Discovery and Geology of Discovery and Geology of Ldic

8/6/2019 Discovery and Geology of Discovery and Geology of Ldic

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CHAPTER17: DISCOVERY AND GEOLOGY OF THE LAC DES ILES PALLADIUM DEPOSITS

M.J.Lavigne,Galantas Gold Corporation, Omagh Minerals Ltd,56 Botera Road Upper, Cavanacaw, Omagh, Co.Tyrone, U.K., BT78 5LH

E-mail: [email protected]

M.J.Michaud,China Diamond Corporation, 1724 Hyde Park Road, London, Ontario, N6H 5L7, Canada

and

J. Rickard,North American Palladium, 710 Norah Cres. Thunder Bay, Ontario, P7C 4T8

Mineralogical Association of Canada Short Course 35, Oulu, Finland, p. 369-390

EXPLORATION AND DEVELOPMENT

HISTORY

Prior to 1963, few were familiar with theLac des Iles area despite its being only 100 km

north of the present city of Thunder Bay, which ison the northwestern shore of Lake Superior (Fig.17-1). The area was difficult to access due to a lackof roads; it had not yet been subjected to logging. Inaddition, the area straddles the height of land andtherefore lacks navigable waterways. The difficultyof access is highlighted by the fact that neither theGeological Survey of Canada, nor the Ontario

Department of Mines had mapped the area insufficient detail to discover the Lac des IlesIntrusive Complex (LDI-IC). A few prospectorshowever, were familiar with the area. An Ontario

Department of Mines report (Pye 1968) documentedthe earliest activity, "Following the discovery, by anaeromagnetic survey in 1958 of a large magneticallyanomalous area at Lac des Iles, F.H. JowseyLimited acquired two groups of 80 claims coveringa large part of the north end of the lake (Lac desIles). Subsequent to both an airborne and groundelectromagnetic and magnetometer

FIG. 17-1 Location of Lac des Iles, Lake Superior Region, North America.


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surveys, five diamond drill holes were bored to testfour zones.

The drillholes did not intersect anything ofimportance and indicated that, in general theanomalies were caused by magnetite in peridotite

layers associated with pyroxenite." The individualresponsible for managing these activities was FredJowsey, better known for his involvement with thediscovery of uranium at Elliot Lake.

Subsequent to the abandonment of theexploration program at Lac des Iles, Fred Jowseyremained optimistic about the areas potential. Insubsequent years he made several attempts togrubstake a prospecting party, specifically a partythat included Walter Baker of Kirkland Lake.Walter did not make himself available until 1963 ashe was preoccupied prospecting in Hemlo, where hediscovered gold west of the now famous Williams

claims. Walter was later recognized for hiscontribution to the eventual discovery of the HemloGold camp by the Prospectors and DevelopersAssociation of Canada who awarded him theProspector of the Year award in 1987. Shortly afterbreakup in May, 1963, a prospecting partycomposed of Walter Baker, his son Clement,George Moore and geologist Bruce Arnott flew toLac des Iles and set up camp on an island at thesouth end of Lac des Iles.

The prospecting party went to Lac des Ileswith a new tool in hand. In 1963 the GeologicalSurvey of Canada and the Ontario Department ofMines released aeromagnetic maps covering the Lacdes Iles area (GSCODM 1962) as part of its

ongoing nation-wide airborne magnetic survey.Magnetic highs became prospecting priorities, thefirst being a prominent magnetic high south of thelake, within sight of their camp, illustrated in Figure17-2 (results of 2004 aeromagnetic survey). Success

was immediate. They discovered coarse-graineddisseminated chalcopyrite and iron sulfidesminerals. Samples were sent to SwastikaLaboratories on the next supply flight, to be assayedfor base metals only. A note to the assayer fromWalter Baker asked him to keep an eye on thebead as he suspected PGE may have been present.In 1963 assaying for PGE was difficult andexpensive. Walters intuition was correct; thesamples sent contained appreciable amounts of Pd,Pt, Au, Cu and Ni. Within weeks, the property wasoptioned to Gunnex Limited. The prospectorsdiscovered 8 mineralized areas, and claims were

staked and recorded on Walter Bakers prospectinglicense.Gunnex documented the mineralized areas

and cored 12 drill holes. The best result, 302 feet(90 m) assaying 4.89 g.t-1 Pd, was from hole #5.Figure 17-3 shows the location of most of themineralized zones that were discovered by Gunnexdrilling. For reference, the Roby Phase 2 pit, circa1999 and the distribution of pyroxenite are shown.The magnetic high that attracted the prospectors iscreated by magnetite-rich gabbronorite, includingnumerous lenses of semi-massive magnetite. Themagnetite-bearing rocks are devoid of mineraliz-ation, and their palladium tenor is usually atbackground levels. In contrast, the mineralized

FIG. 17-2 Lac des Iles magnetic map, based on 2004 detailed airborne survey.


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E

A

B

C

H

F

A

No Window

250

m ret es

0DDH 3: 1.37 g/T PGE,

0.099% Cu, 0.082% Ni/150'

DDH 5: 4.89 g/T PGE,

.11% Cu, .13% Ni / 302.2'

DDH 11: 2.71g/T PGE,

.173% Cu, .166% Ni / 144.7'

DDH 2: 4.63 g/T PGE,

.12% Cu, .128% Ni/ 170'

Camp

Lake

ShortyLake

WalterLake

8

7

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11

3

2

6

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5

NORTH AMERICAN PALLADIUM LTD.

LAC DES ILES MINES LTD.

Gunnex Ltd. 1964

Exploration Highlights

Gunnex Sulfide Zones

DDH Mineralization Intersection

1999 Pit Outlines

LEGEND

Pyroxenite Layer - 1999

Gunnex Ltd. 1964 Grid

Gunnex Ltd. 1964 Drillholes1

N

UTM

FIG. 17-3 Discovery map ca. 1963 with Gunnex drilling, superimposed on Phase 2; Roby Pit and pyroxenite ca.1998.

rocks have low magnetic susceptibility. Subsequentdetailed magnetic surveys, superimposed on theoutline of the currently known Pd mineralization,clearly separate the barren magnetic rocks from the

mineralization of the Roby and Baker Zones (Fig.17-2). The exceptions to this are a narrow zone ofthe most intensely altered pyroxenite where talc andmagnetite occur in addition to other alterationminerals and are associated with high-grademineralization, and the Twilight Zone. The TwilightZone is distinguished from the Roby Zone by thedominance of gabbronorite, and relatively weaksilicate alteration, and the near absence of vari-textured gabbro.

