A PROSPECTOR’S GUIDE TO Mississippi-Valley Type (MVT) Lead-Zinc Deposits in Newfoundland and Labrador Contributed by: Tom Lane Teck Exploration Ltd. Matty Mitchell Prospectors Resource Room Information Circular Number 3 First Floor, Natural Resources Building Geological Survey of Newfoundland and Labrador 50 Elizabeth Avenue, PO Box 8700, A1B 4J6 St. John’s, Newfoundland, Canada prospectors Telephone: 709-729-2120, 709-729-6193 e-mail: [email protected] resource room Website: http://www.nr.gov.nl.ca/mines&en/geosurvey/matty_mitchell/ A PROSPECTOR’S GUIDE TO MVT DEPOSITS IN NEWFOUNDLAND AND LABRADOR What are MVT Deposits? Where do MVT Deposits Occur in Newfoundland VT or Mississippi Valley Type Deposits and Labrador? Mare zinc and lead concentrations in car- bonate sedimentary rocks. The ore minerals are sphalerite (zinc sulphide) and galena (lead VT style mineralization occurs in carbon- sulphide), and these are commonly associated Mate sedimentary rocks from St. George’s with the iron sulphides, pyrite and marcasite. Bay to Cape Norman in western Newfoundland. Minor accessory minerals include barite Carbonate rocks in southeastern Labrador have (barium sulphate), gypsum (calcium sulphate) limited potential. Also, Proterozoic carbonate and fluorite (calcium fluoride). The host rocks rocks from Labrador City to north of to MVT deposits are sedimentary limestones Schefferville may have potential to host this and dolostones. The sulphides are commonly style of mineralization. disseminated and have a preference to occur in open pores, vugs and veins. Where sulphides The largest known MVT deposit in occur in mineable settings, they form massive Newfoundland was near Daniel’s Harbour, to semi-massive beds that partially replace where nearly 7 million tonnes of zinc ore, with limestones and dolostones. a grade of 8% zinc, was mined by Newfoundland Zinc Mines between 1975 and MVT deposits are named after the type area 1990. Several sub-economic concentrations of of the Mississippi Valley in the central United zinc are known in the area west of St. Anthony. States, where many mines have extracted zinc Zinc is the only metal found in MVT-style and lead over the past 100 years. MVT deposits mineralization in the Ordovician carbonate are part of a larger family of Carbonate-hosted rocks of western Newfoundland. Both lead and deposits, all of which contain zinc. The MVT zinc occur in Cambrian-aged carbonates. style of zinc and lead deposits formed at Silurian carbonate rocks south of Hampden and relatively low temperatures, from 90°C to similar Carboniferous rocks near Stephenville 150°C. Other carbonate-hosted deposits can have a significant amount of galena, barite and form at higher temperatures and commonly celestite (strontium sulphate). contain silver. 1 a number of geological settings in which this How do MVT can happen. These include: where formations “pinch out” against a basement high; where Deposits Form? porous carbonate rocks lie beneath imper- meable rock formation barriers such as shales, finely crystalline dolostones or unconformities; VT types are stratabound deposits that and, lastly, along faults and the ends of fracture Mtend to occur in specific carbonate rock zones. Thick porous beds and concentrated layers or beds that have notable porosity and areas of fractured rocks are required to form permeability, in the form of inter-crystalline mineable thicknesses of ore. pores, fractures, breccias and open cavities. The sphalerite–galena–pyrite deposits formed long after deposition and burial of the carbonate sediments. The sulphides crystallized as late What Formations Contain cements in veins and pores and fractures, partially replacing the surrounding rock. Num- Zinc Deposits? erous studies of MVT deposits have found that the deposits formed when large volumes of warm, saline fluids, carrying low concentra- s noted above, zinc deposits occur within tions of metals, passed through the rocks. These Aspecific rock formations, commonly fluids were forced through the rocks when they beneath unconformities and at the base of were folded and faulted, as during development dolostone formations. Here is a list of the of the ancient Appalachian Mountain Belt. formations that contain MVT Pb–Zn occur- Regional deformation of the rocks probably rences in Newfoundland and Labrador. The generated the necessary means of moving heat- distribution of these rock formations can be ed, metal-bearing fluids. Sulphides crystallized found on bedrock geological maps of this area, in significant concentrations in areas character- available at the Geological Survey of ized by abrupt increases in rock porosity. The Newfoundland and Labrador. Occurrences in porosity increase may have been related to western Newfoundland and southernmost