Contributed by: Tom LaneTeck Exploration Ltd.

A PROSPECTOR’S GUIDE TOMississippi-Valley Type (MVT)Lead-Zinc Depositsin Newfoundland and Labrador

Matty Mitchell Prospectors Resource RoomInformation Circular Number 3

Telephone: 709-729-2120, 709-729-6193 • e-mail: matty@gov.nl.caWebsite: http://www.gov.nl.ca/mines&en/geosurvey/matty_mitchell/

First Floor • Natural Resources BuildingGeological Survey of Newfoundland and Labrador50 Elizabeth Avenue • PO Box 8700 • A1B 4J6St. John’s • Newfoundland • Canada

resource roompros pec tor s

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A PROSPECTOR’S GUIDE TO MVT DEPOSITS

IN NEWFOUNDLAND AND LABRADOR

What are MVT Deposits?

MVT or Mississippi Valley Type Depositsare zinc and lead concentrations in car-

bonate sedimentary rocks. The ore minerals aresphalerite (zinc sulphide) and galena (leadsulphide), and these are commonly associatedwith the iron sulphides, pyrite and marcasite.Minor accessory minerals include barite(barium sulphate), gypsum (calcium sulphate)and fluorite (calcium fluoride). The host rocksto MVT deposits are sedimentary limestonesand dolostones. The sulphides are commonlydisseminated and have a preference to occur inopen pores, vugs and veins. Where sulphidesoccur in mineable settings, they form massiveto semi-massive beds that partially replacelimestones and dolostones.

MVT deposits are named after the type areaof the Mississippi Valley in the central UnitedStates, where many mines have extracted zincand lead over the past 100 years. MVT depositsare part of a larger family of Carbonate-hosteddeposits, all of which contain zinc. The MVTstyle of zinc and lead deposits formed atrelatively low temperatures, from 90°C to150°C. Other carbonate-hosted deposits canform at higher temperatures and commonlycontain silver.

Where do MVT DepositsOccur in Newfoundland

and Labrador?

MVT style mineralization occurs in carbon-ate sedimentary rocks from St. George’s

Bay to Cape Norman in western Newfoundland.Carbonate rocks in southeastern Labrador havelimited potential. Also, Proterozoic carbonaterocks from Labrador City to north ofSchefferville may have potential to host thisstyle of mineralization.

The largest known MVT deposit inNewfoundland was near Daniel’s Harbour,where nearly 7 million tonnes of zinc ore, witha grade of 8% zinc, was mined byNewfoundland Zinc Mines between 1975 and1990. Several sub-economic concentrations ofzinc are known in the area west of St. Anthony.Zinc is the only metal found in MVT-stylemineralization in the Ordovician carbonaterocks of western Newfoundland. Both lead andzinc occur in Cambrian-aged carbonates.Silurian carbonate rocks south of Hampden andsimilar Carboniferous rocks near Stephenvillehave a significant amount of galena, barite andcelestite (strontium sulphate).

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How do MVT Deposits Form?

MVT types are stratabound deposits thattend to occur in specific carbonate rock

layers or beds that have notable porosity andpermeability, in the form of inter-crystallinepores, fractures, breccias and open cavities. Thesphalerite–galena–pyrite deposits formed longafter deposition and burial of the carbonatesediments. The sulphides crystallized as latecements in veins and pores and fractures,partially replacing the surrounding rock. Num-erous studies of MVT deposits have found thatthe deposits formed when large volumes ofwarm, saline fluids, carrying low concentra-tions of metals, passed through the rocks. Thesefluids were forced through the rocks when theywere folded and faulted, as during developmentof the ancient Appalachian Mountain Belt.Regional deformation of the rocks probablygenerated the necessary means of moving heat-ed, metal-bearing fluids. Sulphides crystallizedin significant concentrations in areas character-ized by abrupt increases in rock porosity. Theporosity increase may have been related tofracturing, to original rock type (e.g., porosityis higher in lithified reefs and carbonate sandsthan in some other carbonate rocks), or to thedevelopment of ancient weathering surfaces(paleokarst) where dissolution of the carbonaterocks occurred. The folding also helped to formtraps for the metal-rich fluids, where zinc andsulphur could - under special chemical condi-tions - combine to form the sulphide minerals.

Where do SignificantMVT Deposits Occur?

Large MVT deposits occur where buriedfluids were focused and trapped. There are

a number of geological settings in which thiscan happen. These include: where formations“pinch out” against a basement high; whereporous carbonate rocks lie beneath imper-meable rock formation barriers such as shales,finely crystalline dolostones or unconformities;and, lastly, along faults and the ends of fracturezones. Thick porous beds and concentratedareas of fractured rocks are required to formmineable thicknesses of ore.

