Ground IP‐Resistivity, and airborne Spectrem and helicopter ZTEM survey results over Pebble copper‐moly‐gold porphyry deposit, Alaska

Abstract

Survey results from ground IP-Resistivity, Spectrem airborne electromagnetics and magnetics, and ZTEM™ tipper AFMAG helicopter electromagnetics are compared over the Pebble porphyry copper deposit, in southwest Alaska. The IP data display higher amplitudes associated with the increased sulphides over the Pebble alteration halo that vector towards the deposit. Lower resistivities are associated with the chargeable alteration halo and slightly higher resistivities occur in the less chargeable core at Pebble West. The deeper Pebble East was not detected in IP due to insufficient penetration. The airborne EM surveys, Spectrem and ZTEM, appear to map the resistivity contrast with more resolution than the ground IP resistivity survey, due to higher sampling and better depth of investigation. Spectrem conductors coincide with the rich pyrite mineralization, sulphide veins and alteration, but the depth of investigation is at the detection limit of Pebble East. The ZTEM tipper AFMAG survey appears more sensitive to weak resisitivity contrast especially at depth. Both Pebble West and East are well detected in the ZTEM results.

3D joint inversion of magnetotelluric and airborne tipper data: a case study from the Morrison porphyry Cu–Au–Mo deposit, British Columbia, Canada

“…Massive sulphides typically have low resistivity values of 0.1-1 m (Palacky 1988) that are readily detected with the time domain EM method (TDEM). Deposits containing disseminated sulphides, where the sulphide minerals occur as individual grains within the rock matrix, have a higher resistivity and smaller resistivity contrasts with the host rock that may not be easily detected with TDEM since limited electric current is induced in the more resistive target (Kaminski, Legault and Kumar 2010;Paré and Legault 2010). Thus, detection of disseminated sulphides based on resistivity contrasts benefits from the use of frequency domain EM methods that can detect targets with a higher resistivity, which often means a lower resistivity contrast with the host rock.…”

Helicopter AFMAG (ZTEM) Survey Results over the Ad Duwayhi intrusion related gold deposit (IRGD) in the Western Arabian Shield, KSA

“…The weakening response at lower frequencies might reflect limited depth extent to the deposit or to the granitoid intrusive or else reflects weaker resistivity contrasts at depth. However, Ad Duwayhi lacks any well-defined, low resistivity halo due to the surrounding pyrite-phyllic alteration that accompanies more typical ZTEM copper porphyry signatures (Paré and Legault, 2010). Yet there appears to be a weak NE-elongated trend to the Ad Duwayhi resistive TPR feature, particularly at highest frequencies; and, as with the magnetics, the Ad Duwayhi TPR anomaly is open to the south.…”

The application of ZTEM to porphyry copper-gold exploration

“…ZTEM results over the deposit have been presented earlier by Paré and Legault (2010). The geology (plan and section) are shown in Figure 2.…”