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ByMorgan Poliquin, M.Sc., P.Eng.

Geological Engineer and Director

Almaden Mineral s Ltd.

Suite 1103-750 West Pender St, Vanco uver, B.C., Canada, V6C 2T8.

ph: 604 689-7644 fax: 604 689-7645 email: info@almadenminerals.com; www. almadenminerals.com

mailto:info@almadenminerals.commailto:info@almadenminerals.com

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EPITHERMAL SYSTEMS

The association of gold mineralization with volcanic and geothermal hot spring activity has long been recognized by

prospectors and geologists. We now know that this association is a consequence of the hot magmas which not only

produce volcanic eruptions and volcanic rocks but also are the source of the hot fluids that transport gold and other

metals and may in fact be the source of gold itself. Fluids emanating from a molten magma are extremely hot and

under high pressure deep below the surface. As these fluids rise, they mix with surface waters and change the

composition of the rocks with which they come into contact. This process is known as alteration. Eventually the

fluids breach the surface and form either acidic lakes known as fumaroles common in the craters of volcanoes or

dilute, neutral hot springs like those at Yellowstone or the Geysers in California. These two different surface

manifestations acidic lakes or neutral hot springs reflect two different fluid types that each result from the two

different paths taken by the magma as it rises to the surface. Both form gold deposits and are known respectively as

low- and high-sulphidation gold deposits. In both subtypes gold will largely be precipitated from 2.5 kilometers depth

to surface.

Recognizing that gold precipitates

near the surface in these

systems, the great American

geologist Waldemar Lindgren

coined the term epithermal in

1933, epi meaning shallow and

thermal referring to the heated

fluid. The chemist Werner

Giggenbach further subdivided

epithermal gold deposits into low

and high sulphidation types

(illustrated right1). Low and high

do not refer to each types relative

amount of sulphide minerals (metal complexes of sulfur with metals). Rather the distinction is based on the different

sulfur to metal ratio within the sulphide minerals of each subtype. While this discussion deals with high-sulphidation

epithermal systems, it is worth mentioning that low-sulphidation systems also form economic gold deposits although

they develop under vastly different chemical conditions.

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HIGH-SULPHIDATION DEPOSITS

High-sulphidation deposits result from fluids (dominantly gases such as SO2, HF, HCl) channeled directly from a hot

magma. The fluids interact with groundwater and form strong acids. These acids rot and dissolve the surrounding

rock leaving only silica behind, often in a sponge-like formation known as vuggy silica. Gold and sometimes copper-

rich brines that also ascend from the magma then precipitate their metals within the spongy vuggy silica bodies. The

shape of these mineral deposits is generally determined by the distribution of vuggy silica. Sometimes the vuggy

silica can be widespread if the acid fluids encountered a broad permeable geologic unit. In this case it is common to

find large bulk-tonnage mines with lower grades.

The acidic fluids are progressively neutralized by the rock the further they move away from the fault. The rocks in turn

are altered by the fluids into progressively more neutral-stable minerals the further away from the fault. As a result,

definable zones of alteration minerals are almost always are formed in shell-like layers around the fault zone.

Typically the sequence is to move from vuggy silica (the centre of the fault) progressing through quartz-alunite to

kaolinite-dickite, illite rich rock, to chlorite rich rock at the outer reaches of alteration. Alunite (a sulphate mineral) and

kalonite, dickite, illite and chlorite (clay minerals) are generally whitish to yellowish in colour. The clay and sulphate

alteration (referred to as acid-sulphate alteration) in high-sulphidation systems can leave huge areas, sometimes up

to 100 square kilometers of visually impressive coloured rocks.

ALTERATION IN A HIGH-SULPHIDATION SYSTEM:

In contrast, low-sulphidation veins are formed when the fluids interact with greater amounts of groundwater as they

rise from the hot magma. The protracted boiling of the fluids in low-sulphidation systems produces high grade gold

(greater than one ounce gold per ton) and silver deposits. The fluids interact with the surrounding rock for a much

longer period of time than the quickly channeled high-sulphidation fluids. As a result, the fluids become dilute and

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neutralized and the silica dissolves. The silica is later precipitated in the veins as quartz, often sealing the fissure

closed. When this occurs, the pressure of the gases underneath the sealed fault builds until the seal is ruptured,

which provokes catastrophic boiling and the precipitation of gold. After this explosive boiling event, passive conditions

return, and quartz precipitates once again. This cyclical process results in the well-known banded texture of the

quartz-adularia veins typical of low-sulphidation vein systems. Quartz-adularia veins can contain high-grade gold

(greater than one ounce gold per ton) and silver deposits, over vertical intervals of generally 300 to 600 metres.

Within this vertical dimension, high gold grades can make for a large amount of easy to mine gold in a narrow

compact area.

