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Low-Sulphidation Epithermal Quartz-Adularia Gold Silver Veins and

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Low-Sulphidation Epithermal Quartz-Adularia Gold Silver Veins and Powered By Docstoc
					LOW-SULPHIDATION EPITHERMAL QUARTZ-ADULARIA GOLD SILVER VEINS
                       & THE EL FUEGO PROJECT, MEXICO


                             By Morgan Poliquin, M.Sc., P.Eng.
                                Geological Engineer and Director
                                     Almaden Minerals Ltd.




                                             Suite 1103-750 West Pender St, Vancouver, B.C., Canada, V6C 2T8.
   ph: 604 689-7644 fax: 604 689-7645 email: info@almadenminerals.com; www. almadenminerals.com


HORSESHOE GOLD MINING INC.                         1202-1022 Nelson St., Vancouver, B.C. Canada, V6E 4S7.

                                  Contact: Jim McInnes, President.
INTRODUCTION TO LOW-SULPHIDATION EPITHERMAL VEINS
Much of the world’s gold has been produced from quartz veins. Veins are formed when quartz or other minerals
precipitate from a cooling fluid in a planar zone of weakness known as a fault. Quartz can precipitate from several
different types of fluids, one of which is responsible for low sulphidation epithermal gold-silver veins and geothermal
systems such as the hot springs at Yellowstone or the Geysers in California. The fluids are typically a mixture of
groundwater and fluid emanating from molten rock at depths of around 5 to 10 kilometers below surface. These hot
fluids are under very high pressures at those depths, and as they rise along faults to depths of about two kilometers
from the surface, they begin to boil. As the fluids boil, they cool rapidly, causing the quartz to precipitate in the fault,
forming the vein. Calcite and adularia (a feldspar mineral) also precipitate in response to boiling as well as any gold
and silver present in the fluid. Eventually the rising fluids breach the surface and form a hot spring.


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
right 1 ). Low and high do not refer to each
type’s 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 paper deals with low-sulphidation (which are also known as quartz-adularia)epithermal vein
systems, it is worth mentioning that high-sulphidation epithermal systems also form economic gold deposits although
they develop under vastly different chemical conditions.


High sulphidation deposits result from fluids rapidly channeled directly from a hot magma (where often bulk-mineable
porphyry copper deposits form) along a fault where, after interacting with a much lesser amount of groundwater than
low sulphidation veins, highly acidic fluids are formed. These acids rot and dissolve the rock leaving only silica behind
often in a sponge-like formation known as vuggy silica. Metal-rich brines that also ascend from the magma then
precipitate gold and often copper in the spongy vuggy silica. As a result these deposits are commonly broad, bulk-
tonnage mines often with lower grades. In contrast, fluids that form low sulphidation veins interact with the rock for a


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much longer period of time. As a result of the extended duration of the fluids’ interaction with the rock, the fluids
become dilute and neutralized and the silica dissolves (later precipitated as quartz). In low sulphidation veins,
protracted boiling of these fluids produces 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, gold grades can be very high
and result in large amount of easy to mine gold in a narrow compact area.




TEXTURES & GOLD PRECIPITATING PROCESSES
IN LOW-SULPHIDATION EPITHERMAL VEINS
The formation of low-sulphidation veins can be quite
dramatic and results in minerals being precipitated and
transported along several different faults above the
depth at which the fluids start to boil. As quartz crystals
precipitate in a particular fault, the fracture gradually
becomes sealed. When this happens the boiling fluid
finds another fracture along which to rise. In the
meantime gases build up in the fluid underneath the
sealed fault until the pressure ruptures the closure. At
this point the pressure changes rapidly resulting
catastrophic boiling. This type of violent phase
separation results in gold, a distinctive bladed form of
calcite and fine grained gel-like silica (amorphous silica)
all precipitating rapidly and being swept along by the
moving fluids. Eventually the fluids return to equilibrium
and quartz crystals begin to precipitate under passive
conditions, sealing the vein again until the entire process repeats itself.


The episodic nature of quartz precipitation, rupturing followed by gold precipitation, results in banded veins (the
picture above illustrates a banded quartz vein exposure at the Hishikari gold deposit, Japan 2 ) with each band
representing a different phase in the process. The bands of coarse quartz crystals represent passive conditions.
Bands of bladed calcite, fine silica (that has over time turned to quartz), and dark metal rich sludge (containing high
concentrations of gold in the form of electrum), precipitate under conditions of violent boiling and fluid flow. The
catastrophic boiling seems to happen only within a narrow vertical interval, generally about 300 to 600 meters thick.
This is the high grade productive part of the vein system which I will refer to as the ore zone.


