Iodargvrite As an Indigator of Arid Climatic Conditions and Its Association with Gold-Bearing Glacial Tills of the Chibougamau

The Canadian Mineralo gist Vol.35, pp.8-34Q997)

IODARGVRITEAS AN INDIGATOROF ARID CLIMATIC CONDITIONS AND ITS ASSOCIATIONWITH GOLD-BEARINGGLACIAL TILLS OFTHE CHIBOUGAMAU - CHAPAISAREA" OUEBEC*

DANIELR. BOYLE1

Geological Survey of Canada, 601 Booth Street, Ott6wq, Ontario KIA 0E8

ABsrRAgr

The Ag halide mineral, iodargyrite, togetherwith associatedminerals chlorargyrite,bromargyrite, and "embolite", form a group ofsecondaryminerals thal in terrigenoussuprgene environmen8worldwide, areexclusively confned to oxidizedsulfide zonesformed in arid and semi-midclimates. The presenceof iodargyritein gold-bearingglacial sedimentsof the Chibougamau - Chapaisarea of Quebec(first recordedoccurrence of this mineral in Canada),together with ofher indicatorsof an arid climate, provides cogentevidence of a past arid climate in this region. Theseindicators include: a) the existenceof a deepwater table during sulfide ore oxidation of Chibougamaucopper deposits, b) the abundanceof "limonite dice' in glacial tils, indicative of the pseudomorphicreplacement of pyrite grains in bedrockformations under arid conditions,c) the abundanceof spongyand corrodedgrains of gold typical of lateritic terranes,and d) the abundanceof possibly desert-derivedquartzofeldspathic sand formations. The period of aridity is conjectural at this time owing to lack of definitive dating media but the nature and preservationof weatheredprofiles in the region strongly suggesta period of aridity in the Tertiary. Arid climatic conditionsin this region may coincide with a known global warrning period in late Eoceneto middle Oligocenetimes, when the Earth experiencedsome of its warmesttemperatues. However, possiblePliocene arid conditionspreceding ouset of glacial events cannotbe ruled out.

Keywords: iodarryrite, Ag hatides,climatic mineral indicators,arid climate,gold" weathering,heavy minsrals, lsltiary, glacial tills, Chibougamal"Quebec.

Somtans

Le halog6nurede Ag, I'iodargyrite, ainsi que les mineraux qui lui sont associ6s,comme cblorargyrite, bromargyrite et "embolite", constituentun groupede phasessecondaires qui sont,dans les milieux supergdnespartout au monde,exclusivement retrouv6sdans les zonesoxyd6es d'amas de sulfuressujets d un climat aride ou semi-aride.La prdsenced'iodargpite dansles s6dimentsaurifdres provenant des glaciers dar:sla r6gion de Chibougamau- Chapais,au Qudbec(e premier exemplede ce min6ral connuau Canada),consid6r6e i la lumibre d'autresindicateurs d'un climat aride,fournit une indication concluantede la pr6sence climat aridedans cette r6gion parle pass6.Parmi ces indicatews, on trouvea) l'existencede nappesdleauqlofondes d'un *d6s iors de f oxytlationdu minerai sulfir€ desgisements de cuiwe de la r6gion de Chibougamau,b) l'abondancede de limonite" dansles tilis glaciaires,indicatifs du renplacementpar pseudomorphosede grains de pyrite dansles formations du socle en milieux arides,c) l'abondancede particulesd'or spongieuseset corrod6es,typiques de sdquenceslatdritiques' et d) la pr6sence d'amas de sable quartzofeldspathique,possiblement d'origine d6sertique.11 n'y a encoreaucune donn6e radiom6trique apte d prfciser la p6rioded'addit6, faute de mat6riauxdatables, mais la natureet le degr6de prdservationdes profils m6t6oris6sde la rdgionfontloupgonner le Tertiaire.l,es conditionsclimatiques arides dans cette r6gion pourraientcoincider avec une p€riodede r6chauffementd f6chelle du globe ven la fin de I'Eocine jusqu'au milieu de lOligoclne, d une dpoqueof la Tene a subi les climats les plus temp6ds. Il est toutefois possibleque c'est au Pliocine que cettep6riode aride a eu lieu, commepr6lude arx stadesde glaciation. (Traduit par la R6daction)

Mots-clds: iodargyrite,halog6nures d'Ag, indicateursclimatiques, ctimat aride,or, mdt6orisation,min6raux lourds, Tertiaire, tills glaciaires,Chibougamau, Qu6bec.

