Potential For Gem Beryl And Schist Hosted Emerald In .
Ministry of Energy and MinesMining and Minerals DivisionGeological Survey BranchPOTENTIAL FOR GEM BERYL ANDSCHIST HOSTED EMERALDIN BRITISH COLUMBIABy Andrew S. Legun, P. Geo.GEOFILE 2005-19
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POTENTIAL FOR GEM BERYL AND SCHIST HOSTED EMERALD INBRITISH COLUMBIABy: Andrew S. LegunTABLE OF CONTENTSINTRODUCTION .4PHYSICAL PROPERTIES AND ASPECTS OF VALUATION OF GEM BERYL.4COLOR AND CHEMISTRY .4OPTICAL QUALITIES .4DURABILITY .5VALUATION .5FORMATION AND DISTRIBUTION OF BERYL IN THE GRANITIC ENVIRONMENT .5BERYL AND VOLATILE GRANITES.5BERYL AND PEGMATITES .5BERYL IN B.C. .6EMERALD DEPOSIT MODELS.7EMERALD IN WALLROCK.7SIGNIFICANCE OF YUKON DISCOVERIES TO B.C. EMERALD POTENTIAL .7EMERALD AND GEM BERYL POTENTIAL MAP .8COMMENTS ON GENERAL AREAS OF POTENTIAL.8DESCRIPTIONS OF SPECIFIC AREAS OF POTENTIAL .8ATLIN AND JENNINGS RIVER AREA .8SURPRISE LAKE SUITE .8SEAGULL BATHOLITH.9LOGTUNG .9CASSIAR AND CRY LAKE AREA.10CASSIAR BATHOLITH .10SYLVESTER ALLOCTHON.10HORSERANCH RANGE.10MOUNT HASKIN.11WEST WILLISTON .11BAYONNE MAGMATIC ARC, KOOTENAY REGION.11NORTHERN BAYONNE SUITE AND SHUSWAP COMPLEX.11SOUTHERN BAYONNE SUITE AND VALHALLA COMPLEX.11SUMMARY AND CONCLUSIONS .12ACKNOWLEDGEMENTS .12REFERENCES .12Cover photo by Brad Wilson - Aquamarine from Slocan AreaGEOFILE 2005-162
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INTRODUCTIONThis article reviews the gem varieties of beryl, theassociations of beryl with granitic rocks, and theparticular association of emerald with ultramaficwallrocks affected by volatile-rich fluids. The articleincludes an emerald potential map for the province anda general description of areas of gem beryl potential.There has been little systematic work on transparentgemstones in British Columbia in spite of a number ofgem discoveries in the last fifteen years. Thesediscoveries include iolite (gem cordierite), peridot (gemolivine), topaz and aquamarine (Simandl et al., 2000),(Domville, 1998). Discoveries of chromium richemerald at Regal Ridge in south-central Yukon,vanadium rich emerald at Lened near theYukon/Northwest Territories border (Groat et al., 2002)has raised interest in the emerald potential of BritishColumbia. In B.C. poorly formed crystals of emeraldare known (Wilson, 1997).Beryl Be3Al2(Si03)6 is the most common mineral ofberyllium and aquamarine the most abundant gemvariety of beryl. Emerald is gem beryl with traces ofchromium or vanadium as the coloring agent. Otherberyllium bearing minerals that may have sufficientclarity to form gems include chrysoberyl (BeAl2O4),euclase (BeAlSiO4OH) and phenakite (Be2SiO4).Beryllium is associated with other lithophile metals(W, Sn, Mo, U, Nb and Li). These metals, with theexception of lithium and perhaps tin, are wellrepresented by showings, prospects and deposits of B.C.This association suggests more beryllium prospects,including gem beryl, will be found and their paucity isprobably due to a lack of careful exploration. Thejuxtaposition of beryllium bearing granitic rocks withultramafic rocks in B.C. is a geologic setting generallyconducive to emerald formation.Emerald is traditionally found in small volumedeposits, such as wallrock zones of pegmatites.Mineable widths may only be a metre or so. Howeverthere may be many producing pegmatites in an area andthe total volume of emerald-bearing rock may besignificant. The artisan miner or small co-operative hashistorically exploited individual high value but smallvolume emerald deposits.The range in value of emerald varies tremendously,from a few dollars, to tens of thousands of dollars percarat (a fifth of a gram). The value of gold at 400 dollarsa troy ounce translates to about 2.57 dollars per carat.Emerald is more valuable than gold but quality ofmaterial in small volume deposits is of paramountimportance.PHYSICAL PROPERTIES ANDASPECTS OF VALUATION OF GEMBERYLCOLOR AND CHEMISTRYBeryl or beryllium aluminum silicate is colorless inits pure form. In common form it is white and opaquedue to inclusions or alkali content. The appeal of gemberyl is in its transparency