The Montauban and Grand Calumet deposits are geologically similar lead-zinc-gold- silver deposits occurring in the Grenville Province. The lithological, mineralogical and economic similarities are strong enough to suggest that the two deposits were formed under the same general conditions. Their similarities may therefore be used as exploration guides for finding similar lead-zinc-gold-silver deposits in the Grenville. The Montauban and Grand Calumet deposits are located, respectively, about 100 km west of Quebec City and 100 km northwest of Ottawa. Both properties have previously been exploited for zinc, lead, silver and minor gold. Since 1983, Muscocho Explorations has been producing gold and silver from the North Gold Zone of the Montauban deposit. Initial reserves on the property were calculated at 923,000 tonnes of ore grading 3.8 g gold and 13 g silver per tonne. The Grand Calumet deposit is being explored for precious metals by Lacana Mining Corp.
Both deposits are hosted by complexly folded Helikian supracrustial rocks comprising a sequence of hornblende, biotite and calc silicate gneisses and marble. Mineral assemblages are typical of amphibolite grade metamorphism, and primary structures have been largely obliterated, making it difficult to discern the nature of the precursor rocks even by detailed chemical and petrologic study.
The lithologies most closely related to the Montauban and Calumet deposit mineralization differ somewhat from the thick sequences of carbonate rocks which host zinc deposits elsewhere in the Grenville, including those of the Balmat-Edwards type.
What follows are separate descriptions of the geology and mineralization of the two deposits separately. The authors will then compare the differences and similarities.
Recent exploration and mining at Montauban provides information about precious metal horizons, while data on the base metal zones come from previous production data and academic studies. Information concerning Calumet is derived from older research reports, geological level plans and cross-sections of the old lead- zinc-silver workings, as well as from the results of recent precious metal exploration. The Montauban Deposit
The lithologic sequence at Montauban comprises, in ascending stratigraphic order:
* a quartz-biotite gneiss;
* a locally sillimanite-rich quartz- muscovite-biotite gneiss hosting the base and precious metal zones (the Mine Series);
* a quartz-muscovite-biotite gneiss; and
* a meta-basalt.
Amphibolite dikes discordant with the lithologic contacts intrude these units. The quartz-biotite gneiss has been subdivided into a quartz-biotite gneiss and a grey composite gneiss on the basis of mineralogical differences in felsic layers. All units have a well developed foliation trending north and generally dipping at 30 degrees -35 degrees to the east. The Montauban deposit is believed to lie on the eastern limb of a major syncline overturned to the west, so the stratigraphic sequence is inverted in the mine area.
The Mine Series encompasses a wide variety of biotite gneisses and calc-silicate- and magnesium-silicate- rich rocks. It hosts four lead-zinc-silver and two gold-silver orebodies which occur laterally to one another in the same stratigraphic interval.
The base metal zones are enclosed in biotite gneisses and the actual host rock for the mineralization is a marble or a calc-silicate rock composed mainly of tremolite and diopside. The ore consists of the following sulphides in decreasing order of abundance: sphalerite, galena and pyrrhotite. The base metal horizon is folded but seems to be concordant with the foliation of the surrounding gneisses.
Two distinct auriferous units have been located at Montauban: the North and South Gold zones. Only the North Gold zone is discussed here.
The Mine Series hosting the North Gold zone is composed of a weakly foliated quartz-biotite-garnet gneiss, a well foliated quartz-biotite gneiss locally banded with biotite-rich layers, a quartz-biotite-sillimanite gneiss, and a distinctive cordierite-anthophyllite rock characterized by anthophyllite rosettes up to 4 cm long. Biotite-rich schistose lenses less than a metre thick and 5-10 m long occur throughout these lithologies.
The gold mineralization occurs between two bands of quartz-biotite- sillimanite gneiss. It is hosted by quartz-biotite-garnet gneiss and cordierite-anthophyllite rock. The contact between those units appears gradational and interdigitated but always in the same sequence; that is, the cordierite-anthophyllite rock always occurs at the bottom of the auriferous zone. The mineralization is not affected by this transition.
Precious metal zones at Montauban are associated with minor base metal mineralization. Sulphides may form up to 30% of the rock mass and, in order of decreasing abundance, consist of sphalerite, pyrrhotite, chalcopyrite, galena and pyrite. Within these zones, the gold values are independent of the total sulphide concentration and are not sympathetic with variable amounts of any particular sulphide type.