INrRoDUcnoN also occur; ....it seems probable that their abundance can be traced to the effect of the peculiar climatic ooOvera large part of the arid region of the wes! conditions which have prevailed in that region in late lying between the Rocky Mountains and the Sierra geologic times." Nevada ores containing chloride of silver (cermgyrite) are abundant and sometimes the bromides and iodides Penrose (1894' p' 31'4)

1 E-mail address: dboyle@ gsc.nrcan.gc.ca r' GeologicalSurvey of Canadaconuibution number 199U47. 24

"The silver halides include chlorargyrite (cerar- indicatethat gossandeposits in the Bathurstbase-metal gyrite), bromargyrite (bromyrite), miersite, and iodar- metallogenic province of northern New Brunswick gyrite (iodyrite). They are comparatively common formed in a predominantly warm, humid, temperate supergeneminerah in arid regions or where aridity climate during the Pliocene. Saprolite profiles over prevailedin the past." Grenville rocks along the north shore of the Boyle(1968, p. 26) St. l,awrenceRiver Q,asadleet al. 1983,deKimp, et al. 1985), saprotte and laterite soil profiles over rocks of The useof mineralsas indicators of pastclimales has cental Gasp6region @avid & B6dard 1986, Cloutier seena constantincrease in applicationsince the turn of & Corbeil 1986), and Tertiary gold-bearing placer the century.In this respect,certain minerals, as charac- depositsin the EasternTownships region of Quebec teized by their physical form, chemical composition, (sbilts & smirtr 1986,smith & shilts 1987)also signis or relative abundance"can be usedto describeclimatic the existenceof a preglacial hotter and more humid consffaintsat the time of their formation. The present climate during the Tetiary than exists in theseregions work attemptsto showthat a particulargroupof chemi- today. In the Arctic platrorm region of Canada,exten- cally precipitatedminerals, namely the silver halides, sive researchon the late Tertiary sedimentaryrocks, and in particular iodargydte (Agf), can be used,along notably by Wolfe & Poore (1982), Wolfe (1985), and with other climatic indicators,to confirm the existence Mafihews& Ovenden(1990), indicates that the Tefiiary of past axidity in certain geological and geographical forests of this region formed in a warm temperate environments.Recently, the authorhas discovered con- climate similar to that of soutlern Canadatoday. centations ofiodargyrite assosiatedwith large concen- Placing the above-mentionedTertiary climatic regions tations of gold grainsand otherproducts of weathering in today's climatic context,the EastemMaritimes and in glacial tills in the Chibougamau- Chapaisregion of St. LawrenceLowlands regions would be locatedin the north-centralQuebec. This is the first recordedoccur- souttreasternto south-cenfralportion of the United rence of rhis mineral in Canadabut more importantly, States,and the Arctic pla.dormwould be locatedin the it is a "market'' of a period of preglacial aridity that Oregon to south-centralBritish Columbia region. In probably persistedfor some time in the Tertiary in betweenthese two regionsare areas that experiencehot this region. temperate, alpine (including arid alpine), cool temperate, and semi-arid to arid climates.Unfortunately, REvrEwoFPRBvrous WoRK becauseof extensive glaciation, geoscientistsknow very little about the Tertiary climatic conditions that Mineral climatis indicatorscan include the physical prevailed in the large region that extends from the form and relative abundanceof quartz grains in Maritimesto the high Arctic. This region is muchlarger sediments(Dat 1968,D'Orsay & van de Poll 1985), in size than the entire United States,and we can sur- feldspar stabilities and abundances(Todd 1968), miseotherefore, that tle variety of climatesthat prevail clay:quartz ratios @onatti & Gartner 1973), types, over the U.S. subcontinenttoday also occurredover the chemicalcompositions, and abundancesof clay min- Canadianlandmass during the Tertiary. erals (Singer 1984, Mnota & van Reeuwijk 1989, Chamley 1990),relative abundancesofheavy minerals REVmwoF OCCI'RRENCESOFSILVER Hal.uss in recentand ancientsediments (Kurze & Roth 1977), AND THEXR CINVIETTC SE"TItr.TGS specific types of carbonateaccumulations in soils and sediments(Quigley & Dreimanis 1966), the relative Many economicgeologists have recognized the rela- abundances of chemically precipitated minerals tionshipbetween the formationof Ag halideminerals in (Usdowski & Knoke 1970, Chafetz 1980, Semeniuk oxidized sulfide deposits and periods of aridity. In 1986),and the use of isotoperatios in minerals,partic- addition to the quoted referencesat the beginning of ulady those of oxygen and carbon, to map climatic this paper, the confinementof Ag halide minerals to trends (Bowen 1966, Stuiver 1970, Savin 1991, arid regions of the world has been noted by Burgess Winograd et al. 1992). (1911,p. 13),Emmons (L917, p.255-3M), Lindgren For Canadiangeological environments,elucidation (1933,p. 860-868),and Guilbert & Park(1986, p. 89). of Cenozoic,and in particular,Tertiary climatic condi- Some of these investigators noted the presenceof tions is obscuredby thick glacial depositsand the lack chlorargyritein regions with slightly moister climates of complete Cenozoic continental stratigraphic than semi-aridconditions. but occurrencesof the much sections.Nevertheless, partiat stratigraphicsections or less soluble bromide, iodide, and bromide-iodide weatheredprofiles havebeen used to describepossible minerals are almost exclusively confined to arid and preglacialclimatic eventsoverthe Shield Appalachian, semi-aridregions. and Arctic platrorm regions.Boyle (1995) and Symons A surveyofthe relative proportionsofhalide miner- et al. (1996) have usedpaleomagnetic surveys and the als in metallogenicprovinces shows that the bromide absence of certain minerals that occur in massive and iodide minerals are concentratedin deposits sulfide gossansformed in semi-aridto arid climatesto resfficted to extremely arid environments fe.g,, the IODARGYTE IN GOLD.BEARING TILIS. CHIBOUGAMEAU 25

