low-sulphidation epithermal quartz-adularia ... - Almaden Minerals

LOW-SULPHIDATION EPITHERMALQUARTZ-ADULARIA GOLD-SILVER VEINSAND THEIXTACA PROJECT, MEXICO1

Introduction to Low-Sulphidation Epithermal VeinsQuartz veins often contain quantities of gold and silver and have produced much of theworld’s gold. Veins are formed when quartz (and/or other minerals) deposits from acooling hot fluid in a crack in the upper part of the earth’s crust known as a fault. Quartzcan deposit from several different types of fluids, one of which is responsible for lowsulphidation epithermal gold-silver veins and geothermal systems such as the hotsprings at Yellowstone or the Geysers in California. These fluids are likely a mixture ofgroundwater and fluid emanating from molten rock at depths of around 5 to 10kilometers in the earth.At these depths the hot fluids are under very high pressures and rise along faults todepths of about two kilometers from surface, where they begin to boil. Boiling causesthe fluid to cool rapidly, causing quartz to deposit in the fault, forming a vein. Eventuallythe rising fluid breaches the surface forming hot springs. If the fluids contain dissolvedgold and silver, boiling can also cause these metals to deposit in the veins. Thisgenerally happens from 1.5 kilometers depth to just below the surface. Recognition thatgold deposition occurs below hotsprings resulted in the term epithermal (epi meaningshallow and thermal referring to the heated fluid) being coined as a descriptive term in1933 by the great American geologist Lindgren. Epithermal gold deposits have furtherbeen subdivided bythe chemistGiggenbach intolow and highsulphidationtypes(illustrated below 1 ).This terminologydoes not refer to therelative amount ofsulphide minerals(metal complexes ofsulfur with metals)2

in each type. Rather it is based on the chemical nuance that the sulfur to metal ratiovaries in the sulphide minerals found in each subtype. While this discussion deals withlow-sulphidation (which are also known as quartz-adularia epithermal vein systems), itis worth mentioning that the other subtype, high-sulphidation epithermal systems, alsoform economic gold deposits and that these two types form under vastly differentchemical conditions.High sulphidation deposits result from fluids channeled directly from a hot magma(where often bulk-mineable porphyry copper deposits form) along a fault where, afterinteracting with a much lesser amount of groundwater, highly acidic fluids are formed.These acids rot and dissolve the rock leaving only silica behind often in a sponge-likeformation known as vuggy silica. Metal-rich brines that also ascend from the magmathen deposit gold and often copper in the sponge-like vuggy silica. As a result thesedeposits are commonly broad, bulk-tonnage mines with lower grades.In contrast low-sulphidation veins are formed by fluids that originate from the hotmagma that mix with a larger amount of groundwater. The resulting fluids interact withthe rock for much longer than the quickly channeled high-sulphidation fluids, in theprocess dissolving silica, which is later deposited as quartz. Gold is deposited byprotracted boiling resulting in high grade gold silver deposits associated with veins. Highgrade gold (greater than one ounce gold per ton) and silver in these veins can be foundover vertical intervals of generally 300 to 1,000 metres. Within this vertical dimension,gold grades can be very high, resulting in a large amount of easy to mine gold in anarrow compact area.Textures and Gold Precipitating Processes in Low-Sulphidation Epithermal VeinsThe fluids that form low-sulphidation veins encounter fractures that are more or lessopen to surface at about 2 kilometers depth. At this point they begin to boil, depositingquartz, calcite and adularia (a feldspar mineral) along with metals, notably gold andsilver. As quartz crystals grow in a particular fault it becomes sealed. When thishappens the fluid finds another fracture along which to rise. In the meantime underneath3

the quartz seal, gases accumulate until the pressure becomes so great that the sealfractures. At this point the pressure changes rapidly causing catastrophic boiling. Thistype of violent phase separationresults in gold, distinctive bladedcalcite crystals and fine-grainedgel-like silica (amorphous silica)depositing below the seal rapidlyand being swept along by themoving fluids. Eventually thefluids return to equilibrium andquartz crystals begin to depositagain under passive conditions,sealing the vein again until theentire process repeats itself.The episodic nature of quartzdeposition, rupturing and golddeposition results in bandedveins (the picture aboveillustrates a gold-rich banded epithermal quartz vein exposure at the Hishikari golddeposit, Japan 2 ) with each band representing a different phase in the process. Thebands 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 (containinghigh 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 andeconomically most productive part of the vein system. Gravity limits how far the goldrichsludge travels upwards and increasingly smaller amounts of gold are found athigher 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 chalcedonyprecipitated. The highest concentrations of bladed calcite are typically found at the top4

