CBA interpretation is calculated using Nettleton profiles over the local topography of glacial deposits. Even the airborne full tensor and Falcon gradiometry surveys use the glacial deposits / overburden to determine the Bouguer density used in their interpretations. To calculate the RFAA, the expected free-air gravity station data is modeled using an expected 3D geology interpretation, and a density inversion used to estimate the geology densities that minimizes the misfit between the observed and estimated free-air gravity data. The RFAA is the difference between the observed and estimated free-air gravities. In the case of the Athabasca Basin, the overall generic 3D geology is simple; though individual prospects can be challenging. The generic Athabasca Basin geology is; and starting from the topographic surface downward: glacial deposits / overburden, lakes and water bodies, Athabasca Sandstone, then basement. The basement consists of metasediments and granites. Since mining companies often have substantial local drilling information, even at the early stages of a prospect’s exploration life, there exist basic 3D geology data necessary to build one or more reasonable and testable 3D geology interpretations. For the Midwest project, the interpretation is a simple 3 layer 3D model with 5 densities. The geology includes: glacial deposits / overburden, lakes / waterbodies / ice, an assumed unaltered Athabasca Sandstone, and metasediment/granite basement.