30. GEOPHYSICAL EXPLORATION AT THE PINE POINT MINES LID. ZINC-LEAD PROPERTY,NORTHWEST TERRITORIES, CANADA

Jules J. Lajoie and Jan KleinCominco Ltd., Vancouver, Canada

Lajoie, Jules J. and Klein, Jan, Geophysical exploration at the Pine Point Mines Ltd., zinc-leadproperty, Northwest Territories, Canada; ~ Geophysics and Geochemistry in the Search forMetallic 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 ofGreat 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 asopen-space filling in tectonically-prepared ground and solution-collapse structures .. The area is mostlycovered 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 isunsuccessful because of poor electrical continuity of the conducting minerals in the orebodies. Insubsequent years, extensive induced polarization surveys resulted in the discovery of many of the PinePoint orebodies. The majority produce strong IP anomalies due to subcropping mineralization. Thereis 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 arraysfor better target detection and delineation. Exploration case histories and tests are discussed toemphasize particular aspects of exploration in Pine Point with the IP method. Gravity surveys respondto sinkholes and the larger, more massive orebodies. Magnetic surveys produce only weak anomaliesfrom minor pyrrhotite which is sometimes associated with the ore. The seismic reflection method maybe helpful in detecting collapse structures but is not a cost effective tool in comparison with the IPmethod. The background IP geologic noise level is very low but telluric noise is often severe. Carefulanalysis of the IP data aids in lithologic correlation studies.

Resume

Pine Point est une region de carbonates mineralises en plomb et zinc, comprenant environ 40masses mineralisees situees Ie long de la cote sud du Grand lac des Esclaves, dans les Territoires duNord-Ouest au Canada. La roche encaissante est un complexe devonien de recif barriere. Le mineraise compose, par ordre d'abondance relative, de sphalerite, marcasite, galene, pyrite et parfoispyrrhotine. La gangue consiste en dolomie et calcite. La mineralisation se presente comme unre 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 uneepaisseur moyenne de 15 m.

En 1963, des essais comparatifs des methodes electromagnetiques, de polarisation spontanee, etde polarisation provoquee ont montre clairement que la polarisation provoquee est l'outil d'explorationIe plus efficace dans cette region. Les methodes electromagnetiques ne donnent pas de resultatsconcluants d cause de la faible continuite electrique des mineraux conducteurs dans les corpsmineralises. Dans les annees ulterieures, des leves detailles de polarisation provoquee ont abouti d ladecouverte de plusieurs masses mineralisees d Pine Point. La plupart d'entre elles ont cree de fortesanomalies de polarisation provoquee, dues aux mineralisations proches de la surface. La despositiondes electrodes suivant 'la methode du gradient a d'abord eteutilisee, puis, avec les perfectionnements del'appareillage, un mode de disposition des electrodes suivant des lignes multiples a ete adoptee, pourmieux deceler et localiser les corps mineralises. Plusieurs exemples d'exploration et d'essais deprospection sont donner ici pour bien montrer les aspects particuliers de l'exploration pratiquee dPine Point par la methode de polarisation provoquee. Les leves gravimetriques enregistrent enparticulier la presence de dolines et de corps mineralises massifs et de grande taille. Les levesmagnetiques enregistrent seulement de faibles anomalies dues d la presence de petites quantites depyrrhotine parfois associee au mineraL La methode de sismique reflexion peut aider d detecter lesstructures d'effondrement mais elle n'est pas rentable comparee d la methode de la polarisationprovoquee. 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 polarisationprovoquee facilite la correlation des niveaux lithologiqUes.

654 J.J. Lajoie and J. Klein

INTRODUCTION

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The Pine Point zinc-lead district is located on the southshore of Great Slave Lake in Canada's Northwest Territories,approximately 800 km directly north of Edmonton, Alberta(Fig. 30.1). It is accessible by road and railroad and there isdaily air service into the town of Pinc Point with a present(1977) population of obout 2()OO. The regional topographicrelief is about 50 m and much of the area is swampy.

In 1898, the first few claims were staked by a local furtrader as a result of mining interest generated by prospectorsen route to the Klondike gold fields of the Yukon. In 1926, aproperty eXRmination by the Consolidated Mining andSmelting Company, now Cominco Ltd., revealed a geologicalsimilarity to the famous Tri-State zinc-lead district ofsoutheastern Missouri, and an option was secured on somemineral claims. During the late 1940s, a large concession wasacquired by Cominco enclosing the known mineralization. In1951, Pine Point Mines Ltd. was formen to financeexploration on the property. A production decision wasfinally reached in 1963, after successful negotiations with thefederal government for const ruction of a northern milway.

Figure 30.1.

Figure 30.2, Plan map of barrier complex and mining pits.Figure 30.4. Sample of sphalerite andmineralization in a dolomite gangue. (GSC 203492-1)

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Pine Point, NWT 655

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5/16/7Figure 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.

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Figure 30.5. Geophysical test profiles over orebody N-42,line 2+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 oreaveraging 2.6% lead and 5.9% zinc in several orebodies whichhad been found by drilling. The induced polarization (IP)method which was first introduced to Pine Point in 1963,played a very important role in discovering more orebodies,and in significantly extending t.he ore reserves of the dist.rict.The t.otal known ore is now about. 74 million t.ons averaging2.8% lead and 6.3% zinc in about. 40 orebodies. About. half oft.his ore has now (1977) been mined, mostly by open pitmet.hods.