of the ore zone. Beneath the ore zone the veins are generally made up of bands ofcoarse quartz crystals with little to no fine-grained quartz. Finding anomalous, but noneconomicamounts of gold in a vein that has such fine-grained quartz may indicateshallow erosion, a good sign for potential high-grade gold below.When the fluid boils water vapour, CO 2 and H 2 S are the main gases that separate.These gases rise and above the water table H 2 S condenses and naturally formssulphuric acid. Sulphur can be deposited as well, resulting in the foul smelling nature ofmany hot springs. The sulphuric acid at surface renders down many rocks to clay andsulphate 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 thisprocess. The resulting silica-laden fluid transports and re-deposits silica at the watertable where it trickles down to. If a permeable unit exists at the water table, such as avolcanic rock, a large area can be flooded with silica. This type of situation results inwhat many refer to as a silica cap, a resistant quartz-rich rock that occurs above manyvein systems. Silica caps and clay alteration blankets are common expressions ofancient hotsprings and overlie the vein zone which channeled the fluids to the ancientsprings at surface.Since gold is not transported by the gases or sulphuric acid, the silica cap is usuallydevoid 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 andmercury are often widely dispersed into the rocks around the veins. Very often highconcentrations of mercury and arsenic are found in above the ore zone in the clayblanket and silica cap. Gold and silver are highest in the ore zone and lead and zincconcentrations increase with depth, although there are significant exceptions to thisrule. Erratic gold and silver values can be found immediately above the ore-zone in thelattice-textured part of the vein. Sometimes elevated molybdenum can occurimmediately above the ore zone as well.5

Techniques for Looking for Gold in Low-Sulphidation Vein SystemsUltimately the means of discovering ore in a vein system is drilling and it can sometimestake many holes to find the productive ore-zone in a given vein system. In the pastdirecting drill holes was an art however much input is available to the geologist today toguide drilling. The most important is the interpretation of vein textures and thedistribution of metals. The textures of the vein minerals such as quartz, calcite andadularia, vary along the fluid flow path and therefore with respect to depth. By observingthese textures and using the knowledge of the variation of textures with respect to depthand 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 isobserving fluid inclusions. As quartz deposits from the hot fluid, tiny amounts of the fluiditself can be caught in the growing crystals as microscopic bubbles, known as fluidinclusions. Fluid inclusions can be examined under the microscope and observed whilethey are heated and cooled. Using these techniques the temperature and salinity of theoriginal fluid at the time of quartz formation can be estimated. This information can thenbe used to corroborate observations made from vein textures and geochemistry aboutthe depth at which to expect gold mineralization. The deeper the quartz crystal formedthe higher the temperatures will be estimated by observing its fluid inclusions.Low-Sulphidation Gold-Silver Veins in or near Production TodayJuanacipio, MexicoThis recent discovery owes much to geologic sleuthing combined with new technologywhich 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 continuousoperation since 1563 and currently produces approximately 12% of world silver supplypresently operated by Mexican miner Penoles. In the 1990’s a geologist recognized alarge area of silica and clay alteration to the west of the Fresnillo mine and interpretedthe area to represent the preserved upper portion of the same hydrothermal system that6

the Fresnillo vein was a part of. The alteration zone is characterised by a 3km by 8kmare of blanket clay and silica cap alteration about 50-100 metres thick and cut by laterstructures carrying argillic and advanced argillic alteration similar to that observed tomark the upper-most reaches of high-grade veins at the nearby and more deeplyeroded Fresnillo mine. Sampling at surface predictably did not return any significantvalues of silver or gold but a geophysical survey was conducted which showed deepvertical resistive structures beneath the clay alteration which were interpreted torepresent veins. Remarkably the first hole drilled by MAG Silver in 2003 is the discoveryhole, intersecting from 596.45 to 598.45 meters depth 2 meters that averaged 10.8 g/tgold and 200 g/t silver. This success quickly attracted the attention of Peñoles toacquire an option to earn an interest in the property. Drilling by Peñoles in late 2005subsequently discovered the Valdecañas Vein less than 5 kilometres from their mainproduction headframe at Fresnillo. Drilling through the end of 2007 had defined theValdecanas vein over a strike length of 1.6 kilometres, over a width of 5.8 metres, andwith a down dip length of 450 to 500 metres. The average grade of the high-grade zoneis 1,292 g/t Ag, 2.5 g/t Au and 8.4% Pb+Zn. This remarkable discovery of concealedmineralization highlights the necessity of drilling deep holes to intersect mineralizationwhen an epithermal system, like that of Juanacipio, is interpreted to shallowly eroded.The surface clay alteration, overlying banded veins, is a similar setting to that of theIxtaca Zone.7

