A ROCK PHYSICAL PROPERTIES PROXY FOR MINERALIZATION AT SIX PORPHYRY DEPOSITS IN BRITISH COLUMBIA, CANADA

Identification of the geological sources of geophysical anomalies requires knowledge of the physical properties of rock types, formations and their alteration products. Mira Geoscience and the Geological Survey of Canada are building a Rock Properties Database System (rpds.mirageoscience.com). It compiles the most valuable physical properties for mineral exploration (density, porosity, magnetic susceptibility and remanence, electrical resistivity and chargeability) linked with location and lithological metadata. A critical knowledge gap concerns the dependence of physical properties on alteration and mineralization processes. We report on the results of a collaborative University of British Columbia - Geoscience British Columbia project, “Integrated Geological & Geophysical Porphyry Models: Adding Value to Geoscience BC Geophysical Data”. As part of the QUEST and QUEST-West geophysical initiatives, Geoscience BC focused detailed geophysical surveys on a suite of six known porphyry deposits: Mount Milligan, Endako, Huckleberry, Bell, Granisle, and Morrison. Physical properties for a set of 269 samples of characteristic host rocks and their altered equivalents were measured. The causes of physical properties variations were investigated through their correlations with ore and alteration mineralogy determined from thin-section observations and X-ray diffraction analyses. Physical properties of variably altered host-rocks and intrusive rocks vary significantly between different BC porphyry deposits. No simple relationship exists between mineralization and the physical properties measurements due to the wide range of source rocks and alteration processes. However, a promising proxy for mineralization is produced by defining a relationship between the density, electrical resistivity and chargeability for the least mineralized samples using principal component analysis. The concentration of sulphides and oxides is observed to be well-fit by the vector perpendicular to the low-mineralization surface. A magnetic-susceptibility correction removes the concentration of magnetite allowing a determination of the concentration of sulphides. While crude, the mineralization-proxy only uses data which can be determined from above ground using geophysical surveys.