Five Years Later: Progressing the Rhys Model of Fold and Fault Geometry on the Carlin Trend via Integration of Mapping, Sequence Stratigraphy and Lithogeochemistry

Ongoing mapping in new and historic mining areas along the Carlin Trend continues
to test the structural model of deformation and the relation to ore-controlling
structures proposed by Rhys and others (2015). That study proposed a three-phase
contractional history, commencing with the Antler orogeny (Phase 1) and the associated
Roberts Mountains Thrust (RMT) followed by widespread probable Jurassic EESE
verging folding (Phase 2) that preceded Jurassic extension, and finally NW trending
upright folds and reverse faulting (Phase 3). This structural history accounts for
district-wide orientations of fold and fault styles, interpreting previously unrecognized
low-angle structures that repeat and overturn stratigraphy. The study emphasized that
intense mineralization, alteration, and focus of Eocene extensional faulting associated
with mineralization are spatially coincident with earlier areas of structural complexity.
Integrating this structural understanding with sequence stratigraphic analysis
yielded a new mapping and lithostratigraphic approach to building a 3D model capable
of reconciling mapping and drill logging, providing an improved lithostructural
model for exploration. The high degree of tectonism, alteration, and visual similarity
of units along the trend commonly resulted in inconsistent, incorrect, or overly simplified
layer-cake interpretation within drill logs. These interpretations are not compatible
with the complex fold and thrust geometries observed in mapping. The lithogeochemical
approach used geochemical patterns, thresholds, and ratios determined via
empirical observation and geochemical criteria predicted by sequence stratigraphic
principles. Geochemical ratios such as Mg/(Mg + V) and Ni/Co have been utilized to
trace key stratigraphic horizons that highlight deformation styles and architecture,
allowing for unbiased tracing of structural complications predicted by the new structural
model in drill holes. These data have allowed tracking of geochemical marker
units to identify stratigraphic imbrication of Phase 2 thrust faults and the location
and trend of Phase 2 and Phase 3 folds.
Continued mining in new and old surface operations has created exposures north, west, and deeper than previously investigated areas. At the Arturo deposit, mapping, historical
logging, and relogging identified an east-vergent Phase 2 thrust placing Devonian
Bootstrap over Devonian Rodeo Creek, overturning the east-vergent Phase 1 RMT in
the north high wall. An east-dipping normal fault which exploits a steeply overturned
fold limb cuts these features. These observations also establish that Genesis-style fold and
thrust geometry is present in the far northern Carlin Trend at Arturo. In the Genesis area
new mining exposures in the Revelation thrust footwall confirm the overturned nature
of the Genesis anticline, and the presence of a previously inferred thrust below it. A west-vergent
overturned Phase 3 fold in the Bluestar Ridge area indicates Phase 3 deformation
is more widespread than previously recognized. In the Maggie Creek sub district,
new mapping in the historic Tusc pit identified multiple shallow dipping thrust splays of
the Good Hope fault emplacing Roberts Mountain Formation over the Devonian Rodeo
Creek-Popovich contact, and cut by a Hewetite-parallel normal fault. Collectively these
studies have advanced the understanding of the lithostructural architecture of the Carlin
trend and will continue to be utilized in exploration efforts in the district.
Key Words: Carlin Trend, Nevada, Structure, Sequence Stratigraphy