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 geologic observations to better understand the geophysical signature of the Newton epithermal system. Airborne [Formula: see text]-ray data sets indicate elevated emission counts of K, eTh, and eU over the Newton epithermal system, which are caused by hydrothermal alteration. Drillcore [Formula: see text]-ray measurements also indicate 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 the 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 geologic 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 an 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.

3D Geophysical Predictive Modeling by Spectral Feature Subset Selection in Mineral Exploration

“…Geophysical images of Newton deposit were obtained from 3D inversion of d current (DC) resistivity, induced polarization (IP), and magnetic data sets [27,31]. In 2 an 85 line-kilometer DC/IP survey was conducted in the Newton Hill area using a "p Three layered volcano-sedimentary sequences overlain from bottom to top are mafic volcanic rocks (MVR), sedimentary rocks (SR), and felsic volcanic rocks (FVR).…”

“…Geophysical images of Newton deposit were obtained from 3D inversion of direct current (DC) resistivity, induced polarization (IP), and magnetic data sets [27,31]. In 2010, an 85 line-kilometer DC/IP survey was conducted in the Newton Hill area using a "pole-dipole" electrode configuration [27,31]. The DC/IP survey lines were placed at 200m intervals in the east-west direction in the hydrothermal alteration zone.…”

“…The magnetic data were collected along N-S flight lines spaced 200 m apart, and E-W tie lines were flown approximately every 2000 m to 500 m. A total of 7071 line-kilometers of magnetic data were collected, covering an area of 1293 km 2 . The magnetometer sampling rate was 0.1 s, yielding a measurement interval of approximately 10 m along each profile, depending on the helicopter speed [27,31].…”

Electrical Resistivity Surveys and Data Interpretation

“…Since geological structures tend to be three-dimensional (3-D) in nature, a 3-D resistivity survey and interpretation model (Figure 4) should give the most accurate results. Although it has not reached the same level of usage as 2-D surveys, it is now more widely used in very complex areas such as in many environmental (Chambers et al, 2006;Rucker et al, 2010) and mineral exploration surveys (White et al, 2001;Bingham et al, 2006;Abbassi et al, 2018). Many of the early 3-D surveys used the pole-pole array over relatively small grids (up to about 20 by 20 electrodes) with measurements in different directions (Park and Van, 1991;Loke and Barker, 1996b).…”

Magnetic modelling and physical properties of magnesite deposits and regional sequences of Serra das Éguas, Brumado, Brazil

“…Furthermore, the use of physical properties in‐depth has key meaning to use the information as constraints to perform forward/inversion modelling. The joint application was employed to model, for instance, Au–Ag deposits in Canada (Abbassi et al., 2018), for IOCG deposits (Leao‐Santos et al., 2015), for cooper deposits in Brazil (Vasconcelos et al., 2018). Additionally, Baltosser and Lawrence (1970) have described the relationship between radiometric, resistivity and magnetic susceptibility data collected in drill holes and the magnetite per cent for metallic mineral exploitation.…”