ich 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 additionhigh mercury, arsenic and antimony and the fine grained nature of the quartz at surfacesupported the interpretation that the gold mineralization represented a well preservedhydrothermal system. This area had been covered by early IP surveys and this workindicated sulphides and quartz at depth. In 2006 two holes were designed to test theanomalies. The first hole returned only weak gold mineralization (up to 1.23 g/t Au inintersections of 24 m of 0.22 g/t Au and 8 m of 0.49 g/t Au) however the secondintersected clasts of quartz that returned gold values up to 6.08 g/t gold. Another holewas 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 thediscovery of the Fruta del Norte deposit which was rapidly defined by further drillingthereafter.Kupol Deposit, RussiaOne of the most significant recent discoveries is that of the Kupol vein system in Russia.In 2003 Bema Goldannounced a measuredand indicated resource of1.9 million ounces of goldat an average grade of22.3 g/t and an inferredresource of 4.2 millionounces with an averagegrade of 18.4 g/t.This is a spectaculardeposit with somesignificant similarities tothe Ixtaca Zone. One of9

the most important is that, like at Ixtaca, abundant lattice textured calcite has beenidentified in veins on the Kupol property.At Kupol, as with many other vein systems, the lattice textured calcite is distributedgenerally above areas of significant economic gold and silver mineralization. Illustratedis a longitudinal section (a view of the plane of the vein relative to depth) and a crosssection demonstrating the high grade drill intercepts at Kupol. 3The El Penon Gold Deposit, ChileThe El Penon epithermal vein system wasfound and is operated by Meridian GoldCorp. At present the deposit has 1.76 Millionounces of gold at a grade of 9.1 g/t in theproven and probable categories and afurther 0.87 Million ounces of gold at a gradeof 10.0 g/t in the Measured and Indicatedcategories. One of the most intriguingaspects of the exploration and discovery ofthe El Penon deposit is that the vein is notwell mineralized at surface; high gold and10

silver grades were blind and intersected by drilling at depth.Illustrated above are a cross section and a longitudinal section, respectively, showingthe blind nature of the mineralization 4 and the number of holes that were necessary tofind it.Pajingo, AustraliaThis deposit located in Australia has resources andproduction that total 9 million tonnes averaging 12.2g/t for a total of 3.5 million ounces of gold. Ore gradeswere encountered at depth as higher in the veinsystem gold grades diminish greatly. A longitudinaland cross section illustrating the distribution of oregrades are shown 5 . This deposit is an excellentexample of high grades occurring at deeper levelswithin a vein that where gold grades were muchreduced at higher levels and that many holes are11

necessary to find the higher grade portion of a vein system.Hishikari Gold Deposit, JapanThe Hishikari gold deposit was discovered in 1981 by drilling underneath erraticallymineralized narrow banded quartz veins. At surface blanket type clay alteration andhotsprings are also observed, indicating that this epithermal vein system was entirelypreserved. This drill program encountered spectacular high grades at depth, beginningwith a 15 cm intersection of 190 g/t gold and 167 g/t silver in one of the first holes. Thisintersection led to the delination of one of the largest epithermal gold vein deposits inthe world. In 2004 the total contained gold, both mined and in reserve, totalled 264tonnes (8.5 Million ounces) comprising 3.5 Mt @ 60 to 70 g/t Au and 2 Mt @ 20 to 25 g/tAu. The image below illustrates that the high grade veins were intersected at depthbeneath veins that returned low gold grades. In many respects this early discoverydrilling and the well preserved nature of the Hishikari deposit is similar to that of Ixtacaat this early stage of the project’s development.12

Low Sulphidation Epithermal Veins in MexicoMexico is particularly well endowed with epithermal low-sulphidation vein systems. Thisis because there was an abundant source of fluids and metals emanating from hotmagmas 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 wellpreserved. Listed below are some of the most significant vein systems that have beenmined in Mexico. 6 The Ixtaca prospect is thought to be similar in age to these depositsand associated with the same belt of volcanic rocks as that which hosts Pachuca, ElOro and Taxco.13

