To interpret geophysical data most effectively, it is necessary to have an understanding of the characteristic ranges of physical property values of the rocks being surveyed. With knowledge of how physical rock properties vary with rock type, hydrothermal alteration, and mineralization, it is also possible to constrain geophysical inversion calculations to refine 3D models of the Earth's subsurface. This Geoscience BC funded project aims to
1. compile existing property data from rocks representing a selection of central BC mineral deposits,
2. enhance the physical property database with new property measurements made on samples from these deposits, as well as from rock samples collected from outlying areas, and
3. apply acquired property knowledge to constrain geophysical inversions.
The mineral deposits being investigated are those over which Geoscience BC had detailed magnetic and EM surveys flown as part of their QUEST and QUEST-West projects. They represent mainly porphyry-style deposits, and include past producers, current producers, and projects in development stages.
The second phase of the study involves using physical property knowledge to constrain geophysical inversions. Geophysical inversion, the calculation of a subsurface physical property distribution from collected geophysical data, yields improved models when prior geological and physical property information is considered. Downhole property measurements, such as magnetic susceptibility measurements can be directly input as control points. General physical property knowledge, such as expected ranges of property values for certain rock types or altered zones can also be used to guide inversion results. For this second stage of the project, there will be close collaboration with Doug Oldenburg of the Geophysical Inversion Facility (GIF) at the University of British Columbia in assembling all available geological, geophysical, and physical property information, and throughout forward modeling and constrained inversion work.
The physical property data compiled as part of this project, and the geophysical case study demonstrations will serve to aid BC geoscientists in planning geophysical surveys, interpreting geophysical data, and improving subsurface geology models in similar geological settings in BC.
Initial work will involve compilation of four types of rock property data: magnetic susceptibility, density, conductivity, and chargeability. These are the rock properties that correspond to magnetic, gravity, electromagnetic/DC resistivity, and induced polarization geophysical methods, respectively. This data will be statistically analyzed, and mineral analyses will be completed to identify the primary and secondary geologic processes controlling each of the properties. The mineral deposits focused on showcase a range of host rocks and alteration styles, and physical property studies will involve a comparison between behaviors of rock properties in the various settings represented. Unique property ranges found to be associated with mineralized rocks will be sought.