Sleeper(Saunders 1993), Tonopah @urgess 1911), and contains the full complementof Ag halide minerals, Wonder@urgess 1917, Young 1918,Lindgren 1918) and is especiallyrich in iodargyrite;the Murray Brook depositsin the Nevadadesert; the Dusty Mac deposit oxide zoneis devoid of any Ag halideminerals or other (Western Miner 1981) and Exposed Treasure mine secondaryminerals (carbonates,silicates) commonly @mmons l9I7) n the Mohave desert,California, the associatedwif.h dry climates. Lake Valley deposits (Genth & von Rath 1885) of Although there are Ag halidesin oxidized zonesof New Mexico, the Chanarcillo (Whitehead 1919) and polymetallic deposits in the Harz and Erzgebirge Caracoles@oyle 1968)mining disticts in the Atacama regions of Germany (Maqua 1983, von Hoppe & desert, Chile, the Djelambet, Dzhezkazgan, ala.d Damaschun1986) and the Krusne Hory region of the Maikain deposits (Chukhrov 1940) in central Czech Republic @oyle 1968), their occurrencein Kazalfistandesertregion, the Gai pyrite depositsof the metallogenicprovinces of cenffal Europe is very rare. southernUrals, Russia (Sergeevet aL 1994), and the Oxidized sulfide zonesin Germanyand CzechRepu- Broken Hill (Mason 1976), Teutonic Bore (Nickel blic are most likely preglacial in origin. Studies of 1984), and Cobar @ayner 1969) depositsin the arid Tertiary regoliths and sedimentsin this region (e.9., interior of Ausfralial. Most of thesehave been devel- along the Rhine and Loire valleys) indicate a general oped primarily as bonanza secondary Ag halide - overall subtropical climate, with distinct periods of nativeAg deposits,the primary oresgenerally being of aridity in tle Eocene and Pliocene (Wang 1951, much lessereconomic imfortance. Surroundingthese Schwarzbach1968" Staeblein 1972). Given the exfreme exfremelydry regions, oxidized ore depositsin semi- diversificationin climatic zones(tropical to desert)that arid fringes generally contain mostly cblorargyrite, can occur over a landmasslargely characterizedas with few @currencesof Ag bromide-iodide minerals. subnopical(e.g., central Africa today; Tertiary cenfral In conftast fortemperateclimatic areassunoundingAg Europe), it is highly probable tlat arid microclimatic halide provincesosuch as the Great Basin region of regions existedlocally over central Europe during the North America the oxidized sulfide depositsgenerally Tertiary. One such region in Europe that is arid even containonly native silver, Ag sulfosalts,and secondary today is the silver deposit region of Hiendelaencina, argentite as the Ag ore minerals (Emmons 1917, Spain(Calvo & Sevillano L992).Here,the fuIl comple- 'oplata Lindgren 1918,Guilbert & Park 1986).This is alsotrue ment of Ag halide minerals (chlorargyrite: of mineral depositsin small alpine regions within the comea - horn silvet''; bromargyrite: "plata verde - arid Great Basin @mmons 1917), where relativd green silvet''; and iodargyrite) occur with many other moist alpine climateshave existed. secondaryminerals, mostly Ag-As-SH and Cu-S The Ag halide minerals,mainly chlorargyrite,form compounds.The relative abundanceof the Ag halide in high-altituclearid regions such as the Potosi Sn-Ag minerals at Hiendelaencinadecreases significantly in metallogenic province of Bolivia and the Tintic the order presented above, indicating that extreme polymetallic province of Utah @mmons1917). periods of aridity, such as occurredin parts of cental Further evidenceof the resriction of the Ag halide U.S. and the AtacamaDesert region of Chile, where mineralsto arid regionscan be gleanedfrom examina- iodargyriteis relatively more abundant,did not occurin tion of oxidized sulfide zoneslocated in regionswhere central Spain during oxidation of the Hiendelaencina Tertiary clinates are known or suspectedof being deposits. lempe,rateor Oopical,with varying degreesof rainfall. In the deep-seamarine environment"chlorargyrite, Supergenedeposits formed over sulfide deposits in associatedwith atacamite and complex Cu-Pb-Ag temperateregions are largely devoid of Ag halide min- sulfosalts,has been discovered in the oxidizedportions erals,most certainlyso in topical climates.Thus, in the of black smoker sulfide deposits on the Axial Sea rich Ag-Pb veins of Keno Hill - GalenaHill, Yukon, Mount and Explorer Ridge (. Jonasson,pers. cornm.; where oxidation exceedsdepths of 100 metersand the Hannington1993). Iodyargyrite has not yet beenfound late Tertiary climatewas largely temperateClarnocai & in this environment.The solutionsin thesedeposits are Schweger1991), many secondaryminerals of Ag, Pb, probably supersaturatedwith respectto Ag andCl (and As, and Sb are developed but no Ag halides @oyle possibly Br and D, and formation of Ag halidescan be 1965).The TeutonicBore gossandeposit in the Cenral expected.It is higttly unlikely that theseminerals would Arid Basin of Austalia (Nickel 1984) and the Murray survive later diageneticprocesses, since primary Ag Brook depositin the Bathurstregionof New Brunswick halide minerats have never been found in sulfide (Boyle 1995) are both massivesulfide Cu-Pb--Zn-Ag deposits. deposits with very similar primary mineralogy and geology, but their oxide zones have formed under FoRMATToNAND HABn oF Srvm. HALDEMINmers contrasting climatic conditions. The Murray Brook oxide zonefomed underwarm, moi$t tgmpsralg condi- Silver forms tbree end-memberminerals with tne tions (Symonset aL 1996, Boyle 1995), whereasthe halides Cl, Br, and L chlorargyrite (AgCl, formerly TeutonicBore depositformed under a prolongedperiod cerargyrite),bromargyrite (AgBr, formerly bromyrite), of aridity (Quilty 1984). The Teutonic Bore deposit and iodargyrite (AgI, formerly iodyrite). Thereis no F 26 TIIE CANADIAN MINERALOGIST

