Epithermal Ixtaca

7/27/2019 Epithermal Ixtaca

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Introduction to Low-Sulphidation Epithermal Veins

Quartz veins often contain quantities of gold and silver and have produced much of the

worlds gold. Veins are formed when quartz (and/or other minerals) deposits from a

cooling hot fluid in a crack in the upper part of the earths crust known as a fault. Quartz

can deposit 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. These fluids are likely a mixture of

groundwater and fluid emanating from molten rock at depths of around 5 to 10

kilometers in the earth.

At these depths the hot fluids are under very high pressures and rise along faults to

depths of about two kilometers from surface, where they begin to boil. Boiling causes

the fluid to cool rapidly, causing quartz to deposit in the fault, forming a vein. Eventually

the rising fluid breaches the surface forming hot springs. If the fluids contain dissolved

gold and silver, boiling can also cause these metals to deposit in the veins. This

generally happens from 1.5 kilometers depth to just below the surface. Recognition that

gold deposition occurs below hotsprings resulted in the term epithermal (epi meaning

shallow and thermal referring to the heated fluid) being coined as a descriptive term in

1933 by the great American geologist Lindgren. Epithermal gold deposits have further

been subdivided by

the chemist

Giggenbach into

low and high-

sulphidation types

(illustrated below1).

This terminology

does not refer to the

relative amount of

sulphide minerals

(metal complexes of

sulfur with metals)


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the quartz seal, gases accumulate until the pressure becomes so great that the seal

fractures. At this point the pressure changes rapidly causing catastrophic boiling. This

type of violent phase separation

results in gold, distinctive bladed

calcite crystals and fine-grained

gel-like silica (amorphous silica)

depositing below the seal rapidly

and being swept along by the

moving fluids. Eventually the

fluids return to equilibrium andquartz crystals begin to deposit

again under passive conditions,

sealing the vein again until the

entire process repeats itself.

The episodic nature of quartz

deposition, rupturing and gold

deposition results in banded

veins (the picture above

illustrates a gold-rich banded epithermal quartz vein exposure at the Hishikari gold

deposit, J apan2) 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), deposit 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 top to bottom. This is the high grade and

economically most productive part of the vein system. Gravity limits how far the gold-

rich sludge travels upwards and increasingly smaller amounts of gold are found at

higher elevations. Above the ore zone the bands of quartz are much finer grained

(smaller crystals) as different forms of silica other than quartz like opal and chalcedony

precipitated. The highest concentrations of bladed calcite are typically found at the top


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of the ore zone. Beneath the ore zone the veins are generally made up of bands of

coarse quartz crystals with little to no fine-grained quartz. Finding anomalous, but non-

economic amounts of gold in a vein that has such fine-grained quartz may indicate

shallow erosion, a good sign for potential high-grade gold below.

When the fluid boils water vapour, CO2 and H2S are the main gases that separate.

These gases rise and above the water table H2S condenses and naturally forms

sulphuric acid. Sulphur can be deposited as well, resulting in the foul smelling nature of

many hot springs. The sulphuric acid at surface renders down many rocks to clay and

sulphate and in the process can dissolve any silica that may be present in the rocks. Alarge bleached area of clay altered rocks often overlies many hotsprings reflecting this

process. The resulting silica-laden fluid transports and re-deposits silica at the water

table where it trickles down to. If a permeable unit exists at the water table, such as a

volcanic rock, a large area can be flooded with silica. This type of situation results in

what many refer to as a silica cap, a resistant quartz-rich rock that occurs above many

vein systems. Silica caps and clay alteration blankets are common expressions of

ancient hotsprings and overlie the vein zone which channeled the fluids to the ancient

springs at surface.

Since gold is not transported by 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. Very often high

concentrations of mercury and arsenic are found in above the ore zone in the clay

blanket and silica cap. 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

immediately above the ore zone as well.


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Techniques for Looking for Gold in Low-Sulphidation Vein Systems

Ultimately the means of discovering ore in a vein system is drilling and it can sometimes

take many holes to find the productive ore-zone in a given vein system. In the past

directing drill holes was an art however much input is available to the geologist today to

guide drilling. The most important is the interpretation of vein textures and the

distribution of metals. The textures of the vein minerals such as quartz, calcite and

adularia, vary along the fluid flow path and therefore with respect to depth. By observing

these textures and using the knowledge of the variation of textures with respect to depth

and gold content described above, gold mineralization can be targeted and predicted.

Another technique that can aid in the interpretation of depth in a vein system is

observing fluid inclusions. As quartz deposits from the hot fluid, tiny amounts of the fluid

itself can be caught in the growing crystals as microscopic bubbles, known as fluid

inclusions. Fluid inclusions can be examined under the microscope and observed while

they are heated and cooled. Using these techniques the temperature and salinity of the

original fluid at the time of quartz 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. The deeper the quartz crystal formed

the higher the temperatures will be estimated by observing its fluid inclusions.

