sign in sign up
<<
Home , Cathedral Formation, Stephen Formation

GEOLOGY OF THE MOUNT BRUSSILOF DEPOSIT, SOUTHEASTERN (825/12, 13) By George J. Simandl and Kirk D. Hancock

KEYWOKLIS: Industrial , rconomic geology. mag- nesitr. . Middle . dolumitiz- tion, porosity. base metal association, deposit model.

INTRODUCTION Magnesite (M&O,) is an induswial that can bc converted into either caustic, fused or dead-burned magne- sia. Dead-burned magnesia is used mainly in the manufac- ture of refractory products; caustic magnesia is used in treatment of water, in animal feedstuffs, fertilizers. magne- sia cements, insulating boards and wood-pulp processing, in chemicals and pharmaceuticals and us a curing agent in rubber (Coope. 19X7). Magnesium metal is produced either from magnesite or from caustic mapnesia. In the short-term future. production of dead-burned mag- nesia is cxpectcd to remain constant. however, demand for caustic magnesia is increasing (Duncan, IYYO). With the increasing trend toward the use of high-perfomxmce “mag- carbon” refractories. future demand for fused magnesia looks promising. A number of magnesite deposits are known in British Columbia (Grzant, 19X7). the most important of these is the Mount Brussilof orebody. It is hosted by dolomites of the TECTONIC SETTING Middle Cambrian Cathedral Formation. The Mount Brussilof deposit is located in the Forekm3 rectonostratiEnlphic belt and within the “Kicking Horse HISTORY Rim”, as defined by Aitken (1971, 1989). It is situated east of a Cambrian bathymetric feature commonly referred to as The Mount Brussilof deposit was discovered during the Cathedral escarpment (Fritz. 1990: Aitken xnd regional mapping by the Geological Survey of Mcllreath, 19X4, 1990). Existence of the escarpment is (Leech. 1965). Baykal Minerals Ltd. and Brussilof challenged hy Ludwigscn (19X9, 1990) who suggests that Resources L&l. staked and explored the dcpasit. In 1971, the this feature is a -carbonate facie chance on a ramp two companies mrrgcd to form Beymag Mines Co. Ltd. In Leech (1966) described the wne feature in the Mount lY79, Refratechnik GmhH. acquired Baymag Mines (Mac- Brussilof mine area (Figure 3-2-2) as a “faulted facies Lean, IYXX). In 19X0, proven and probable geological change”. In any event, the carbonate rocks east of this reserves were 0.5 million tonnes grading over 95 per cent feature, which host the magnesite mineralization, were magnesia in the calcined product and 13.6 million tonnes of deposited in a shallower mwinr environmcnl than their 93 to 95 percent magnesia in calcined product. Possible stratigraphic equivalents to tht: west. reserves were estimated at 17.6 million tonnes averaging Y2.44 per cent magnesia in calcinated product (Schultes, AND LITHOLOGY 19X6). Previous investigations, including mapping, are described in detail by MacLcan (198X). The stratigraphic relationsh~lp between rocks east of the Cathedral escarpment, and their deeper water equivalents to the west, commonly referred to as the Chancellor Form- LOCATION tion, is described by Aitken and Mcllreath (I!)841 and Stew The Mount Brussilof deposit is located in southeastern art (1989). British Columbia, approximately 3S kilometres northeast ot All known occurrences of sparry carbonate, other than Radium Hot Springs. It is accessible from Highway 93 by veins of or .I few centimetres thick, alp an all-weather unpaved road (Figure 3-2-l). Elevations in located east of the Cathedral exarpment. A composite strati the arca range from 1250 to 3045 mefres ahove see level. igraphic section of this area is shown on Figure 3-2-3

5

.

6

7 1 0 1 2 ---

Figure 3-2-2. Geology of the Mount Brussilof area.