equivalent, since AgF is extremely soluble. There the rela.tionshipbetween sources of the halidesand the would appearto be complete solid-solution between precipitating environment. Penrose (1894), Burgess chlorargyrite and bromargyrite, and limited solid- (lgLI, 7917),21dF.mmons (1917) have noted the close solution between chlorargyrite and iodargyrite, and proximity betweenthe Ag halideoxide depositsand the betweenbromargyrite and iodargyrite, resulting in a formation through time of salt lakesin regionsof New confrrsing array of minerals under the generic name Mexico, Nevada Utab, and Aizona. Theseareas were "embolite' with prefixeschlor-, iod-, andbrom-. Boyle once covered during the early Tertiary by numerous (1968)has indicated that the Ag halide group of miner- large bodiesof waterowhich graduallydried up, result- als shouldmore appropriatelybe given namessuch as ing in a large supply of halide ions to groundwater bromianiodian chlorargyrite,chlorian bromian bromar- oxidation systems. For the Chanarcillo Ag halide gyite, etc. deposits in the Atacama Desert, Chile, Whitehead 'Silver iodidecrysmllizes in both the hexagonal(iodar- (1919) suggesteda windborne source off the Paciflc gyrite: common) and isometric (mienite: very rare) Oceanfor the halides,with evaporativeconcenfration at systems.Miersite has been found only in the Broken the surface.hesumably thesesalts would move down Hill (Ausfralia) and Bisbee (Arizona) deposits,and is into the supergenezone during the few periods of metastablennless it containsa small proportion of Cu precipitationin this region to form concentratedalkali ions (Williams 1990). Three other non-Ag-bearing salt solutionscontaining dissolvedAg. The Ag halide halide minerals are commonly found with the Ag minerals associatedwith Austalian deposits such as halides,namely marshite,CuI, which forms a solid- Broken Hill, TeutonicBore, and Cobaralso are located solution series vrith miersite, nantokite, CuCl, and in areasof extensivedevelopment of salinebasins and calomel,HgCl. Like the Ag halides,all threeare found a long period ofTertiary aridity. only in oxidized zonesformed in arid climates. The approximatesolubilities of the Ag halide miner- occuRRm:.TcEoF Iooancynnt nt TLLS oF THB als are asfollows: AgCl (8.9 x LF gtL),AgBr (8.4 x CmOUGAMAU _ CHAPAIS REGION 10-t CIL), and AgI (2.8 x 10{ glt.). Despite these observedsolubilities (in deionizedwater), the order of Iodargyritehas been found in heavy-mineralconcen- precipitation of the Ag halide minerals, as deduced trates collected during a till exploration program from various zoning relationshipsin deposits@urgess covering a large shear-zonesfructure south of Chapais, LgLl, 1.977,Whitehead 1919, Lindgren 1933),is Quebec(Fig. 1). Till cover in this region is quite thick revetsed (AgCl > AgBr > Agt). Thre€ main factors (up to 40 m), and no in situ remnantsof the preglacial account for this unexpectedreversal: a) chloride is weatheredzone have been found. Three main areas much more abundantthan Br and I, and the solubility where the mineral is