Under Cover Carlin-type Gold Deposit Exploration: South Grass Valley, Lander County, Nevada

Carlin-type gold deposits (CTGDs) cluster within north-central Nevada and cannot exist without affecting large volumes of adjacent bedrock. The exposed bedrock
in the Basin and Range topography of Nevada has been thoroughly explored since the
discovery of CTGDs. Mineralized bedrock undoubtedly exists beneath the cover of
alluvium in the valleys separating the ranges, but the process of winnowing the larger
areas of interest (AOI) into progressively smaller AOIs is required to make exploration economically feasible.
If large, oxidizing CTGDs are present beneath the alluvial cover, it is expected
that gold and CTGD pathfinder elements derived from oxidizing mineralized pyrite
will be present in the groundwater and provide a unique chemical signature. Such a
signature was identified in the groundwater during reconnaissance sampling across
Grass Valley, south of Barrick’s Cortez Hills and Gold Rush complexes. Nevada Exploration Inc. (NGE) has developed and continues to use a low-cost, repeatable analytical method that can be used to reduce the regional AOI of north-central Nevada
into smaller alluvium-covered AOI’s. The anomaly was detected and traced up the
direction of groundwater flow to the alluvium-covered South Grass Valley.
South Grass Valley (SGV) is an under-cover gold exploration project discovered
by NGE, 40 km northeast of Austin, NV. Favorable Ordovician through Silurian carbonate slope facies host-rocks occur in the nearest exposed bedrock and are overlain
by siliciclastics of the Roberts Mountains allochthon, which in turn are overlain by
Tertiary lavas and tuffs. A gravity geophysical survey conducted over SGV provided approximate depths to bedrock (i.e. thickness of alluvium), in order to partition
the groundwater anomaly into smaller AOIs with relatively shallow bedrock. NGE
also commissioned an aeromagnetic survey to further partition the larger AOI into
magnetic and non-magnetic AOIs. Geologic mapping, combined with the gravity and
magnetic data, help predict the location and thickness of post-mineral volcanic rocks,
pre-mineral granitic rocks, favorable target Paleozoic host-rocks, and a fabric of major faults that may have channeled CTGD hydrothermal fluids. Lastly, a soil survey
delineated a 1,700 m diameter Hg and As anomaly.
NGE drilled a series of initial stratigraphic test core holes spaced 400 m apart along
east-west fences spaced 1,200 m apart and encountered carbonate bedrock with widespread alteration characteristic of CTGD mineralization and geochemistry indicative
of the margins of a large CTGD hydrothermal system and/or proximity to several separate CTGD deposits. Three of the eastern-most holes encountered a sequence of postmineral volcanic rocks which broadly bracket mineralization to the Late Eocene. Structural measurements from outcrop and oriented core show southeast plunging folds in
Paleozoic rocks, as well as northwest-trending structures, one of which is buried and
parallel to a gold in groundwater anomaly. The gold in groundwater is relatively high
for the low gold in core (tens of ppb to date), which argues that a higher-grade, more
efficient gold depositing part of the system may be located nearby.