Previous art.icles have presented geophysical surveyresult.s in t.he Pine Point dist.rict., mainly IP, using t.imedomain (Seigel et. a!., 1968; Pat.erson, 1972) and frequencydomain (Hallof, 1972) met.hods. This paper describes t.hegeophysical work done at Pine Point from an historicalperspecti ve, and discusses some geophysical aspect.s ofpart.icular interest.

GEOLOGY

The host of the zinc-lead mineralization is a Devonianbarrier reef complex (Skall, 1975) which subcrops (i.e.outcrops below the overburden) on the south shore of Great.Slave Lake, approximately as shown in Figure 30.2. The reefstrikes east.-northeast. and dips about. 4 metres per kilometret.o t.he sout.hwest, where it. is overlain by more recentDevonian format.ions. The overburden consist.s of glacial dri ft.

and muskeg whose dept.h varies from about. 5 t.o 30 m wit.h anaverage of about. 15 m over the area. Figure 30.2 shows thelocat.ion of t.he pit.s which now exist. on the propert.y. Sulphidemineralizat.ion subcropped in all but. one of the orebodiesshown here.

In cross-sect.ion, t.he reef exhibit.s t.he majorcharact.erist.ics of a c1ast.ic reef complex as shown inFigure 30.3. The environment.al facies encountered fromsout.h t.o north are evaporites, tidal flat. sediments, organicbarrier, forereef arenite facies, offreef facies, and basinalmarine shales. Note that. t.here is a vertical exaggerat.ion of100 in Figure 30.3. The development. of the barrier reefduring t.he Devonian was accompanied by a great.er rate ofsedimentat.ion in t.he evaporit.e basin t.o t.he sout.h. This wasat. least. partly responsible for fault.ing and fract.uring withinthe reef complex, parallel t.o t.he st.rike of t.he reef. Lat.er,t.hese fract.ures served as conduits for magnesium-rich fluidswhich dolomit.ized part.s of t.he reef t.o a coarse, vuggydolomit.e, referred to locally as t.he Presqu'ile facies.

The t.ect.onic activit.y acted as ground preparat.ion fort.he deposit.ion of Mississippi Valley-t.ype deposits. There is avast. supply of sulphur in t.he evaporit.e basin t.o the sout.h.Sources of met.als are post.ulat.ed to be t.he basinal shales t.ot.he nort.h, t.he carbonat.e pile itself, or some other deepersource. Wit.hin the reef complex, the porous carbonate rocksallow easy migrat.ion of met.al brines and sulphur-bearingsolut.ions.

656 J.J. Lajoie and J. Klein

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SURVEY LINEFEET

marine regressions; b) faulting and fracturing and especiallythe intersection of fault zones; and c) dolomitization whichproduces a rock with high porosity. Traps created by pinchouts and facies changes also playa role in ore deposition.Figure 30.3 shows the original stratigraphic positions of fourmajor, partially-eroded, sUbcropping orebodies.

Most of the orebodies can be subdivided into two basicshapes: massive orebodies which are roughly equidimensionaland tend to have a somewhat higher grade, and tabularorebodies which are restricted vertically and extendhorizontally either in one horizontal direction (run type) ortwo horizont31 directions (blanket type).

The ore mineralogy is simple. It consists of crystallineand colloform sphalerite, marcasite, galena, pyrite, undoccasional pyrrhotite. The zinc to lead ratio uverages abouttwo to one, and varies widely for different orebodies. Thegangue consists of calcite and dolomite. A typical sample ofore is shown in Figure 30.4, where galena mineralization issurrounded by eolloform sphaleri te und the gangue isdolomi teo A common physical property of the ore is the l3ckof electrical continuity of the conducting minerals: galena,pyrite and mnrcasite. In Figure 30.4, the galena is surroundedby sphaleri te which is 3 nonconducti ve and nonpolarizablemineral. Generally, however, the conducting paths areinterrupted by calcite 3nd dolomite gangue. This explainswhy the orebodies do not respond to standSI'd electromagneticexploration methods.

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The prime requirement for the formation of the orcs isthe availability of open spaces in the rock for precipitation ofsulphides. At Pine Point, the main causes of open spaces 3re:a) karsting and solution breccias which developed during

Figure 30.7. Chargeability contour map of 1964 gradient-array IP survey (Huntec MK 1).

Figure 30.8. Plan of mineralized holes on line Pand positions of current electrodes (AA' and BB')for gradient-array survey on line P.

Pine Point, NWT 657

GEOPHYSICS

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is no evidence of em EM anomaly over the mebody. Furtherreconnai~;~Jance work wi th EM produced weak anomaliessimilar to that on line 2W and these corrclated wcll wit hswampy ground. Seigel (19{,S) showed Turam results over thePyramid No. One orebody which confirm the lack of responseto electromagnel ic methods.