concenffaledwithin the survey of AgCl increasesmore rapidly thanAgBr andAgI with region havebeen outlined (areasl-3, Fig. l). Approxi- increasing temperature;thus after initial infilradon, mately 1200 heavy-mineral sampleshave been warm, near-surfacewaters will becomesaturated with obtainedfrom the tills of this areausing lnfling, super AgCl, and with further descent, chlorargyrite will panning, and heavy liquid separation.Portions of half precipitateowing to lowering of groundwatertempera- of theseheavy mineral separates,distributed over tle tures and oversaturation,b) both AgBr and AgI are entire are4 weremounted on stubsand examinedusing more solublein alkali chloride solutions,thus requiring secondaryand back-scattertechniques with a scanning more Br andI to entersolution to reachsolubility limits electronmicroscope (SEM). Approximarelyll%o of the for bromargyriteand iodargyrite @mmons1917), and samplescontain iodargyrite. Owing to the small grain- c) the ferric ion exertsa much stonger oxidizing effect size of the iodargyrite (<10 pm) and its friable naturg on Br and f (with formation of Br2 and I) than Cl-, a$emptsto obtain polishedmounts in epoxy for quan- thus lowering the concentrationof bromide and iodide titative chemicalanalysis were unsuccgssfirl.However, ions that cancomplex with Ag (Knopf 1918). slow counting of flat crystal faces of some the larger A distinct vertical zonationof Ag halideminerals has ioda$nite grains allowed reasonablesemiquantitative beennoted for thosedeposits that havebeen studied in analyseswith regardto Cl and Br cont€nts. detail. For the Tonopah deposit in Nevada @urgess Iodargyrite occursas: a) single and aggregalecrys- 1911)and the Chanarcillosilver campof Chile (White- tals on surfacesand in voids of gold grains@g. 2a), b) head 1919), chlorargyrite is located near the surface, encrustationson gold grains @ig. 2b), allrdrarely, on and gives way with depthto bromargyriteand, finally, limonite grains, and c) individual loose grains within to iodargyrite. Transitional "embolite"-Epe minerals the heavy-mineralconcenhates. The gold grains with [Ag(Cl,BrJ)] form tbroughoutthe oxidized zones.The which the mineral is associatedare commonly'oamoe- Ag halideminerals can persistin oxidizedsulfide zones boid" in fom" typical of hydrosolgrains of gold formed to well over 500 metersin depth. in laterites or gossansover auriferous mineralization Perhapsthe most cogentindicator linking the forma- (Fig. 3a). The mineral may be presentas rosetteson tion of Ag halide minemls to arid regions that have highly conoded grains ofgold (Fig. 3b), but generally experiencedhigh mJssofevaporation and salinationis it is presentas dispersedindividual hexagonalcrystals IODARGYTE IN COLD-BBARING TILI.S. CHIBOUGAMEAU 27 aftachedto (Fig. 3c) and "impregnating"gold @g. 3d). (Fig. 3c). Goethitecommonly forms as crustson crys- Crystals of secondarygold are occasionallyfound talsandencrustationsofiodargydte.Loosecrystalsof precipitated on individual grains of iodargyrite iodargyrite in the concentratesgenerally display a

