30. GEOPHYSICAL EXPLORATION AT THE PINE POINT MINES LID. ZINC-LEAD PROPERTY, NORTHWEST TERRITORIES, CANADA Jules J. Lajoie and Jan Klein Cominco Ltd., Vancouver, Canada Lajoie, Jules J. and Klein, Jan, Geophysical exploration at the Pine Point Mines Ltd., zinc-lead property, Northwest Territories, Canada; ~ Geophysics and Geochemistry in the Search for Metallic Ores; Peter J. Hood, editor; Geological Survey of Canada,Economic Geology Report 31, p. 653-664, 1979. Abstract Pine Point is a zinc-lead carbonate camp with about 40 known orebodies on the south shore of Great Slave Lake, Northwest Territories, Canada. The ore host is a Devonian barrier reef complex. The ore mineralogy consists, in order of relative abundance, of sphalerite, marcasite, galena, pyrite, and occasional pyrrhotite. The gangue consists of dolomite and calcite. Mineralization occurs as open-space filling in tectonically-prepared ground and solution-collapse structures .. The area is mostly covered by glacial till and muskeg which averages 15 m in thickness. 1n 1963, comparative tests of electromagnetic (EM), self-potential, and induced polarization (IP) methods clearly demonstrated that IP is the most powerful exploration tool for this districL EM is unsuccessful because of poor electrical continuity of the conducting minerals in the orebodies. In subsequent years, extensive induced polarization surveys resulted in the discovery of many of the Pine Point orebodies. The majority produce strong IP anomalies due to subcropping mineralization. There is often little or no coincident resistivity anomaly. The gradient array was commonly used at first, and, subsequently, improvements in instrumentation permitted the use of multiple separation arrays for better target detection and delineation. Exploration case histories and tests are discussed to emphasize particular aspects of exploration in Pine Point with the IP method. Gravity surveys respond to sinkholes and the larger, more massive orebodies. Magnetic surveys produce only weak anomalies from minor pyrrhotite which is sometimes associated with the ore. The seismic reflection method may be helpful in detecting collapse structures but is not a cost effective tool in comparison with the IP method. The background IP geologic noise level is very low but telluric noise is often severe. Careful analysis of the IP data aids in lithologic correlation studies. Resume Pine Point est une region de carbonates mineralises en plomb et zinc, comprenant environ 40 masses mineralisees situees Ie long de la cote sud du Grand lac des Esclaves, dans les Territoires du Nord-Ouest au Canada. La roche encaissante est un complexe devonien de recif barriere. Le minerai se compose, par ordre d'abondance relative, de sphalerite, marcasite, galene, pyrite et parfois pyrrhotine. La gangue consiste en dolomie et calcite. La mineralisation se presente comme un re mplissage de fissures dans un terrain fracture et dans des structures d'effondrement par dissolution. La region est principalement couverte de depots glaciaires et de sol de marais, qui totalisent une epaisseur moyenne de 15 m. En 1963, des essais comparatifs des methodes electromagnetiques, de polarisation spontanee, et de polarisation provoquee ont montre clairement que la polarisation provoquee est l'outil d'exploration Ie plus efficace dans cette region. Les methodes electromagnetiques ne donnent pas de resultats concluants d cause de la faible continuite electrique des mineraux conducteurs dans les corps mineralises. Dans les annees ulterieures, des leves detailles de polarisation provoquee ont abouti d la decouverte de plusieurs masses mineralisees d Pine Point. La plupart d'entre elles ont cree de fortes anomalies de polarisation provoquee, dues aux mineralisations proches de la surface. La desposition des electrodes suivant 'la methode du gradient a d'abord eteutilisee, puis, avec les perfectionnements de l'appareillage, un mode de disposition des electrodes suivant des lignes multiples a ete adoptee, pour mieux deceler et localiser les corps mineralises. Plusieurs exemples d'exploration et d'essais de prospection sont donner ici pour bien montrer les aspects particuliers de l'exploration pratiquee d Pine Point par la methode de polarisation provoquee. Les leves gravimetriques enregistrent en particulier la presence de dolines et de corps mineralises massifs et de grande taille. Les leves magnetiques enregistrent seulement de faibles anomalies dues d la presence de petites quantites de pyrrhotine parfois associee au mineraL La methode de sismique reflexion peut aider d detecter les structures d'effondrement mais elle n'est pas rentable comparee d la methode de la polarisation provoquee. Le bruit de fond resultant du milieu geologique est tres faible en polarisation provoquee, mais Ie bruit tellurique est souvent genanL L'analyse detaillee des donnees de la polarisation provoquee facilite la correlation des niveaux lithologiqUes. 