Low-Sulphidation Gold-Silver Veins in or near Production Today

Juanacipio, Mexico

This recent discovery owes much to geologic sleuthing combined with new technology

which enabled targeting of this deep and concealed new silver gold quartz-vein system.

This project is located about six kilometres from the historic Fresnillo silver-gold mine,

the largest producing silver mine in the world. Fresnillo has been in continuous

operation since 1563 and currently produces approximately 12% of world silver supply

presently operated by Mexican miner Penoles. In the 1990s a geologist recognized a

large area of silica and clay alteration to the west of the Fresnillo mine and interpreted

the area to represent the preserved upper portion of the same hydrothermal system that


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the Fresnillo vein was a part of. The alteration zone is characterised by a 3km by 8km

are of blanket clay and silica cap alteration about 50-100 metres thick and cut by later

structures carrying argillic and advanced argillic alteration similar to that observed to

mark the upper-most reaches of high-grade veins at the nearby and more deeply

eroded Fresnillo mine. Sampling at surface predictably did not return any significant

values of silver or gold but a geophysical survey was conducted which showed deep

vertical resistive structures beneath the clay alteration which were interpreted to

represent veins. Remarkably the first hole drilled by MAG Silver in 2003 is the discovery

hole, intersecting from 596.45 to 598.45 meters depth 2 meters that averaged 10.8 g/t

gold and 200 g/t silver. This success quickly attracted the attention of Peoles toacquire an option to earn an interest in the property. Drilling by Peoles in late 2005

subsequently discovered the Valdecaas Vein less than 5 kilometres from their main

production headframe at Fresnillo. Drilling through the end of 2007 had defined the

Valdecanas vein over a strike length of 1.6 kilometres, over a width of 5.8 metres, and

with a down dip length of 450 to 500 metres. The average grade of the high-grade zone

is 1,292 g/t Ag, 2.5 g/t Au and 8.4% Pb+Zn. This remarkable discovery of concealed

mineralization highlights the necessity of drilling deep holes to intersect mineralization

when an epithermal system, like that of J uanacipio, is interpreted to shallowly eroded.

The surface clay alteration, overlying banded veins, is a similar setting to that of the

Ixtaca Zone.


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Fruta del Norte, Ecuador

This area was identified as prospective by Aurelian Resources after it was explored by

an Australian consortium from 1996 to 1998. There was some small scale surface

mining by locals in the area as well. Before drilling, work consisted of surface mapping

and sampling as well as IP geophysics. Silicified conglomerate, later known to exist

immediately above the deposit, was found in the work program and samples were taken

that returned up to 101 ppb. At this time the conglomerate was thought to predate the

mineralization and since the mineralization was weak, no attention further attention was

given to this area. In 1999 the Australian firm abandoned the property and Aurelian

optioned it and began work in 2002.

For three years work on the property focused in areas away from the eventual discovery

but in 2005 the same conglomerate was resampled returning similar gold results. This

time the conglomerate was correctly interpreted to have been deposited subsequent to

mineralization and therefore to be obscuring a potential gold deposit, meaning that gold-


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rich quartz in the conglomerate were fragments torn up from buried mineralization.

Further more detailed surface sampling returned gold values up to 1.92 g/t. in addition

high mercury, arsenic and antimony and the fine grained nature of the quartz at surface

supported the interpretation that the gold mineralization represented a well preserved

hydrothermal system. This area had been covered by early IP surveys and this work

indicated sulphides and quartz at depth. In 2006 two holes were designed to test the

anomalies. The first hole returned only weak gold mineralization (up to 1.23 g/t Au in

intersections of 24 m of 0.22 g/t Au and 8 m of 0.49 g/t Au) however the second

intersected clasts of quartz that returned gold values up to 6.08 g/t gold. Another hole

was drilled deeper from the same location as the second and from 199.45 meterintersection 237.25 meters averaging 4.14 g/t gold. This intersection marks the

discovery of the Fruta del Norte deposit which was rapidly defined by further drilling

thereafter.

Kupol Deposit, Russia

One of the most significant recent 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 Ixtaca Zone. One of


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the most important is that, like at Ixtaca, 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

is a longitudinal section (a view of the plane of the vein relative to depth) and a cross

section demonstrating the high grade drill intercepts at Kupol.3

The El Penon Gold Deposit, Chile

The El Penon epithermal 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


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silver grades were blind and intersected by drilling at depth.

Illustrated above are a cross section and a longitudinal section, respectively, showing

the blind nature of the mineralization4

and the number of holes that were necessary tofind it.

