Geophysical inversions applied to 3D geology characterization of an iron oxide copper-gold deposit in Brazil

Abstract

Mineral exploration dynamics often requires an efficient and objective means of evaluating a prospect in early exploration stages, when few holes have been drilled. In the case of deep prospects or prospects under cover, this evaluation will mostly be based on geophysical data. To develop an objective interpretation method capable of combining all the information available, we have developed an integrated interpretation scheme of geophysical models and sparse geologic data. Our method is based on the relationship between recovered physical properties obtained from 2D and 3D inversions, aiming to find patterns associated with geologic units, such as iron formation, copper ore, and host rock. The interpretation is guided by theoretical relations of the minerals of interest (chalcopyrite and magnetite) and the sparse geologic information available. It is suitable for prospects in the initial stages of exploration when only limited mineralogical information is available from, say, one drillhole. We have demonstrated the success of the method using magnetic and DC resistivity data from the Cristalino iron oxide copper-gold deposit, located in northern Brazil, which is covered by a thick soil overburden. The theoretical behavior of the physical properties of chalcopyrite and magnetite was first combined with the rock types identified in the drill cores to find groups or classes associated with different amounts of these minerals. Then, these relative relations between units were applied to define four classes in the scatterplot of recovered susceptibility and conductivity values from 2D inversions. These four classes are associated with iron formation, copper ore, and two types of host rocks. After the validation with the known geology, the same interpretation scheme was applied to the scatterplot of recovered susceptibility and conductivity values from 3D inversions. The final interpreted volume allows the explorationist to have an approximate estimate of the copper body extent.