ALTERATION IN A HIGH-SULPHIDATION SYSTEM:

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HIGH-SULPHIDATION GOLD-SILVER DEPOSITS IN OR NEAR PRODUCTION TODAY

Yanacocha, Peru

The Yanacocha deposit is operated by Newmont Gold Corp. It is one of if not the largest high-sulphidation deposit in

the world and one of the largest gold deposits of any geologic type. Reserves stand at 128 Million Tonnes of 1.0 g/t

or roughly 34.2 Million ounces of gold. The deposit is hosted by volcanic rocks, largely fine grained andesitic tuffs

and flows. The figures below and on the following page illustrate a plan and cross section view respectively of the

mineralized areas of the Yanacocha deposit. The cross section illustrates diamond drill intersection in grams per

tonne (g/t) gold.2Note how the gold is distributed within areas of vuggy silica and quartz-alunite alteration (red and

pink colours) which are surrounded by clay alteration.

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3400

FreshRock

SilicaGranularSilica

Granular

A A

SilicaMassive

Clay

COR-69COR-56COR-59

COR-66

Clay

3200

Meters

0 100

Oxide

Sulfide

44m at 17.7g/t

18m at 8.5g/t

42m at 13.18g/t

+0.2g/t Au

24m at 10.9g/t

10m at 9.1g/t

Intrusive

Pierina, Peru

The Pierina gold deposit is located in Peru and is operated by Barrick Gold Corp. Gold is found in a succession of

volcanic rock rocks that consist of andesite lavas overlain by rhyodacite pumice and lithic tuffs (permeable rocks).

The gold and silver is predominantly found in the permeable pumice tuff, with lesser amounts in the overlying lithic

tuff and the underlying andesite. The high-grade areas of the deposit are associated with vuggy silica alteration. This

alteration type is surrounded by quartz-alunite and argillic alteration. Ore-grade mineralization in the pumice tuffoccurs over intervals of more than 260 meters. The area of currently known mineralization at Pierina measures

approximately 450 meters wide by 1,200 meters long and is presently open to the southeast. Over 95% of the known

mineralization at Pierina is oxide. However, sulphide feeder zones have been intersected at the bottom of the

deposit. At present Barrick Gold Corp. has defined reserves of over 65 Million Tonnes at an average grade of 1.3 g/t

gold.

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High-Sulphidation Epithermal Deposits in Mexico

Mexico is particularly well endowed with epithermal low-sulphidation vein systems. In contrast, high-sulphidation gold

deposits have not been as well developed. This is likely due in large part to the fact that high-sulphidation systems

tend to occur as large bulk-tonnage deposits, which requires advanced technology to discover and mine, whereas

the high-grade narrow veins typical of low-sulphidation deposits were more easily explored during Mexicos early

mining history. However, in the last decade two significant high-sulphidation systems have been identified in Mexico

and are entering production. Reserves are tabularized below.

Mine Name, State Estimated Produc tion Gold Grade Silver Grade

El Sauzal, Chihuahua 18.19 Mt 3.3 g/t 3.7 g/t

Mulatos, Sonora 36.4Mt 1.64 g/t

*Note Mt denotes million tonnes, g/t grams per tonne

THE CABALLO BLANCO HIGH-SULPHIDATION EPITHERMAL SYSTEM

The property is located 75 kilometers north-northwest of the city of Veracruz on the Gulf Coast of Mexico and is

roughly 10 kilometers by 15 kilometers in size. Logistically, it is extremely well situated with the Pan-American

highway running through the east end of the property and ready access to power (Laguna Verde Nuclear Power

Plant) and ocean vessel port facilities. In January, 2003, Comaplex signed a Letter of Agreement with Almaden

Minerals Corporation whereby Comaplex can earn a 60% interest in the Caballo Blanco gold property in eastern

Mexico by incurring exploration expenditures of $2.0 million US over 4 years.

The property is largely underlain by a sequence of volcanic rocks consisting of andesitic to dacitic lithic tuffs, crystal

tuffs and volcanic breccias. These volcanic centers are intruded by fine-grained, magnetic monzonite and diorite

intrusions and dykes have been identified in several locations on the property. Large areas of clay and sulphate

alteration cored by vuggy and massive silica have been identified on the property. Three main areas of alteration and

mineralization have been mapped and are referred to as the Northern Zone, the Central Grid Zone and the Highway

Zone. The Central Grid Zone is interpreted to represent an area of deeper erosion where copper porphyry style

mineralization has been identified while the Northern and Highway Zones are interpreted to represent large, well

preserved high-sulphidation epithermal systems.

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Previous Work

From 1995 to 1998, Almaden Minerals Ltd. completed extensive exploration that concentrated on porphyry Cu-Au

and Au-Ag vein targets in the Central Grid Zone (CGZ). The 1998 work included 17 RC drill holes (2390 meters) that

tested soil geochemical and IP geophysical anomalies spatially associated with mineralized float and outcrop.