The discovery of palladium at Lac des Ilesprompted the Ontario Department of Mines toinitiate regional-scale mapping (Pye & Fenwick

1965, Pye 1968) and attracted the interest of a largemining company, Anaconda American BrassLimited, who optioned the property. Anacondaconducted an exhaustive examination of the areaover 3 years, mapped the LDI-IC and conductedgeophysical surveys. This was followed by drilling13 more core holes, mostly on the existingmineralization, with similar results. Anacondadrilled geophysical targets as well, which was

unsuccessful at discovering additionalmineralization. Palladium at this time was worthapproximately $35 per ounce, and themineralization, despite its volume potential, was

deemed uneconomic. Anaconda dropped the optionand the property became dormant.In 1973, Gunnex held and administered

Walter Bakers prospecting license. This commonpractice provided assurances to mining companiesthat mining claims held under an individualprospectors license werent cancelled if aprospector failed to renew his/her license. WhileWalter was out prospecting in northern Canada, theanniversary date of his license passed withoutrenewal, and all the mining claims attached to itwere cancelled. This went unnoticed until ThunderBay prospector Knut Kuhner asked a clerk at the

mining recorders office for the Lac des Milles Lacsclaim map. The clerk gave him the Lac des Iles mapby mistake. Kuhner noticed a block of cancelledclaims and recognized the property. Kuhner foundfinancial backers, organized a grubstake and flew toLac des Iles to re-stake the claims. It subsequentlytook a year to find a company interested in theproperty as palladium prices were still low. In 1974,the property was optioned to Boston Bay Mines


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Limited, a company controlled by Patrick Sheridan.Yet another grid was established and moregeophysical surveys conducted. The individual withthe on-site responsibility was Phillipe Roby. Thediscovery of high-grade mineralization is attributed

to him, and he was responsible for orienting twokey holes in this campaign. Holes # P014 and #P015 were oriented to intersect newly discoveredmineralization in highly altered pyroxenite east ofthe Gunnex E-Zone (Fig. 17-4). Hole # P015returned 8.37 g.t-1 Pd, 0.247% Cu and 0.245% Niover 180 feet (55 m). This drilling led to therealization that a lenticular zone of higher-grademineralization occurred on the margin of a broaderzone of lower grades. Subsequent systematicfollow-up drilling defined an ore body named theRoby Zone.

The discovery of both higher base and

precious metal values in a coherent ore bodyattracted Texasgulf Inc., discoverers of the KiddCreek Mine in Timmins. Texasgulf optioned theproperty in 1975, accelerated the resource definitiondrilling program, re-examined the entire LDI-IC,examined many other intrusions in the area, and

conducted more geophysical surveys. As the resultof a negative scoping study on production in 1976,the option was abandoned. The property then laydormant until 1985.

Palladium in 1976 was valued at less than

$100 per ounce, a consequence of oversupplycreated by incidental palladium production as by-product of copper-nickel mining at Norilsk, (asmuch as 3 million ounces a year), and Sudbury andfrom the Bushveld platinum mines. It was theenergy crisis and the development of alternateenergy sources such as PGE-based fuel cells in the1980s that led the speculative market to drivepalladium prices to $150 per ounce. On this basis,Patrick Sheridan was able to finance renewedactivity at Lac des Iles. Madeleine Mines Ltd.acquired 50% ownership of the leased claims fromThe Sheridan Platinum Group in 1986 and

continued to delineate the Roby Zone. MadeleineMines also built a 2400 ton per day plant which wasin operation for 3 months in 1990. In 1991, KaiserFrancis Oil Company Limited gained control ofMadeleine Mines Ltd., which changed its name toNorth American Palladium Ltd in 1993. A

No Window

250

m reet s

0 CampLake

ShortyLake

WalterLake

p - 1.58 g/T PGE, .30 % Cu, .20 % Ni/45'

l - 3.77 g/T PGE, .96 % Cu, 2.92 % Ni/1.1'

c - 10.53 g/T PGE, .25 % Cu, .19 % Ni/14'

r - 10.05 g/T PGE,

.31 % Cu, .18 % Ni/4.5'

f - 6.79 g/T PGE, .16% Cu, .15% Ni/4.5'

DDH P12: 6.57 g/T PGE,

.149% Cu, .151% Ni/ 130'


.093% Cu, .138% Ni/200'


.25% Cu, .092% Ni/ 40'


.247% Cu, .245% Ni/180'

DDH P5: 3.77 g/T PGE/160'

E

11

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10

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2

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12

15

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P011

P014

P015

P013P012

P016

P003P007

P008

A

B

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H

F

5

NORTHAMERICAN PALLADIUM LTD.

LAC DES ILES MINES LTD.

Boston Bay - 1975

Early ExplorationBoston Bay 1974-75 Drillholes

DDH Mineralization Intersection

Boston Bay Surface Showings

1999 Pit Outlines

Gunnex Sulfide ZonesA

LEGEND

Gunnex Ltd. 1964 Drillholes

Anaconda American 1966 Drillholes

Pyroxenite Layer - 1999

Gunnex Ltd. 1964 Grid

12

1

N

UTM

FIG. 17-4. Diamond drill holes traces (circa 196466 and 1974) and mineralized areas as mapped in 1963, are shown relative

to the position of a current pit and a pyroxenite unit as mapped in 1992 and 1998.


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significant investment in infrastructure from 1991onwards led to the start of continuous production inDecember 1993.

The mine struggled with profitability, and in1997, the potential residual profit in the existing

open pit operation was less than the existing debt.Fortunately, rising palladium prices in 1998 (nowdriven by increased use of palladium in catalyticconverters) led to a scoping study which examinedthe potential profitability of mining the morevoluminous low-grade ore. The result of the scopingstudy was positive, contingent on the delineation ofadditional ore. This precipitated a drilling campaignin 1999 that doubled the total amount of drillingdone to date to 100,000 metres. The result of thedrilling, in combination with the lowering of miningcut-off grades led to increasing proven and probablereserves from 1.3 to 5 million ounces of palladium

and a positive feasibility study on establishing a15,000 tpd operation. Concurrent with increasingreserves at the Roby Zone, the entire Lac des Ilescomplex, and all other significant intrusions in thearea were subjected to another round of exploration.Continued step-out drilling on the Roby Zonefurther expanded the resources. The discovery ofthe fault-offset high-grade zone at depth was madepossible by geological interpretation based on corelogging. Subsequent to 114,000 m of drilling in2000, the total resource grew to 11.3 million ouncesof Pd.

The key elements that were the basis ofstrategy development for the 19982004 explorationcampaign are:1) the Roby Zone is composed of variable-grade,

irregularly distributed, vertical breccia pipessurrounded by lower-grade vari-texturedgabbros (as opposed to being layer-controlled);

2) with few exceptions, all rock types in the LDI-IC can be ore or waste regardless of sulfidecontent (ore control by simple rock types is notpossible);

3) the mineralization commonly containsdisseminated sulfide, but some significantmineralization is almost devoid of sulfide or anyother visual markers;

4) although the mineralization is usually associatedwith variable alteration of silicates, some areasof altered silicates are unmineralized;

5) the Roby Zone has a surface area greater than1 km2;

6) the intrusion had been thoroughly mapped andprospected on two occasions, and variably

explored on at least three other occasions;7) a variety of geophysical surveys, both ground

and airborne had been conducted;8) more than 50% of the target area has less than

1 m of overburden;

9) palladium values are elevated throughout theLDI-IC (mean >30 ppb Pd), but is less than 7ppb Pd in barren intrusions .

