“Round

“Round Mountain”
Gold Mine in Nevada, U.S.A.
and Potential for Exploration
of Similar Deposits in the Andes
Alberto Lobo-Guerrero Sanz
Consulting Geologist
M.Sc.,
., MinEx

INTRODUCTION
• Round Mountain is one of the largest known volcanic
rock-hosted gold deposits
• Disseminated, epithermal low sulfidation deposit
-In 1999, owned by Echo Bay Inc. (50%), Case, Pomeroy
and Co., Inc. and Homestake Mining Co. (each w/ 25%).
-Mined intermittently since 1905.
-Reserves > 500 tons of metallic Au
Visit on May 5 th , 1999, guided by Chris Ekstrom, mine
geologist, who provided detailed field descriptions.
Brief version of mine’s more relevant issues & comments on the
exploration of similar deposits in the Andean Cordillera.

ABSTRACT 1
• Permeable pyroclastics bound by impermeable
rocks host mineralization + alteration.
• Mineralizing fluids migrated along concentric &
radial fractures related to a caldera.
• Mineralization + alteration processes took

ABSTRACT 2
• hydrothermal alteration & mineralization spreads
from thin fractures
• propyllitic, phyllic, silicification and argillization
• no direct relation between alteration and grade
Occasional Au nuggets, sought w/ metal detector.
Some weigh > 1 lb.
Grade may vary up to 4 orders of magnitude in 10 cm.
The mine has re-usable heap leach pads.

ABSTRACT 3
Throughout the Andean Cordillera there is an
important potential to find deposits confined in
volcanic layers similar to those of Round
Mountain.
Several hundred Quaternary calderas have
been detected in Colombia, Ecuador and
Peru, and many of them contain precious
metal disseminations.

Location Map
(Oligocene – Miocene)

TOQUIMA
CALDERA
COMPLEX
MULTIPLE
CALDERAS
AROUND
ROUND
MOUNTAIN
MINE
Great Basin
Region,
Nevada.
Normal faults

CONCENTRIC
CALDERAS
AROUND
ROUND
MOUNTAIN
DEPOSIT

REGIONAL GEOLOGY
‘ROUND MOUNTAIN’ MINE
NEVADA, U.S.A.

LITHOLOGICAL UNITS
AT ‘ROUND MOUNTAIN’ MINE
ROCK
DESCRIPTION
Type 1 Densely welded fall tuff; aquiclude. 60 m
Type 9 Transitional zone to poorly welded tuff;
aquiclude. 27 m
Type 2 Porous, poorly welded tuff, with large
pumice fragments; host rock. 100-120
Type 3 Lithic-rich, dense tuff; low permeability.
Base of volcanic sequence.
Type 4 Paleozoic metasediments; impermeable.
Basement

Type 9 Transitional zone to poorly-welded tuff, 27 m
thick; eutaxitic texture due to compression, 9:1 to 2:1
aspect ratio in amygdules. Transitional welding inhibited
mineralizing fluid ascention and contributed to gold
dissemination in rock type 2. Acted as “cap rock” or
aquiclude. Rare mineralization. Hydrothermal alteration
restricted to fracture walls. At first sight, it differs from
other lithologies at the mine.
Type 2 Porous, poorly-welded tuff, 100-120 m thick;
with large pumice lapilli fragments and coarse quartz
phenocrystals. It is the best host rock; acts as a sponge
and received most of the Au. Intensely kaolinized and
with dense normal, Basin and Range fractures.

STRUCTUAL ASPECTS (1)
• important role in mineralization and aid in
metal extraction
• fractures served as conduits for migration
of Au-rich fluids towards Type 2 porous
rocks
• at least 2 types of fractures
1. cooling of tuffs
2. caldera collapse
3. late Basin and Range type (NW)

STRUCTURAL ASPECTS (2)
• Preconditioned rocks for mineralization in caldera
margin
• Richer zones = rock suffered > fracturing
• Routes of eruption and fluid conducts = intersection
of
-main anular fractures
-radial fractures produced during system
cooling
Pipes or feeder tubes were emplaced in the
intersections

LOCATION OF
GEOLOGICAL SECTIONS
DETAILED GEOLOGY
ROUND MOUNTAIN MINE
NEVADA, U.S.A.

ABUNDANT RADIAL
FRACTURES IN THE CALDERA

A-B B vertical section controlled by drillholes,
with sub-horizontal
pyroclastic layers

C-D D vertical section controlled by drillholes,
with sub-horizontal
pyroclastic layers

MINERALIZATION (1)
-electrum w/ Au:Ag ratio = 65:35
-free particles & inclusions in py
-generally very fine Au diss; ocasional 6.5 kg masses
-Free Au in qz-adularia-py veins, also py encapsulated
-traces of several Au-Ag tellurides and other sulfides
-Py = dominant sulfides
-Main part oxidizedan and free; electrum = free
-non-oxidated. pyritic ore is refractory
-Particle Au size = 30[m – 20cmØ
-Gold nuggets occasional.

Typical light gray Au mineralized tuff (oxidized).
Ore-grade rock does not look unusual.

Auriferous breccia.
Several brecciation
events.