Pajingo, Australia

This deposit located in Australia has resources and

production that total 9 million tonnes averaging 12.2

g/t for a total of 3.5 million ounces of gold. Ore grades

were encountered at depth as higher in the vein

system gold grades diminish greatly. A longitudinal

and cross section illustrating the distribution of ore

grades are shown5. This deposit is an excellent

example of high grades occurring at deeper levels

within a vein that where gold grades were much

reduced at higher levels and that many holes are


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necessary to find the higher grade portion of a vein system.

Hishikari Gold Deposit, Japan

The Hishikari gold deposit was discovered in 1981 by drilling underneath erratically

mineralized narrow banded quartz veins. At surface blanket type clay alteration and

hotsprings are also observed, indicating that this epithermal vein system was entirely

preserved. This drill program encountered spectacular high grades at depth, beginning

with a 15 cm intersection of 190 g/t gold and 167 g/t silver in one of the first holes. This

intersection led to the delination of one of the largest epithermal gold vein deposits in

the world. 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 below illustrates that the high grade veins were intersected at depth

beneath veins that returned low gold grades. In many respects this early discovery

drilling and the well preserved nature of the Hishikari deposit is similar to that of Ixtaca

at this early stage of the projects development.


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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 theformation 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.6The Ixtaca prospect is thought to be similar in age to these deposits

and associated with the same belt of volcanic rocks as that which hosts Pachuca, El

Oro and Taxco.


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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

The Ixtaca Low-Sulphidation Vein System

Almadens 100% owned Ixtaca zone is part of the Tuligtic project, located within the

Trans Mexican Volcanic Belt about 120 kilometres southeast of the Pachuca gold/silver

deposit which has reported historic production of 1.4 billion ounces of silver and 7

million ounces of gold. The Ixtaca zone is located along a trend of shallowly eroded

epithermal systems that Almaden has identified in eastern Mexico. Almaden has several

other projects staked along this trend. A separate porphyry copper zone, located 3 km

north of Ixtaca, was the focus of exploration at Tuligtic by Almadens partners for many

years until Almaden drilled theIxtaca zone discovery hole in 2010.

The surface manifestation of the Ixtaca zone is very obscure because the region is

almost completely covered with a thin layer of recent volcanic ash. Reports of historic

clay mines brought Almadens attention to the area. These kaolinite and replacement

silica alteration zones are typical of the surface manifestation of an ancient hotspring

environment, the top of a low-sulphidation epithermal vein system. In an arroyo beneath

the kaolinite and silica alteration, some very narrow (0.1 to 3 centimetre) veins with

epithermal textures occur in a small (about 2 metres by 5 metres) outcrop. These veins

assayed up to 1 g/t gold and 110 g/t silver. Small cobbles of float in the creek returned

assays of up to 600 g/t silver and another such cobble assayed 6.0 g/t gold. Work prior

to drilling included a single Induced Polarization geophysical line across this area which


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detected a resistivity anomaly and several short geochemical soil sample lines showed

coincident anomalous gold and silver values.


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The first hole drilled, TU-10-1, intersected multiple quartz-carbonate-sulphide vein

zones over its entire length, averaging 1.01 g/t gold and 48 g/t silver over 302.41 meters

from the base of overburden at 47.50 meters depth to the bottom of the hole at 349.91

meters depth. Vein intersections include several very high grade intervals such as 0.70

meters of 129 g/t gold and 4288 g/t silver. The veins are composed of banded fine

grained quartz, calcite, rhodochrosite and sulphides which display textures typical of

classic low sulphidation epithermal veins. The Ixtaca zone is a blind drilling discovery as

there is very little surface manifestation of the veins. The discovery is the result of the

Companys interpretation of the surface geology and utilising epithermal models of

mineralization in particular recognizing that the system is very shallowly eroded with theremains of the ancient hotspring preserved. Below is a conceptual model of the Ixtaca

zone showing the interpretation that the veins intersected in hole TU-10-1 are the

shallow portion of a major vein systemwhich must be explored to depth.


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The Ixtaca zone mineralization has many similarities with other vein deposits worldwide.

For example the Hishikari, J uanacipio and Fruta del Norte vein deposits were all found

by drilling beneath the upper and barren parts of a vein system correctly interpreted to

exist below. Hishikari and Juanacipio were found beneath silica cap and blanket clay

hotspring alteration and began with the discovery of several narrow but high grade veins

that related to a significant vein system at depth. As at Ixtaca, the Hishikari veins are

hosted by carbonaceous sedimentary rocks including shale units. Illustrated below is a

sample taken from the compared to that of a high grade ore sample from the Hishikari

deposit, J apan7 with very similar textures.


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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 Japans publication the story of a Successful gold exploration, the

Hishikari gold deposit, 1990.3 These figures are taken from Bema golds website: www.bema.com4 Taken from Meridian Golds website: www.meridiangold.com5 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.htm6 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

Epithermal Ixtaca

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