The existence of PGE-rich rock withoutappreciable sulfide was seen as an explorationopportunity, as none of the previous programs weredesigned to discover such an ore type. To this end,the entire LDI-IC was sampled systematically atspacings ranging from 5 to 20 ft (1.5 to 6 m) alongoverburden trenches whose density wascommensurate with the discovery potential. Outcropwas sampled at the highest possible and practicaldensity. More than 20,000 samples were collected,

and areas with anomalous PGE values werefollowed up with further trenching and sampling(Fig. 17-5). This exploration technique wasconsidered cost-effective (approximately $12,000CAD per linear trench km) and provided two-dimensional information (as opposed to one-dimensional drill core). Where significantmineralization was discovered, extensiveoverburden removal was conducted (Fig. 17-6).This was also done in areas of previously knownmineralization with the objective of better definingthe nature and the boundaries of the mineralizedzones as it allowed assays to be superimposed onlithology. These large, washed bedrock exposureswere especially valuable in demonstrating thechaotic nature of the rocks associated withmineralization. The knowledge gained fromexamination of large bedrock surfaces wastransferred to core logging which then led torealistic three dimensional geological models. Inareas covered with greater thicknesses ofoverburden or lakes, induced polarization (IP)electro-magnetic surveys were conducted.Concurrent magnetic surveys helped to screen outIP anomalies created by disseminated magnetite.Areas with known and well-exposed mineralizationwere also surveyed. Previously, IP had seen limited

application and its effectiveness was unknown.Anomalies were investigated by trenching andsampling.

Detailed sampling and assaying of newlycreated bedrock exposures led to many newdiscoveries. The most significant discovery was theTwilight Zone, separated from the Roby Zone by


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FIG. 17-5. 2000 North American Palladium Ltd trench location map.

FIG. 17-6 1999 overburden removal at Twilight Zone (picture)

the barren East Gabbro. Isolated, low-grade Pd

assays had been known since 1963, and althoughsubsequent diamond drilling did intersectmineralization, it was not deemed significantenough to be followed up by definition drilling.Trench sampling uncovered significant amounts ofhigher grade palladium mineralization. Mineralizedzones were subsequently expanded in twodimensions by extensive overburden removal and inthe third dimension by diamond drilling. Similarly,

trenching and sampling of the Baker Zone

demonstrated that a significant volume ofmineralized rock existed. This was subsequentlyaugmented by diamond drilling. Numerousdiscoveries were made throughout the complex, allof which were evaluated on surface for volume andgrade, thus avoiding costly diamond drilling.Included in this were a dozen targets generated byIP. Most of the high-chargeability anomalies weregenerated by zones of silicate alteration, where


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pyroxenes and olivine were converted to hydroussilicates such as actinolite, talc, and serpentine. Insome cases, minor amounts of magnetite alsocreated anomalies.

The superposition of IP results on detailed

surface sampling maps of the Baker Zone showedweak chargeability anomalies over the sulfide-richzone, surrounded by numerous, stronger anomaliesthat were not demonstrably associated withmineralization (Fig. 17-7). A Titan 24 magneto-telluric (MT) survey also produced similar results.The limiting factor in utilizing techniques thatmeasure chargeability and resistivity is thatanomalies are manifestations of the mostvoluminous minerals. The presence of chargeableand variably resistive silicates and the production ofnumerous anomalies that are not associated withmineralization distract attention away from more

legitimate targets. Despite this understanding, someanomalies under deep cover were tested by drilling,with negative results.

The use of indirect geochemical surveys(sampling of media other than bedrock) was alsoinvestigated, but only to a limited extent by theprivate sector. The Ontario Geological Survey(Dyer & Russel 2002, Searcy 2001) and university-based researchers (Cameron & Hattori 2003, Hattori& Cameron 2004) have examined Pd distributionand mobility in the surficial environment. The

preponderance of situations that create falseanomalies, combined with palladiums highmobility in the surficial environment, prevents theuse of soil and lake sediment sampling as aneffective exploration technique. Trial surveys,

conducted by North American Palladium Ltd.,involving bark sampling were carried out alonglinear bedrock trenches that traversed mineralizedand unmineralized rock. Poor correlation existedbetween bedrock and bark assays. In fact, thehighest assay came from an area of barren rock,whereas trees close to and downslope from theBaker Zone returned much lower assays. Airborne,Pd-bearing dust created by the mining operationmay have masked any real anomalies. Hattori andCameron (2004) did find anomalous Pd in humusdownslope from the Baker Zone. Interpretation ofthese results must take into account factors such

overburden source and groundwater flow.In summary, Pd mineralization wasdiscovered by traditional prospecting techniques.Magnetic surveys will discover mafic intrusions,and cursory traversing, sampling and assaying willdiscriminate between pregnant and barrenintrusions. Systematic bedrock sampling is the mostpragmatic discovery tool, as the targets are large.This deposit type is not well-disposed to beingdiscovered by geophysical and geochemicalsurveys.

FIG. 17-7 Baker Zone IPEM on Pd grade contour.


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REGIONAL GEOLOGY

The Neoarchean Lac des Iles IntrusiveComplex (LDI-IC) lies within the Eastern Marmionterrane of the Wabigoon Subprovince, immediatelynorth of the Wabigoon-Quetico Subprovince

boundary, along which mafic and ultramaficintrusions are common, extending 300 km fromRainy Lake to Lake Nipigon (Fig. 17-1). TheMarmion terrane was defined by Tomlinson et al.(in press) as the south-central WabigoonSubprovince and contains juvenile 3.0 Ga crust(Marmion batholith) and minor youngerMesoarchean and Neoarchean volcanic and plutonicrocks which dominantly yield 3.0 to 2.8 Ga Ndmodel ages. The LDI-IC is the largest of a series ofmafic to ultramafic intrusions defining a circularpattern in the LDI area that is approximately 30 kmin diameter (Fig. 17-1). The LDI-IC had previously

been sub-divided into three distinct chambers, theultramafic North LDI-IC, the Mine Block Intrusion,and the Camp Lake Intrusion to the south (Fig.17-8). The LDI suite consists of the Taman,Demars Lake, Buck Lake, Dog River, Tib Lake,

North LDI and the Mine Block intrusions. Whereasseveral other mafic intrusions nearby were formerlyconsidered part of the LDI suite, recent mapping,age dating and lithogeochemistry by the OntarioGeological Survey, (Hart et al. 2000a,b, Hart et al.2001a,b, Stone 2002) and subsequent age dating,lithogeochemistry and compilation (Stone et al.2003) has led to significant new realizations. Thenew mapping has defined the monzodioritic ShelbyLake batholith of the sanukitoid suite directly southof the LDI-IC. Early gabbroic phases occur locallyat the rim of the Shelby Lake batholith, at localitiessuch as Wakinoo Lake, Camp Lake and Legris

FIG. 17-8 Geology of the Lac desIles Intrusive Complex. (Mine

Block Intrusion and Camp LakeIntrusion modified after Sutcliffe& Sweeney 1986, Macdonald &Lawson 1987. North Lac des IlesIntrusion mapped B. Nelson, M.MacIsaac, A. MacTavish and J.Rickard for North AmericanPalladium Ltd, 19992001).


