23

The C anadian Mineralo gistVol.35, pp.8-34Q997)

IODARGVRITE AS AN INDIGATOR OF ARID CLIMATIC CONDITIONSAND ITS ASSOCIATION WITH GOLD-BEARING GLACIAL TILLS

OF THE CHIBOUGAMAU - CHAPAIS AREA" OUEBEC*

DANIELR. BOYLE1

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

ABsrRAgr

The Ag halide mineral, iodargyrite, together with associated minerals chlorargyrite, bromargyrite, and "embolite", form agroup ofsecondary minerals thal in terrigenous suprgene environmen8 worldwide, are exclusively confned to oxidized sulfidezones formed in arid and semi-mid climates. The presence of iodargyrite in gold-bearing glacial sediments of the Chibougamau- Chapais area of Quebec (first recorded occurrence of this mineral in Canada), together with ofher indicators of an arid climate,provides cogent evidence of a past arid climate in this region. These indicators include: a) the existence of a deep water tableduring sulfide ore oxidation of Chibougamau copper deposits, b) the abundance of "limonite dice' in glacial tils, indicative ofthe pseudomorphic replacement of pyrite grains in bedrock formations under arid conditions, c) the abundance of spongy andcorroded grains of gold typical of lateritic terranes, and d) the abundance of possibly desert-derived quartzofeldspathic sandformations. The period of aridity is conjectural at this time owing to lack of definitive dating media but the nature andpreservation of weathered profiles in the region strongly suggest a period of aridity in the Tertiary. Arid climatic conditions inthis region may coincide with a known global warrning period in late Eocene to middle Oligocene times, when the Earthexperienced some of its warmest temperatues. However, possible Pliocene arid conditions preceding ouset of glacial eventscannot be ruled out.

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

Somtans

Le halog6nure de Ag, I'iodargyrite, ainsi que les mineraux qui lui sont associ6s, comme cblorargyrite, bromargyrite et"embolite", constituent un groupe de phases secondaires qui sont, dans les milieux supergdnes partout au monde, exclusivementretrouv6s dans les zones oxyd6es d'amas de sulfures sujets d un climat aride ou semi-aride. La prdsence d'iodargpite dans less6diments aurifdres provenant des glaciers dar:s la r6gion de Chibougamau - Chapais, au Qudbec (e premier exemple de cemin6ral connu au Canada), consid6r6e i la lumibre d'autres indicateurs d'un climat aride, fournit une indication concluante de lapr6sence d'un climat aride dans cette r6gion parle pass6. Parmi ces indicatews, on trouve a) l'existence de nappes dleauqlofondesiors de f oxytlation du minerai sulfir€ des gisements de cuiwe de la r6gion de Chibougamau, b) l'abondance de *d6s de limonite"dans les tilis glaciaires, indicatifs du renplacement par pseudomorphose de grains de pyrite dans les formations du socle enmilieux arides, c) l'abondance de particules d'or spongieuses et corrod6es, typiques de sdquences latdritiques' et d) la pr6senced'amas de sable quartzofeldspathique, possiblement d'origine d6sertique. 11 n'y a encore aucune donn6e radiom6trique apte dprfciser la p6riode d'addit6, faute de mat6riaux datables, mais la nature et le degr6 de prdservation des profils m6t6oris6s de lardgion fontloupgonner le Tertiaire. l,es conditions climatiques arides dans cette r6gion pourraient coincider avec une p€riode der6chauffement d f6chelle du globe ven la fin de I'Eocine jusqu'au milieu de lOligoclne, d une dpoque of la Tene a subi lesclimats les plus temp6ds. Il est toutefois possible que c'est au Pliocine que cette p6riode aride a eu lieu, comme pr6lude arxstades de glaciation.

(Traduit par la R6daction)

Mots-clds: iodargyrite, halog6nures d'Ag, indicateurs climatiques, ctimat aride, or, mdt6orisation, min6raux lourds, Tertiaire, tillsglaciaires, Chibougamau, Qu6bec.

INrRoDUcnoN also occur; ....it seems probable that their abundancecan be traced to the effect of the peculiar climatic

ooOver a 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' Geological Survey of Canada conuibution number 199U47.

24

"The silver halides include chlorargyrite (cerar-gyrite), bromargyrite (bromyrite), miersite, and iodar-gyrite (iodyrite). They are comparatively commonsupergene minerah in arid regions or where aridityprevailed in the past."