270 British Columbia Geo/nRica~ Survey Branch oblique to bedding. Near the Cathedral escarpment this LEGEND shale may become grey or parti.rlly converted to talc and serpentine. Middle Cambrian The Cathedral Formation, wlhich hosts the magnesite deposits, is also Middle Cambri.m in age. It is about 140 Chancellor Formation: Argillaceous metres thick and consists of buff, white and grey and . Basinal equivalent and dolomites. Laminations, ripple marks, intraformational of the Pika, Eldon, Stephen and breccias, yoholaminites (Mcllrealh and Aitken, 1976). algal Cathedral formations mats, ooliths, pisolites, fenestrae and burrows we well pre- served. Pyrite is common either as disseminations or pods Arctomys Formation: Purple and red and veins. shales with beige dolomite. Overlain by the Waterfowl and Sullivan formations. The consists of tan to grey, thinly bedded to laminated shale about I6 metres thick, with a Eldon and Pika formations (undivided): cleavage subparallel to bedding. It is of Middle Cambrian Buff, grey and black massive dolomite, age and contains abundant fossil fragments and locally well- argillaceous dolomite and limestone. preserved and inarticulate brachiopodls. The Eldon and Pika formations cannot be subdivided in Stephen Formation: Brown and tan the map area. The lowermost beds of the Eldon Formation, shales. Fossiliferous. overlying the Stephen Formation, are black limestones approximately 50 metres thick. This basal unit is very dis- Cathedral Formation: Buff and grey dolomite and limestone tinctive, containing millimetre to centirnetre-scale argillaceous layers that weather to a red, msty colour; Naiset Formation: Thin-bedded, brown elsewere these formations cannot be readily distinguished and green shale. from the Cathedral Formation, texcept by fosi.il evidence. The Arctomys Formation, also Middle Cambrian in age, Lower Cambrian is characterized by green and purple shales and siltstones interbedded with beige, fine-grained dolomites. Mud cracks I Gag Formation: Massive, tan, sandstone. and halite crystal prints are commonly preserved. The thick- ness of this formation was not determined, as the base marked the limit of mapping. SYMBOLS All the formations are well exposed over the area, except the recessive Stephen Formation, which was no,: observed in the southern part of the map area. It is not clear if this lack of exposure is due to lack of ou!:crops or to nondeposition. Magnesite STRUCTURE Sparry carbonate Rocks west of the Cathedral1 escarpment are strongly Magnesite ( Lzech, 1966 ) deformed. The deformation is characterized ksy numerous small-scale folds with subhorirontal fold axes oriented 160’ Minor thrust faults, and a well-developed steeply dipping Cathedral Escarpment cleavage striking 160” are other typicel features. Along the I I I I r Cathedral escarpment, cleavage is subvertical, closely _,- _).... Geological contact: defined, approximate, assumed spaced and injected by dolomite, calcite an

Eldon and Pika White, buff, grey and black, formations mottled, fenestral laminated to ma&e (undivided) dolomite.

Stephen Formation Brown, tan and grey calcareous, fossiliferous shale.

Cathedral Formation Buff, white and grey, massive to laminated limestone and dolomite. Contains magnesite mineralization and the Mt. Brussilof mine. Mt. Brussilof mine

Naiset Formation Green and red, cleaved thin-bedded shales.

LOWER Gog Formation Orange to buff, massive to crudely thick-bedded, CAMBRIAN matore, quartz sandstone.

272 MAGNESITE DEPOSITS crystals (Plate 3-2-6) or a mixture of light grt~y and white magnesite crystals. Common impurities in magnesite Sparry carbonate rocks occur within the Cathedral, Eldon are isolated rhombohedral dolomite crystals, salcite veins. and Pika formations (Figure 3-2-2). They consist mainly of pyrite veins (Plate 3-2-7). suhvertical fractures filled by a cwrse dolomite and magnesite crystals in varying propor- mixture of beige ankerite, ulcilt: and chlorite,

Plate 3-2-4. Discordant. sharp and irregular urntact between sparry carbonate (SC1 and fine-grairwd dolomite (DO).