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Lake, all of which host PGE mineralization.Although these gabbroic bodies were previouslythought to belong to the LDI suite, it now appearsmore likely that they are the early phases of theShelby Lake batholith. The Shelby Lake batholith is

dated at 26900.9 Ma (Kamo 2004) which is similarto an age date of 2689.01 for gabbro pegmatitefrom the Roby Zone in Mine Block intrusions of theLDI suite (Davis 2003).

Stone (2003) mapped regional-scalemylonitic zones associated with major intrusions ofthe LDI suite, the largest of which is the ShelbyLake fault, which extends 75 km southwest fromthe Mine Block Intrusion to the boundary of theQuetico Subprovince at Lac des Mille Lacs. Othermylonitic zones are spatially associated with theBuck Lake and Tib Lake Intrusions. The maficintrusions themselves are not strained, although

repeated magmatic injection and brecciationobserved at the Buck Lake Intrusion can beattributed to episodic magmatic activity along acrustal-scale fault.

Geology of the Lac des Iles Intrusive Complex

(LDI-IC)

The following description of the geology andits interpretation is based on observations madeduring the course of an intense exploration programthat lasted six years. This was the third significantexploration program conducted at Lac des Iles bywell funded companies at the direction of qualifiedgeologists. Most of the observations made by thefirst two programs have been lost and the followingis an attempt to bring into the public domain someobservations and thoughts of geologists who are toobusy (at earning a living trying to find mines) towrite papers. The observations were made duringdetailed mapping and assaying of several thousandsquare metres of outcrop generated by extensiveoverburden removal, the examination and assayingof 200,000 metres of core, for the sole purpose ofdiscovering mineralization. The interpretation ofthese observations was conducted for the purpose ofdeveloping a pragmatic exploration model. As such,this was not an academic study and this paper does

not compare the observations with previous work,or similarities elsewhere. The most significantdistinction between the data gathering of thisexploration program and a typical academic study isthe scale and coverage. The entire LDI-IC wasmapped with the aid of overburden removal, andsampled excessively (>20,000 samples), and themineralized zones were almost completely stripped

of overburden, and mapped and sampled in detail.They were also defined in the third dimension bydrilling. A companion study (Rickard et al, in prep.)was the first mineralogical study to examine theentire deposit comprehensively and systematically.

This study was driven by the needs of themetallurgist. In addition to mineralogy, this paperwill also bring into the public domain an analysis ofthe mine-planning assay database which revealeddeposit-wide geochemical trends that shed muchlight on the genesis of the deposit. The results of themost recent academic study were published byHinchey et al. (2005).

In addition to geological mapping conductedby various mining companies, mapping andpetrographic analysis of the LDI-IC has beenundertaken by university-based geologists andgovernment geological surveys including Pye

(1968), Guanera (1967), Linhardt & Bues (1987),Michaud (1998), Sutcliffe (1986), Sutcliffe &Sweeny (1985, 1986), Sweeny (1989) andWatkinson & Dunning (1979).

The LDI-IC has two chambers, each adistinct lithological domain (Fig. 17-8). Ultramaficintrusions are centered on Lac des Iles (North Lacdes Iles Intrusion, NLDI-I). Chaotic gabbroicintrusions occur immediately south of Lac des Iles(Mine Block Intrusion, MBI). These two intrusionsare partially separated from each other by tonalitesepta. The presence of widespread breccia and therarity of rhythmic layering suggest a dynamicintrusive environment with disruptive magmaticpulses.

The North Lac des Iles Intrusion (NLDI-I) isultramafic in composition with a minor maficcomponent. On the basis of mapping by NorthAmerican Palladium (Lavigne and Michaud, 2001)the NLDI-I can be subdivided into four domainsbased on lithological variations and structure. Thefour domains are:1) massive to broadly layered, east-trending

clinopyroxenite, with lesser websterite andminor olivine-bearing units underlying theextreme north-northwestern portion of theNLDI-I;

2) massive to locally well-layered, north-trendingclinopyroxenite, websterite, thin olivine-richunits and underlying gabbro in the northeasternportion of the NLDI-I;

3) east-trending, massive gabbro, gabbronorite,vari-textured gabbro/gabbronorite, andheterolithic gabbro/gabbronorite breccia,interfingered with massive clinopyroxenite and


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minor websterite and olivine-bearing units in thesouth-central portion of the NLDI-I;

4) clinopyroxenite with less-abundant, olivine-bearing units, websterite and gabbro, underlyingthe southern portion of the NLDI-IC, whose

geological contacts and geophysical trendsdefine a circular structure.

In all four domains, outcrop-scale rhythmiclayering is uncommon and weak. Detailed mappinghas revealed that proportions of orthopyroxene,clinopyroxene and olivine vary considerably,randomly and gradually, thus rendering theextrapolation of geological units from outcrop tooutcrop difficult. The most common style ofmineralization in the NLDI-I consists of zones up to10 m in thickness of disseminated chalcopyrite,containing up to 1.0 g.t-1 Pd + Pt.

The Mine Block Intrusion (MBI) is

texturally and compositionally complex (Fig. 17-9).Its composition ranges from anorthosite toclinopyroxenite, leuco-gabbronorite to melanonorite

and includes magnetite-rich gabbronorite. Texturesinclude equigranular, fine- to coarse-grained,porphyritic, and pegmatitic, vari-textured units andheterolithic gabbro breccia. These last three texturaltypes are the most common hosts to PGE

mineralization, including the Roby Zone.The MBI consists of two lithologically

distinct domains. The oval-shaped domainimmediately south of Lac des Iles is lithologicallycomplex and contains widespread PGEmineralization, whereas the domain further to thesouth is dominated by massive, medium-grained,PGE-barren gabbronorite. Systematic surfacesampling of the massive gabbronorite hasdemonstrated its PGE content to be anomalouslylow, as no samples exceeded the analyticaldetection limit of 30 ppb. Extensive stripping hasrevealed that the interior of the oval-shaped domain

south of Lac des Iles has an abundance ofmonolithic and heterolithic breccia with an averagecomposition of gabbronorite. Within this area,

FIG. 17-9 Simplified geology of the Robyand Twilight zones (LDI-IC).(Modified after Sutcliffe & Sweeney1986; Macdonald & Lawson 1987,Michaud 1998).


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individual lithological units are not laterallyextensive but rather chaotically distributed. Themost laterally continuous unit is a massive,medium-grained gabbro, referred to as the EastGabbro. It is adjacent to a vari-textured gabbro

rim to the west and more equigranulargabbronorite to the east. The vari-textured rim ishost to the Roby Zone palladium deposit whereheterolithic gabbro breccia is common, occurring aspipes and pods, and large blocks (~60 m) of varyingcomposition.