                                                              3
By virtue of the fact that the gold is transported, increasingly smaller amounts of gold are found at elevations above
this level. Finding anomalous, but non-economic amounts of gold in a vein that is clearly eroded to a level above that
of the ore horizon is viewed as a good sign of the potential for high-grade gold below. Above the ore zone the bands
of quartz are much finer grained (smaller crystals) since different forms of silica precipitated other than quartz, such
as opal and chalcedony. In addition, the highest concentrations of bladed calcite are typically found at the top of the
ore zone, while mercury and arsenic are found in higher grades above the zone. Beneath the ore zone the veins are
generally made up of bands of coarse quartz crystals with little to no fine-grained quartz present. Gold and silver are
highest in the ore zone and lead and zinc concentrations increase with depth, although there are significant
exceptions to this rule. Erratic gold and silver values can be found immediately above the ore-zone in the lattice-
textured part of the vein. Sometimes elevated molybdenum can occur above the ore zone as well.


When the fluid boils along with water vapour, CO2 and H2S also separate. These gases rise vertically. H2S
condenses above the water table where it naturally forms sulphuric acid. Sulphur can precipitate as well, resulting in
the foul smell of many hot springs. At the surface, the sulphuric acid reduces many rocks to clay and sulphate, and in
the process the acid can dissolve any silica that may be present in the rocks. The resulting silica-laden fluid trickles
down to the water table and re-precipitates the silica. If a permeable unit (such as a volcanic rock) is situated at the
water table, a large area can be flooded with silica. This process results a resistant quartz-rich rock that occurs above
many vein systems, commonly known as a silica cap. Since gold is not transported by either the gases or sulphuric
acid, the silica cap is usually devoid of gold although generally highly elevated in mercury, arsenic and antimony.
Antimony tends to occur in and within close proximity to the veins while arsenic and mercury are often widely
dispersed into the rocks around the veins.


TECHNIQUES FOR LOOKING FOR GOLD IN LOW-SULPHIDATION VEIN SYSTEMS
Ultimately, drilling is the means to discover ore in a vein system, and multiple holes are sometimes necessary to find
the productive ore-zone. In the past, directing drill holes was a rarified art. Today, however, much input is available to
geologists to guide drill programs, of which the interpretation of vein textures and the vein geochemistry are the most
important. The textures of the minerals that form the veins (dominantly quartz, calcite and adularia) vary along the
fluid flow path and, therefore, also vary with respect to depth. By observing these textures and understanding their
variation according to depth and gold content as described above, gold mineralization can be targeted and predicted
with accuracy. Observing fluid inclusions is another technique that can aid in determining depth of a vein system.
When quartz precipitates from the hot fluid, tiny amounts of the fluid itself can be caught in the forming crystals as
microscopic bubbles. These are known as fluid inclusions. If fluid inclusions are examined under a microscope as
they are heated and cooled, the temperatures at which they freeze and at which they becomes a homogenous fluid
can be determined. From this test, the temperature and salinity of the original fluid at the time of the inclusion’s


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formation can be estimated. This information can then be used to corroborate observations made from vein textures
and geochemistry about the depth at which to expect gold mineralization.


LOW-SULPHIDATION GOLD-SILVER VEINS IN OR NEAR PRODUCTION TODAY
Kupol Deposit, Russia
One of the most significant recent low sulphidation discoveries is that of the Kupol vein system in Russia. In 2003
Bema Gold announced a measured and indicated resource of 1.9 million ounces of gold at an average grade of 22.3
g/t and an inferred resource of 4.2 million ounces with an average grade of 18.4 g/t. This is a spectacular deposit
with some significant similarities to the Fuego prospect.