and wide variety in color.The color variations include:Emerald: leaf green to deep bluish green;Morganite: pink, apricot;Goshenite: colorless;Heliodor: pale yellow to yellow-orange to yellowgreen;Aquamarine: watery blue, bluish-green to greenishyellow;Bixbite: an informal name to a very rare variety ofred beryl.The particular hue of emerald is the result of traceamounts of chromium or vanadium that substitute forAl3 in the beryl ring silicate structure. Iron isresponsible for hues in the less valuable but morewidely distributed aquamarine. The color variations ofaquamarine depend on the oxidation state of the iron(Fe3 or Fe2 ) and whether the ion occupies a channelposition in silicate rings or substitutes for aluminum inoctahedral sites. The yellow of heliodor is a result ofFe3 substituting for aluminum in the octahedral site.Manganese is responsible for violet-pink hues inmorganite and probably the red of bixbite.Gem crystals of beryl may also be color zoned,either normal or parallel to the c-axis of the crystal. Theresult is color banding along the length of the crystal, orcrystals with a visibly paler core.Natural irradiation may lead to color effects. Theintense blue of a rare "Maxixe" beryl is due to naturalradiation and it fades with time. Artificial gammaradiation is applied on some colorless beryl to inducecolor with temporary or permanent effect.OPTICAL QUALITIESThe optical qualities of emerald (refractive index 1.57-1.59, dispersion 0.014) represent only modestbrilliance and sparkle (fire). The saturated tones of“leafy green” in quality stones nevertheless translateinto a value per carat that may exceed diamond (Hall,2004).Inclusions are common in gem beryl and emerald inparticular. In schist-hosted emerald it may be needles ofactinolite or tremolite. Trains of bubbles of gas and fluidmay form inclusions and internal fractures aresometimes described as inclusions. Flawless, inclusionfree crystals of emerald are not known. The emeraldGEOFILE 2005-164
clarity scale begins at very, very slightly included ABILITYEmeralds and other gem beryls are not as durable aseither diamond or sapphire. The hardness of beryl at 7.5to 8 is less than that of sapphire (9) or diamond (10).The presence of inclusions makes gem beryl moresusceptible to abrasion and disintegration duringtransport. Alluvial placers of emerald are rare.VALUATIONMany aspects govern valuation of a transparentgem deposit. This includes ease of extraction of thecrystals, weight and size of crystals, clarity, color hueand saturation, fractures and inclusions, and thechemical nature of the coloring agent. In most depositsonly a portion of the crystalline material is worth cuttingand a further portion is lost in the faceting process. It isthus difficult to assign a net value to a deposit. InCanada's climate abundant fractures in elluvial emeraldsreduce the available size for cutting and are a concern indeposit development (ibid). In the individual stone valueescalates rapidly with faceted size.The value of vanadium-bearing emerald is less thanthat of chrome-bearing variety though the color is verycomparable. Inclusions may add value and character toemerald (a natural birthmark) but value is diminished ifinclusions mark the surface. Valuation of gemstonesrequires consideration of competing stones. Heat-treatedtopaz provides competition for aquamarine of the samecolor.FORMATION AND DISTRIBUTION OFBERYL IN THE GRANITICENVIRONMENTBERYL AND VOLATILE GRANITESBeryl is associated with volatile-bearing phases ofleucocratic granites and syenites. According to Pell andHora (1990) volatile granites may form anywhere theSr87/86 ratio exceeds 0.704. The granites can be eitherorogenic, resulting from compression, crustal thickeningand melting of metasediments (S-type granites) orrelated to rifting (anorogenic/anhydrous A-typegranites).Concentrations as low as 4-8 ppm beryllium in theparent magma may lead to beryl bearing pegmatitethrough fractionation (London and Evensen, 2001). Astemperatures decline beryl becomes insoluble in themelt. Volatiles facilitate this process via fluxing - i.e.reducing the temperature of crystallization. The