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Summary Report
on the
LIPANGUE BRECCIA ZONE
and the relationship to
LAS DOS MARIAS REPLACEMENT DEPOSIT
by
Gordon D. House, P.Geo.
for Medinah Energy Inc. 7th August 1997
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The
Alto de Lipangue Breccia Zone and Las Dos Marias "Stockwork"
have been fully described in the "Report on the
Evaluation of the Santiago Project, Metropolitan Region,
Chile, for Medinah Energy Inc. dated 25th July 1998".
The relationship between the two properties was indicated,
and recommendations were made to follow up this relationship
during the mapping programs recommended for both properties.
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Lipangue Breccia Zone
The Lipangue Breccia Zone is a large hydrothermal,
diatreme breccia pipe intruded at the contact between
the Granodiorites of the Central Batholith and the
intermediate volcanics of the Cretaceous "Formación
Lo Prado". The granodiorite batholith is of early
to mid Cretaceous age and may have been contemporaneous
with the volcanics of the Formación Lo Prado, the
hydrothermal breccia pipe was intruded later and includes
brecciated fragments of both rock types.
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Hydrothermal breccia pipes of this
type are the surface expression of a late stage intrusive
event which failed to reach the paleosurface, but the
contained pressures in gases and hydrothermal fluids
exploded to the paleosurface in a much smaller pipe
punching up through the rocks above. Breccia pipes are
associated with highly mineralized
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copper-molybdenum-gold
porphyry deposits in many places, and are considered
reliable indicators of mineralized intrusives at depth.
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In
the case of the Lipangue Breccia pipe it is evident
that the temperatures and pressures involved caused
alteration of the fragments in the breccias. The relief
of these pressures after the pipe had exploded onto
the paleosurface resulted in complex physicochemical
reactions within the hydrothermal gases and fluids
venting to the surface. The breccia fragments were
coated with crystals of quartz as the pressure dropped,
the gases and fluids continued venting with hydrothermal
fluids passing more slowly through the breccia pipe
depositing contained sulphides in the form of pyrite
and chalcopyrite. The hydrothermal fluids carried
other sulphide minerals as well as gold, silver and
mercury in solution. These minerals were deposited
in the spaces within the breccia pipe eventually cementing
the whole into a solid rock mass.
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The
mineralization present in breccia pipes is not always representative
of the mineralization in the source intrusive body. In many
cases trace element geochemistry will indicate associations
with mineralized copper-molybdenum-gold porphyry deposits.
Such geochemical analyses were not carried out during the
evaluation of the Santiago Project properties but samples
taken will be re-assayed in the future. The existence of a
hydrothermal breccia pipe is considered indicative of a mineralized
porphyry copper-molybdenum-gold deposit at depth, which caused
the breccia pipe by explosive degassing.
Las Dos Marias Replacement Deposit
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The
deposit known as Las Dos Marias is located on Quebrada
Durazno, about 2.0 to 2.5 kilometres to the east of
the Lipangue Breccia pipe and lies within the same
Cretaceous Formación Lo Prado of intermediate volcanic
tuffs and lavas. Las Dos Marias has been variously
described as a "stratiform Skarn" deposit
and a "stockwork" deposit. However, the
underground exposures in the old workings on the east
side of the creek are clearly stratabound replacement
deposits where hydrothermal solutions penetrated through
porous or receptive horizons within the volcanic stratigraphy.
These stratabound deposits are well known in Chile
where they are called "mantos". Such "manto"
deposits were and are the source of much of the copper
and gold mined in the Andacollo area in Region IV
near La Serena and Coquimbo.
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The
Las Dos Marias deposit is a manto type deposit, a replacement
deposit where hydrothermal solutions emanating from a late
stage intrusive event, such as the Lipangue Breccia pipe,
percolated through porous or permeable volcanic tuffs or lavas
and deposited the contained copper and other metal sulphides
by replacing minerals in the host rock. The hydrothermal solutions
were able to replace permeable volcanic rocks without causing
extreme alteration of the less permeable or impermeable rocks
within the stratigraphic column, thus there are barren horizons
between the "mantos" in the deposit.
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The
source of the hydrothermal fluids at the Las Dos Marias
deposit is not known at present, but the relatively
close proximity of a late stage intrusive event as
represented by the Lipangue Breccia pipe suggests
that the intrusive under and causative of the breccia
pipe may be the source. Las Dos Marias is located
2.0 to 2.5 kilometres east of Lipangue Breccia pipe
and is from 500 to 700 meters lower in elevation,
and perhaps the lower levels of the breccia pipe,
if not the intrusive body itself, was the source of
the hydrothermal fluids.
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It
is worth noting that in the Andacollo area the "manto"
deposits are located in favourable horizons up to 5.0 kilometres
distant from the Andacollo porphyry copper deposit, which
is believed to have been the source of the hydrothermal fluids
which deposited the "mantos". The distance of "manto"
deposits from the source of hydrothermal fluids is not a problem,
it is the availability of favourable permeable volcanic horizons
in the country rock that is essential to allow the hydrothermal
fluids to move away from the source. The contained sulphide
minerals are deposited from solution when the physico-chemical
conditions change sufficiently through cooling or release
of pressure, replacing the rock forming minerals to form the
"mantos".
The
relationship of the Lipangue Breccia pipe to the "manto"
deposits at Las Dos Marias is not proven, but it is strongly
suggested by the proximity of the one to the other and the
hydrothermal breccia pipe representing the top of a late stage,
mineralized intrusive event. In our Report, we recommended
that the geological mapping and geochemical sampling on the
Las Dos Marias deposit attempt to determine if there is a
link between the two deposits. The geophysical program recommended
for both properties may show that the "mantos" at
Las Dos Marias do extend to the west towards the Lipangue
Breccia pipe, in which case it would be worth while carrying
out diamond drilling to trace out the manto deposits to the
west.
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The
two deposits, at the Lipangue Breccia pipe and Las
Dos Marias "manto", appear to be related
and future exploration programs should be used to
confirm this relationship. The Altos de Lipangue plateau
is a well mineralized district, and there are many
small quartz vein deposits within the 4.5 x 1.5 kilometer
extent of the plateau. Most of these deposits
do have an affinity with classical porphyry copper-molybdenum-gold
deposits. This indicates that there was a strong hydrothermal
mineralizing event in the district and the Lipangue
Breccia pipe is the marker part of this event.
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August
7, 1998
"Gordon D. House"
Gordon D. House, P.Geo.
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