The IP data in Figures 30.5 and 30.6' were acquiredusing a gradient array with a potential elect rode spocing of200 feet (61 m). It is evident Ihat strong chargeabilityanomalies coincide directly wit h the orebody on both lines.The background chargeability level is between 2-3 ms and isremarkably flat; thus, the anomalircs are about seven timesbackground. A distinct resistivity low occurs on line 2Wwhere a rlrillhole shows a combined lead-iron grade of 21 %.e)n line 5W, where the combined lead-iron grClde is only about12%, a weak resistivity anomaly is only bClrely del ectableover the background vari at ions.

Initial High Success Rate

The geophysical test of 1963 marked the beginning of anew exploration era at Pine Point. The following year, about250 line miles of time-domain gradient-array IP weresurveyed. Figure 30.7 shows an example of IP discoveriesmade during the extraordinarily high success periodexperienced during 1964. The lines are about 5UO feet(J 52 m) apart. There are four char<]eability anomalies witharnplitude~; greater than 5 ms. Each is caused by an orebody.

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First Geophysical TestsComprehensive geophysical tests were performed during

the spring of 1963. Hunting Survey Corporation Limited wascontracted to carry out tests of self-potential,electromagnet ic, and induced polarization <]80physicalmethods over orebody N-42. The results of these tests on twolines three hundred feet (91 m) apart are shown inFigures 30.5 and 30.6. Representative sections through theorebody are drClwn to scale at the bottom of these figures.The station interval for the data points is ] 00 feet 00 m).

The self-potential results shown at the top ofFigure 30.5 show moderate activity on both lines. However,there is no anomaly which could be attributed tomineralization.

For the horizontal-loop elect romagnel ic (HLEM) data, 3

coil separation of 200 feet (61 m) <md a frequency of 2400 Hzwere used. There is possibly 0 weak quadraturc anomalynear the base line on line 2W (Fig. 30.5) where a drillholehas a high grade 75-foot (23 m) intersection of16% Pb/20% Zn/5% Fe. On line 5W (Fig. 30.6) however, there

Figure 30.9, Gradient-array IP profiles obtained in 1964and 1970 on line P (Huntec MK 1). f.I, is the current electrodeseparation. Grade is in per cent Pb, J:n, and Fe.

Figure 30.10. Pole-dipole array IP profiles on line P(Huntec MK 1), Grade is in per cent Pb, Zn, and Fe,

* In figure captions denoting "rluntec MK 1", the data were 3cquired with a Huntee MK 1time-domain receiver which integrates the secondary voltage from 15 ms to 415 ms aftercessation of the 2 s ON current pulse. This is then normalized to the received voltage ofthe current pulse. The chargeability uoits are therefore mvolts-second per volt or, simplyms. In figure captions denoting "Huntec MK 3", t he data were acquired wit h a HuntecMK 3 time domain receiver which integrates the secondary volta<]e from 120 ms to1020 ms after cessation of a 2 s ON current pulse.

65A J.J. Lajoie and J. Klein

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Example of an Indirect Discovery

ThR examples previously discussed dRmonstrate theideal one-to-one correlation between IP anomalies andeconomiR mineralization. Actually, during the initial 1963and 1964 surveys, such correlation was very common andeveryone expected the first hole drilled into an anomaly tointersect an orebody. In some cases, however, IP anomaliesled to discoveries indirectly and this section describes anexample of this.

In Figure 3lJ.ll, the iron sulphides nearly subcropwhereas the zinc-lead sulphides are deeper. The data online M are from a gradient-array IP survey with a potentialelectrodR spacing of 100 feet 00 m). A weak but definiteanomaly coincides directly with the iron sulphidemineralization. The first drillhole confirmed the source ofthis anomaly. However, due Lo favourable geologicalindicators, drilling continued and economic mineralizat.ionwas eventually found. Careful analysis of the detailed IPdata CLln aid in guiding drilling in the search for hiddeneconomic mineralization.

of weak mineralization than the gradient array. Also, theymake it easier to interpret the depth and the lateral limits ofpolarizable sources. The multi-scparation arrays becamepractical with improvements in instrumentation in both timedomain and frequency domain methods, which allowedmeasurements of the lower signal levels at higherseparations.

IP Anomaly Over a Small. Massive. Shallow Orebody

The larger orebodies at Pine Point mLlY be discoveredrelatively easily with multi-separation lP surveys, especiallyif they are at or near subcrop. The smaller are bodies,however, offer a little more challenge and this exampleillustrat es the discovery of a small massive-type orebodywhich is a very difficult drill t.argRt. On a property the sizeof Pine Point, there are many drillholes with intcrestingintersertionsof mineralization and it is difficult to decidewhich ones to follow-up first. Diamond-drill hole (DOH) A

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Note the very low variation in background level ofchargeability; this is typical for much of the Pine Pointdistrict. The anomaly in the centre of Figure 3D.7 is the N42orebody on which the first IP tests were carried out. It hadbeen previously found by grid drilling. The other threeorebodies occurred nearby within 8reas which had beenpreviously tested with drillholes spaced roughly 1000 feelU05 m) ap8rt. However, none had intersected" the orebodies.They were discovered directly as a result of the survey shownin Figure 30.7, actu811y within a few days of each other.

Figure 30.11. Example of an indirect discovery using time­domain IP on line M (Huntec MK-l) showing apparentresistivity and chargeability profiles. OB is overburden.