Boyle (1968, p. 26)

The use of minerals as indicators of past climales hasseen a constant increase in application since the turn ofthe century. In this respect, certain minerals, as charac-teized by their physical form, chemical composition,or relative abundance" can be used to describe climaticconsffaints at the time of their formation. The presentwork attempts to show that a particulargroup of chemi-cally precipitated minerals, namely the silver halides,and in particular iodargydte (Agf), can be used, alongwith other climatic indicators, to confirm the existenceof past axidity in certain geological and geographicalenvironments. Recently, the author has discovered con-centations ofiodargyrite assosiated with large concen-tations of gold grains and otherproducts of weatheringin glacial tills in the Chibougamau - Chapais region ofnorth-central Quebec. This is the first recorded occur-rence of rhis mineral in Canada but more importantly,it is a "market'' of a period of preglacial aridity thatprobably persisted for some time in the Tertiary inthis region.

REvrEw oFPRBvrous WoRK

Mineral climatis indicators can include the physicalform and relative abundance of quartz grains insediments (Dat 1968, D'Orsay & van de Poll 1985),feldspar stabilities and abundances (Todd 1968),clay:quartz ratios @onatti & Gartner 1973), types,chemical compositions, and abundances of clay min-erals (Singer 1984, Mnota & van Reeuwijk 1989,Chamley 1990), relative abundances ofheavy mineralsin recent and ancient sediments (Kurze & Roth 1977),specific types of carbonate accumulations in soils andsediments (Quigley & Dreimanis 1966), the relativeabundances of chemically precipitated minerals(Usdowski & Knoke 1970, Chafetz 1980, Semeniuk1986), and the use of isotope ratios in minerals, partic-ulady those of oxygen and carbon, to map climatictrends (Bowen 1966, Stuiver 1970, Savin 1991,Winograd et al. 1992).

For Canadian geological environments, elucidationof Cenozoic, and in particular, Tertiary climatic condi-tions is obscured by thick glacial deposits and the lackof complete Cenozoic continental stratigraphicsections. Nevertheless, partiat stratigraphic sections orweathered profiles have been used to describe possiblepreglacial climatic events overthe Shield Appalachian,and Arctic platrorm regions. Boyle (1995) and Symonset al. (1996) have used paleomagnetic surveys and theabsence of certain minerals that occur in massivesulfide gossans formed in semi-arid to arid climates to

indicate that gossan deposits in the Bathurst base-metalmetallogenic province of northern New Brunswickformed in a predominantly warm, humid, temperateclimate during the Pliocene. Saprolite profiles overGrenville rocks along the north shore of theSt. l,awrence River Q,asadle et al. 1983, deKimp, et al.1985), saprotte and laterite soil profiles over rocks ofcental Gasp6 region @avid & B6dard 1986, Cloutier& Corbeil 1986), and Tertiary gold-bearing placerdeposits in the Eastern Townships region of Quebec(sbilts & smirtr 1986, smith & shilts 1987) also signisthe existence of a preglacial hotter and more humidclimate during the Tetiary than exists in these regionstoday. In the Arctic platrorm region of Canada, exten-sive research on the late Tertiary sedimentary rocks,notably by Wolfe & Poore (1982), Wolfe (1985), andMafihews & Ovenden (1990), indicates that the Tefiiaryforests of this region formed in a warm temperateclimate similar to that of soutlern Canada today.Placing the above-mentioned Tertiary climatic regionsin today's climatic context, the Eastem Maritimes andSt. Lawrence Lowlands regions would be located in thesouttreastern to south-cenfral portion of the UnitedStates, and the Arctic pla.dorm would be located in theOregon to south-central British Columbia region. Inbetween these two regions are areas that experience hottemperate, alpine (including arid alpine), cool tem-perate, and semi-arid to arid climates. Unfortunately,because of extensive glaciation, geoscientists knowvery little about the Tertiary climatic conditions thatprevailed in the large region that extends from theMaritimes to the high Arctic. This region is much largerin size than the entire United States, and we can sur-miseo therefore, that tle variety of climates that prevailover the U.S. subcontinent today also occurred over theCanadian landmass during the Tertiary.

