LEAD ISOTOPE EVIDENCE REGARDING ARCHEAN ANO PROTEROZOIC METALLOGENY Ln CANADA
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Revista Brasileira de Geociências 12(1·3): 510·521, Marc-Set., 1982 . Silo Paulo LEAD ISOTOPE EVIDENCE REGARDING ARCHEAN ANO PROTEROZOIC METALLOGENY lN CANADA RALPH I. THORPE* AB8TRACT A new lead evolution model (Ao = 10.242, 80 = 11.9556, Co - 30.37 and To = 4,060 Ma) has been derived, in part, from a steep linear pattern defined by lead isotope . data for deposits from the Superior Province of the Canadian Shield. Application ef this model in Superior Province provides evidence that: a) supports widespread formationofmassive sulphtde deposits at 2,700 to 2,750 Ma; b) some deposits in northwestern Ontario may have formed at 2,Soo to 2,S50 Ma; c) deposition was synvolcanic or nearly so for a few gold deposits; and d) many gold deposits were probably formed, and a few massive sulphide deposits remobilized, at 2,656 to 2,700 Ma, a period of major deformation and emplacement of syntectonic to post-tectonic plutons in the greenstone belts containing the depcsits. The pattern for Slave Province deposits is similar, but massive sulphide deposits are generally 30w60 Ma younger. Modellead ages for Pro terozoic massive sulphide deposits in the Churchill, Southern, Grenville and Cordilleran provinces are generally in good accord with other geochronological evidence. The sarne also appears to be true for silver-arsenide veins at Great Bear Lake, Bear Province, and in the Cobalt area, Southern Province. and for certain other epigenetic deposits. Lead isotope data help define a I ,200~1 ,400 Ma metallogenic epoch in the central Yukon, northern Canadian Cordtllera. INTRODUCTION Prior lo 1966 lead ísotope data for deposits in lhe Canadian Shield had been published by Russell and Farquhar (1960), Kanasewich and Farquhar (1965) and Roscoe (1965), and by a few other workers. Howev,er, more analyses have been accumulated in lhe subsequenl 17 years, many of them by lhe Geological Survey of Canada. Although some of these have been pub lished in various papers and two major data sets are contained in papers now in press (Franklin et al., in press; Franklin and Thorpe, in press), a significant proportion have not yet been published. This paper attempts lo presenl some of lhe general geochronological and melallogenetic conclusions and interpretations that can be reached on lhe basis of ali these data. Reference is also made to analyses available for deposits in Precambrian rocks within lhe Cordilleran Province. GENERAL GEOLOGV AND METALLOGENV OF THE CANADIAN 8HIELD A reviewofCanadian Shield metallogeny has recently been prepared by Franklin and Thorpe (in press), who summarized lhe pertinent geology as well as inforrnation on deposits of iron, nickel, gold and volcanic-associated massive sulphide deposits for lhe main geological provinces. Therefore, only a brief'sketch ofShield geology and melallogeny will be presented here. The small Nutak Province, along lhe coast of Labrador Figure I - Index map showing geological provinces o/lhe Canadian (Fig. I), contains some of lhe oldesl rocks of lhe Canadian Shield. Ruled areas represem Phanerozoic cover Shield. These are mainly gneissic rocks and are of both plutonic and supracruslal origins, Superior and Slave provinces, lhe main Archean blocks belts, sedimentary rccks predominale in lhe belts of south of lhe Canadian Shield (Fig. I), contain greenslone belts em Slave Province. Rocks in greenstone belts are com and intervening granitic-gneissic belts similar lo those in monly at greenschist facies, whereas those in gneiss belIs, other Archean cratons. Although volcanic rocks generally in many cases including abundanl metasedimentary supra predominate over sedirnentary rocks in these greenstone crustal rocks, are aI higher metamorphic grades. "Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario KIA OES, Canada R~lI ista Brasileira ae Geoci ências, Volume 12 (1.3). 1982 511 Volcanic cyc1es in lhe greensto ne belts consist, in many de formed , and in part highl y metamorphosed , sedirnenta ry cases; of basal ultramafic (kornatiitic) lavas, overlain by rocks that grade westward into deeper water fades. an extensive and thick " plate" of pillowed basaltic lavas Rocks o f the Southern Province are dorn inated by an and capped by areally restricted felsic volca nic rocks. ea rly Aphebian (Hur onian) sedi rnenta ry succession co n Most major greenstone belts contain two to five volcaníc sisting o f conglomerate, sands to ne, argillite, mudstone, and cyc1es. ln greenston e belt s in lhe Slave Province, however , mino r limestone. Early sedimentatio n was confined to ba relatively few ultramafic rocks are known, no ult ramafic sins, troughs an d valleys, but later spread lo blanket