273 Plate 3-2-5. White and grey, layered “perry magnesite ore Plate 3-2-h. Randomly oriented sparly magnesite crys- from the Mount Brussilof mine. tals, Cathedral Formation, 100 mettes cast of the Mount Brussiluf mine. ing or single quartz crystals and coarse pyrite pyritohedrons Dolomite veins cutting magnesite ore occur at the mme. and octahedrons disseminated within sparry magnesite. however, magnesite veins were never observed to cut sparry Chalcocite, fersmite, phlogopite, talc and coarse, white. dolomite. acicular palygorskite were also observed in the mine. Boulangerite, huntite and brucite were reported from labo- CHEMISTRY OF CARBONATE ROCKS ratory analysis by White (1972). Analyses of I9 samples of magnesite and dolomite- bearing rocks were available in time for this publication. Where fine-grained dolomite is not entirely converted to These samples were analyzed for MgO, CaO, FeO, SiO, magnesite. replacement features such as coarse, white car- and AI,O,. The major constituents are MgO and CaO, bonate crystals growing perpendicular to fracture planes wjhich are negatively correlated (Figure 3-2-4). (Plate 3-2-8) or partings (Plate 3-2-9) and lenses of fine- The magnesium content of the carbonate rocks varies grained dolomite enclosed by sparry carbonates are com- continuously from dolomite to magnesite. Stoichiometric mon. Bipolar growths of zoned magnesite crystals (Plate dolomite and magnesite are given for reference. Fine- 3-2-10). magnesite pinolite (Plate 3-2-l I), rosettes and grained massive or laminated carbonates are dolomitic in coarse carbonate crystals having lozenge-shaped cross- composition. Coarse and sparry carbonates have higher sections (PIa 3-2-12). All these features are interpreted as magnesia contents than fine-grained carbonates. replacement textures. Some long magnesite crystals are deformed, suggesting that at least some magnesite predates ELEMENTS OF THE GENETIC MODEL or is penecontemporaneous with the last period of Several elements of a genetic model explaining the origin deformation. of the Mount Brussilof deposit are indicated by the tectonic, Sparry dolomite rock consists mainly of dolomite stratigraphic and structural setting, secondary porosity fea- rhombs. It forms lenses, veins or irregular masses in fine- tures, replacement textures. paragenesis and absence of grained dolomite and is believed to occur at the same fine-grained magnesite, protodolomite or hydromagnesite, stratigraphic horizons and to contain the same impurities as The presence of huntite (White, 1972) remains to be coarse sparry magnesite. explained.

274 Plate 3-2-l. Pyrite veinlets cutting magnesite rock: Mount Plate 3-2-K Bipolar growth of sparry carbomtes from a Brussilof mine. fracture plane in fine-grained dolomite that hosts the Mount Bmssilof deposit. It is suggested that the magnesite postdates early Mg2+ ratio is illustrated in Figure 3-2-5, high temperature diagenesis of the Cathedral Formation and probably of the and low mole CaZ+/mole MgZ+ ratio increases the potential Stephen, Eldon and Pika formations as well. Widespread of the fluid to convert carbonate:s to magnesia:. dolomitization and subsequent fracturing and brecciation Predictions based on this model suggest that the highest contributed significantly to an increase in porosity. Some of grade magnesite deposits should be located along the edge the fracturing may be due to reactivation of a pre-Cathedral of the Cathedral escarpment, within the reef facies. Lower escarpment fault or to a difference in competence of deep grade magnesite deposits and sparry d&mites would be and shallow-water sediments during the post-Middle located at a greater distance up-dip from the Cathedral Cambrian tectonic activity. However, most of the breccias escarpment along the same permeable zones, or adjacent to were probably produced by a partial dissolution and col- the escarpment but in the zones of lesser pemteability. lapse of the carbonate hostrock, caused by incursion of This model conforms well to the field obwvations. It meteoric water or hydrothermal solutions in the manner requires confirmation and integration with the results of described by Sangster (1988). future petrographic work, geochemical (isotopic, REE, Fluids responsible for crystallization of coarse sparry minor and major element) amdysis, fluid inclusion and carbonates reacted with dolomitized, permeable and fract- crystallinity studies. mass balance determinations and fur- ured reef facies along the Cathedral escarpment and moved ther thermodynamic considerations. Future studies will up-dip along the permeable zones. The fluid cooled and focus on constraints on the ori,& temperature and com- evolved chemically along its path due to interaction with position of the mineralizing fluid, geochemical gradients, dolomitic hostrock. The most important parameters deter- paragenetic relationships and fluid/rock ratios. mining the ability of the fluid to increase the magnesium content of are temperature, the mole Ca2+1 mole Mg2f ratio of the solution, the fluid/rock ratio and the ECONOMIC CONSIDERATIONS salinity of the fluid as well as the permeability, porosity and physical and chemical characteristics of the protolith. The Several new magnesite showi~ngsthat are part of a contin- relationship between temperature and mole Ca*+/mole uous sparry carbonate belt parallel to the Cathedral escarp-