REVmw oF OCCI'RRENCES OFSILVER Hal.ussAND THEXR CINVIETTC SE"TItr.TGS

Many economic geologists have recognized the rela-tionship between the formation of Ag halide minerals inoxidized sulfide deposits and periods of aridity. Inaddition to the quoted references at the beginning ofthis paper, the confinement of Ag halide minerals toarid regions of the world has been noted by Burgess(1911, p. 13), Emmons (L917, p.255-3M), Lindgren(1933, p. 860-868), and Guilbert & Park (1986, p. 89).Some of these investigators noted the presence ofchlorargyrite in regions with slightly moister climatesthan semi-arid conditions. but occurrences of the muchless soluble bromide, iodide, and bromide-iodideminerals are almost exclusively confined to arid andsemi-arid regions.

A survey ofthe relative proportions ofhalide miner-als in metallogenic provinces shows that the bromideand iodide minerals are concentrated in depositsresfficted to extremely arid environments fe.g,, the

IODARGYTE IN GOLD.BEARING TILIS. CHIBOUGAMEAU 25

Sleeper (Saunders 1993), Tonopah @urgess 1911), andWonder @urgess 1917, Young 1918, Lindgren 1918)deposits in the Nevada desert; the Dusty Mac deposit(Western Miner 1981) and Exposed Treasure mine@mmons l9I7) n the Mohave desert, California, theLake Valley deposits (Genth & von Rath 1885) ofNew Mexico, the Chanarcillo (Whitehead 1919) andCaracoles @oyle 1968) mining disticts in the Atacamadesert, Chile, the Djelambet, Dzhezkazgan, ala.dMaikain deposits (Chukhrov 1940) in centralKazalfistan desertregion, the Gai pyrite deposits of thesouthern Urals, Russia (Sergeev et aL 1994), and theBroken Hill (Mason 1976), Teutonic Bore (Nickel1984), and Cobar @ayner 1969) deposits in the aridinterior of Ausfralial. Most of these have been devel-oped primarily as bonanza secondary Ag halide -native Ag deposits, the primary ores generally being ofmuch lesser economic imfortance. Surrounding theseexfremely dry regions, oxidized ore deposits in semi-arid fringes generally contain mostly cblorargyrite,with few @currences of Ag bromide-iodide minerals.In conftast fortemperate climatic areas sunoundingAghalide provinceso such as the Great Basin region ofNorth America the oxidized sulfide deposits generallycontain only native silver, Ag sulfosalts, and secondaryargentite as the Ag ore minerals (Emmons 1917,Lindgren 1918, Guilbert & Park 1986). This is also trueof mineral deposits in small alpine regions within thearid Great Basin @mmons 1917), where relativdmoist alpine climates have existed.

The Ag halide minerals, mainly chlorargyrite, formin high-altitucle arid regions such as the Potosi Sn-Agmetallogenic province of Bolivia and the Tinticpolymetallic province of Utah @mmons 1917).

Further evidence of the resriction of the Ag halideminerals to arid regions can be gleaned from examina-tion of oxidized sulfide zones located in regions whereTertiary clinates are known or suspected of beinglempe,rate or Oopical, with varying degrees of rainfall.Supergene deposits formed over sulfide deposits intemperate regions are largely devoid of Ag halide min-erals, most certainly so in topical climates. Thus, in therich Ag-Pb veins of Keno Hill - Galena Hill, Yukon,where oxidation exceeds depths of 100 meters and thelate Tertiary climate was largely temperate Clarnocai &Schweger 1991), many secondary minerals of Ag, Pb,As, and Sb are developed but no Ag halides @oyle1965). The Teutonic Bore gossan deposit in the CenralArid Basin of Austalia (Nickel 1984) and the MurrayBrook depositin the Bathurstregion of New Brunswick(Boyle 1995) are both massive sulfide Cu-Pb--Zn-Agdeposits with very similar primary mineralogy andgeology, but their oxide zones have formed undercontrasting climatic conditions. The Murray Brookoxide zone fomed under warm, moi$t tgmpsralg condi-tions (Symons et aL 1996, Boyle 1995), whereas theTeutonic Bore deposit formed under a prolongedperiodof aridity (Quilty 1984). The Teutonic Bore deposit

contains the full complement of Ag halide minerals,and is especially rich in iodargyrite; the Murray Brookoxide zone is devoid of any Ag halide minerals or othersecondary minerals (carbonates, silicates) commonlyassociated wif.h dry climates.