large lavas have beco documented, and o nly in a few cases is areas of Archea n roc ks of lhe Superior Pro vince. Th e there more than Doe volcanic .cycle. Volcanic-derived Gowganda Fo rrna tio n in lhe eastern part of So uthern epiclastic rocks, greywacke and finer-grained turbidites Province has been interpreted lo be of glaeial origino Sedi co nstitute lhe maio sedimentary rocks in greensto ne belts, mentation was probably confined lo lhe period 2,500 lo whereas iro n-fo rrnation and carbonate units are less com 2,300 Ma . ln lhe Sudbury lo Cobalt region thick undula mo no lron-forrnation is locally ab"u nda nt ln the Superio r ting sheets of gabbro (Nipissing diabase), yielding a Rb-Sr Province and within Archea n terranes in lhe Churc hill isochron age of abo ul 2,120 Ma (Fairbairn et al., (969) , Province, Nort hwest Territories, but it forros o nly minor have int ruded lhe Aphebian seq uence and older basement occurrences in Slave Province. Carbonate units are locally rocks. Relatively littl e deforrned clastic and chemica l sedi present in Superior Province belts (F ra nklin and T horpe, mentary rocks of the Gunflint and Rove lormatio ns form a in press), but are co mparative ly common in SIave Province homoelinal seq uence in lhe Thunder Bay area. T he White belt s. water Group o f tufTs, carbonaceous and pyriti c siltstones The bulk of lhe volcanic rocks in Superior Province were an d greywacke is confined lo lhe Sudbury Basin , extruded between 2,700 and 2,750 Ma ago (Krogh (' I al., The Grenville Province is similar in many respects to the 1982a ; Nunes and Pykc , 1980 ; Nunes a nd Jcnsen, 1980 ) Churchill Province. The rocks are, in many places, gneissic according to zirco n U-Pb data. However, some greenstone a nd inelud e highl y deformed and rneta mo rphosed sedi belts in no rthwestern Ontario also contai n sequences 3,015 mentary rocks. Volcanic rocks are abundant in some parts lo 2.9 15 Ma and abo ut 2.776 to 2,794 Ma old (Nu nes of the Province arid marble is abundant in oth ers, Granitic, and T hurs to n, 1980 ; Nunes and Wood, 1980 ; Dav is et al., and in sorne areas anorthositic, pluton s are relatively com 1982). A V· Pb zirco n age of 2,670 ±4 Ma was recently mo n. Supracrustal rocks are in part known and /or presumed obtained for volcanic rocks in the Back River area , eastern lo have ·been deposited in lhe intcrval 1.400 to 1.200 Ma . SIave Province (La mbert and Henderson, 1980 ). Th is date but some older rocks hav e been identified, pa rticular ly is in cxccllent agreernent with a date of about 2,666 lo in lhe Grenv ille Tectonic Zo ne flânking Superior Province. 2,669 Ma fo r a subvolcanic intrusion in lhe Hackett River Rocks in lhe G renville Province were generally highly area a Iittle farthcr no rth, and with prior zircon data for de forrned and metamorphosed during lhe Grenville oro- so uthem Slave Province (F rith and Loveridge, 1982). geny at about 1,050 lo 1,000 Ma (Stockwell, 1973). Volcanic rock s are similar in age, about 2,680 Ma , in lhe T he differenl structural-geologica l provinces of lhe Ca na Po inl Lake area , north western Slave Provi nce (Krogh, dian Shield have distinctive metallogenetic characteristics. pers. co rnmun.], where olde r basement rocks, 2,870 lo Superior and Slave prov inces both co ntain irnportant Cu-Zn 3,155 Ma , are kno wn (Krogh and Gi bbons, 1978 ). Zircon massive sulphide an d gold deposits, However, while Suo ages of about 2,677 ànd 2,953 Ma have been reported. res perior Province co ntains importam iron and nickel deposits, pec tively, fo r volcanic roc ks in lhe Rankin -Enn adai belt, these are lackin g in Slave Province. The absence of nickel Churchill Province and for paragneiss associated with Prince deposits in SIave Pro vince is no doubt related lo the pa ucity Albert Group supracrusta l rocks on Melvi lle Peninsula , o f ultramafic rocks. The Bear Province is characterized by District of Franklin (Wanless, 1979 ). deposits of uranium, copper and silver-a rsenide veins, and by lhe striking laek of signifi cant go ld o r massive sulphide Churchill Provínce co ntains Proterozoi c volcanic-sedi deposits. Churchill Province co ntains abundant volcanic menta ry sequences, belts of shelf-facies sedimenlary rocks, -associated massive sulphide deposits, similar to those of gneissic rocks o f various co mpos itio ns and of varied sour Superior Province, in Aphebian terrane in lhe circum ce lith ologies und ages. and also, as noted abov e, so me -Kisseynew (Flin Flon, Snow Lake) region , and oec urrences major belts o f Archean rocks. The latter are in part Inter ofthis type in Archean terranes in the Northwest Territories. folded with Proterozoic strata, and are also in part ai low Major iron deposit s and a few rnassive sulphide depo sits are metamorphic grade.