Integrated geophysical imaging of a concealed mineral deposit: A case study of the world-class Pebble porphyry deposit in southwestern Alaska

Abstract

We combined aeromagnetic, induced polarization, magnetotelluric, and gravity surveys as well as drillhole geologic, alteration, magnetic susceptibility, and density data for exploration and characterization of the Cu-Au-Mo Pebble porphyry deposit. This undeveloped deposit is almost completely concealed by postmineralization sedimentary and volcanic rocks, presenting an exploration challenge. Individual geophysical methods primarily assist regional characterization. Positive chargeability and conductivity anomalies are observed over a broad region surrounding the deposit, likely representing sulfide minerals that accumulated during multiple stages of hydrothermal alteration. The mineralized area occupies only a small part of the chargeability anomaly because sulfide precipitation was not unique to the deposit, and mafic rocks also exhibit strong chargeability. Conductivity anomalies similarly reflect widespread sulfides as well as water-saturated glacial sediments. Mineralogical and magnetic susceptibility data indicate magnetite destruction primarily within the Cu-Au-Mo mineralized area. The magnetic field does not show a corresponding anomaly low but the analytic signal does in areas where the deposit is not covered by postmineralization igneous rocks. The analytic signal shows similar lows over sedimentary rocks outside of the mineralized area, however, and cannot uniquely distinguish the deposit. We find that the intersection of positive chargeability anomalies with analytic signal lows, indicating elevated sulfide concentrations but low magnetite at shallow depths, roughly delineates the deposit where it is covered only by glacial sediments. Neither chargeability highs nor analytic signal lows are present where the deposit is covered by several hundred meters of sedimentary and volcanic rocks, but a 3D resistivity model derived from magnetotelluric data shows a corresponding zone of higher conductivity. Gravity data highlight geologic features within the deposit, including shallow diorite sills that locally contain higher-grade mineralization. The results thus show ways in which an integrated survey approach might be used to distinguish zones of potentially economic mineralization.

Three-dimensional shape and structure of the Susitna basin, south-central Alaska, from geophysical data

“…This region was referred to as the "Southern Alaska Magnetic High" by Saltus et al (2007) because applying long-wavelength filters to these anomalies suggests a broad magnetic high. A similar high-low anomaly paired with a total gradient high was observed over a Jurassic pyroxenite body to the southwest near the Pebble porphyry deposit (Shah et al, 2013), also within that belt.…”

Application of BEMD in Extraction of Regional and Local Gravity Anomalies Reflecting Geological Structures Associated with Mineral Resources

“…It has been illustrated that the ASRR fault cuts through the crust and controls the regional distribution of both intrusions and related mineralization [22,23], which may have caused the mantle and/ or the basement uplift with distinct positive gravity anomaly [13,24,25]. The negative gravity zone is inferred to be associated with the mantle and/or the basement depression (II) [13,[24][25][26]. The Gejiu tin-copper polymetallic deposits and the related granites that are characterized by initial 87 Sr/ 86 Sr ratios ranging from 0.7097 to 0.7130 suggesting mixed crust-mantle sources [6], are located at the transitional zone (III), whereas the Bozhushan silver-lead-zinc polymetallic deposits and the related granites with initial 87 Sr/ 86 Sr ratios ranging from 0.7126 to 0.7170 (unpublished data from authors), are situated within the zone of mantle depression.…”

Gravity method for investigating the geological structures associated with W–Sn polymetallic deposits in the Nanling Range, China

“…For instance, while potential field gradients are capable of horizontal delineation of faults and lithological contacts by using edge detection technique, the 2D gravity anomaly data could help map 3D density distributions of buried ore bodies or other concealed geological features by using inversion approaches. The gravity and magnetic exploration methods have shown great prospects in the study of ore genesis (e.g., Lü et al, 2012;Wannamaker et al, 2009), metallogenic regularity (e.g., Blakely and Jachens, 1991;Gunn and Dentith, 1997) and mineral resource prediction (e.g., Shah et al, 2013;Wang et al, 2011) as evidenced from various case studies.…”