Mine Name, State Estimated Production Gold Grade Silver GradeTayoltita, Durango >19Mt 8 g/t 500 g/tFresnillo, Zacatecas >6.2 Mt 0.56 g/t 780 g/tGuanajuato, Guanajauato 40 Mt 4 g/t 850 g/tPachuca, Hidalgo 80 Mt 2.5 g/t 500 g/tTaxco, Guerrero >30 Mt 0.3 g/t 240 g/tZacatecas, Zacatecas >20 Mt 2.5 g/t 900 g/tEl Oro, Mexico 43.3 Mt 7 g/t 100 g/tNatividad, Oaxaca 1.7 Mt 20 g/t 300 g/t*Note Mt denotes million tonnes, g/t grams per tonneThe Ixtaca Low-Sulphidation Vein SystemAlmaden’s 100% owned Ixtaca zone is part of the Tuligtic project, located within theTrans Mexican Volcanic Belt about 120 kilometres southeast of the Pachuca gold/silverdeposit which has reported historic production of 1.4 billion ounces of silver and 7million ounces of gold. The Ixtaca zone is located along a trend of shallowly erodedepithermal systems that Almaden has identified in eastern Mexico. Almaden has severalother projects staked along this trend. A separate porphyry copper zone, located 3 kmnorth of Ixtaca, was the focus of exploration at Tuligtic by Almaden’s partners for manyyears until Almaden drilled theIxtaca zone discovery hole in 2010.The surface manifestation of the Ixtaca zone is very obscure because the region isalmost completely covered with a thin layer of recent volcanic ash. Reports of historicclay mines brought Almaden’s attention to the area. These kaolinite and replacementsilica alteration zones are typical of the surface manifestation of an ancient hotspringenvironment, the top of a low-sulphidation epithermal vein system. In an arroyo beneaththe kaolinite and silica alteration, some very narrow (0.1 to 3 centimetre) veins withepithermal textures occur in a small (about 2 metres by 5 metres) outcrop. These veinsassayed up to 1 g/t gold and 110 g/t silver. Small cobbles of float in the creek returnedassays of up to 600 g/t silver and another such cobble assayed 6.0 g/t gold. Work priorto drilling included a single Induced Polarization geophysical line across this area which14

detected a resistivity anomaly and several short geochemical soil sample lines showedcoincident anomalous gold and silver values.15

The first hole drilled, TU-10-1, intersected multiple quartz-carbonate-sulphide veinzones over its entire length, averaging 1.01 g/t gold and 48 g/t silver over 302.41 metersfrom the base of overburden at 47.50 meters depth to the bottom of the hole at 349.91meters depth. Vein intersections include several very high grade intervals such as 0.70meters of 129 g/t gold and 4288 g/t silver. The veins are composed of banded finegrained quartz, calcite, rhodochrosite and sulphides which display textures typical ofclassic low sulphidation epithermal veins. The Ixtaca zone is a blind drilling discovery asthere is very little surface manifestation of the veins. The discovery is the result of theCompany’s interpretation of the surface geology and utilising epithermal models ofmineralization in particular recognizing that the system is very shallowly eroded with theremains of the ancient hotspring preserved. Below is a conceptual model of the Ixtacazone showing the interpretation that the veins intersected in hole TU-10-1 are theshallow portion of a major vein system which must be explored to depth.16

The Ixtaca zone mineralization has many similarities with other vein deposits worldwide.For example the Hishikari, Juanacipio and Fruta del Norte vein deposits were all foundby drilling beneath the upper and barren parts of a vein system correctly interpreted toexist below. Hishikari and Juanacipio were found beneath silica cap and blanket clayhotspring alteration and began with the discovery of several narrow but high grade veinsthat related to a significant vein system at depth. As at Ixtaca, the Hishikari veins arehosted by carbonaceous sedimentary rocks including shale units. Illustrated below is asample taken from the compared to that of a high grade ore sample from the Hishikarideposit, Japan 7 with very similar textures.17

Footnotes and References1 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, theHishikari gold deposit”, 1990.3 These figures are taken from Bema gold’s website: www.bema.com4 Taken from Meridian Gold’s website: www.meridiangold.com5 Taken from Butler, I, Murphy, T, and Parks, J, 1999, Vera South: Discovery History, Sydney Mineral ExplorationDiscussion 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-SulphidationEpithermal Deposits in Mexico: Constraints from Fluid Inclusion amd Stable Isotope Data, In: Albinson, T. andNelson, 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 SpecialIssue, No.14, 199318