49'20' 49"20', 75'20' 74"35'

T--:-- Masslve and foliated Geological boundary . . . /'-\---/ l"l granitic intrusive Fault. . . Mafic, ultnmaficand Mineraloccunence, ...... cul tI ultabasic meta-intrusive E Metasedlment E Felsb metavolcanic Maflc and lntermediate E metavolcanic

Frc. l. Location andgeology of till-prospectingsurvey in the Chibougamau- Chapaisarea of centralQuebec. Locations 1, 2 and 3 re,?resentsit€s in the surveyarea where iodargyrite has been found, associafedwith gold in dlls. 28

Ftc. 2. Two main forms of iodargyrite associatedwith gold grains in tills of the Chibougamau- Chapaisregion: A) single aa4 aggregatecrystals on surfacesand in voids of gold grains,B) encrustations(aggregates of fine crystals)of iodmgyrite (I) on gold grainswith associatedgoethite (G) and clay (Cl) minerals.Scales: I cm = 4 U,min Ao 6 gm in B.

hexagonaltabular habit with the pronouncedhemimor- may be relatedto the depth of glacial erosion and the phism characteristicof this mineral (Kraus & Cook fact that iodargyrite is generally found at the deepest 1909).Individual crystalsrarely exceed10 pm in size. levels of the oxidation zone. The upper zones wore Chemically, all of the grains examineddisplay an probably subjectedto cyclical advancesofice; material almost pure AgI composition on energy-dispersion comprising them may have been diluted and nans- specta. It is possible,however, tlat Ag(I,Br) minerals ported farther to the south.Many of the heavy-mineral may be present,since fine till fractionsrich in gold are samplesfrom this areacontain large concentrationsof also anomalousin bromine. euhedral grains of pyrite, suggestingglacial erosion In additionto gold, ofher secondaryminerals nssoci- down to at least the supergenesulfide zone of the ated q/ith iodargyrite include kaolinite, massive to weatheredprofile. Allard & Cimon (1974)have argued vesiculargoethite, and'limonite dice" (pseudomorphs for minimal glacial erosion in the Chibougamau- after pyrite). The primary heavy minerals associated Chapaisregio4 the preservationoflarge concentations with bedrockmineralization include euhedralto subhe- of secondaryminerals in dlls, and other features of dral pyrile an4 more rarely, sideriteand ankerite.In a weatheringmentioned below, supporttheir hypothesis. few cases,iodargyrit€ was found attachedto grains of pyrite. Concenhationsof pyrite grainsin the tills repre- EVIDB{CE FoR PREGI.A.CIAL ARDTTY INTTIE sent downcuttingof the supergene sutfide zone of the Cnrnoucauau - CHApArsRsdon oF QUEBEC Tertiary oxidation profile in this region. In other oxidized deposits,such as Chanarcillo and Tonopah, iodaxgyritreis more concenftatedin the lower oxide and A compilationof preglacialweathering profiles that underlying supergenesulfide zones,where it is present have been preservedin the Quebechecambrian and as crystal massesand coatingson supergenesulfides, Quebec - Northern Maritime Appalachian regions mainly pyrite and Ag sulfosalts. showstbree predominant areas where Tertiary climatic Chlorargyrite was not identified in any of the conditionscan be studied:a) the LabradorTrough area samplesof the Chibougamau- Chapaisstudy area,and (A, Fig. 4), b) the northernAppalachian - St. Lawrence the presenceof bromargy'iteis only suspectedfrom the Riverregion@,Frg.4), andc) theChibougamauto Val anomalouslevels of bromine in t'lls associatedwith d'Or region (Abitibi Subprovince), stretching into high concentrationsof gold. Absenceof chlorargyrite Ontario(C, Fig.4). IODARGYTE IN COLD-BEARING TILTS. CHIBOUGAMEAU 29

Ftc.3. Crystalhabits andrelationship of iodmgyriteto bothprimary and secondaryforms of gold. A) Iodargyritecrystals growing on "amoeboid' gold typical ofhydrosol gold that forms in lateritesor gossansover auriferousmineralization. The grain has beenabraded during glacial transpoft.Iodargyrite crystals on abradedsurfaces have been removed whereascrystals in voids within abradedareas (e.9., far left) remain.B) Rosetteof iodargyritecrystals on gold grain with associatedgoethite. Angular voids in gold are probably pyrite casts. C) Hexagonal crystal of iodaqyrite with secondarygrowth of gold (sAu). D) Iodargyritecrystal impregnatinggold grain; overlappinggold on crystal may be secondary.Scales: L cm = 20 Um in A, 10pm in B, 5 pm in C and 1.5pm. in D.

