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The Walker Lane
gold belt is a 50-mile wide zone of widely disrupted
right lateral shears and associated normal faults.
These structures have tapped deep-seated magmatic
and hydrothermal fluids. Basement rocks
to this structural zone are complexly folded,
sheared and thrusted Paleozoic sedimentary rocks.
Intruded into these basement rocks are intermediate
composition Mesozoic plutonic rocks. Locally,
thick sequences of Miocene volcanic rocks cover
the structural zone.
Quaternary gravel and alluvial fan deposits cover
most of the Castle/Black Rock property.
The oldest unit on the property is the Ordovician
Palmetto Formation, a black limey shale and chert.
This is overlaid by porphyritic andesite flows
of the Miocene Blair Junction Andesite Unit.
Intercalated in the andesite units are rhyolite
tuff units that include both non-welded and welded
textures. Rhyolite dikes and dome features
cut the section.
Hydrothermal alteration in the volcanic rock
is focused on structures and zoned vertically
and laterally from these structures. Alteration
mineralogy is quartz-adularia associated with
fracture and breccia-matrix filling. Vertical
zonation of the alteration sequence has created
an intense argillic cap above the gold bearing
structures, from 3 to 30 meters thick. Lateral
zonation grades away from quartz-adularia filled
structures to pervasive sericite-clay outward
to a propylitic assemblage in andesite.
Hydrothermal alteration is most restrictive in
the sedimentary rocks, more dispersed in andesite
and extensive in rhyolite tuff.
Gold is concentrated in 3 identified zones; Castle,
Black Rock and Berg-Boss. In each zone,
gold is concentrated in structures hosted by sedimentary
rocks, andesite and rhyolite. Gold is also
distributed away from the structures in andesite
and rhyolite. Gold-bearing fluids are thought
to have moved vertically and laterally on the
structures. The gold-bearing fluids were
ponded by argillicly altered rhyolite tuff that
forced the fluids to spread laterally into the
andesite and lower rhyolite tuff units.
In the Castle zone, high angle structures localize
the highest-grade gold concentrations. In
these zones, multiple episodic boiling of hydrothermal
fluids produced veins and breccia zones that concentrated
gold. Adjacent to these structures, fluids
invaded the rock and created a lower-grade halo
of gold concentrations.
At the Berg-Boss zone, gold concentrations are
centered on a northwest trending fault and the
contact zones between rhyolite and andesite.
The rhyolite units project as intrusive bodies,
leading to the interpretation that fracturing
on the margin of the intrusions created the permeability
for gold-bearing fluids to invade the andesite
host rock.
The Black Rock zone contains a high-angle northwest
structure similar to the Berg-Boss zone.
Gold concentrations are associated with this northwest
structure and hosted by both sedimentary rocks
and volcanic rocks. Gold-bearing fluids
seem to have spread laterally along the contact
between shale and rhyolite tuff.
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