Advantages of Multi-separation Arrays

The high success rate of the eilrly surveys eventuallyslowed down with the completion of the drill testing of allobvious anomalies; experimentation then began to determinemore diagnostic survey parameters for ore delineation. Thefollowing example illustrates a field experiment where thetarget consisted of short minerali?ed intersections cdongline P with grades of about 3.6% Pb/1.5% Zn/O.'j% Fe in threeholes 100 feet 00 m) apart. They are shown in plan inFigure 30.B and in section in Figures 30.9 and 30.10. Theupper profile in Figure 30.9 is a 1964 gradient-array surveywith a potential elecl.rode separation of 200 feet (61 m). Thecurrent electrodes were located at A and A', 3400 feet(1034 m) opart and BOO feet (244 m) away from the surveyline. There is no indication of an ilnomaly to hint at themineraliZed ion. As shown irl the cent.re profile, the surveywas repented in 1970, using the identi cal parameters and theresults were essentially the same. For the lower profile inFigure 30.9, the CUlTent elertrodes wcre moved to Band B',direcUy on the survey line, 2000 feet (610 m) Clpart, and thepotential electrode spacing was reduced to 100 feet (30 m).As can be seen on Figure 30.9, this electrode configurationproduced a definite anomaly roincident with themineralizution. However, surveying large areas with suchsmall elertrode spLlcings and survey blocks is costly.

Next, a two-separation pole-dipole array was tried withLln electrode spacing of 200 feet (61 m). rhe resultantanomaly shown in Figure 30.10 is stronger and more definitethan that obtained with t he gradient array; On secondseparation (N=2), the expected double peaking of the anomalywi th the strongest peak on the side of the current electrodecan be scen. Surh field tests showed that, here at least,multi-sep8ration mrays are better focused for the detection

Pine Point, NWT 659

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Figure 30.14. IP-resistivity data obtained on line A oforebody R (Huntec MK 3).

20m-65m @ 3.4/9.1/7.4

Figure 30.16. IP-resistivity data obtained on line B oforebody R (Huntec MK 3). Grade is in per cent Pb, Zn, andFe.

locat.ed near orebody R as shown in Figure 30.12 is t.ypical.At a depth of 65 m, it intersected 0.5 m with a grade of0.5% Pb/17.B% Zn/0.4% Fe. The base line was surveyed withiJ pole-dipole IP array and the results are shown inFigure 30.13 in pseudosection form. The pole (moving currentelectrode) is to the right and the electrode spacing is 75 m.The top pseudosection shows apparent resistivity data whilethe boUom shows t.he apparent chargeability data. DOH Awit.h its intersection is shown at. the bottom of Figure 30.13.An IP anomaly is evidenl and it has a few importantcharacteristics. First, t.he shape of the anomaly indicates ashallow source. However, the st.rongest chargeabilities do notoccur on first separation (N=l) as might be expected.Instead, the strongest charge abilities occur at sepmations ofN=2, 3 and 4, on the pole-side "pant-leg" of the anomaly, t.hehighest being 9.7 ms. Second, t.he mineralized intersection inDOH A does not explain the high chargeabilities of over 9 ms.Third, t.he cent.re of t.he anomaly is displaced from DOH A.An anomaly with t.hese characteristics, that is, a shapeindicating a shallow source but wit.h t.he strongestchargeabilities occurring on higher separations, is recognizedas evidence of a subcropping body of polarizable materialwhich is just. off t.he survey line to one side or the ot.her.

Subsequently, lines A, B, and C which are about 100 mapart., were surveyed. Figure 30.14 shows the data obtainedon line A. There is an apparent chargeabilit.y high of greaterthan six ms in a background of about. t.wo ms and it has apoorly developed iJnomaly shape. The anomaly is centred justnorth of t.he baseline.

Figure 30.15 shows the IP-resist.ivit.y data obtained online C. The results are similar to those obtained on line A,t.hat is, a broad region of moderate chargeabilit.y with thehighest readings on second and third separations.Figure 30.16 shows the IP-resistivity data obtained on line B,the centre line. This is a typical Pine Point IP anomaly oversubcropping mineralization with the pole-dipole array. Theanomaly shape indicates a shallow source with the strongestchargeability value occurring on first separation, on the poleside, and with t.he chargeability amplitudes decaying downeach "pant-leg" of the anomaly from 9.3 to 7.3 ms. Notethat there is no significant apparent. resistivity anomalycoincident with the IP anomaly on this line. It isapparent that line B passes directly over the polarizablesource. DOH 1 intersected 45 metres of mineralization

660 J.J. Lajoie and J. Klein

grading 3.4% Pb/9.1% Zn/7.4% Feat subcrop. Subsequentdrilling outlined a small but high grade orebody whosehorizontal dimension along line B was less than 60 m.

An example such as this shows that IP is a verypractical tool for finding orebodies of small lateraldimensions at Pine Point. Geological exploration throughdrilling is useful to define areas of good potential. However,when it comes to pinpointing the target for drilling, IP is themore cost-effecti ve exploration tool.