Geolo,qical Fieldwork 1990, Paper 1991-l 275 Plate 1-2-10. Bipolar magncsite pinolitr: vestigial silty dolomitic protolith (hlack). zoned magnesitc crystals (white merit were identified in the course of fieldwork. Magnesia and light grey): Mount Brussilof mine. content varies considerably within this belt. About I kilo- metre north of the mine the favorable horizon of the Cathe- chalcocite, the previously reported occurrence of dral Formation is covered by barren Eldon Formation. boulangerite (White, 1972) and abundant pyrite in the However the continuity of the mineralization beneath this Mount Brussilof mine further encourage exploration pro- cover is proved by sparry carbonate showings along the grams in the Mount Brussilof area. Assiniboine Creek valley. The belt may extend south of the known Miller Pass showings (Figure 3-2-2). Very littlc is ACKNOWLEDGMENTS known about the grade of these occurrences and further Baymag Mines Co. Ltd. kindly provided access to com- exploration is justified. paoy documents and a field office during the lengthy rainy Mapping confirmed that magnesite is not confined to a periods of the 1090 field season. Twenty major element single stratigraphic horiron within the Cathedral Formation. analyses were performed by Baymag free of charge. Special Sparry dolomite is widespread throughout the formation. thanks are extended to H. Fergen, Mine Manager, and I. and also occurs within the Eldon Formation. The possibility Knuckey, Mine Geologist. ft)r their cooperation and assis- that the Cathedral escuqxnent is not the only permeable tance. R. Matthew. Consulting Engineer, directed our atten- zone that was open to magnrsite-forming fluids should he tion to the mineralogical particularities of the Mount considered by prospectors. Brussilof mine. Dr. S. Paradis of the Geological Survey of The known association of base metal deposits with the Canada in Quebec, performed most of the density studies Cathedral escarpment (Aitken and Mcllreath.l9X4), sim- wed to determine the proportions of magnesite and do- ilarities between the dolomitiration styles in the Kicking omite. Z.D. Hors of tlte British Columbia Ministry of Horse mine (Rasetti, I95 I) and in the Mount Brussilof area Energy, Mines and Petroleum Resources proposed the pm indicate that exploration should not be restricted to ject and kindly proof-read an earlier version of this man- magnesite. uscript. Enriching discussions with Dr. J.M. Aitken, Dr. I. Discovery of a fluorapatite float on Mount Brussilof. the Jonasson and Dr. D. Sangster from the Geological Survey of identification of fersmite (a niobium-hearing mineral) and Canada are acknowledged.