Enlargment of
silicified and mineralized
portion in previous shot.
Note clast-support,
angularity and clast
compositional variety.

Note clast packing & angularity.
Free Au in this view.

Note packing degree
and angular clast variety

1.9 cm Ø
Vein with breccioid infill.
Penny for scale.
Matrix is opaline silica and contains free Au.
Note clast rounding and variety.

Enlargement of previous image.
Red poriton with re-brecciation and transport.
Note coarse matrix, > porosity and non-oriented clasts.
Compare with next.

Note evidence of flow in black
ortion and orientation of angular
clasts in the red portion.

Fracture intruded by Au-mineralized micro-breccia.
Rounded white clasts and quartz-adularia dark gray matri
with minor proportrion of py + Au.
Locally called “cockscomb breccia”.

Au nuggets
found in
hydrothermal bxs.

Gold foil
Gold nuggest
found in
hydrothermal bxs.
Metal detectors in zones where bonanzas are expected,
to recover most free Au and keep nuggets away from the
leach piles.
Coin 2.8 cm Ø

Nickel
Nugget that weighs more than a pound.
Found in vein associated with micro-breccias.

ALTERATION
All the range of hydrothermal alteration from a potassic
nucleus to a propylitic halo, through high level silicic
alteration and intermediate argillization
Alteration by hydrothermal fluids that migrate along a
fracture, and away from it along permeable rocks.
Disseminated py and precious metals extend 9-15 m
away from main fractures.
See next figure

”SANDWICH”
MODEL

Panoramic view of open pit, mid summer.

Panoramic view of re-usable leach pads.
Smoky Valley in background.

‘ROUND MOUNTAIN’ ECONOMIC PARAMETERS
MINE RESERVES (May, 1999)
RESERVES TONS (X10 3 ) gAu/ton TOTAL (tons Au)
Open Pit
233,638 0.6500 151.864
Stock Piles
127,271 0.3110
39.581
Indicated resources of 142 million tons not yet added to reserves.
Estimated mine life: 12-15 years
Milliing and leaching will ocntinue 8 years after mine
-average grade: 0.7465 g Au/ton.
-production costs 05’99: US$205-$210/oz (US$6.59-$6.75/gAu)
-average strip ratio: 2:1
-approximate open pit dimensions: 1700 x 1500 m
-Open pit depth: 460 m

FINAL OBSERVATIONS
• Rhyolitic tuffs (of the “ash fall” type) occur
in volcanic environments that tend to be
extremely explosive.
• In general such rocks are porous and
conform permeable layers.
If they are covered by lava flows (common trait in
bimodal volcanic systems) setting similar to those of
Round Mountain may take place, where lava acts as
an aquiclude.

ANDEAN PROSPECTS
Colombia, Ecuador and Perú have a reasonable
potential for gold deposits hosted in volcanic
strata such as those present at Round Mountain.
Several hundreds of Quaternary calderas have
been detected in the Nariño deparment,
Colombia, and many of them contain precious
metal dissemination. Repetitive explosive activity
in southern Colombian volcanic systems has
exposed numerous mineralized tuffs and some
welded tuffs.

Cerro Bravo Volcano, Tolima and Caldas, Colombia
Dome and five concentric calderas
4000 m.o.s.l., Colombian Central Cordillera

Galeras Volcano, one of the best studied in the region,
expells 0.5 kg of gold per day to the atmosphere in its
fumaroles, and is probably depositing more than 0.06
kg Au/day in the volcanic edifice (GOFF et al [1994]).
If such rates remain constant, a moderately sized gold
deposit (>200 tons of contained Au) may form in only
10,000 years. If an equivalent amount is left behind in
porous volcanic rocks, a short lapse of hydrothermal
activity may produce deposits such as Round Mountain.
These observations do not take into account moments of
great activity and explosive vulcanism, when fumarolic
activity increases and several type of hydrothermal
breccia are formed.

Nevado del Ruiz, in the axis of the Colombian Central
Cordillera, is another Quaternary volcanic system
studied by the author. It contains important epithermal
gold dissemination associated with tuff layers limited by
andesite lava aquicludes.
Mineralization is conditioned to more than eight porous,
permeable, pumice-rich pyroclastic layers. The system
is of the high sulfidation type, since alunite conforms a
large portion of the matrix in breccias and mineralized
tuffs.

ECUADOR, LATERAL EXPLOSIONS
Ecuador has numerous recent volcanic edifices,
that are well exposed after lateral explosions.
Several volcanoes in the Interandean Graben, such
as Hilaló and Guagua-Pichincha are open to the
west due to preferential collapse of volcanic
edifices in that direction, where unidirectional wind
regimes tend to erode away the ash and other
components.
Cerro Bravo and Nevado del Ruiz volcanoes in
Colombia display clear evidence of lateral
explosions.

PERUVIAN ANDES
Large extensions of Eocene to Oligocene
volcanic rocks outcrop in the Peruvian Andes.
-older
-more eroded than in the Northern Andes
-may offer ideal conditions for entrapment of
precious metals in Round Mountain style.
Numerous high explosivity rhyolitic events,
intercalated w/ pyroclastic, cineritic & welded tuff
events produced monotonous sequences of
porous & non-porous rocks.

Time for questions