Example of Lithologic Correlation

So far, the discussion hCls been mainly concerned withthe location of IP allOmalies imd their subsequent drilling todiscover economic mineralization. Another use for thegeophysical data is in lithologic correlation studies.Figure 30.17 shows a pseudosection of app8rent resistivity,chargeability, and metal faetor* data on line N. /\ pole­dipole array was u~;ed, the pole being to the north. Theelectrode spacing was 75 m. One call easily pick out twodi fferent regions from these data. In the northern hal f of thesection, both the apparent resistivity and chargeability dataappear to indicate a two-layer earth, while, in t he southernhalf, n hal rspace appears to be a more appropriate model. Inthe northern region, there is a good positive correlationbetween the apparent resistivity alld chargeability data whichwould lead one to suspcct that the variations observed inthese data are due to the same geological units. Because ofthis positive correlation, the metal factor data acts as afilter of the layer effect.

In the northern region, an average of the apparentresisti vities and chargeabilil ies from A to B, denoted inFigure 30.17, for each separation, yielc!s the values shown inFigure 30.18. Figure 30.18 81so shows a simple two-layerresistivity and chargeability model which gives a reasonablefit with the observed data. The thickness of the top layer inthe numerical model is 80 m.

A correlCltion with drillhole information (Fiq.30.19)~;hows that the high resistivity and chargeability layer may beexplained by a facies which could be described as a fine,sucrosic, and argillaceous dolomite with poorly developedbedding. The high resistivit.y is most likely the result ofpoorer permeability and t.he higher chargeability is probably afunction of the clay and iron content in this facies.

Limits of Detectability and Usefulness of the InducedPolarization Method in Pine Point

The following two examples demonstrate t.he limits ofdetectabilit.y of the IP method at Pine Point.

In the first, hole no. 1 (Fic]. 30.20) was drilled in anarea of favourable geology Clnd intersected 56 feet. (17 m)grading 3.5% Pb/6.4% Zn/7.6% F ~ at a c!epth to top of203 feet (62 m). However, 16 more holes were drilled ono grid of 100 feet UO m) and no sulphides were intersected.A 4-separCltion pole-dipole IP survey was carried out on line Land the data presented in Fiqure 30.21, with the drillintersection shown at the bottom. The background inchargeabilit.y is about 1.8 ms and a weak anomaly with a peakof 2.8 ms, only 1 ms above background, occurs on secondseparation. Note that the contour interval is only 0.25 ms.From the shape and amplitude of this anomaly, one can safelyconclude that it i~; caused by the mineralization found inDOH 1 at a depth of 200 feet (61 m). This is a good field

* Metal factor is defined herein as apparent chargeability dividedby apparent resistivity, multiplied by 1000.

example for demonst.mting how small a target can bedetected Ht moderate depths in areas of very low backgroundvariations in chargeabilit y.

The second example demonstrAtes how theeffecti veness of the IP method drops off completely when theorebody contains no signi ficant omounts of the conductingminerals - galena, marcasite, and pyrite. Figure 30.22 showsem IP and apparent resisti vi ty pseudoseetion over a "run-type"urebody which pas~;es under line S and consists mostly of~;phalerite which does not produce an IP effect. Theelectrode interval chosen for this test survey was Drlly 25 min an atternpt 10 optimi7e the detection and resolution of thisshalluw orebody whi ch is at a depth of 30-40 m. There is nonnomaly coinciding with the ore zone. This can be explainedhy the grade of combined conducting sulphides of lead andiron which is only about 1.3%, and not significantly anomalousin this particular area of the property. Thus the onlypossibility of mapping Ihe ~xtension of the ore zone is bystudying its relation to patterns in the goephysical data.

The GraVity Method

After IP, the next lOCJical geophysical tool to usc isgravity. Seigel (1%8) quoted densities of 2.65 and 3.95 gm/ccfor the host limestone and ore in the Pyramid orebody. Therehas been no attempt to further establish rock c!ensit ies,however, because of the widely varying relative amounts ofsphalerite, galena and pyrite from one ore body to the other.The dem;ilies of these minerals are 5.7,7.5 and 5.0respectively. Also, the degree of porosity can varysignificantly in the host rock in the vicinity of an orebody.The following examples will attempt 10 show the advantClgesand limitHtions of the gravity method.

Figure 3D.23 shows the residual Bouguer gravity and IPdata over orebody A. There is a charge<lbility anomaly ofabout 10 ms above background within a broad region of lowapparent resistivities of about 100 ohm-metres. Coincidentwith the IP high is a residual 110uguer gravity anomaly of0.5 mgals. There is obviously very little doubt t.hat theseanomalies are due to underlying mineralization. One of themost interesting holes drilled on this orebody intersected72 feet (22 m) of ore grading 8.9% Pb/17.5% Zn/13.4°;b F-e.Orebody A has 3.5 million tons of ore grading 4.2% Pb and9.5% Zn. This example illu~;trates that gravity data forrn anexcellent complement to the IP data. The gravi t.y methodresponds not only to the conductive ore minerals but also tosphalerite and may therefore indicate concentrations of zincore which do not show up on the IP data.

Figure 30.24 shows IP Clnd gravity data over a sinkholeon line U. Sinkholes occur very frequently at Pine Point andare usually filled with sand, grnvel, granite boulders, andlimestone breccia whose average density might therefore beexpected to be les~; than the host limestones and dolomites.This sinkhole is characl erized by a moderate chargeabilityanomaly of about 4 ms above background and resistivityanomaly of 200 ohm-metres below background. The residualgravity data show a weak negative anomaly of 0.4 mgals.Two holes did not intersect solid bedrock after drilling about150 feet (46 m) in an area where the overburden thir.kness isknown from nearby drilling to be about 40 feet (12 m).

