|
Regional
Geology
The Grassy Mountain Property is located in the
Oregon Plateau portion of the northern
Great Basin and is characterized by abundant
Cenozoic volcanism. The flat-lying to
gently dipping volcanics and volcanic
sediments were deposited over wide areas during
this time of crustal extension. The region
is dominated structurally by N-NE trending normal
and to a lesser extent W-NW right lateral and
late E-W faults.
Multiple basaltic flows and shallow rhyolitic
bodies were intruded during Miocene time.
This formed the foundation for the present Oregon
Plateau region. During the middle to late-Miocene
period the volcanic platforms were deformed
into elongated basins and ridges by north-trending
normal faults. Hot spring activity was
common along these structures and is contemporaneous
with sedimentary deposition in the restricted
basins.
Property Geology
The rocks exposed at Grassy Mountain are part
of the late to middle-Miocene Grassy Mountain
Formation, a sequence of volcanic and volcaniclastic
rocks made up primarily of olivine-rich basalt
and intercalated tufaceous siltstones, sandstones,
and conglomerates. The rocks have been
dated through fossil evidence and K-Ar methods
to be approximately 10 Ma. The sediments
are primarily flat-lying with a slight regional
dip to the east. The structural trend
of the area is N10°W to N30°E. These features
were probably cut by later post-mineralization
east-west faulting.
Mineralization is associated with a low grade
gold-silver bearing siliceous hot springs system
with enrichment along multi-stage quartz-adularia
veins and favourable lithologies. Explosive
brecciation and over pressuring of the rock,
common in these systems, was minimized due to
the un-lithified nature of the sediments.
The mineralized rock is oxidized and highly
silicified and locally brecciated in the vicinity
of the feeder structures. As silicification
decreases so does grade. Away from the
feeder zones, lithology also plays an important
role in gold deposition. The finer grained
siltstones contain the bulk of the lower grade
material, while the higher grades are found
in the coarser arkosic sandstones. The
feeder or vein zones contain grades as high
as 20 ounces of gold per ton.
|