Geology of the Roby Zone

The Roby Zone is a bulk-mineable depositwith a minimum north to south length of 950 m anda width of 815 m (including the Twilight Zone). Itsdimensions are much greater if mineralized rockthat is below the mining cutoff grade of 0.7 g.t-1 Pd

is included. The southern boundary to themineralization as seen on Figure 17-9 extends toinclude the Powerhouse and Moore zones adjacentto the massive barren gabbronorite, while itswestern boundary is basement tonalite, and itseastern boundary is the East Gabbro. A tail ofmineralization extends northwards within the rim ofvari-textured gabbro pinched in between the EastGabbro and basement tonalite. Similarly,mineralization extends eastward from the south-eastern corner within the rim of vari-texturedgabbro. The Roby Zone has been intersected to adepth of 1000 m by core drilling. For miningpurposes, the Roby Zone comprises three distinctore types (Fig. 17-9): High Grade Ore (7.6% ofvolume), North Roby Ore (5.3% of volume) andBreccia Ore (87.1% of volume), all of whichcontain multiple styles of mineralization.

The volumetrically most significant host tomineralization is vari-textured gabbro andheterolithic gabbro breccia. With the exception ofthe East Gabbro, every equigranular rock typewithin the MBI has a vari-textured counterpart. As aresult, the term vari-textured gabbro covers abroad range of both textures and composition. Itvaries from leucocratic to ultramafic (pyroxenitic),fine-grained with medium-grained patches and

veinlets, medium-grained with coarse-grained andpegmatitic patches and veinlets, and coarse-grained with pegmatitic patches and veinlets(Figs. 17-10a and b). The coarser patches andveins have nearly identical mineralogy(plagioclase + clinopyroxene + orthopyroxene) tothe equigranular host. The terms vein andveinlets are used to describe thin planar features,

FIG. 17-10. a, vari-textured gabbro consisting of coarse-grained patches in medium-grain equigranular matrixeach with similar mineralogy; b, vari-textured gabbroconsisting of coarse-grained patches and veins hostedby fine-grained gabbro with similar mineralogy; c,comb layering.

and the use of these terms in this case does notimply an introduction of material. In this case, thetransition from wallrock to veins is simply achange of grain size without any disruption ofinterlocking minerals.

Randomly oriented, pegmatitic gabbro podsup to several metres in diameter occur throughoutthe Roby Zone. The pegmatitic pods occur as layer-like bodies, apparently crosscutting dikes, cuspatelenses interleaved with gabbro, and sub-sphericalpods commonly with quartz + tourmaline-bearingcores (Fig. 17-10c). The gabbroic pegmatites may


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owe their origin to the same fluids thought to beresponsible for the development of vari-texturedgabbros in the MBI. As seen on Figure 17-8, vari-textured gabbro defines an ovoid rim on the MBI,which is coincidentally the principal host to Pd

occurrences and deposits, including the Roby Zone.

High-Grade Ore

High-Grade Ore is mostly hosted by andadjacent to a planar unit of pyroxenite andmelanogabbro 15 to 25 m thick. It is located in theeast-central portion of the Roby Zone, bounded bythe PGE-barren East Gabbro hanging wall (Fig.17-11) and Breccia Ore footwall to the west. TheHigh-Grade Ore deposit is planar, trends at 341and dips nearly vertical to a depth of 250 m, andsubsequently dips less steeply at greater depth, tothe east. It is confined along strike to a 400 m

length of the pyroxenite; the on-strike extensions ofthe pyroxenite are barren to low-grade. Acrossstrike, only the western side of the pyroxenite is ore,with much of the higher grades occurring within theadjacent wallrock. The composition of the wallrockalong the western contact of the pyroxenite variesalong strike and with depth, and it includes mostrock types present in the MBI.

High-Grade Ore is composed almost entirelyof secondary silicates, including amphibole andchlorite. The restriction of well-developed, planarfabric to the most altered portions of the pyroxenite,and its lack of lateral continuity is consistent withthe fabric being the product of volume loss duringcompression, as opposed to simple shear. (Figs.17-12a, b and c). The host pyroxenite is acompositionally uniform, medium- to coarse-grained unit of primarily cumulus and intercumulusclinopyroxene. The cumulus clinopyroxene grainsform a mesocumulate texture or, more rarely, anadcumulate texture. Cumulus plagioclase andorthopyroxene account for only approximately 5volume percent. Although clinopyroxene displaysisomodal layering, very subtle grain size-gradedlayering has been observed. Coarser crystals, up to1.5 cm, occur along the western side of the unitcreating a vari-textured zone, whereas equigranular,

medium-grained clinopyroxenes occur along theeastern side. Thus, the western side of thepyroxenite, that which has a higher Pd tenor and isclosest to the Breccia Ore, is vari-textured. Theequigranular, eastern portion of the pyroxenite,furthest removed from Breccia Ore, is lower-grade.In addition, the pyroxenes in the western half arestrongly altered and a minor amount of relict,

unaltered clinopyroxene, orthopyroxene andfeldspar is preserved. These relict grains ofclinopyroxene have been completelypseudomorphed by actinolite and hornblende andare commonly granulated and brecciated.

Sulfide mineralization within the pyroxeniteconsists of 0.25% to 3.0% fine-grained,disseminated sulfides with local net-texturedpatches containing up to 10% sulfide minerals. Thedominant sulfide minerals are pyrrhotite,

FIG. 17-11 Geology of the north-central portion of theRoby Zone, mapped by Michaud (1998).


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FIG. 17-12. a, High Grade Ore in the Roby Pit showing fissile nature of rocks, photograph taken facing south; b, hand

specimen of High Grade Ore (22 g.t-1 Pd), low sulfide, and foliated; c, photomicrograph of High Grade Ore showingpseudomorphing actinolite alteration of clinopyroxene grains and development of chlorite [crossed polars]; d,photomicrograph of sulfides (light colored) as fine inclusions within secondary silicates and paralleling the long axes ofactinolite grains [polished section].

chalcopyrite, pyrite and pentlandite. Palladium andplatinum mineralization within the High-Grade Oreconsists predominantly of fine-grained PGE-bearingsulfide minerals; these are predominantly braggite(Pt,Pd,Ni)S) and the telluride minerals merenskyite(Pd,Pt)(Te,Bi)2 and kotulskite Pd(Te,Bi) (Sweeny1989), which occur interstitial to cumulus grains oras inclusions within secondary silicates (Fig.17-12d).