One of the most important similarity is
that, like at Fuego, abundant lattice
textured calcite has been identified in
veins on the Kupol property. At Kupol, as
with many other vein systems, the lattice
textured calcite is distributed generally
above areas of significant economic gold
and silver mineralization. Illustrated above
is a longitudinal section (a view of the
plane of the vein relative to depth) and to
the right a cross section demonstrating the
high grade drill intercepts at Kupol. 3




                                                            5
    The El Penon Gold Deposit, Chile
    The El Penon epithermal banded quartz vein system was
    found and is operated by Meridian Gold Corp. At present
    the deposit has 1.76 million ounces of gold at a grade of
    9.1 g/t in the proven and probable categories and a
    further 0.87 million ounces of gold at a grade of 10.0 g/t
    in the measured and indicated categories. One of the
    most intriguing aspects of the exploration and discovery
    of the El Penon deposit is that the vein is not well
    mineralized at surface; high gold and silver grades were
    blind and intersected by drilling at depth. Illustrated right
    is a cross section and below, a longitudinal section
    respectively that show the blind nature of the
    mineralization 4 indicating the number of holes that were
    necessary to find ore deposit.




6
Pajingo, Australia
The Pajingo deposit has resources and production that total
9 million tonnes averaging 12.2 g/t for a total of 3.5 million
ounces of gold. High gold grades were encountered at
deeper depth, and diminished closer to surface. A cross
section (left) and longitudinal section (below) illustrate the
distribution of ore grades. 5 This deposit is an excellent
example of how high grades frequently occur at deeper
levels within a vein while near the surface little indication
may be evident of the high grade deposit below. The
longitudinal section below demonstrates how many drill
holes (hollow circles) were necessary to define the deposit
since many holes missed the ore zone altogether, which is
shaded from lowest (grey) to highest (black) grade.




7
Hishikari Gold Deposit, Japan
The Hishikari gold deposit is one of the
largest epithermal gold vein deposits in the
world. It was discovered in 1981 by drilling
underneath erratically mineralized quartz
veins. This drill program encountered
spectacular high grades at depth. In 2004,
the total contained gold, both mined and in
reserve, totalled 264 tonnes (8.5 Million
ounces) comprising 3.5 Mt @ 60 to 70 g/t Au
and 2 Mt @ 20 to 25 g/t Au.


The image to the right illustrates that the high
grade veins were intersected well beneath
veins which had returned low gold grades.




Low Sulphidation Epithermal Veins in Mexico
Mexico is particularly well endowed with epithermal low-sulphidation vein systems. This is because there was an
abundant source of fluids and metals emanating from hot magmas over a long period of time. In addition there has
been little erosion since the formation of the vein deposits. This means that veins in Mexico are often well preserved.
Listed below are some of the most significant vein systems that have been mined in Mexico. 6 The Fuego prospect is
thought to be similar in age to these deposits and associated with the same belt of volcanic rocks.


Mine Name, State                        Estimated Production              Gold Grade             Silver Grade
Tayoltita, Durango                      >19Mt                             8 g/t                  500 g/t
Fresnillo, Zacatecas                    >6.2 Mt                           0.56 g/t               780 g/t
Guanajuato, Guanajauato                 40 Mt                             4 g/t                  850 g/t
Pachuca, Hidalgo                        80 Mt                             2.5 g/t                500 g/t
Taxco, Guerrero                         >30 Mt                            0.3 g/t                240 g/t
Zacatecas, Zacatecas                    >20 Mt                            2.5 g/t                900 g/t
El Oro, Mexico                          43.3 Mt                           7 g/t                  100 g/t
Natividad, Oaxaca                       1.7 Mt                            20 g/t                 300 g/t
*Note Mt denotes million tonnes, g/t grams per tonne


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THE FUEGO LOW-SULPHIDATION VEIN SYSTEM
The Fuego property was found during a helicopter reconnaissance program carried out by Almaden Minerals Ltd. in
2003. An area of hydrothermal alteration and veining was recognized from the air and the initial prospecting returned
gold grades as high as 29.9 g/t and silver values up to 881 g/t silver. The Fuego Project is optioned to Horseshoe
Gold Mining Inc. (Horseshoe) who can earn a 60% interest in the property by spending US$3 Million exploring the
project and issuing 1,000,000 shares of Horseshoe to Almaden. Upon earning a 60% interest in the property,
Horseshoe would have 120 days to acquire Almaden's remaining 40% interest in the property in return for a 40%
interest in the issued capital of Horseshoe, to be issued by Horseshoe to Almaden at that time.