The last two examples suggest that it is entirely logicalto conclude that, given an J[' anomaly, a coincident positivegravity anomoly indicates mineralization while a negativegravity anomaly indicate~; a sinkhole. Figure 30.25demonstrates the danger of being restricted by any model,

Pine Point, NWT 661

0+00. 1+50N. 3+00N 4+50NI I

6+00N 7+50NI I

9+00NI

10+50N 12+00NJ J

13+50N, 15+00NI

16+50N, 18tOON 19+50NI ,

N-I

N-2

N-3

N-4

RESISTIVITY (APP) OHM-METERS

.oo~. ,,~··;~c\ I• ~ .'" .~~ • • . • • • • ..~ ~ 800

DDH4

N -I

N-2

N-3

N-4

CHARGEABILITY (APP) MILLISECONDS

... f·~·"")..· ... -\.

~'~'.~.. '~\.~'.. _/~\.. '.':---1,P 2-_0 ..... ...·0

.):~~ (0.0

. . .. . . ..~.~o

.:.:.:.:.:~~O.'~:

.--:-.' .~.' .~'O~-r'-""'" • ~2;:---

METAL FACTOR (APP)..,0 "0 "Q..J ~~ « 0

N-I-.:'·' 'v"-:./) '<. "'-."':J\' . o'--..y .'

N-2~'~"'".N-3 0 . . . . 0 .N-4 '0 ... .,. ..~~~.

50 ~o -10 ~o

IP-resistivity data obtained on line N (Huntec MK 3).

I- a-+- N a""i

r§h ~, /

" /"" /'/a-75m

Figure 30.17.

oI

100 200 300I

METRES

N - 1 1635 1635TWO-LAYERAPPARENT

N-2 1105 II I 5 MODELRE SISTIVITY

N-3 850 795 ./I-2000Jtm t(OHM-METERS) N _4 625 640

80mm = 5 M S EC 1- f..= 475Jl.m

N - 1 4.5 4.6 2APPARENT m . 2.1 MSECN -2 3.9 3.8CHARGEABILITY N _3 3. I 3.0(MSEC) N-4 2.4 2.5

(NORTH HALF)

SEPARATION OBSERVEDAVERAGE (AIoB)

Model fitting of IP-resistivity data obtained

.)~(@e-~

7 l•15A

16 8

17A

9•f

SURVEY

LINE L

Plan of drillholes in the vicinity of line L.

FEET

o 200, ,

2 5•10 13A

3 IIBASELINE

12 14..4 6•

o 203'-259'@ 3.5/6.917.6• 200' DRILL GRID

.. 100' DRILL GRID

Figure 30.20.2

TWO-LAYERMODEL

34

Figure 30.18.on line N.

S NO~E:;:S=E=T;:Of:S=U::;:C=R=O=S=IC::::;;:Z::=::::'fS=U=C=R=O~S=IC+D=O::::L=O:::;M=IT:;:E::'fA:J~=G=I=L=LA=CE~~:""'uJDOLOMITE. THINLY POORLY DEVELOPED BEDDING 'IBEDDED. VARVED 80m

(TIDAL FLAT) (SUBTIDAL BACK REEF) r

Geological cross-section of line N.

N.I 209'-!!l~229 244 282 250 248 390~302 262N=2 "3i2"-302-"318 352 377 327 351 ~446'-36L

N.3 344 385 364 37 17 353 374/406 443 412 474N.4 419 418 435 439 91 390 400 451 409/516,

I I I I I I I I I

4+00N StOONI I I I

8+00S 4+00S 0I I I I I I

o 200400L........L-....

FEET

CHARGEABILITY (APP) MSECNol 1.8/ 1.6 1.~/2.1 25 2.5 ~2/ 18 \SI 19 19N-2 ~;2~ (6~ 2. (17 18 20;-'}0No 3 1.8 1.8 2.0 2.5~ 2.4 2.0 1.7 1.9 2.0 19N·4 'IiJ 4 2.4 2.4 2.3 2 \ 1.9 1,9 2.0 2.0

...No..... a ...

08 mrr6.

,

:. ' •.13.5 ~ r, /

I o~o;OO'

203'-259'@ 3.5/6.9/7.6

RESISTIVITY (APP) OHM-METERS

DOLOMITE

POROSITY

ANDFACIES)

SUCROSIC

INTERSTIT

BARRIERARENITE

DENSE T

WITH HIG

(ORGANICCLEAN

Figure 30.19.

250m

~r

Figure 30.21. Time-domain IP-resistivity data on line Lover deep and localized mineralized intersection (HuntecMK 3), Grade is in per cent Pb. Zn. and Fe.