Although there are Ag halides in oxidized zones ofpolymetallic deposits in the Harz and Erzgebirgeregions of Germany (Maqua 1983, von Hoppe &Damaschun 1986) and the Krusne Hory region of theCzech Republic @oyle 1968), their occurrence inmetallogenic provinces of cenffal Europe is very rare.Oxidized sulfide zones in Germany and Czech Repu-blic are most likely preglacial in origin. Studies ofTertiary regoliths and sediments in this region (e.9.,along the Rhine and Loire valleys) indicate a generaloverall subtropical climate, with distinct periods ofaridity in tle Eocene and Pliocene (Wang 1951,Schwarzbach 1968" Staeblein 1972). Given the exfremediversification in climatic zones (tropical to desert) thatcan occur over a landmass largely characterized assubnopical (e.g., central Africa today; Tertiary cenfralEurope), it is highly probable tlat arid microclimaticregions existed locally over central Europe during theTertiary. One such region in Europe that is arid eventoday is the silver deposit region of Hiendelaencina,Spain (Calvo & Sevillano L992).Here, the fuIl comple-ment of Ag halide minerals (chlorargyrite: 'oplata

comea - horn silvet''; bromargyrite: "plata verde -green silvet''; and iodargyrite) occur with many othersecondary minerals, mostly Ag-As-SH and Cu-Scompounds. The relative abundance of the Ag halideminerals at Hiendelaencina decreases significantly inthe order presented above, indicating that extremeperiods of aridity, such as occurred in parts of centalU.S. and the Atacama Desert region of Chile, whereiodargyrite is relatively more abundant, did not occur incentral Spain during oxidation of the Hiendelaencinadeposits.

In the deep-sea marine environment" chlorargyrite,associated with atacamite and complex Cu-Pb-Agsulfosalts, has been discovered in the oxidized portionsof black smoker sulfide deposits on the Axial SeaMount and Explorer Ridge (. Jonasson, pers. cornm.;Hannington 1993). Iodyargyrite has not yet been foundin this environment. The solutions in these deposits areprobably supersaturated with respect to Ag and Cl (andpossibly Br and D, and formation of Ag halides can beexpected. It is higttly unlikely that these minerals wouldsurvive later diagenetic processes, since primary Aghalide minerats have never been found in sulfidedeposits.

FoRMATToN AND HABn oF Srvm. HALDE MINmers

Silver forms tbree end-member minerals with tnehalides Cl, Br, and L chlorargyrite (AgCl, formerlycerargyrite), bromargyrite (AgBr, formerly bromyrite),and iodargyrite (AgI, formerly iodyrite). There is no F

26 TIIE CANADIAN MINERALOGIST

equivalent, since AgF is extremely soluble. Therewould appear to be complete solid-solution betweenchlorargyrite and bromargyrite, and limited solid-solution between chlorargyrite and iodargyrite, andbetween bromargyrite and iodargyrite, resulting in aconfrrsing array of minerals under the generic name"embolite' with prefixes chlor-, iod-, and brom-. Boyle(1968) has indicated that the Ag halide group of miner-als should more appropriately be given names such asbromian iodian chlorargyrite, chlorian bromian bromar-gyite, etc.

'Silver iodide crysmllizes in both the hexagonal (iodar-gyrite: common) and isometric (mienite: very rare)systems. Miersite has been found only in the BrokenHill (Ausfralia) and Bisbee (Arizona) deposits, and ismetastable nnless it contains a small proportion of Cuions (Williams 1990). Three other non-Ag-bearinghalide minerals are commonly found with the Aghalides, namely marshite, CuI, which forms a solid-solution series vrith miersite, nantokite, CuCl, andcalomel, HgCl. Like the Ag halides, all three are foundonly in oxidized zones formed in arid climates.

The approximate solubilities of the Ag halide miner-als are as follows: AgCl (8.9 x LF gtL), AgBr (8.4 x10-t CIL), and AgI (2.8 x 10{ glt.). Despite theseobserved solubilities (in deionized water), the order ofprecipitation of the Ag halide minerals, as deducedfrom various zoning relationships in deposits @urgessLgLl, 1.977, Whitehead 1919, Lindgren 1933), isrevetsed (AgCl > AgBr > Agt). Thre€ main factorsaccount for this unexpected reversal: a) chloride ismuch more abundant than Br and I, and the solubilityof AgCl increases more rapidly than AgBr and AgI withincreasing temperature; thus after initial infilradon,warm, near-surface waters will become saturated withAgCl, and with further descent, chlorargyrite willprecipitate owing to lowering of groundwater tempera-tures and oversaturation, b) both AgBr and AgI aremore soluble in alkali chloride solutions, thus requiringmore Br and I to enter solution to reach solubility limitsfor bromargyrite and iodargyrite @mmons 1917), andc) the ferric ion exerts a much stonger oxidizing effecton Br and f (with formation of Br2 and I) than Cl-,thus lowering the concentration of bromide and iodideions that can complex with Ag (Knopf 1918).

