.. Gisements alluviaux d’or, La Paz.1-5juin 1991 YVONT. MAURICE Geological Surveyof Canada, 601 Booth Street, Ottawa, Ontario, Canada,K1A OE8 ABSTRACT Heavy mineral concentrates were systematically collected and analyzed for gold and about30 other elementsin the Estrie-Beauce region of southern Quebec,in an effort to map andinterpret the distribution of alluvialgoldin this historically important placerregionof eastern Canada. The primary objective wasto outline dispersal patterns that could help explorationists lmate undiscovered primary- or placer-type gold mineralization. Gold is widely scattered throughout the regionit is and evident that glacial dispersal played a key role in shaping the present geochemical landscape. The paper describes the methods used to sample and analyse the heavy minerals, and presents a compilationof the gold data. Some - examples on how to interprettheresults arepresented along withsomd cautionary notes regarding their significance. Morphological features of gold grains foundin southern Quebec suggest that some gold particles may have formed in the glacial detritus from gold mobilized during weathering and soi1 profile development. RÉSUMÉ Des concentrés de minéraux lourds ont été recueillis de façon systématique et analysés pour l’oret une trentaine d’autresCléments dans la région de l’Estrie-Beauce (sud du Québec), afin de cartographier et d’interpréter la répartition de l’or alluvionnaire dans cette région de placers historiquement importante dedu l’est Canada. Le but principal du projet deétait délimiter des trdinées de dispersion pouvant servirh l’exploration pour de nouveaux gisements primaires d’or ou des pl-acers. L’or se retrouve dispersé largementà travers toutela région etil ne fait aucun doute que la glaciation ajoué un rôle majeur dansle façonnement du paysage géochimique actuel.Le texte décrit les méthodes utilisées pour la cueilletteet l’analyse des minéraux lourds, et présente une compilation des résultats d’or.On donne également quelques exemples la de façon d’interpréter les résultats ainsi que des en mises garde au sujet de leur signification. Un examen morphologique de paillettes d’or provenantdu sud du Québec suggère que certaines ont pu s’être formées dans le détritus glaciaire à partir d’or mobilisé durant l’oxydation de ces dépôts et la formation du SOI. 231 Gisements alluviaux d'sr, La Paz 1-5 juin 1991 The gold placers in the Estrie-Beauce regionof southern Quebcc have been known since 36's. Their discovery ledto one of the first g~ldrushes on the North mericm continent, which lastedintemittently until theKlondike at the end of the century. The last placer opemtion in southern Quebec ended in 1964 and it is estimated that about 4 tonnes of placer gold were extractecl from the region. Severn%awmd-winning nuggets were found in the last century, the record being one weighing 2.2kg (71 oz). There havebeen reeeent anempts to recommence dluvid mining in parts thearea; eunent memes for one of the earliest known and most promising prospects, on Gilbert fiver, are estinslatesi at slightly over 1.5 tonnes of recoverable gold at abut 1.1 dm3. Cher the years, the wi read ~CCUIT~~G~of alluvial gold in southem Quebec atmcted the interest of many geologists se reported on the small-scale mining opemtions that flourished mtil the 1926'~~but no comprehensive studyof the phenomenonwas ever cmiedout. This lack of basic knowledge,eombined with a growinginterest in goldexploration in the 1980'~~prompted an investigation by the Geological Surveyof Canada (GSC). One important aspect of this project was te produce a regiond geochemicd map thatwouPd show the distributionof gold in modem alluvium throughout southern Quebec.It was hoped that sueh a map would reveal dispersal patterns that could lead not only to exploration tmgets for coneeded econo~cdeposits, but also to a better understm&ng.ofthegenesis of the alluvial gold * thesespecific objectives in mind, it was thought that heavy mineral concentrates systematically extractecl from largevolumes of strem dluvium would constitute m ideal geoehemicdsmple. Figure 1, fiom Boyle (1979),is a compilation from histoldeal records that shows the overdl distribution of gold-beardng strearns in a 15 608 km2 xea of southem Quebec. In most of the awiferous stream segments shown on the map9 the gold occurs in the Quatemmy and Recent stre s, and these were rarelyworked at a profit. onally, and this was the case in the GilbertRiver md in a few other locations, rich amiferous Tertiq gavePs kere found undemeath the Quatemwy cover at the bo incised vdleys. These gravels are the remnmt of a long period of weathei processes. "Flheir position at the bottom of the valleys protected them from erosion by the Bleistocene glaciations. Figure 2, afterShilts