What Formations ContainZinc Deposits?

As noted above, zinc deposits occur withinspecific rock formations, commonly

beneath unconformities and at the base ofdolostone formations. Here is a list of theformations that contain MVT Pb–Zn occur-rences in Newfoundland and Labrador. Thedistribution of these rock formations can befound on bedrock geological maps of this area,available at the Geological Survey ofNewfoundland and Labrador. Occurrences inwestern Newfoundland and southernmostLabrador occur in carbonate rock formations ofCambrian and Ordovician age. Those inwestern Labrador occur in much olderProterozoic rocks.

Catoche Formation (Ordovician)Dolostones at the top of the CatocheFormation contain the Daniel’s Harbourzinc deposit. At the mine, sphalerite isdistributed over 33 m of vertical strati-graphy; ore commonly was mined over 5 to10 m thicknesses in dolostones above acontact with underlying limestones. Theyellow and brown sphalerite is associatedwith coarsely crystalline, white dolomitethat forms a “cement”, filling in the openspaces and fractures that cut the tan to greydolostone. The sphalerite also occurs in

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large bodies of carbonate “collapse”breccias that lie close to high-angle faults.

Boat Harbour and Watts Bight formations(Ordovician)

Occurrences near St. Anthony commonlyoccur in the carbonate rocks in both theBoat Harbour and Watts Bight formations,near the contact of these units. Themineralization is associated with whitecrystalline dolomite in zones of carbonate“collapse breccia”, and in open spaces indolostone beds below an unconformity.

Table Point Formation (Ordovician)Sphalerite occurs in grey porous dolostonesthat replace limestones at the base of theformation at St. John’s Island

Petite Jardin and March Point formations(Cambrian)

These Cambrian dolostones commonlycontain scattered occurrences of galena andsphalerite, mainly in fractures and veins.

Sops Arm Group (Silurian)Minor carbonate rocks associated withvolcanic rocks contain galena in breccias atTurner’s Ridge.

Big Cove Formation, Codroy Group(Carboniferous)

Fossil-rich carbonate mounds in narrowpaleokarst valleys contain mineralized andcemented breccias and open spaces withsphalerite, galena, calcite, barite andcelestite. Most of this mineralization occurson the Port-au-Port Peninsula.

Denault Formation (Proterozoic), westernLabrador

The same carbonate sedimentary rocks thatoccur near the iron formations of westernLabrador are known to contain galena andsphalerite farther north in Quebec.

What to Look for on Mapsand in the Field

The carbonate terrane of westernNewfoundland have been the focus of

several generations of mapping and pros-pecting. Even though many occurrences arerecorded, the region still remains prospective.Large parts of the area are extensively coveredby till. Zinc deposits can easily be hiddenbeneath barren rock, because they are non-magnetic and lack electromagnetic conduc-tivity (and are thus hard to detect using mostgeophysical instruments). As a result, carefuldetective work could pay off in the discovery ofnew targets. A variety of important guides canbe used. These are listed below and can serve asa handy checklist.

° Mineral occurrences: Minor mineraloccurrences commonly surround majormineral deposits. Also, the presence of min-eral occurrences indicates prospective rockunits.

° Surficial geochemistry: It works. Lakesediment geochemistry and soil geochem-istry effectively revealed covered zincmineralization in the Daniel’s Harbour area.

° Mineralized boulders: Boulder tracing ledto the discovery at Daniel’s Harbour.Recent studies of glacial ice flow can helpyou find the source of boulders.

° Geologic maps: Extensive bedrockmapping of western Newfoundland differ-entiates key mineralized formations and hasidentified a number of important faults.Utilize geological maps, along with mineraloccurrence maps, to define prospectiveformations.

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° Magnetic and gravity maps: Magnetic andgravity maps usefully indicate basementstructures that were important fluid path-ways for metal-bearing fluids, as well assites of mineralization. These maps shouldbe used, hand-in-hand, with geologic maps.

° Dolomites: Most MVT zinc deposits occurin dolostones and in formations charac-terized by regional dolomitization. Morespecifically, deposits occur at transitionsbetween limestones and dolostones. Inwestern Newfoundland, coarse-grained greyand white dolomites occur at theseboundaries.Coarse-grained white dolomites,in mottled black and white rocks, called“pseudobreccia”, host the zinc ore atDaniel’s Harbour. Black dolostones arecommon around orebodies.

° Veins and Breccias: The dolostones arounddeposits contain abundant veins and brec-cias cemented by coarse-grained, crystallinewhite dolomite. In Carboniferous rocks nearStephenville, mineralized veins are cemen-ted by coarse-grained calcite.

