Variability of outcrop magnetic susceptibility and its relationship to the porphyry Cu centers in the Highland Valley Copper district

The Highland Valley Copper porphyry Cu-(Mo) centers compose Canada’s largest porphyry Cu mining district, and are hosted in plutonic rocks of the Late Triassic Guichon Creek batholith in the Quesnel terrane, British Columbia. Here we evaluate the relationship between variations in magnetic susceptibility and alteration in order to define a footprint around the porphyry Cu centers using a district-scale dataset of 1610 minimally weathered outcrops and 11,432 magnetic susceptibility measurements. Statistical analysis of the outcrop-based measurements revealed changes in outcrop geometric mean magnetic susceptibility and the coefficient of variation (extent of variability in relation to the mean) are proportional to vein frequency and altered rock volume estimates. At a confidence interval of 95% and 9 degrees of freedom, the average standard error (uncertainty) of the geometric mean and coefficient of variation from 10 magnetic susceptibility outcrop measurements is 7.3% and 13.2%, respectively. Modeling of outcrop magnetic susceptibility data shows that domains of coefficient of variation of >0.3 extend 1–4 km away from the porphyry centers, whereas pronounced susceptibility lows of <10 × 10−3 SI have a smaller footprint centered on the Cu-mineralization. The magnitude of change of magnetic susceptibility coefficient of variation between protolith and altered rocks in the porphyry Cu footprint is greater than the corresponding change in susceptibility geometric means. Thus, with some caveats, variability in magnetic susceptibility provides a good signal that corresponds to vein and alteration intensity, and can potentially be used to map ore-forming processes and to vector to porphyry Cu centers.