md Smith(198 ection of a Tetiargr placer in southernQuebec.TertiaryThe gravels cont by a few tens of rnetres of heavylaminated whichclays w s at the oniet of glaciation. The Idces formed as a result of the northwarddrainage system being damed by the advsuacing ice sheet comingfrom the north. Glaciation eroded most of the unprotected saprolites that covereda large proportionof the temne, and spread the debris down-ice depositingit as till. Because it contains a high proportion of oxidizd saprolitie matesial, this tends till to be brownand darker thhan'de overlying till,which ispales~dgre~(ShiltsandS~ith,1986.1988). Thelater~eytillismadeupsfsefslobilizedolder 232 Gisements alluviauxd'or, La Paz 1-5 juin 1991 Fig. 1. Distribution of alluvial goldin southern Quebec from historical records (from Boyle,1979) till diluted by relatively fresh bedrock material and lamiiiated lake sediments which were deposited in proglacial lakes during short glacial retreats and longer ioterstadial periods.2 Figure is a simplified section and does not showal1 the complexity of the various glacial advances and retreats. \ The present drainage flows on a recent alluvial floodplainit has carved but valleys into the underlying till sheets, mostly into the grey tills but alsooccasionally into the olderIn darker tills. .places where the eroded saprolites were auriferous, the tills deposited down-ice tend to contain a certain amount of gold. The gold concentrationsof course, will, be highestin the lower, darker tills but the youngertills also contain some gold, One of the main objectives of theGSC project consistedin mapping the gdd dispersal at a regional scale across the entire region, to help minera1 exploration. It w,as .hoped that the dispersal patterns would point to areas where auriferous saprolite had been 'eroded. Then exploration companies could use this information to explore forgold-bedng uneroded saprolites or Tertiaryplacers, or primary gold mineralization, assuming that auriferous saprolite would have ken eroded from the exposed portions of such deposits. The GSC project in southern Quebec produced regional distribution ma@ for many elements other than gold, someof which are valuable pathfinders for gold mineralization, while others are usefulto explore for other commodities. This paper, however,is concerned exclusively 233 .. Fig. 2. Cross section of a typicplacer in southem Quebec (after Shilts and Smith, 1988) rnG rnrnWQwE employed in smpling the heavy lninerds wqs developed by Ma~ceand Mercier (1984) specifically for the pbmpose of mapping the regional bueion of dluvid gold in ssuthem.Quebec. Ssme of the impomnt factors that had to be into csnsidemtion in designhg the samiling pro ere: (1) the existence of a good ~oadnctwork providing easy access to nealy al1 sectors umey wea; (2)the fact that much of the alluvial gold wcurs as coarse native goldputicles; md (3) relatively Qowgold concentrations in the suficialdeposits as a mult of dilution by glacial processes. The cowseness md the reQativelgrlow ncy ofgokl pdcles necessitated extraction of the mcs from large volumes of dluvium entation smdy (Ma~ce,1986) dso sbowd that smples should be obtaind fiom below the uppemost layer of active strem sediments, avhichitids not to retain native gold particles.~he prwedure findly adoptd is c~edout in mo stages: (1) a heavy mineral preconcentrate is obtained at the sample site using a portable suctisn dredge. (Fig. 3); (2) the final heavy mineral concentrateis produced-in the labsratory with a rotary spiral concentrator (Fig. 4). The dredging speration is time-controlled, lasting usuallyone hdf hour. The volume of dhvium treated duringthat period of time varies from site ts site depending on local conditions, but averages about 260 litres. The suction enables samglingdeeper into the sediment pile thm would otheawisebe possible, thereby increasingthe likelihsod of esllectingparticles of precious meds and of other high densitymineAs if they are present. ' The rotary bpifal concentrator was found to be the ideal instrument to e&act the heavy 1' 234 Gisements alluviaux d'or, Lcj Paf 1-5 juin 1991 Fig. 3. Portable suction dredge used to coUect a preconcentrate of heavy minerals in the field Fig. 4. Rotary spiral concentrator used to produce pure heavy mineral conccntrates in the labratory 235 Gisements alluviaux d'or, La P To overcome the nugget effect and ensure that the gold malyses weae representative of the original volume of dluvium tre with the suction ge, it would have ken desirable to malyse the entirenon-magnetic mineml fraction each sample site. However, this was nst done kcause of the necessity to perfom oher malyses on the smples, md the need to preseme s~meof the concentrate as referen al md for minerdogical examinations. As of the