° Grey breccia bodies: Ore deposits atDaniel’s Harbour are clustered arounddolostone “karst” breccia bodies with a greymatrix. These bodies vary from metres to 2km in diameter, are closely related to an un-conformity and deeply penetrate 200 m intounderlying rocks. Mineralization in theWatts Bight and Boat Harbour formations isalso related to unconformity-relatedbreccias.

° Fault systems: Dolomites, breccias andmineralization occur close to faults, inparticular ones oriented northeast–southwest.

° Fold domes: The zinc deposits at Daniel’sHarbour occur within a fold dome.Potentially, ore deposits formed within

structural highs where trapped fluids mayhave precipitated ores.

Field Recognitionof Dolostones andSulphide Minerals

Limestones and dolostones are commonlydistinguishable by their colour difference.

Limestones weather light grey to darker“battleship” grey. Finely crystalline limestoneweathers smooth, as if polished. Limestone alsoeffervesces strongly in weak hydrochloric acid.The dolostone units that host the mineralizationare typically made of fine- to medium-grainedcrystalline, “sucrosic”(or sugary-textured) dolo-stones that weather tan to brown. Coarse-grained dolostones that contain sulphide oresweather dark brown. Dolostones only effer-vesce in acid if they are ground to a powder.

Zinc and lead sulphides are not readilyvisible because they do not rust. Sphalerite isparticularly tricky (the Greek meaning of itsname). It is commonly honey brown, but maybe pale yellow and indistinguishable from thedolostones. Its other colours are dark brown,red and black. On weathered surfaces, it formsa greenish zinc carbonate, smithsonite, in highrelief to surrounding carbonates. The crystalsare angular and pyramid-like in form. Checkcare-fully for sphalerite crystals among thecoarse dolomite crystals. If in doubt, you canuse “zinc zap” solutions (available from mostcommercial labs) that turn red in the presenceof zinc. Also, a white hydrous zinc powder mayencrust surrounding carbonate rocks.

Galena is easier to identify. The cubiccrystals are shiny and silvery on a brokensurface. The crystals on weathered rock are adull blue-grey, in high relief relative to car-bonate hosts.

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What is an AttractiveTarget?

Many zinc–lead occurrences contain lessthan 1% zinc, in general. A viable mine

requires an overall grade greater than 8% zinc,with ore beds of greater than 15% zinc, andmining thicknesses greater than 3 m. Suchtargets are buried, but may be indicated byminor min-eralization and dolomitization inoverlying rocks and in adjacent beds overthicknesses of 3 or more metres. Such targetsare close to faults that displace stratigraphicunits. The presence of dolomitization alongfaults and along fold domes may suggest thepresence of buried targets.

Location close to economic infrastructurefavours mine development. The carbonate rocksin western Newfoundland are well situated,close to roads, power lines and seaports.

Who Would be Interested?

Junior and major mining companies areinterested in MVT zinc deposits because of

the good value of the clean zinc concentrate(with low iron content) and the inexpensivemining and milling costs. Preferred target areaswould have the potential to host a depositcontaining greater than 10 million tonnes of orewith grades of 8 to 10% zinc or greater.

What Does the BuyerWant to See?

Buyers are interested in a target that couldcontain a deposit with the above para-

meters. The features should fit the checklist ofkey characteristics. A target is most likely to beburied in a favourable formation and near afault structure. Prospector’s are advised to visitknown occurrences, see mineralized rocksaround the former Daniel’s Harbour mine,examine drill cores in the Pasadena core libraryand talk to provincial geologists familiar withwestern Newfoundland.

HAPPY PROSPECTING!

Tom LaneSenior GeologistTeck Exploration Ltd.

Appendix: Some Terms Defined

dolomite: a common rock-forming mineral,consisting of calcium, magnesium and carbonate(CO3). The term “dolomite” is also used to referto a sedimentary rock, of which more than 50% ismade up of the mineral dolomite.

dolomitization: the process by which limestone ispartially or wholly converted to dolomite rock ordolostone. Buried, magnesium-bearing waterpercolating through limestones, changes thecalcium carbonate (calcite) to magnesium-richcarbonate (dolomite).

dolostone: another name for the sedimentary rockdolomite; a carbonate rock composed primarily ofthe mineral dolomite.

limestone: a sedimentary rock, of which morethan 50% is made up of the mineral calcite (CaCO3.

karst: a topography formed on limestone andrelated rocks exposed at or near the surface, bydissolution of these rocks in groundwater. It ischaracterized by caves, sink-holes andunderground drainage channels. (A paleokarst isan ancient example of this style of topography.)