654 J.J. Lajoie and J. Klein INTRODUCTION The Pine Point zinc-lead district is located on the south TIDAL FLAT shore of Great Slave Lake in Canada's Northwest Territories, EVAPORITES SEDIMENTS g~~~~~ BASINAL approximately 800 km directly north of Edmonton, Alberta (Fig. 30.1). It is accessible by road and railroad and there is daily air service into the town of Pinc Point with a present s':':~'.~------'--'----'I!JAi'"''Ijj''''''''~~'"""'"'' '~f'v~~:~~' (1977) population of obout 2()OO. The regional topographic relief is about 50 m and much of the area is swampy. * * tr.- N In 1898, the first few claims were staked by a local fur ** * I // __..+.i.(...:..:..:.:.. trader as a result of mining interest generated by prospectors ESHALE .)-_•••••• ··f···• •.••_.-...·OFFREEF en route to the Klondike gold fields of the Yukon. In 1926, a ••••••••••• ••••••••••• FOREREEF FACIES property eXRmination by the Consolidated Mining and ARENITES Smelting Company, now Cominco Ltd., revealed a geological similarity to the famous Tri-State zinc-lead district of FAULT southeastern Missouri, and an option was secured on some []] COARSE DOLOMITE (PRESQU'ILE) mineral claims. During the late 1940s, a large concession was D 0 REB 0 DIE S (STRATIGRAPHIC LOCATIONS) acquired by Cominco enclosing the known mineralization. In 1951, Pine Point Mines Ltd. was formen to finance 50mL exploration on the property. A production decision was finally reached in 1963, after successful negotiations with the 5Km federal government for const ruction of a northern milway. Figure 30.3. Simplified geological cross-section of barrier complex, Note the exaggerated vertical scale. ~ ALASKA ./ FAIRBANKS / / / YUKON N. W. T. YELLOWKNI FE MAN. / ./ LIN FLON / / / ( i I MILES ~ o 200 Figure 30.1. Location map o -0- J \ ~ OVERLYING , FORMAT'ONS ~'" ..'".. • EXISTING PITS (19~"') /~'" Km I i o 10 Figure 30.4. Sample of sphalerite and galena Figure 30.2, Plan map of barrier complex and mining pits. mineralization in a dolomite gangue. (GSC 203492-1) Pine Point, NWT 655 LINE 2 +OOW LINE 5+00W S.P. MV ./.,......./.\......_., /. .fIII' .,. .,. __._., 1O0 _--t_-+----lI----+----lI---.... 01 I "I' II \'----1 1 H.L.E.M. 1 H.L.E.M. F=2400Hz CS=200 F=2400Hz CS=200 +10% +10% IN PH A~E, /'_'_' ~~.~-:~;. ,-JI ,._. .----f--+.....;:a..,.!-. 1 I o .----1--+'---',·-i- II ° ~ QUADRATURE e('QUADRATURE _.--.._. -10% _.-......."'.~. "'''''-'. -10% ._ 1 I P& RES. GRADIENT ARRAY a =200 I P& RES. GRADIENT ARRAY a =200' RES.~ /.\ /...... I RES, MSE~. ·;~~~·-7'._X J~O~O 10 1000 '-'-"./'-'-' ._.,......._. \ I I IP \" Slm MSEC. ........ ° I ,/r 0 ._._.--0, --.- /\ FEET 409ct=rS.,.SL, 400N OL----t--4---l--+----t- 08 -75' @ FEET 400S 16/20/5 61'@ Figure 30.5. Geophysical test profiles over orebody N-42, 5/16/7 line 2+00W, 1963 (Huntec MK 1). Ore grade is in per cent Pb, Zn, and Fe. Figure 30.6. Geophysical test profiles over orebody N-42, line 5+00W, 1963 (Huntec MK 1). Ore grade is in per cent Pb, Zn, and Fe. At this time, the company had 8.8 million tons of ore and muskeg whose dept.h varies from about. 5 t.o 30 m wit.h an averaging 2.6% lead and 5.9% zinc in several orebodies which average of about. 15 m over the area. Figure 30.2 shows the had been found by drilling. The induced polarization (IP) locat.ion of t.he pit.s which now exist. on the propert.y. Sulphide method which was first introduced to Pine Point in 1963, mineralizat.ion subcropped in all but. one of the orebodies played a very important role in discovering more orebodies, shown here. and in significantly extending t.he ore reserves of the dist.rict. In cross-sect.ion, t.he reef exhibit.s t.he major The t.otal known ore is now about. 74 million t.ons averaging charact.erist.ics of a c1ast.ic reef complex as shown in 2.8% lead and 6.3% zinc in about. 40 orebodies. About. half of Figure 30.3. The environment.al facies encountered from t.his ore has now (1977) been mined, mostly by open pit sout.h t.o north are evaporites, tidal flat. sediments, organic met.hods. barrier, forereef arenite facies, offreef facies, and basinal Previous art.icles have presented geophysical survey marine shales. Note that. t.here is a vertical exaggerat.ion of result.s in t.he Pine Point dist.rict., mainly IP, using t.ime 100 in Figure 30.3. The development. of the barrier reef domain (Seigel et. a!., 1968; Pat.erson, 1972) and frequency during t.he Devonian was accompanied by a great.er rate of domain (Hallof, 1972) met.hods. This paper describes t.he sedimentat.ion in t.he evaporit.e basin t.o t.he sout.h. This was geophysical work done at Pine Point from an historical at. least. partly responsible for fault.ing and fract.uring within perspective, and discusses some geophysical aspect.s of the reef complex, parallel t.o t.he st.rike of t.he reef.