662 J.J. Lajoie and J. Klein

0+50S 0 0+50N I+OON 1+50N 2+00N 2+00N RESiDUAL BOU GU ER GRAVITY

lJl-,0.5«Cl

:!:• 25 RESIDUAL BOUGUER GRAVITY

o'----+----j----+----+---f....J 0FEET 0 400N BOON 1200N 1600N 2000N

3 UUl

2 lJl

:!:

200e

1000/

ARRAY4

ARRAY300

BOO N 1200N 1600 N 2000 N400N

RESISTIVITY GRADIENTAPPARENT APPARENT a- 200'CHARGEABILI-Y' RESISTIV~T/.~

'+/. ". ~/~.- ,,:_.

0--./

.-., ...--'--.........-..... /. ._.

"...../

SINK

o l...---+----t-__-+__-+__--;....J 0400N BOON 1200N 1600N 2000N 2400N

HOLE~tt1.r· :; .:.~::7i.7)::;'):SECT 10 N PLAN

4

a

o(/)

~-.2Cl

=<_.4

Ul1J(/)

=<

-.6FEET 0

IP AND12

IP AND RESISTIVITYAPPARENT GRADIENT

400T/'-'\RESISTIVITY a- 200'

~ 30~f\-~:~~~~Il,n200 '\ \ ,.......':::::::._.100 .,:-/ ._.

+0.2

r~ 0 . ·_·....,1 / .....1

~ -0.2 \.._.

-0.4FEET 400N aOON 1200N 1600N 2000N 2400N

~.•...•..•.•.•• 08

\

<- ".... . "..•.. . LITTLE OR NO

. .• RECOVERY IN D.D.H'S(SINK HOLE)

Figure 30.24. Time-domain IP-resistivity and Bouguergravity data over a sinkhole on line U (Huntec M K 1)./~.

. \.I \/ .

._.,,/'\.! \ .....,o ._/ 1 1 1 j".......__._,

FEET 0 400N BOON 1200N 1600N 2000N

RESIDUAL BOUGUER GRAVITY

IP AN 0 RESISTIVIT Y POLE-DIPOLE ARRAY

12 APPARENT ,. 200,",: roo• CHARGEA8IL1TY//'""~PPARENT

U B \ "'/ RESISTIVITY 200 E

~ 4 ._>/. __' ~. 100 cj

o . 0FEET 0 400N BOON 1200N 1600N 2000N

'"~106'- 17B' ,@ B_9 / 17.5 /13.4

Figure 30.23. Time-domain IP-resistivity and Bouguergravity data over orebody A (Huntec MK 1). Grade is in percent Pb, Zn, and Fe.

RESISTIVITY (APP) OHM - METRESN" I 283 279 277 283 294 291 267 342 331 366 355 374 359

N'2 "435-37~2A~7~389 389--473/50~!_500-521r-N'3 ~507,442 475---s47525~575 564 576 567__ 605

N-4 528 555......... 499/555 .... 603..... 583 517 519 599-6Ir~t3

Figure 30.22. Time-domain IP-resistivity data on line Sover run-type orebody consisting mostly of sphalerite (HuntecMK 3). Grade is in per cent Pb, Zn, and Fe.

n··2

" .•... ly :...o 25 50L-..l....-.JMETRES

33m-36m @ .7/5.71.7 40m-48m @ .7/3.71.6

CHARGEABILITY (APP) MSECN=I

N=2

N·3N·4

even one as simple as that jusl described. In ~- igure 30.Z5,there is a chargeahility anomaly of about 2 ms ahovebackground, a resistivity anomaly of about 300 ohm-metresbelow background and a negati ve residual gravity anomaly of0.2 mgals. As might be expected, the three drillholes shownin section intersected sinkhole material down to aboul150 feet:. Drillhole no. 4, however, shown in plan view andonly 100 feet 00 m) to the east of the other three,intersected 177 feet grading 8.4% Pb/Z.l % Zn/1.5% Fe.Further drilling outlined u small ore body adjacent to thesinkhole. The drilling was guided by thal part of thechargeability anomaly which was intermediate in amplitudebetween background and the peak over the sinkhole in theremainder of this survey area. Actually, in this case, thegravity data were acquired on a test basis after all drillingwas completed.

Figure 30.25. Time-domain IP-resistivity and Bouguergravity data over a sinkhole and nearby orebody W (HuntecMK 1). Grade is in per cent Pb, Zn, and Fe.

A moderate amount of exploration experience usinggravity has shown that most anomalies were due to varyingoverburden thickness. Therefore, to be truly effective,gravity surveying should be accompanied by refractionseismic surveying to determine the variations in overburdenthickness in order to apply terrain corrections to the gravitydata. However, this considerably lowers the cost­effectiveness of the gravity method in reconnaissance surveysat Pine Point in comparison with the IP technique and maynot be feasible for general use.

Pine Point, NWT 663

43 40 30 20" 10 2.SHOT POINTS.......•...••............••..•...••.•......TOTAL FIELD MAGNETIC DATA

40(f)

<l: 30::E::E

20<l:C)

/'10

0 .~ I I IFEET a005 4005 0 400N aOON 1200N

I P AND RESISTIVITY DIPOLE-DIPOLE

3 % APPARENT % ~~~~'t~~~Y O' 200 N'I J600FREQUENCY " _

2 % EFFECT ~:L"' y .""'" 400 E.-'_. ._--- -- '"'0/0 ._•.".". .- ........._. 200 .,

O'----+----+---!------if-----+---loFEET a005 4005 0 400N aOON 1200N

08 ~RCA5ITE WITH....TRAcE PYRRHOTITE

MINERALIZATION

Figure 30.26. Frequency-domain IP-resistivity andmagnetic data over iron sulphide deposit on line Y (McPharP660, 0.3-5 Hz).