Higher PGE grades (mean: 7.89 g.t-1 Pd,maximum: 55.95 g.t-1 Pd) occur in those portions ofthe pyroxenite that are altered to an assemblage ofamphibole (anthophyllite actinolite hornblende) talc chlorite. The PGE tenor is not proportional

to the sulfide content and samples free of visiblesulfide commonly contain more than 10 g.t-1 Pd.The sulfide-poor sample of Figure 17-12c contains22 g.t-1 Pd. This is reflected by a poor correlationbetween base metal and precious metal contents.The high-grade mineralization is located primarilywithin the western, highly altered portion of thepyroxenite, as much of the pyroxenite in betweenthe barren East Gabbro and High Grade Ore is low-grade. Within the pyroxenite, the intensity ofsilicate alteration decreases eastward towards theEast Gabbro contact, as does the Pd tenor. Inaddition, a 6 m selvage of alteration on the westernborder of the East Gabbro, along the pyroxenite


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contact, is coincidental with anomalously high Pdtenor when compared to East Gabbro further fromthe contact. This tenor/alteration relationship isgraphically demonstrated by plotting the averagegrades for the entire deposit against distances from

the pyroxenite contacts (Fig. 17-13). Pd gradesincrease within the breccia ore towards thepyroxenite contact, with a dramatic increase in thewestern portion of the pyroxenite and a gradualdecrease to the east, into East Gabbro. The higher-

grade High Grade Ore is not restricted to thepyroxenite but commonly straddles its westerncontact. At depths exceeding 250 m, the volume ofhigh-grade ore outside the pyroxenite is greater thanwithin; high-grade ore may attain a thickness of

50 m within some sections (Fig. 17-14).Occasionally drilling fails to intersect thepyroxenite as expected. However, typical Pd gradesare still present in other host rocks such as brecciaand vari-textured gabbro.

FIG.17-13. Roby Zone palladium grade profile (based on averaging all assay data equidistant from contact).

Pd Grade

> 2.5 g/tonne

0.7 to 2.5 g/tonne

< 0.7 g/tonne

Outline of Phase 3 pit

Surface circa 2001Outline of pyroxenite unit

1000

m

500

m

0 m

500 m

E 1000

m

500

m

W0

m

FIG. 17-14. Cross-section of contouredPd grade block model,

Roby and TwilightZones (see Fig. 17-15for plan view ofsection location).


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Pd Grade

> 2.5 g/tonne0.7 to 2.5 g/tonne

Outline of Phase 3 pit

Outline of pyroxenite unit

FIG. 17-15 Plan of contoured Pd grade block model,Roby and Twilight Zones

As seen on plan view of the contoured grademodel (Fig. 17-15), the higher grade segment of theHigh Grade Ore is juxtaposed with the highergrades found in the adjacent Breccia Ore. The PGEtenor of the pyroxenite declines dramatically to thesouth and it also declines coincidentally in theadjacent Breccia Ore. The juxtaposition of the

highest grade Breccia Ore, with the highest gradeHigh Grade Ore is consistent with the interpretationthat the pyroxenite was mineralized during theevent that created the Breccia Ore and that thepyroxenite, backed by the thick and rigid EastGabbro, acted as a barrier to the mineralizing fluid.The fact that a vari-textured equivalent to the EastGabbro does not exist suggests that it was the firstlithologic unit to fully crystallize and was thusrendered impervious to the fluids fluxing throughthe crystal mush (resulting in vari-textured rocks)that dominated the rest of the chamber.

North Roby OreNorth Roby Ore is a tabular zone that is 20to 40 m thick and 200 m long. The zone strikes at020, with the footwall dipping variably to the eastat 45 to 60, with a somewhat steeper hangingwall. It is hosted by a wide variety of lithologicunits and at surface is dominated by coarse-grainedleucogabbro containing irregular masses of vari-

textured gabbro and medium- to coarse-grainedgabbronorite and clinopyroxenite.

The contacts between gabbronorite andclinopyroxenite and the encompassing coarse-grained leucogabbro are very sharp, lack mineral

zonation and often have a dimpled texture (Fig.17-16a and b), indicating that several of theadjacent layers may have co-existed as a semi-solid,crystal mush. However, intrusive pulsesoccasionally resulted in brittle deformation of theadjacent rocks (Fig. 17-16c). Silicate alteration ismoderate to weak near the mineralization, however,the most widespread and intense zone of alterationoccurred in the footwall.

Sulfide mineralization occurs primarilywithin gabbronorite and clinopyroxenite. Totalsulfide mineral volumes range from trace amountsto 4%, and are typically less than 0.25%. The

dominant sulfides are pyrrhotite, pentlandite,chalcopyrite and pyrite. Net-textured sulfides arecommon in gabbronorite and clinopyroxenite andoccur locally within the gabbroic matrix of theheterolithic gabbro (Fig. 17-17a, b).

PGE mineralization of the North Roby Zoneoccurs primarily within gabbronorite andclinopyroxenite, and consists mainly of braggite andvyskoskite ((Pd,Ni)S). These are considered to behigh-temperature minerals that form early in theformation of many magmatic deposits (Cabri &Laflamme 1979) and their presence was interpretedby Sweeny (1989), to be indicative of a magmaticorigin. The distribution of PGE is erratic, with amean of 1.7 g.t-1 Pd and a maximum assay value of39.7 g.t-1 Pd within the gabbronorite, generallydecreasing to the northeast. A central core of highergrade rock is bounded by a broad halo of lowergrade material, itself containing tabular zones ofmoderate grade. Although the PGE mineralization istypically associated with sulfides, it is often difficultto visually distinguish the sulfide-poor ore from thebarren wallrock.

Breccia Ore

Southwest of the High-Grade Ore, thecentral mass of the Breccia Ore is contained within

a mineralized complex that measures 550 m northsouth by 350 m wide. An arm of this mineralizationhas been traced for an additional 250 m to thesoutheast. Rock composition ranges fromclinopyroxenite to anorthosite to norite. Texturesinclude equigranular, fine- to coarse-grained,porphyritic, pegmatitic, vari-textured and


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Figure 17-16. a, Modal layering within gabbronorite defined by varying proportions of feldspar and clinopyroxene/orthopyroxene grains. The long axis of the hammer is oriented approximately northsouth, with the head of the hammertowards the north; b, clinopyroxenite-East Gabbro dimpled contact; c, clinopyroxenite in the central portion of the RobyZone containing numerous angular fragments of leucogabbro (East Gabbro).

heterolithic gabbroic breccia, of which the latterthree textures are host to the PGE mineralization.The eastern boundary to the central portion of theBreccia Ore is well-defined by the sub-verticalcontact with the pyroxenite that is host to the High-Grade Ore. The other boundaries are moregradational and typically the variability incomposition and texture decreases to the west andsouth.