The LabradorTrough, with its rich iron-formations, tion) of the iron-formation in this region exceeds has undergoneweathering (silica leaching)and oxida- 200 metersin depth. The bodies of secondarylimon- tion from at least the Cretaceous(Dorf 1959, Gross ite-hematite display similarities to both tropically 1968).The zone of completeoxidation (and benefac- weatherediron-formation and arid weatheringprofiles 30 TIIE CANADIAN MINERALOGIST

QUEBEC

-r".")

Fro. 4. Map of Quebec and nortlem New Brunswick outlining the main areaswhere preglaciat\reathering profiles have been discovered.(A) LabradorTrougb region: 1. Schefferville, 2. Labrador Ctty; @) Northern Appalachians- Lower St. Lawrence region: 1. Mont Jacques-Cartierregion aud Gasp6Cu deposits.2. CMteau-Richerand Chadesbourgregions, 3. Beauce- ChaudidreRiver gold placer region; (C) Abitibi Subprovince:1. Icon-Sullivan Cu deposil 2-4. CopperRand Henderson,and otherCu sulfide deposits.5. Chibougamau- Chapaistill surveyarea (this study), 6. Iac Shortt rare-earth-enrichedlateritic carbonatite,7. SelbaieCu (chalcocite--covellite)deposit; @) massivesulfidegossans andBigBaldMountainarea, Batlu$tregion, NewBrunswick

such .!s the Tertiary Hammersley-typesecondary iron In the Chibougamzu- Val d'Or region,major Tertiary depositsin nortlwestem Ausfralia. weatheringprofiles arerepresented by: a) the Cu oxide In the Northern Appalachian- St. Lawrenceregion, zone over the Icon-Sullivan deposit, Lac Mistassini lateritic, saprolitic, and gossanousbedrock profiles (Troop & Darcy 1973),b) deeplywealhered (down to occur near Quebec City [saprolitesi Lasalle et aI. 500 m) CopperRand and Henderson ore depositsof the (1983),de Kimpe et aL (1985)1,Gasp6 Peninsula [oxi- Chibougamaumining camp, as well as other, smaller dized Gasp6Copper deposifi Ford (1959), saprolites: deposits(Panish 1968, Allmd & Cimon 1974, Allard Cloutier & Corbeil (1986)1,throughout the Bathurst 1976), c) lateritization of anorthosile in the Chibou- mining camp of New Brunswick [massivesulfide gamau area down to hundreds of meters (Allard & gossans:Boyle (1995), saprolit€ and grus formations: Cimon 1974, Allard L976), d) rare-earth-bearing Warg et al. (1981, 1982)1,and in the EastemTown- laterite and saprolite developedover the Lac Shorft shipsof Quebec[gold-bearing laterites: Shilts & Smit] carbonatite(Quirion 1989), an4 d) the secondary (1986),Smith & Shilts(1987)1. covellite--chalcocitezone at tle Selbaiecopper deposit IODARGYTE IN GOLD.BEARING TILI,S, CHIBOUGAMEAU 3l