volatilecomponents (besides water) may include CO2, F, B, Li,P, and Cl.5The crystallization of pegmatite from such magmasmay be followed by development of coarse micaceousaggregates rich in tourmaline, topaz, fluorite, rutile,cassiterite, wolframite, beryl and lithium mica. Themica rich aggregates are known as greisen and formfrom high temperature volatile-rich (particularlyfluorine rich) aqueous solutions. Thus more than onegeneration of beryl may form as temperatures fall andphysiochemical conditions change - a first generation inpegmatite, and a second at lower temperature whengreisen fluids affect pegmatite (Markl and Schumacher,1997).Beryl may also crystallize from late liquid and gasrich pockets or miaroles within the essentially solidifiedhost intrusive.Volatile granites are often geochemically distinctand characterized by names such as "tin granites" or"topaz granites". However simple granites may be localenriched in beryllium and beryl due to volatile activity(Rainer, 2001). Many granites in B.C. are simple butcrystallized in the presence of active volatilecomponents - shown in presence of tourmaline, fluoriteand apatite.BERYL AND PEGMATITESCerny (1991) classified pegmatites according totheir metamorphic depth environment of formation. Theabyssal and muscovite class represent melts that havenot migrated far from source and crystallise in highgrade metamorphic environments of amphibolite andgranulite grade. They tend to form poor gemstock.The rare element class represents melts that havemigrated to shallower levels and fractionated en route.They are subdivided into the LCT (Li-Cs-Ta) and NYF(Nb-Y-F) families. Enrichment in beryllium is commonto both families. The LCT type is derived from orogenicplutonic suites that tend to be peraluminous and potassic(e.g. Bayonne suite in B.C.) while the NYF type isanorogenic, related to rifting or post orogenreactivation, and derived from source rocks affected bydeep mantle-related metasomatism (e.g. Surprise Lakesuite in B.C.). NYF magma tends to be peralkaline tometaluminous.Both LCT and NYF pegmatites may form goodgemstock depending on whether gem minerals are freegrowing or in interlocking texture with other minerals.The subvolcanic miarolitic class offers the bestpotential for extractable gems. High quality crystalsfrom open space fillings or weakly bonded matrix areavailable in symmetrically zoned "fissure fill"pegmatites in this class. Such pegmatites tend to format shallow levels within crustal blocks undergoinguplift. The uplift may relate to cordilleran tectonics orarching of roofs above subvolcanic intrusions.In contrast to aquamarine, emerald is notcommonly found in pegmatite. It is found in wallrock topegmatite. Most pegmatites are crystalline at the timeaqueous fluids are expelled into wallrock (Morgan andLondon, 1987). Thus there is little opportunity forelement exchange in the pegmatite with trace elementsGEOFILE 2005-16
such as chromium in wallrock. There are only a few(rather intriguing) geologic references to emerald inpegmatite (e.g. Brown, 2004).BERYL IN B.C.In the mid 1990's there were approximately 23documented showings of beryl in the province (Minfilerecords) and 2 of the beryllium mineral danalite-helvite.Presently there are 40 showings of beryl and 3 of otherberyllium minerals that are discussed in this article (seetable 1). By comparison there are only 10 officiallydocumented occurrences of beryl in the Yukon (Lewis,Hart and Murphy, 2003).Most beryl in B.C. is found in pegmatitesassociated with leucocratic granites (alaskites, quartzmonzonites), and in late quartz veins that cut the hostintrusion or country rock. Beryl has also beententatively identified in skarn (Bohme, 1984) and inquartz veins cutting skarn at Mt. Haskin (Gower et al.,1985).Beryl bearing pegmatites in B.C. are usually inborder phases or close to the periphery of a largeintrusive body (White Creek batholith) but they arefound internally as well (Laib claims). The pegmatitesvary from massive and homogenous to zoned bodies.Holland (1956) described a zoned beryl-bearing dike inthe Horseranch Range area. It had a 2-inch marginalzone