Preliminary surface work was completed in 1999 on the Highway and Northern Zones. Several zones of gold and

silver mineralized quartz-barite veins were intersected. Two shallow drillholes into an intrusive in the Central Grid

Area intersected 107 meters of 0.25 gmt gold and 0.18% copper in one hole and 40 meters averaging 0.39 gmt gold

and 0.15% copper in another. Based on this work, it was interpreted that these holes were just touching the top of the

system.

In 2001, Noranda optioned the Caballo Blanco property from Almaden and drilled 7 diamond drillholes totaling 1,641

meters. No significant copper mineralization was intersected and despite significant alteration and anomalous gold

mineralization in several holes, Noranda terminated its option in the fall of 2002. It is noteworthy that Noranda wassingularly looking for a large copper deposit and their focus was not on gold.

Recent Exploration

After optioning the property from Almaden, Comaplex completed three exploration programs on the property during

the spring and summer of 2003. These programs concentrated on the Highway and Northern Zones and included

detailed alteration mapping, aided by the use of Almadens portable infrared mineral analyzer (PIMA), rock sampling,

an induced polarization-restivity (IP-R) survey and a magnetic survey. This work outlined two areas of extensive

alteration and mineralization characteristic of high-sulphidation epithermal gold systems. Concurrent with this

program, noted explorationist, Jeff Hedenquist visited the property on Comaplex's request to review the geology and

alteration of the prospects and to assist Comaplex in best focusing their exploration efforts on the property.

ALTERATION AND MINERALIZATION

Central Grid Zone

The CGZ is mainly a zone of clay - silica alteration. Numerous zones of quartz vein float have been identified in the

area in the past and the distribution of the vein float was mapped and sampled over the entire area, which is

approximately 3km by 2 km. Comaplex completed additional sampling of the vein float with the vast majority of the

samples assaying greater than 1 g/t and up to 25 g/t Au. Silver is locally present and most samples were highly

anomalous in Cu, Pb and Zn. The mineralized quartz-vein float in the CGZ shares characteristics and settings that

are typical of intermediate sulfidation veins, commonly associated with and adjacent to lithocaps of high-sulfidation

deposits. The vein float may be indicative of a larger vein deposit at depth and their structural control and depth

potential are being assessed.

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Highway Zone

In the Highway Zone is a 3km x 4km area which exhibits a zone pattern of alteration and mineralization from

innermost multiple silica core zones (vuggy quartz and silicified vuggy quartz), to quartz-alunite (advanced argillic), to

kaolinite and/or dactite to illite (argillic), to propylitic margins. Rock samples from the Highway Zone are generally low

(

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Northern Zone

In the Northern Zone, exploration has defined a large (roughly 6 km x 6 km) area of silica to advanced argillic to

argillic alteration. Multiple large zones of vuggy to silicified, locally brecciated vuggy quartz, may represent multiplefeeders to the alteration system. Past sampling by Almaden in this area returned isolated gold values up to 11 g/t.

However, very little work was carried out at that time.location of geophysical IP

line 85+500 (shownbelow) and surface

samples up to 4.7 g/t gold

Rock samples collected by Comaplex from one of the vuggy silica core zones located in the eastern end of the zone

in July, 2003 returned gold values up to 1.2 g/t gold. A preliminary IP-R survey completed in this area outlined a

large, very high resistivity feature that extends to depth. Proximal low chargeability suggests oxidation of the system.

In January, 2005 Almaden and Comaplex staff have sampled one of several areas of outcrop of vuggy silica and

quartz-alunite acid sulphate alteration. Thirty-two rock chip samples were collected over a roughly 35 by 100 meter

area of vuggy silica. These samples averaged 0.62 g/t gold and ranged from 0.01 to 4.67 g/t gold. Eleven samples

returned gold grades above 0.50 g/t gold and six above 1.00 g/t gold.

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IPSECTION84500NORTHERN ZONE:

hese results, which are interpreted to represent the gold content of a very high level in a well preserved high-

sulphidation epithermal system, are considered by Almaden to be very encouraging. The area sampled on surface is

Surfasample

ces up to

4.7 g/t gold

Chargeability high

Largeresistivity high

T

spatially immediately above significant resistivity highs identified in a ground geophysical induced polarization (IP)

survey previously carried out by Comaplex. The IP survey also identified high chargeability responses associated

with the high resistivity responses at depth. This data suggests that resistive, vuggy silica material, similar to that

sampled in outcrop, is oxidized at surface and may extend to considerable depth.

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Footnotes and References:

uist,J W, 1994, Epithermal environments and styles of mineralization; variations and their causes, and

mal gold mineralization of the Circum-Pacific; geology, geochemistry, origin and exploration; II.Siddeley-G

ditor), Journal of Geochemical Exploration. 36; 1-3, Pages 445-474. 1990.

1Taken from White, N C and Hedenq

delines for exploration, In: Epithergui

(e

2Taken from Newmont Gold Corp.s website, www.newmont.com.

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