Heterolithic gabbro breccia is composed ofnumerous, irregularly shaped, angular, rounded and

sub-rounded fragments ranging from severalcentimetres to several metres in size that arerepresentative of most of the lithologic units in thecomplex, as well as a few exotic clasts, within agabbroic to melanogabbroic matrix (Fig. 18a, b andc). This breccia commonly consists of sub-angularfragments crowded together with very little matrixand grades locally to a matrix-supported breccia

consisting of 10% sub-rounded fragmentsinterpreted as digested xenoliths and 90% matrix.The superposition of detailed sampling on mappingat the southern end of the Roby Zone has shownthat higher grades are returned in the matrix-supported breccias, which also are interpreted torecord the greatest degree of digestion of clasts,based on their rounded shapes. Sampling of matrixand clast in the southern portion produced averageassays of 8 g.t-1 Pd and 0.8 g.t-1 Pd, respectively.The various fragments often have convoluted edges,

with numerous tongues or lobes that protrude intothe adjacent matrix, suggesting that the fragmentsand the encompassing magma co-existed in a liquidor crystal mush state. The gabbroic composition ofthe matrix in many locations is interpreted as acompositional change due to digestion of less maficxenoliths.

All rock types within the Breccia Ore also


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Figure 17-17. a, photomicrograph of gabbronorite showing unaltered cumulate texture consisting of subhedral to euhedralorthopyroxene grains and albite twinning of plagioclase feldspar grains. [crossed polars]; b, photomicrograph ofgabbronorite containing net-textured sulfides (opaque). [plane-polarized light].

FIG. 17-18. a, Photograph looking west across contact from matrix rich, high grade heterolithic melanogabbro brecciatowards matrix poor, low grade heterolithic gabbro breccia located near northwestern corner of the South Roby Pit, as seenon Figure 17-15; b, low-grade heterolithic gabbro breccia from South Roby Zone; c, high-grade heterolithic gabbrobreccia from the north-central Roby Zone.

occur as irregularly shaped domains with maximumdimensions of 60 m with chaotic distribution andbrecciation at a larger scale. A large block ofequigranular leucogabbro has a rim of vari-texturedleucogabbro. The transition from equigranularprotolith to vari-textured rim is interpreted as theincomplete impregnation of the deuteric fluid into

the more massive unit. The mineralogy remains thesame, indicating that the fluid was a catalyst forcrystal growth only.

Alteration within the Breccia Ore consists ofmoderate to strong, pervasive uralitization ofclinopyroxene, producing actinolite pseudomorphs.Plagioclase has undergone saussuritization, forming


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patches of zoisite and epidote within the center ofthe grain and along the grain boundaries.Orthopyroxene has been altered to talc,anthophyllite and serpentine. More intensivealteration typically occurs as isolated patches,

suggesting that deuteric fluids have modified theoriginal magmatic textures and compositions.

Sulfide mineralization within the BrecciaOre consists of up to 5% intergrown chalcopyrite,pentlandite, pyrrhotite and pyrite. Sulfides occur asfine-grained, disseminated grains throughout, and asirregularly shaped blebs up to 1.0 cm in diameter,interstitial to clinopyroxene and plagioclase grains(Figs. 19a and b). Sulfides also occur as inclusionsand thin streaks parallel to the long axis of thesecondary silicate pseudomorphs, suggesting thatthe sulfides may have been remobilized duringalteration or were introduced during the alteration

of pyroxene to actinolite.PGE mineralization within the Breccia Oreis associated with heterolithic gabbro breccia andaccompanying vari-textured gabbro. Althoughgabbronoritic and pyroxenitic fragments (some in

FIG. 17-19. Samples of net textured sulfide in gabbroic(above) and sulfide (below) pods in the pegmatiticgabbro.

the 10s of metres) can be of ore grade, the matrixto the breccia typically contains the bulk of the PGEmineralization. The mean grade of the Breccia Oreis 1.2 g.t-1 Pd, with a maximum of 36 g.t-1 Pd. Thesouthern and western boundaries of the Breccia Ore

are defined by a gradual decrease in PGE content.As defined by the 0.7 g.t-1 Pd assay cutoff, thewestern boundary is sub-vertical and the southernboundary dips steeply to the north, whereas thesoutheastern extension of the Breccia Ore remainsopen.

The platinum group minerals within theBreccia Ore consist of predominantly kotulskite,merenskyite, braggite and vysotskite, and areassociated with fine-grained, disseminated andirregularly shaped blebs of pyrrhotite, pyrite,pentlandite and chalcopyrite (Sweeny 1989).

Although certain patterns of mineralization

distribution have been documented, extensivesampling of outcrop and drill core has shown thatevery rock type in the Roby Zone, regardless ofsulfide content, can either be ore or waste. Thisobservation is not unexpected since the Roby Zonewas created by an evolving process with successivesuperimposition of related, but distinct processes, asexplored in the discussion.

Geology of the Twilight Zone

The Twilight Zone, most recently studied byDionne-Foster (2001) (Fig. 17-9), is a small pod ofgabbronoritic breccia found on the eastern side ofthe Roby Pit. Itis separated from the Roby Zone onits western boundary by the 5070m thick EastGabbro and separated from the southeasternextension of the Roby Zone by a 100 m thickbarren, post-mineralization gabbro dike. Thebreccia is characterized by large, sub-rounded,noritic clasts set within a matrix ofmelanogabbronorite. Unlike the Roby Zone, theserocks are almost devoid of vari-textured/pegmatitictextures. The alteration is variable and in someplaces intense, but for the most part the rocks areweakly altered to fresh. Orthopyroxene is mostcommonly fresh, but may be altered along grainboundaries or completely pseudomorphed by

amphibole and talc. Plagioclase is more resistant toalteration and is typically only weakly altered.

Macroscopically, the mineralization appearsprimary, dominated by large sulfide blebs.However, the Pegs are associated with both theprimary sulfides and secondary silicate mineralssuch as amphibole, chlorite, and to a lesser extenttalc. Palladian tellurides (kotulskite, merenskyite)


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and arsenides (stillwaterite, isomertieite) are themost abundant PGM, with a lesser amount ofvysotskite. Many of the PGM are found as discretegrains completely enclosed by silicates, but they arestill in close proximity to the sulfides. These rocks

contain 0.250.5 vol.% pyrrhotite and chalcopyrite.A unique feature of Twilight Zone, as

compared to the Roby zone, is the strong correlationof the precious metals with the base metals andsulfur. This geochemical evidence, together withmicroscopic observations and the near absence ofvari-textured/pegmatitic textures and silicatealteration, suggests that only minor remobilizationof the PGM has occurred in the Twilight Zonebreccias.

Baker Zone

The Baker zone is a lower grade deposit

situated approximately 1 km east of the Roby ZonePit. The zone consists mostly of equigranularnorite/gabbronorite that has been intruded byheterolithic melanogabbro breccia, melanogabbro,and a small amount of vari-textured gabbro. Thebreccia zone strikes east-northeast and dips atapproximately 50 to 90 degrees to the south-southeast. Both surface mapping and diamond drillcore intersections show that the vari-texturedgabbro is often at the margins of the breccia zone.Sub-vertical pyroxenitic dikes and more shallowlydipping late mafic dikes cross-cut the other units inthe outcrop.

