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Hog Ranch is an
epithermal low-sulfidation gold deposit with similarities
to the Midas, Comstock and Sleeper mines in Nevada
and El Penon and El Indio in Chile. It is
hosted by middle Miocene rhyolite volcanic and
volcanoclastic rocks that produced mineral deposits
at Sleeper, Grassy Mountain, National and Buckskin,
Delamar and McDermitt. The deposits at Hog
Ranch are localized by eruptive volcanic features
formed along the northeast trending Black Rock
Structural Boundary.
In the mine area, rhyolite tuffacous rocks are
represented by poorly welded to densely welded
ash units and incestuous volcanic dome complexes.
Interbedded with these tuffs are lacustrine rocks,
made up of volcanic detritus and water lain ash
and abundant plant remains. Ore zones exploited
by Ferret and Western Mining were principally
disseminated occurrences hosted by poorly welded
tuff and lacustine rocks. Veins are better
developed in the component densely welded tuff.
Volcanic centers and the historical open pits
are aligned in a northeast trend coincident with
the Black Rock Structural Boundary. This
structural feature represents the northwestern
boundary of a large and long lived volcanic depression
in Northern Nevada and is considered a major crustal
break. During middle Miocene time, northeast-southwest
directed extensions produced northwest trending
dilation zones, which are best preserved in the
Black Rock Structural Boundary. Gold-bearing
fluids utilized these northwest trending dilatational
faults to produce the ore deposit at Hog Ranch.
The identified high-grade vein system is contained
in the northwest trending faults.
Acid hydrothermal alteration is ubiquitous on
the property and grades laterally and vertically
around the northwest trending fault system.
Silica and potassium-bearing minerals are the
characteristic alteration minerals, with alteration
intensity and mineral temperatures increasing
toward faults that host veins and at depth.
Below the level of fluid boiling, the alteration
assemblage is indicative of hydration reactions
by fluids in equilibrium with the rock.
This alteration style is identified about 1000
feet below the present surface and laterally several
miles from the deposits.
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