276 25

50

011 1.b Id.0 - ( MCa*‘/ MM;) 0 5 10 15 20 25 30 ‘70 cao REFERENCES MacLean, M.E. jl988): Mount Brussilof Magnesite Project. Aitken, J.D. (1971): Control of Lower Paleozoic Sedimen- Southeast British Columbia (82113E); B.C. Minist!:~ ifl tary Facies by the Kicking Horse Rim. Southern Rocky Ene/;y.v, Minrs md Pen-ol~um Rmnurces, Geological Mountains, Canada; Bullerin qf Cunodian Pm&urn Fieldwork 1988, Paper 1989.1, pages 507.510. Geolng~, Volume 19, pages 557.569. Mcllreath, 1.A and Aitken. J.D. (1976): Yoho/aminim (Mid- dle Cambrian) Problematic Calcareous Sediment- Aitken, J.D. (1989): Birth, Growth and Death of the Middle stabilizing Organism: Gwlogical A.ssok~fion of Cm- Cambrian Cathedral Lithosome, Southern Rocky Mountains; Bullrrin r,f’Conodian Petmlmnt GEO/O(C\.. ada, Program with Abstracts, 1976 Annual Meeting, Volume 37, pages 316-333. page 84. Aitken, J.D. and Mcllreath, LA. (1984): The Cathedral Reef Rasetti, F. (1951): Middle Cambrian Stratigraphy and Fauna Escarpment, a Cambrian Great Wall with Humble of the Caadian Rocky Mountains; Smirhsoniurl Mis- Origins: G~0.s. Volume 13, pages 17.19. ndlmeorrs Colkriom. Volume 1 16. Number 5, 277 pages. Aitken, J.D. and Mcllreath (1990): Comment on “The Bur- gess Shale: Not in the Shadow of the Cathedral Escatp Rosenberg. P.E., Burt, D.M. and Holland, H.D., (1967): merit”; Geoscience Canada, Volume 17, pages Calcite - Dolomite - Magnesite Stability Relations in 111-115. Solutions: the Effect of Ionic Strength; Grochimica (,f Coope, B.(l987): The World Magnesia Industry; Indrrswkd Cosmochimica Acra, Volume 3 I, pages 39 l-396. Mineds, Number 223, pages 2 l-3 I. Sangster, D.F. (1988): Breccia-hosted Lead-Zinc Deposits Duncan, L.R. (1990): Magnesite; Mining Eqginewing, Vol- in Carbonate Rocks; in Paleocarst, N.P. James, and ume I9 I, Number 6. page 569. P.W. Choquette, Editors, Sprinp--l/w/q NPM. York Fritz, W.H. (1990): Comment: In Defence of the Escarp- Inc., pages 102-I 16. merit near the Fossil Locality: Gcn- Schultes. H.B. (1986): Baymag High Purity MgO from s&we Cunada, Volume 17. pages I Oh- I IO. Natural Magnesite; Cawarlrun Institute $ Mining and Grant. B. (1987): Magnesite, Brucite and Hydromagnesite M~rallqy, Bulletin, May 1986, pages 43-47. Occurrences in British Columbia; B.C. Minisrr~ of Stewart, W.D. (1989): A Preliminary Report on Stratigraphy Energy, Mines and Perroleum Kesourws, Open File and Sedimentology (Middle to Upper Cambrian) in the 1987.13, 68 pages. Zone of Facies Transition, Rocky Mountain Main Leech, G.B. (1965): Kananaskis Lakes, W l/2 Area; in Ranges, Southeastern British Columbia; in Current Report of Activities, May to October, 1965; Grologi- Research, Pan D, Geol~,~ix~l Survey of Cnnada, Paper cd SWvey of Canada, Paper 66 I, pages 65-66. 89.ID, pages 61-68. Leech, G.B. (1966): Kananaskis Lakes; Geologirnl Suwey Wilson, E.M., Hardie, A.L. and Phillips, O.M. (1990): Dol- of Canada, Open File 634. omitiration Front Geometry, Fluid Flow Patterns, and Ludwigsen, R. (1989): The Burgess Shale : Not in the the Origin of Massive Dolomite: The Latemar Shadow of the Cathedral Escarpment; Geoscience Buildup, Northern Italy; American Journal of Science. Canada, Volume 16, pages 139.154. Volume 290, pages 741.796. Ludwigsen. R. (1990): Reply to Comments by Fritz, and White, G.P.E. (1972): Mineralogy of the Baymag Mines Aitken and Mcllreath; Geoscience Canada, Volume Ltd. Magnesite Prospect, South Kootenay Area, B.C.; 17, pages 116.118. Acrers Wesrern Limited, unpublished report, 17 pages.

278 British Columbia Geological Survey Branch