A distinct vertical zonation of Ag halide minerals hasbeen noted for those deposits that have been studied indetail. For the Tonopah deposit in Nevada @urgess1911) and the Chanarcillo silver camp of Chile (White-head 1919), chlorargyrite is located near the surface,and gives way with depth to bromargyrite and, finally,to iodargyrite. Transitional "embolite"-Epe minerals[Ag(Cl,BrJ)] form tbroughout the oxidized zones. TheAg halide minerals can persistin oxidized sulfide zonesto well over 500 meters in depth.

Perhaps the most cogent indicator linking the forma-tion of Ag halide minemls to arid regions that haveexperienced high mJss ofevaporation and salination is

the rela.tionship between sources of the halides and theprecipitating environment. Penrose (1894), Burgess(lgLI, 7917),21d F.mmons (1917) have noted the closeproximity between the Ag halide oxide deposits and theformation through time of salt lakes in regions of NewMexico, Nevada Utab, and Aizona. These areas wereonce covered during the early Tertiary by numerouslarge bodies of watero which gradually dried up, result-ing in a large supply of halide ions to groundwateroxidation systems. For the Chanarcillo Ag halidedeposits in the Atacama Desert, Chile, Whitehead(1919) suggested a windborne source off the PaciflcOcean for the halides, with evaporative concenfration atthe surface. hesumably these salts would move downinto the supergene zone during the few periods ofprecipitation in this region to form concentrated alkalisalt solutions containing dissolved Ag. The Ag halideminerals associated with Austalian deposits such asBroken Hill, Teutonic Bore, and Cobar also are locatedin areas of extensive development of saline basins anda long period ofTertiary aridity.

occuRRm:.TcE oF Iooancynnt nt TLLS oF THBCmOUGAMAU _ CHAPAIS REGION

Iodargyrite has been found in heavy-mineral concen-trates collected during a till exploration programcovering a large shear-zone sfructure south of Chapais,Quebec (Fig. 1). Till cover in this region is quite thick(up to 40 m), and no in situ remnants of the preglacialweathered zone have been found. Three main areaswhere the mineral is concenffaled within the surveyregion have been outlined (areas l-3, Fig. l). Approxi-mately 1200 heavy-mineral samples have beenobtained from the tills of this area using lnfling, superpanning, and heavy liquid separation. Portions of halfof these heavy mineral separates, distributed over tleentire are4 were mounted on stubs and examined usingsecondary and back-scatter techniques with a scanningelectron microscope (SEM). Approximarely ll%o of thesamples contain iodargyrite. Owing to the small grain-size of the iodargyrite (<10 pm) and its friable naturga$empts to obtain polished mounts in epoxy for quan-titative chemical analysis were unsuccgssfirl. However,slow counting of flat crystal faces of some the largerioda$nite grains allowed reasonable semiquantitativeanalyses with regard to Cl and Br cont€nts.

Iodargyrite occurs as: a) single and aggregale crys-tals on surfaces and in voids of gold grains @g. 2a), b)encrustations on gold grains @ig. 2b), allrd rarely, onlimonite grains, and c) individual loose grains withinthe heavy-mineral concenhates. The gold grains withwhich the mineral is associated are commonly'oamoe-boid" in fom" typical of hydrosol grains of gold formedin laterites or gossans over auriferous mineralization(Fig. 3a). The mineral may be present as rosettes onhighly conoded grains ofgold (Fig. 3b), but generallyit is present as dispersed individual hexagonal crystals

IODARGYTE IN COLD-BBARING TILI.S. CHIBOUGAMEAU

aftached to (Fig. 3c) and "impregnating" gold @g. 3d). (Fig. 3c). Goethite commonly forms as crusts on crys-Crystals of secondary gold are occasionally found talsandencrustationsofiodargydte.Loosecrystalsofprecipitated on individual grains of iodargyrite iodargyrite in the concentrates generally display a

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

27

T--:--l " l

tIEEE

Frc. l. Location and geology of till-prospecting survey in the Chibougamau - Chapais area of central Quebec. Locations 1, 2 and3 re,?resent sit€s in the survey area where iodargyrite has been found, associafed with gold in dlls.