fracturing, to original rock type (e.g., porosity Labrador occur in carbonate rock formations of is higher in lithified reefs and carbonate sands Cambrian and Ordovician age. Those in than in some other carbonate rocks), or to the western Labrador occur in much older development of ancient weathering surfaces Proterozoic rocks. (paleokarst) where dissolution of the carbonate rocks occurred. The folding also helped to form Catoche Formation (Ordovician) traps for the metal-rich fluids, where zinc and Dolostones at the top of the Catoche sulphur could - under special chemical condi- Formation contain the Daniel’s Harbour tions - combine to form the sulphide minerals. zinc deposit. At the mine, sphalerite is distributed over 33 m of vertical strati- graphy; ore commonly was mined over 5 to Where do Significant 10 m thicknesses in dolostones above a contact with underlying limestones. The MVT Deposits Occur? yellow and brown sphalerite is associated with coarsely crystalline, white dolomite that forms a “cement”, filling in the open arge MVT deposits occur where buried spaces and fractures that cut the tan to grey Lfluids were focused and trapped. There are dolostone. The sphalerite also occurs in 2 large bodies of carbonate “collapse” breccias that lie close to high-angle faults. What to Look for on Maps Boat Harbour and Watts Bight formations and in the Field (Ordovician) Occurrences near St. Anthony commonly occur in the carbonate rocks in both the he carbonate terrane of western Boat Harbour and Watts Bight formations, TNewfoundland have been the focus of near the contact of these units. The several generations of mapping and pros- mineralization is associated with white pecting. Even though many occurrences are crystalline dolomite in zones of carbonate recorded, the region still remains prospective. “collapse breccia”, and in open spaces in Large parts of the area are extensively covered dolostone beds below an unconformity. by till. Zinc deposits can easily be hidden beneath barren rock, because they are non- Table Point Formation (Ordovician) magnetic and lack electromagnetic conduc- Sphalerite occurs in grey porous dolostones tivity (and are thus hard to detect using most that replace limestones at the base of the geophysical instruments). As a result, careful formation at St. John’s Island detective work could pay off in the discovery of new targets. A variety of important guides can Petite Jardin and March Point formations be used. These are listed below and can serve as (Cambrian) a handy checklist. These Cambrian dolostones commonly contain scattered occurrences of galena and ° Mineral occurrences: Minor mineral sphalerite, mainly in fractures and veins. occurrences commonly surround major mineral deposits. Also, the presence of min- Sops Arm Group (Silurian) eral occurrences indicates prospective rock Minor carbonate rocks associated with units. volcanic rocks contain galena in breccias at Turner’s Ridge. ° Surficial geochemistry: It works. Lake sediment geochemistry and soil geochem- Big Cove Formation, Codroy Group istry effectively revealed covered zinc (Carboniferous) mineralization in the Daniel’s Harbour area. Fossil-rich carbonate mounds in narrow paleokarst valleys contain mineralized and ° Mineralized boulders: Boulder tracing led cemented breccias and open spaces with to the discovery at Daniel’s Harbour. sphalerite, galena, calcite, barite and Recent studies of glacial ice flow can help celestite. Most of this mineralization occurs you find the source of boulders. on the Port-au-Port Peninsula. ° Geologic maps: Extensive bedrock Denault Formation (Proterozoic), western mapping of western Newfoundland differ- Labrador entiates key mineralized formations and has The same carbonate sedimentary rocks that identified a number of important faults. occur near the iron formations of western Utilize geological maps, along with mineral Labrador are known to contain galena and occurrence maps, to define prospective sphalerite farther north in Quebec. formations. 3 ° Magnetic and gravity maps: Magnetic and structural highs where trapped fluids may gravity maps usefully indicate basement have precipitated ores. structures that were important fluid path- ways for metal-bearing fluids, as well as sites of mineralization. These maps should Field Recognition be used, hand-in-hand, with geologic maps. of Dolostones and ° Dolomites: Most MVT zinc deposits occur Sulphide Minerals in dolostones and in formations charac- terized by regional dolomitization. More specifically, deposits occur at transitions imestones and dolostones are commonly between limestones and dolostones. In distinguishable by their colour difference. western Newfoundland, coarse-grained grey L Limestones weather light grey to darker and white