5

FEET, io 1000

0

UILlUl

!ILl

::::E

200 I-

..J

ILl

>!OO <It

cr:I-

~

<It400 ~

IILlZ0

1100

N..MINERALIZED ZONE WITHIN

A LARGER UNDEFINED

COLLAPSE STRUCTURE

The Magnetic Method

Small amounts of pyrrhotite, the magnetic ironsulphide, are sometimes found with pyrite and marcasitemineralization which, in turn, occur wi th the ore.Figure 30.26 shows frequency-domain, IP-resistivity andmagnetic profiles over one extremity of a noneconomicdeposit consisting mainly of barren iron sulphides. Thefrequency-domain IP results were obtained using aMcPhar P660 syst em. Of the holes drilled into this deposit,the drill hole shown had the highest concentration ofpyrrhotite. Coincident with thc IP anomaly is a magneticanomaly of about 20 gammas which was confirmed byrepeating the survey. However, since Pine Point is in theauroral zone, the area is subjected to severe magnetic stormactivity, and since only a 20 gamma anomaly was obtainedover a deposit with highcr amounts of pyrrhotite than usual,it is unlikely that the magnetic method can be a significantexploration tool at Pine Point in comparison with IP.

The Reflection Seismic Method

Some experimentation has been carried out with theseismic reflection method in order to determine its possibleusefulness as a mapping tool, a direct ore finder, and to finddisturbances in the bedrock such as faults and collapsestructures which may lead to mineralization. Figure 30.27shows a seismic section on a line whieh passes over orebody Awhose lateral extent is shown by the thick arrow. Theorebody is contained within a large area of collapse. Thedata were processed wi th a 25-75 Hz bandpass filter. Severalgood reflections appear at the ends of the line and there is adefinite depressiorl from shot points 13 to 27. The shallowestcontinuous reflection ap[Jears at a one-way travel time ofabout 160 ms and this definitely puts it below the ore zone.It is possible that the depression in the reflected events is avelocity slow down anomaly; that is, reflections from deeperhorizons were slowed down in passing I hrough a region oflower velocity in the collap~;ed area surrounding themineralization. However, Figure 30.27 shows the best resultsof the experiment. The remaining seismic data acquiredduring the test were not as encouraging.

25 Hz TO 75 Hz BANDPASS FILTER

Figure 30.27. Reflection seismic data over orebody A. Shotspacing = 110 feet (34 m). Group offset = 1320 feet (402 m).

It is possible that very high resolution seismic reflectionsurveys such as those carried out for nuclear si teinvestigation may be more successful. Again, however, thccost effectiveness of such surveys would not compare withthat of the IP method on the Pine Point Mines property.

CONCLUSIONS

Most of the Pine Point Mississippi Valley-type depositscontain sufficiently high concentrat ions of the conductingminerals galena, marcasite, and pyrite, to be good IP targets.The poor electrical continuity in bulk is a commoncharacterisl ic of these ores and explains why the orebodies donot respond to standard electromagnetic methods. Thechargeability anomalies produced by the ore are usually ofthe order of 10 ms (although chargcability values in excess of30 ms were observed in one case). This is not very large whenone considers that normal background variations inchargeabilit y in many volcanic terranes arc often as high asthis. Therefore, the success of the IP method at Pine Pointdepends largely on the very low and uniform chargeability ofthe host limestones and dolomites. Weak variations in thecharqeability can be measured which CiJn sometimes becorrelated to the lithology. Production surveying is howeveroften hindered by high telluric activity since Pine Point is inthe auroral zone. The gravity method is useful as acomplementilry tool especially in anomaly del ailing, andresponds to sphalerit e mineralization which is not polarizable.At the present time, EM, magnetic and seismic methodsappear to have little application as direct ore-finding tools incomparison with the IP method. Thus, the IP method is by farthe most cost-effecti ve geophysical exploriltion tool on thePine Point Mines property.

664 J.J. Lajoie and J. Klein

ACKNOWLEDGMENTS

We wish to thank Cominco's Northern Group based inYellowknife, N.W.T. and Pine Point Mines Ud., forpermission to publish this paper. The geological dat a wereacquired from the geological staff of Pine Point Mines Ltd.The IP data was takerl from surveys performed for Pine PuintMines Ltd. by Canadian geophysic81 contractors.

Paterson, N.R.1972: The applications and limitations of the IP method

-- Pine Point 8rea, N.W.T.; Can. Min. J., v. 93 (8),p. 44-50.

Seigel, H.D., Hill, H.L., and Baird, J.G.1968: Discovery case history of the Pyramid are bodies,

Pine Point, Northwest Territories, Canada;Geophysics, v. 33, p. 645-656.

REFERENCES

Hallof, P.G.1972: The induced polarization method; in Proc. 24th

Int. Geol. Cong., Montreal, Sec. 9, p.64-81.

Skall, H.1977: The paleoenvironment of the Pine Point lead-zinc

district; Econ. Geo!., v. 70, p. 22-47.