Masslve and foliatedgranitic intrusive

Mafic, ultnmaficandultabasic meta-i ntrusive

Metasedlment

Felsb metavolcanic

Maflc and lntermediatemetavolcanic

Geological boundary . . . /'-\---/Fault. . .Mineral occunence, . . .. . . .cul

28

Ftc. 2. Two main forms of iodargyrite associated with gold grains in tills of the Chibougamau - Chapais region: A) single aa4aggregate crystals on surfaces and in voids of gold grains, B) encrustations (aggregates of fine crystals) of iodmgyrite (I) ongold grains with associated goethite (G) and clay (Cl) minerals. Scales: I cm = 4 U,m in Ao 6 gm in B.

hexagonal tabular habit with the pronounced hemimor-phism characteristic of this mineral (Kraus & Cook1909). Individual crystals rarely exceed 10 pm in size.

Chemically, all of the grains examined display analmost pure AgI composition on energy-dispersionspecta. It is possible, however, tlat Ag(I,Br) mineralsmay be present, since fine till fractions rich in gold arealso anomalous in bromine.

In addition to gold, ofher secondary minerals nssoci-ated q/ith iodargyrite include kaolinite, massive tovesicular goethite, and'limonite dice" (pseudomorphsafter pyrite). The primary heavy minerals associatedwith bedrock mineralization include euhedral to subhe-dral pyrile an4 more rarely, siderite and ankerite. In afew cases, iodargyrit€ was found attached to grains ofpyrite. Concenhations of pyrite grains in the tills repre-sent downcutting of the supergene sutfide zone of theTertiary oxidation profile in this region. In otheroxidized deposits, such as Chanarcillo and Tonopah,iodaxgyritre is more concenftated in the lower oxide andunderlying supergene sulfide zones, where it is presentas crystal masses and coatings on supergene sulfides,mainly pyrite and Ag sulfosalts.

Chlorargyrite was not identified in any of thesamples of the Chibougamau - Chapais study area, andthe presence of bromargy'ite is only suspected from theanomalous levels of bromine in t'lls associated withhigh concentrations of gold. Absence of chlorargyrite

may be related to the depth of glacial erosion and thefact that iodargyrite is generally found at the deepestlevels of the oxidation zone. The upper zones woreprobably subjected to cyclical advances ofice; materialcomprising them may have been diluted and nans-ported farther to the south. Many of the heavy-mineralsamples from this area contain large concentrations ofeuhedral grains of pyrite, suggesting glacial erosiondown to at least the supergene sulfide zone of theweathered profile. Allard & Cimon (1974) have arguedfor minimal glacial erosion in the Chibougamau -Chapais regio4 the preservation oflarge concentationsof secondary minerals in dlls, and other features ofweathering mentioned below, support their hypothesis.

EVIDB{CE FoR PREGI.A.CIAL ARDTTY INTTIECnrnoucauau - CHApArs Rsdon oF QUEBEC

A compilation of preglacial weathering profiles thathave been preserved in the Quebec hecambrian andQuebec - Northern Maritime Appalachian regionsshows tbree predominant areas where Tertiary climaticconditions can be studied: a) the Labrador Trough area(A, Fig. 4), b) the northern Appalachian - St. LawrenceRiverregion @,Frg.4), and c) the Chibougamauto Vald'Or region (Abitibi Subprovince), stretching intoOntario (C, Fig.4).

IODARGYTE IN COLD-BEARING TILTS. CHIBOUGAMEAU

Ftc.3. Crystal habits andrelationship of iodmgyrite to bothprimary and secondary forms of gold. A) Iodargyrite crystals growingon "amoeboid' gold typical ofhydrosol gold that forms in laterites or gossans over auriferous mineralization. The grain hasbeen abraded during glacial transpoft. Iodargyrite crystals on abraded surfaces have been removed whereas crystals in voidswithin abraded areas (e.9., far left) remain. B) Rosette of iodargyrite crystals on gold grain with associated goethite. Angularvoids in gold are probably pyrite casts. C) Hexagonal crystal of iodaqyrite with secondary growth of gold (sAu).D) Iodargyrite crystal impregnating gold grain; overlapping gold on crystal may be secondary. Scales: L cm = 20 Um in A,10 pm in B, 5 pm in C and 1.5 pm. in D.