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10
The Fuego project is road accessible and located in Oaxaca State, roughly 140 kilometers southeast of Oaxaca City.
The property covers an area of low-sulphidation epithermal veining exhibiting classic low vein textures
commensurate with a highly preserved dynamic mineralizing environment. The textures include colloform banded
fine grained quartz which has replaced non-crystalline amorphous silica, quartz which has replaced lattice-textured
calcite and sulphide-rich bands containing electrum which exhibits graded bedding. These textures are interpreted to
represent a high level of exposure within the mineralised portion of a large epithermal vein system. The veins on
average are very wide, and locally up to 10 meters in width. Several parallel veins have been identified along the
roughly 1,000 meters of strike length that is presently known. Based on these observations, along with fluid inclusion
studies and geochemistry, the Fuego vein system is interpreted to represent a level of erosion immediately above an
ore forming environment. Significant gold grades have already been encountered at surface, indicating that
consistent, high-grade gold-silver ore zones should be sought at shallow depths beneath the exposures of the vein
with drilling.


In 2004 and 2005, Horseshoe completed a surface geologic mapping and rock and soil sampling program on the
Fuego property. This program outlined the main vein zone which has been traced in outcrop roughly 1,000 meters
along strike. Along this exposed strike length vein widths vary from 3 to 10 meters wide. Several parallel veins were
identified in outcrop as well. Where exposure allowed, continuous chip sampling was carried out on the main vein.
The results included a 4 meter exposure averaging 4.52 g/t Au and 86 g/t Ag with individual samples up to 10.6 g/t
Au and 162 g/t Ag and a 3 meter sample that returned 7.8 g/t Au and 191 g/t Ag. Soil sampling highlighted the main
vein zone with elevated Au, Ag, Sb and As values in samples collected. Additional zones of elevated Au, Ag, Sb and
As in soil samples spatially unrelated to the main vein zone suggest that unexposed parallel structures may exist. A
small Induced Polarization (IP) geophysical survey was carried out to test the effectiveness of this methodology in
identifying vein structures that are not exposed. This work identified the known veins as resistivity and chargeability
highs. Additional resistivity and chargeability highs were identified in this work which suggests that further veins may
exist. Additional geologic mapping, sampling and IP surveys are planned to better define these results and
Horseshoe has informed Almaden that permits are presently being acquired for a planned diamond drill program to
commence as soon as possible. Samples were analysed by ALS Chemex Labs of North Vancouver and taken under
the direction of qualified persons Mr. H. L. King, P.Geo., Mr. William Wengzynowski, P.Eng., Mr. Andris Kikauka,
P.Geo and the author.


Illustrated below is a sample taken from the Fuego project (sample assayed 29.9 g/t gold and 550 g/t silver)
compared to that of a high grade ore sample from the Hishikari deposit, Japan 7 . Similar features are evident;
however the Fuego specimen contains lattice-textured calcite (replaced by quartz) which is indicative of a level
immediately above an ore zone.



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Sulphide and gold-rich bands             Calcite Blades (lattice texture)




 Fine-grained Silica Gel (now quartz)




                                        12
Footnotes and References:


1   Taken from White, N C and Hedenquist,J W, 1994, Epithermal environments and styles of mineralization; variations and their causes, and
guidelines for exploration, In: Epithermal gold mineralization of the Circum-Pacific; geology, geochemistry, origin and exploration; II.Siddeley-G
(editor), Journal of Geochemical Exploration. 36; 1-3, Pages 445-474. 1990.
2   Taken from the Metal Mining Agency of Japan’s publication “the story of a Successful gold exploration, the Hishikari gold deposit”, 1990.
3   These figures are taken from Bema gold’s website: www.bema.com
4   Taken from Meridian Gold’s website: www.meridiangold.com
5   Taken from Butler, I, Murphy, T, and Parks, J, 1999, Vera South: Discovery History, Sydney Mineral Exploration Discussion Group,
http://www.smedg.org.au/Sym99vera.htm
6   Taken from: Albinson, T, Norman, D.I., Cole, D., Chomiak, B, 2001, Controls on Formation of Low-Sulphidation Epithermal Deposits in
Mexico: Constraints from Fluid Inclusion amd Stable Isotope Data, In: Albinson, T. and Nelson, C.E., eds., Society of Economic Geology
Special Publication 8, p. 1-32.
7   Taken from: High Grade Epithermal Gold Mineralization-The Hishikari Gold Deposit, Resource Geology Special Issue, No.14, 1993




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