Geophysical properties of an epithermal Au-Ag deposit in British Columbia, Canada

Abstract

The Newton Au-Ag deposit is an intermediate sulfidation state epithermal system in British Columbia, Canada. Multiple types of geophysical data are interpreted and evaluated with drillcore petrophysical, geochemical and geological observations to better understand the geophysical signature of the Newton epithermal system. Airborne γ-ray datasets show elevated emission counts of K, eTh, and eU over the Newton epithermal system that are caused by hydrothermal alteration. Drillcore γ-ray measurements also show high potassium concentrations related to the K-rich phyllosilicates in the form of argillic and quartz-sericite alteration assemblages. Magnetization vector inversion (MVI) is used to recover an unconstrained 3D magnetization vector model of the system on regional and deposit scales. The regional MVI has resolved a deep concentric shaped low magnetic zone that is interpreted as a porphyry system beneath the epithermal deposit. At the deposit scale, 3D direct current (DC) resistivity and induced polarization (IP) inversion, and unconstrained MVI revealed finer details of epithermal system architecture. Cooperative DC/IP and magnetic inversion, at the deposit scale, constrained the magnetic susceptibility model and recovered a more precise susceptibility image of the epithermal system that is well-matched with borehole geology. The integrated geophysical interpretation helped to resolve several 3D latent geological features in places without direct access to drillcore samples. We identified four petrophysical domains based on the three cooperatively inverted physical properties, including electrical resistivity, IP chargeability, and magnetic susceptibility. The combined geophysical models differentiated porphyritic intrusions (chargeability/susceptibility lows), disseminated sulfides (resistivity lows and chargeability highs), a Cu-rich zone in mafic volcanic rocks (susceptibility/chargeability highs and resistivity lows), and a Au-Ag-Cu-rich zone with silicification in felsic volcanic rocks (chargeability/susceptibility lows and resistivity highs). These petrophysical domains also provide useful exploration vectors for identification of similar epithermal systems.