29

The Labrador Trough, with its rich iron-formations,has undergone weathering (silica leaching) and oxida-tion from at least the Cretaceous (Dorf 1959, Gross1968). The zone of complete oxidation (and benefac-

tion) of the iron-formation in this region exceeds200 meters in depth. The bodies of secondary limon-ite-hematite display similarities to both tropicallyweathered iron-formation and arid weathering profiles

30 TIIE CANADIAN MINERALOGIST

Fro. 4. Map of Quebec and nortlem New Brunswick outlining the main areas wherepreglaciat \reathering profiles have been discovered. (A) Labrador Trougb region: 1.Schefferville, 2. Labrador Ctty; @) Northern Appalachians - Lower St. Lawrenceregion: 1. Mont Jacques-Cartier region aud Gasp6 Cu deposits. 2. CMteau-Richer andChadesbourg regions, 3. Beauce - Chaudidre River gold placer region; (C) AbitibiSubprovince: 1. Icon-Sullivan Cu deposil 2-4. Copper Rand Henderson, and other Cusulfide deposits. 5. Chibougamau - Chapais till survey area (this study), 6. Iac Shorttrare-earth-enriched lateritic carbonatite, 7. Selbaie Cu (chalcocite--covellite) deposit;@) massive sulfidegossans andBigBaldMountainarea, Batlu$tregion, NewBrunswick

such .!s the Tertiary Hammersley-type secondary irondeposits in nortlwestem Ausfralia.

In the Northern Appalachian - St. Lawrence region,lateritic, saprolitic, and gossanous bedrock profilesoccur near Quebec City [saprolitesi Lasalle et aI.(1983), de Kimpe et aL (1985)1, Gasp6 Peninsula [oxi-dized Gasp6 Copper deposifi Ford (1959), saprolites:Cloutier & Corbeil (1986)1, throughout the Bathurstmining camp of New Brunswick [massive sulfidegossans: Boyle (1995), saprolit€ and grus formations:Warg et al. (1981, 1982)1, and in the Eastem Town-ships of Quebec [gold-bearing laterites: Shilts & Smit](1986), Smith & Shilts (1987)1.

In the Chibougamzu - Val d'Or region, major Tertiaryweathering profiles are represented by: a) the Cu oxidezone over the Icon-Sullivan deposit, Lac Mistassini(Troop & Darcy 1973), b) deeply wealhered (down to500 m) Copper Rand and Henderson ore deposits of theChibougamau mining camp, as well as other, smallerdeposits (Panish 1968, Allmd & Cimon 1974, Allard1976), c) lateritization of anorthosile in the Chibou-gamau area down to hundreds of meters (Allard &Cimon 1974, Allard L976), d) rare-earth-bearinglaterite and saprolite developed over the Lac Shorftcarbonatite (Quirion 1989), an4 d) the secondarycovellite--chalcocite zone at tle Selbaie copper deposit

Q U E B E C

- r " . " )

IODARGYTE IN GOLD.BEARING TILI,S, CHIBOUGAMEAU 3 l

(Sinclair & Gasparini 1980, Bouillon 1990). Similarenvironments of deep oxidation in Ontario and thenortheastern United States have been described byMoore (1938). In one of these areas, the Cobalt silvercamp, oxidation in the Keeley mine is present down toat least 200 meters. Unfortunately, no detailed minera-logical studies were carried out on this deposit to deter-mine presence or absence of Ag halide minerals. Thedeep oxidation, however, does signify the presence of avery deep water table.

Tertiary profiles in the Northern Appalachians -SL Lawrence region suggest that a subtopical oosavanna-

type" climate prevailed over this region. The gold-bearing laterites and derived placers are strongindicators of this type of climate. Tertiary profiles atsome of the sites in the Chibougamau - Val d'Or regionare more indicative of a much drier climale than existedfarther to the south, in the St. Lawrence Lowlands.Depths of oxidation at the Copper Rand and Hendersondeposits (5@ m or more) suggest that the waler table inthis region was very deep. Weathering of sulfideorebodies to these depths has only been recorded invery dry regions, such as the Great Basin area of thewestem United States @urgess 1911, Emmons 1917,Lindgren 1933). The very deep weathering of ttreCopper Rand and Henderson deposits would suggestthat the period of aridity over which they formedpersisted for some time. These oxidized zones are veryporous, indicating that a considerable amount of mate-rial has been carried downward in solution, eventuallyexiting the system in deep groundwaters. There is notectonic distirbance of the oxide mineral fabrics inthese deposits, suggesting that they probably formed inTertiary or Cretaceous times.