(Sinclair& Gasparini 1980,Bouillon 1990).Similar Fe basis),some introduction of Fe is requiredfor final environmentsof deep oxidation in Ontario and the formation. Limonite dice are abundantin oxide zones northeasternUnited States have been described by of arid regions (Blanchard 1968). The mineralLed Moore (1938). In one of theseareas, the Cobalt silver bedrock underlying the iodargyrite-bearingtills in the camp,oxidation in the Keeley mine is presentdown to Chibougamau - Chapais region contains abundant at least200 meters.Unfortunately, no detailedminera- cubesof pyrite in metasediments.If topical or temper- logical studieswere carriedout on this depositto deter- ate conditions prevailed, these formations would mine presenceor absenceof Ag halide minerals.The simply leave"pyrite casts"in the weatheredrockowing deepoxidation, however, does signify the presenceof a to rapid dissolution and mobilify of ferrous and ferric very deepwater table. ions. Tertiary profiles in the Northern Appalachians- SL Lawrenceregion suggestthat a subtopical oosavanna- DtscussroN type" climate prevailed over this region. The gold- bearing laterites and derived placers are strong On the basisof its confinedworld-wide disribution indicators of this type of climate. Tertiary profiles at in oxidized sulfide depositsformed in arid regions,the someof the sitesin the Chibougamau- Val d'Or region presenceof iodargyrite in glacial sediments of the aremore indicativeof a much drier climale than existed Chibougamau- Chapaisarea of Quebec,together with farther to the south, in the St. Lawrence Lowlands. other indicators of an arid climale, provides cogent Depthsof oxidationat the CopperRand and Henderson evidenceof a past arid climate in this region. These deposits(5@ m or more) suggestthat the waler table in indicatorsinclude: a) the existenceof a deepwater table this region was very deep. Weathering of sulfide during oxidation of sulfide ore in the Chibougamau orebodiesto these depths has only been recorded in copperdeposits, b) the abundanceof "limonite dice" in very dry regions, such as the Great Basin area of the glacial tills, indicative of the pseudomorphicreplace- westemUnited States@urgess 1911, Emmons 1917, ment of pyrite cubesin bedrockformations under arid Lindgren 1933). The very deep weathering of ttre conditions, c) the abundanceof spongy and corroded Copper Rand and Hendersondeposits would suggest grains of gold typicat of lateritic terranes,and d) the that the period of aridity over which they formed abundanceof possibly desert-derivedquartzofeldspa- persistedfor sometime. Theseoxidized zonesare very thic sandformations. porous,indicating that a considerableamount of mate- The exact timing of Tertiary aridity for the Chibou- rial has been carried downwardin solution, eventually gamau- Chapaisregion is highly conjectural,given the exiting the systemin deep groundwaters.There is no lack of suitable preglacial material for dating. It is tectonic distirbance of the oxide mineral fabrics in known that on a world scale,the late Eoceneto middle thesedeposits, suggesting that they probably formed in Oligocenewas characterizedby some of the warmest Tertiary or Cretaceoustimes. global temperaturesobserved (Shackleton 1978). If the Furtherevidence of a period of aridity in tle Chibou- iodargyrite was formed at this time, either aridity per- gamau- Chapaisregion during the Tertiary lies in an sistedtbroughout much of the late Tertiary, or elsethe explanationfor the greatexpanse and thicknessof fine weatheringprofile was protectedby overlying deposi- quartzofeldspathicsaads efthis are4 which stetch as tion of post-Oligocenesediments thai werelater removed far west as the Kapuskasingarea in Ontario.Although by erosion. During the Pliocene, the climate in the reworked by glacial processes,these sandsmay have CanadianArctic was largely temperate.However, be- been derived from windblown desert materials or causeof the lack of suitablefossil materials,very little matrix-weatheredzones in semi-aridlateritic terranes. is known aboutPliocene climale ftends over the south- Corestonescharacteristic of the lower weatheringzones ern CanadianShield. Studiesof coresfrom the coastal of lateritesare commonlyfound in exposedsections of Atlantic Ocean produce general clim*ic trends of a till within the region. In lateritic and saproliticterranes, semiglobalnature, but thesecan hardly be extrapolated the maffix, or exfoliated products of weathering, of thousandsof kilometersinland to predict regional mi- tlese corestone horizons usually consist of quartz croclimatic trends. It is conceivable,therefore, that grainsand clays. aridity in the Chibougamau- Chapaisarea may have The two main mineralogicalindicators of aridity in prevailedin Pliocenetimes. the Chibougamau- Chapaisregion me the presenceof iodargyrite preservedin tills discussedabove and the ACKNOWI,EDGEMENTS common@currence of 'limonite dic€" in heavy-mineral concentrates.To form pseudomorphsafter pyrite Detailed 5gl\d slaminations of the Chibougamau - requiresa very low water-to-rockratio during oxidation Chapais heavy-mineral concentrates were performed to compensatefor a reduction in unit-cell dimensions with the he$ of David Walker, Mineral Resources (pyite versaslimonite) and to maxim2e the retention Division, Geological Survey of Canada This work of Fe. In addition, since the molar volume of pyrite is forms part of a broader geochemical study of till being 239 cfr comparedto 20.8 cm3for goethite (constant carried out in this region by the author and Marcel 32 TIIE CANADIAN MINERALOGIST

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