of muscovite, a 2-inch intermediate zone rich intourmaline and garnet, and a 4-inch centre of coarselycrystalline feldspar and quartz with beryl. Stockscheiderfabric often accompanies zoning. This is minerallayering with unidirectional crystal growth resultingfrom heat loss to walls during crystallisation. Suchfabric is often seen in smaller pegmatite sills and dikesof the Hellroaring Creek body. Accessory minerals inB.C. beryl-bearing pegmatites may include fluorite,tourmaline, garnet, magnetite, scheelite, andmolybdenite.granitic bodies. Aquamarine crystals on the B.Q. claimsnear Salmo are found in a pegmatite ascribed to theEocene Ladybird suite (Gauthier and Dixon, 1997). Theaquamarine locale named "Mt. Foster" (Wilson, 1997)may be related to Eocene leucocratic granite of theBennet Lake cauldron complex, and the localitydescribed as "East of Atlin" by Wilson (ibid) may beassociated with a topaz bearing satellite body of thenearby Late Cretaceous Surprise Lake batholith. Thegeologic settings are guesses by the writer as truelocation is proprietary info. Smaller aquamarinegemstones have also been extracted from pegmatiteswhere the beryl is in intergranular texture with otherminerals (e.g. the Eocene pegmatites in the HorseranchRange).Prospectors have found new showings in the matureexploration ground of the Kootenay region ofsoutheastern B.C. This includes the Laib Creek, BlueHammer and Topaz Creek showings (Brown, 2004).Most crystals are translucent but occasionally atransparent gem quality crystal is found:(http://www.creamminerals.com/cream/cma aquamarine details.htm)Wilson (19997) provides cursory documentation ofthe single vanadium emerald occurrence. It is describedas occurring in narrow quartz-calcite-pyrite veinscutting volcaniclastic rocks adjacent to the quartzmonzonite intrusive on Red Mountain near Stewart.Rb-Ba-Sr relations derived from whole-rockgeochemical data points to beryl-bearing plutons in B.C.Values near the Rb apex in triangular plots suggestplutons that are more evolved and fractionated. Plutonswith high Rb/Sr or Rb/Ba ratios include Logtung andStorie in B.C. and Seagull, Pluto in the Yukon (seefigures 8,14 in Sinclair, 1983). All these plutons haveassociated beryl showings. Data from HorseranchRange pegmatites (Simandl, 1998) also follow the Rbrich trend. Such triangular plots may assist in choosingplutons of interest in B.C.Cretaceous to Eocene ages of intrusion is typicalbut the Hellroaring and Greenland Creek suites in theCranbrook area represent pegmatite of Proterozoic age.There is little info on pegmatites of other ages.Known beryl-bearing pegmatites have beententatively placed in the LCT class under Minfile. TheHellroaring pegmatite is probably a good example of theberyl-columbite subtype of the LCT class (Brown,2004). Beryl bearing NYF pegmatites have not beenformally documented in B.C. though granites parental tothat type are known.It appears B.C. has a lack of lithium-richpegmatites that give rise to polychrome beryl (seePezzotta and Williams, 2001; Sinkankas and Read,1986). Mt. Begbie (Minfile 082LNE015) is the onlydocumented locale of lithium bearing pegmatite in B.C.However intrusives with elevated trace amounts oflithium suggest potential for this type of pegmatite.They include Logtung at 180 ppm (Kirkham andSinclair, 1985, table 2).Figure 1. Faceted crystals of aquamarine from undisclosedlocale near Bennet B.C. (NTS 104M14). Photo is courtesy ofBrad Wilson.To date most beryl gemstone has been extractedfrom miarolitic cavities within pegmatites or epizonalGEOFILE 2005-166
EMERALD DEPOSIT MODELSIn general the geologic settings for emerald can becategorized by how the incompatible elementsberyllium and chrome (or vanadium) are broughttogether (Schwarz and Guilani, 2001). The most famous(and probab
Mining and Minerals Division Geological Survey Branch GEOFILE 2005-19 By Andrew S. Legun, P. Geo. 1 GEOFILE 2005-16. POTENTIAL FOR GEM BERYL AND SCHIST HOSTED EMERALD IN . Aquamarine from Slocan Area . 3 GEOFILE 2005-16. INTRODUCTION This article reviews the gem varieties of beryl, the
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