Further evidence of a period of aridity in tle Chibou-gamau - Chapais region during the Tertiary lies in anexplanation for the great expanse and thickness of finequartzofeldspathic saads efthis are4 which stetch asfar west as the Kapuskasing area in Ontario. Althoughreworked by glacial processes, these sands may havebeen derived from windblown desert materials ormatrix-weathered zones in semi-arid lateritic terranes.Corestones characteristic of the lower weathering zonesof laterites are commonly found in exposed sections oftill within the region. In lateritic and saprolitic terranes,the maffix, or exfoliated products of weathering, oftlese corestone horizons usually consist of quartzgrains and clays.

The two main mineralogical indicators of aridity inthe Chibougamau - Chapais region me the presence ofiodargyrite preserved in tills discussed above and thecommon @currence of 'limonite dic€" in heavy-mineralconcentrates. To form pseudomorphs after pyriterequires a very low water-to-rock ratio during oxidationto compensate for a reduction in unit-cell dimensions(pyite versas limonite) and to maxim2e the retentionof Fe. In addition, since the molar volume of pyrite is239 cfr compared to 20.8 cm3 for goethite (constant

Fe basis), some introduction of Fe is required for finalformation. Limonite dice are abundant in oxide zonesof arid regions (Blanchard 1968). The mineralLedbedrock underlying the iodargyrite-bearing tills in theChibougamau - Chapais region contains abundantcubes of pyrite in metasediments. If topical or temper-ate conditions prevailed, these formations wouldsimply leave "pyrite casts" in the weatheredrock owingto rapid dissolution and mobilify of ferrous and ferricions.

DtscussroN

On the basis of its confined world-wide disributionin oxidized sulfide deposits formed in arid regions, thepresence of iodargyrite in glacial sediments of theChibougamau - Chapais area of Quebec, together withother indicators of an arid climale, provides cogentevidence of a past arid climate in this region. Theseindicators include: a) the existence of a deep water tableduring oxidation of sulfide ore in the Chibougamaucopper deposits, b) the abundance of "limonite dice" inglacial tills, indicative of the pseudomorphic replace-ment of pyrite cubes in bedrock formations under aridconditions, c) the abundance of spongy and corrodedgrains of gold typicat of lateritic terranes, and d) theabundance of possibly desert-derived quartzofeldspa-thic sand formations.

The exact timing of Tertiary aridity for the Chibou-gamau - Chapais region is highly conjectural, given thelack of suitable preglacial material for dating. It isknown that on a world scale, the late Eocene to middleOligocene was characterized by some of the warmestglobal temperatures observed (Shackleton 1978). If theiodargyrite was formed at this time, either aridity per-sisted tbroughout much of the late Tertiary, or else theweathering profile was protected by overlying deposi-tion of post-Oligocene sediments thai were later removedby erosion. During the Pliocene, the climate in theCanadian Arctic was largely temperate. However, be-cause of the lack of suitable fossil materials, very littleis known about Pliocene climale ftends over the south-ern Canadian Shield. Studies of cores from the coastalAtlantic Ocean produce general clim*ic trends of asemiglobal nature, but these can hardly be extrapolatedthousands of kilometers inland to predict regional mi-croclimatic trends. It is conceivable, therefore, thataridity in the Chibougamau - Chapais area may haveprevailed in Pliocene times.

ACKNOWI,EDGEMENTS

Detailed 5gl\d slaminations of the Chibougamau -

Chapais heavy-mineral concentrates were performedwith the he$ of David Walker, Mineral ResourcesDivision, Geological Survey of Canada This workforms part of a broader geochemical study of till beingcarried out in this region by the author and Marcel

32 TIIE CANADIAN MINERALOGIST

Durocher of Batle Mountain - Hemlo Inc. Reviews ofan earlier draft of this paper by Yves Mo€lo of theInstitut des Mat6riaux, Universit6 de Nantes, andE.H. Nickel of CSIRO, Division of Mineral Products,Western Ausfali4 and an anonymous referee havehelped to further elusidate the author's inlerpretations.

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Received. May 21, 1996, reviseil manuscript accepted October6. 1996.