The Role of Geophysics in Exploration of the Gold Bar District, Cortez Trend, Eureka County, Nevada

Since discovery of Carlin-type gold mineralization in the Gold Bar district in
1983, previous owners conducted gravity, airborne magnetic and CSAMT surveys
over fragmented portions of the district. Significant gaps in data coverage were filledin
with additional surveys in 2018 by McEwen Mining Inc. (McEwen) to complete
data coverage over the entire property area. The McEwen exploration team assembled
all geophysical data into a comprehensive package and integrated them with
the geologic mapping, surface geochemistry and drill data. This built the geologic
foundation and understanding that, combined with regional and detailed geologic
cross section construction plus 3-D modeling, resulted in the identification of several
well-supported, high-quality drill targets.
McEwen’s geophysical acquisition and data compilation, completed with the
guidance and interpretation of Wright Geophysics, consists of district-scale gravity,
CSAMT (controlled-source audio-frequency magnetotellurics) and airborne magnetic
surveys. Gravity gradients define major structural zones where higher-density
carbonate rocks are juxtaposed against less-dense siliciclastic and volcanic rocks and
alluvium. CSAMT sections refined the definition of contacts between high- and lowresistivity
rocks and, in places, identified alteration within carbonate rocks along
district-scale faults. Detailed analysis and 3D inversion-modeling of magnetic data
suggest the presence of a previously unrecognized, large, deeply buried intrusion.
The unified geophysical data package is an integral component of the districtand
local-scale data set that resulted in the identification of numerous brownfield
and greenfield drill targets across the district. Faults identified by geophysics have
assisted in understanding the district-scale structural architecture at Gold Bar. Drilling
of priority targets in some cases returned variably argillized or silicified host rock
units carrying trace to ore-grade gold mineralization. As part of the process, some
of the geophysical anomalies initially interpreted as gold-related decarbonatization,
silicification, argillic alteration or faults were drill-tested and subsequently explained
by geological features such as unmineralized mafic dikes and argillized post-mineral
Tertiary volcanic rocks rather than alteration associated with gold mineralization.
Key Words: Gold Bar district, Carlin-type gold, gravity, CSAMT, airborne magnetics
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