Mineralized rock is continuously distributedthroughout the heterolithic melanogabbro brecciaand melanogabbro units, but is distributedsporadically throughout the surrounding norite andgabbronorite. The highest grading unit is a smallexposure of leucogabbro breccia that ischaracterized by gabbro and melanogabbro clastsset within a matrix of leucogabbro. Themelanogabbro breccia predominantly contains clastsof vari-textured gabbro and equigranular norite andgabbronorite derived from the surrounding rocks,with smaller amounts of various other gabbroicrocks. Both the matrix and some of the clasts hostsulfide and PGM mineralization. Precious metal

grades in all of the mineralized units show a strongcorrelation with the base metal content. PGMidentified within these rocks are merenskyite andkotulskite. Some narrow, massive sulfide stringerscross-cut the norite/gabbronorite and melanogabbro.

This zone, which exhibits sub-horizontaligneous layering, has been affected by twodeformational events. The first is manifested as a

north-trending foliation, produced by a minorshearing event that occurred after the intrusion ofthe breccias and melanogabbro. The second event ismanifested as northeast-trending, tightly spacedshear fractures with significant sinistral offsets,

which has resulted in sinistral transposition of themineralized zones into an east-trending orientationfrom an original north-trending orientation.

DISCUSSION

Several new observations were madepossible as a result of creating, mapping andthoroughly assaying large exposures of themineralization and country rock throughout theLDI-IC and examining the entire Roby Zonethrough logging and assaying of core. Keyobservations that support the proposed processesare:

1) Gabbro breccia is high grade ore (matrix has thehighest metal content), vari-textured gabbro ismedium to low grade, and equigranular gabbrois low grade to barren.

2) The above pattern is overprinted with thecommon, but not universal association of highgrade and silicate alteration.

3) Breccia and vari-textured gabbro and Pdmineralization have a close spatial association

4) The mineralogy of veins is commonlyidentical to wallrock.

5) Within the Roby Zone, there is poor correlationbetween sulfide content and grade whereas inthe neighboring weakly altered Twilight Zone,correlation is good.

6) There is a spatial relationship between vari-textured gabbro and silicate alteration. This islargely based on the near absence of both in theTwilight Zone, and their near universality in theRoby Zone.

7) Breccia fragments are round and lack reactionrims.

8) Grade is correlated with grain size and alterationin the High-Grade zone.

9) The Roby Zone contains multiple styles of PGEmineralization.

PGE are associated with CuNi-sulfide in

the matrix of magmatic breccia and in vari-texturedto pegmatitic gabbro (which together representthe Breccia Zone) and in the pyroxenite unit that ispart of the High-Grade Zone. PGE also occur withsulfide-poor, vari-textured to pegmatitic gabbro thatis found throughout the Roby Zone (and is thedominant style in the North Roby Zone) and occurwith sulfide-poor mineralization associated with


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strong silicate alteration found throughout the RobyZone (exemplified by the portions of the High-Grade Zone).

The implication of these observations is thatthe Roby Zone is the product of multiple stages of

intrusion, alteration and mineralization. It isenvisioned that the mineralization sequence wasinitiated by the energetic injection of sulfide- andPGE-enriched magma into a partially crystallizedchamber, creating intrusive breccia. This energeticintrusion is a result of an abundance of dissolvedvolatiles. These volatiles resolved from the magmaand fluxed through the crystal mush, acting as acatalyst, accelerating crystallization in its path,creating vari-textured gabbro, and pegmatite.These volatile plumes would differentially transportand deposit metals, resulting in a broad range ofPGE and base metal ratios, including sulfide-poor

mineralization. The intergrowth of pyroxenes withthis mineralization implies high temperatures offormation and the evacuation of water and othervolatiles. In this scenario, the already crystallizedand rigid, sub-vertical sheet of East Gabbro acted asa barrier to the volatiles emanating from the brecciato its west, resulting in greater localizedaccumulation of PGE, creating the High-GradeZone. The unconfined volatiles fluxing towards thenorth created the North Roby Zone and eventuallydispersed. The widespread association of PGE withlower temperature silicate alteration is interpreted torepresent continued fluxing, or a lagging volatileplume fluxing through the crystal mush atprogressively lower temperatures. At the TwilightZone, the positive correlation between base andprecious metals and the virtual absence of both vari-textured gabbro and lower temperature alterationboth support the contention that the high-temperature fluid thought to be responsible forgenerating the vari-textured gabbros also worked tomobilize metals and overprinted the magmatic metalratio signature at lower temperatures.Mineralization associated with low-temperaturesilicates is the result of continued circulation andcooling of its high-temperature precursor thatcreated vari-textured gabbro. The PGE and base

metals at Lac des Iles were carried in an energeticmagma and were subsequently remobilized by itsdeuteric fluids.

This model can be used to evaluateexploration targets. The great volume of breccia,geological chaos, evidence of fluids, and anomalousmetal content are features that can be observed byreconnaissance. An examination of bedrock at LDI

gives one the impression of a very dynamic system.It is unlikely this type of system would be present ata significant scale in a massive or layered intrusion,especially one with less than 7 ppb Pd content.Early recognition of potential based on the above

features is key to evaluating intrusions. Chaosdictates that subsequent more detailed activities beunconstrained by preconceived geological control,and that they are applied broadly and evenly,sampling and survey spacing dictated by expecteddimensions. Direct exploration, i.e., samplingbedrock is the most efficient method as indirectmethods, such as sampling overburden andconducting geophysical surveys, amount to beingdistracting activities that produce false anomalies.

ACKNOWLEDGEMENTS

A better understanding of this deposit is the

result of many who dug, washed, sampled anddocumented geological relationships on theextensive bedrock exposures and 200,000 metres ofdrill core created during the recent explorationprogram. In addition to the authors this includesexploration team members Scott Burgess, MikeMacIsaac, Al McTavish and Brian Nelson.Interpretation was made possible by those whocompiled the database and generated two and three-dimensional images, a fraction of which is includedherewith. Key to these efforts are Douglas Kim,mine geologist, Paul Nielsen, Karen Kettles andGerry Katchen.

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SUTCLIFFE, R.H. & SWEENY, J.M. (1986):Precambrian Geology of the Lac des IlesComplex, District of Thunder Bay, Ontario.Ontario Geological Survey, Map 3047,


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Geological Series Preliminary map, scale1:15840.

SWEENY, J.M. (1989): The geochemistry and originof platinum group element mineralization of thehybrid zone, Lac des Iles Complex, Northwestern

Ontario. M.Sc. Thesis, University of WesternOntario, London, Ontario, 185p.

TOMLINSON,K.Y.,STOTT,G.M.,PERCIVAL,J.A.&STONE, D. (in press) Basement terrane

correlations and crustal recycling in the westernSuperior Province: Nd isotopic character ofgranitoid and felsic volcanic rocks in theWabigoon Subprovince, N. Ontario, Canada.Submitted to Precambrian Research.

WATKINSON, D.H. & DUNNING, G.R. (1979):Geology and platinum-group mineralization, Lacdes Iles complex, northwestern Ontario. Can.Mineral. 17, 453-462.

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Discovery and Geology of Discovery and Geology of Ldic