The North Gold zone occurs as a steeply dipping elongate body with a strike length of about 750 m, a maximum depth of 70 m and thickness averaging 2-3 m with local widening up to 6 m. Both the base metal zone and the North Gold zone are deformed by a succession of “Z”-type assymetric folds with sub-horizontal axes. These folds are observed at different scales and can produce displacement of several metres in the stopes, as shown above, at an elevation of 265 m. They also concentrate metallic mineralization by greatly increasing the thickness of the zone along the hinge, forming ore shoots within the favorable horizon. Grand Calumet Deposit
The lithologic sequence at the Grand Calumet mine is represented on page 17. In ascending stratigraphic order, it is composed of a hornblende gneiss; intercalated hornblende and biotite gneiss; a carbonate-rich amphibolite; the Mine Series; a leucocratic siliceous biotite gneiss containing lenses of hornblende-biotite gneiss; and a marble which tops the sequence. The country rocks enclosing the Mine Series are moderately foliated and generally trend northwest, dipping about 30 degrees northeast. Deformation has resulted in the overturning of the entire package of rocks in the mine area, so that the marble now underlies all units.
The Mine Series is made up of a siliceous biotite gneiss hosting the base metal mineralization and a distinctive auriferous quartz-biotite- garnet-anthophyllite gneiss (Gold Migmatite). It varies from 40-60 m thick and has a strike length of at least 1,200 m. The siliceous biotite gneiss is grey, fine-grained and often has lenticular quartz segregations. Sillimanite is locally present. The base metal zones occur near the hangingwall contact with overlying carbonate amphibolite and are conformable with it. The main auriferous horizon, the Gold Migmatite, underlies the base metal horizon, although smaller auriferous units have been found lateral to base metal horizons. A 1-5-m thick, fine- grained magmatite-rich amphibolite is usually present at the base of the Mine Series.
Base metal zones are tabular in nature and are composed of pods, lenses, bands and intercalations of the following:
* Marble — coarse white calcite.
* Calc-silicate rock — fine to coarse grained tremolite/actinolite and diopside.
* Coarse biotite — biotite-phlogopite.
* Massive sulphides — mainly sphalerite, pyrrhotite and galena.
The massive sulphides generally occur within the calcareous units, but concentrations may also be present in the other units. Cordierite is also common to the coarse biotite and calc- silicate-rich rocks.
The Gold Migmatite (quartz-biotite-garnet-anthophyllite gneiss) hosts the most widespread precious metal mineralization located to date. It consists of fine-to-medium-grained, well foliated, siliceous biotite gneiss with unevenly distributed bands and lenses of coarse biotite, pink garnets, anthophyllite and lenticular quartz segregations. Fine-grained magnetite and cordierite are also common c
onstituents. Sulphides, mostly pyrite with lesser chalcopyrite, occur in low concentrations within the Gold Migmatite, averaging between 2% and 5%. Minor base-metal-bearing zones are also found in this unit and can contain up to 15% sulphides.
The main Gold Migmatite body has minimum strike and dip dimensions of 100 m and 425 m respectively. It contains between 150 and 3,500 parts per billion gold over widths varying from 12 m to 18 m. Silver averages 12 g per tonne but can run as high as 110 g per tonne where base metals are present. Within it, gold values range from 3.8 to 12 g gold per tonne over widths of 1-6 m.
An interesting feature of the Grand Calumet mineralization is the presence of the thin amphibolite unit in the footwall of the Mine Series which contains 1% to 10% fine-grained blebs of magnetite where it underlies the auriferous quartz-biotite-garnet gneiss. Sillimanite is also common in the leucocratic biotite gneiss where it is found underlying the Mine Series. Similarities
The major similarities between the Grand Calumet and Montauban lead- zinc-silver and gold-silver orebodies are:
* Regional geologic setting.
* Occurrence in strongly deformed Helikian supracrustial rocks which have been metamorphosed to the amphibolite facies.
* Association of the lead-zinc-silver mineralization with a thin horizon of marble, calc-silicate or biotite-rich rocks enclosed in biotite gneiss.
* Size, grade and sulphide petrology of the base metal zones.
* Association of gold-silver mineralization with quartz-biotite-garnet gneisses containing bands of coarse biotite and magnesium-enriched silicates (i.e. cordierite and anthophyllite).
* Close proximity to the precious and base metal horizons. In the case of Montauban, the North Gold Zone occurs laterally to, but within, the base metal hosting stratigraphy, and at Calumet, the Gold Migmatite Zone occurs in the footwall of the units hosting the base metal mineralization. Differences
The most obvious differences are:
* The presence of a strongly sillimanite enriched biotite gneiss within the Moutauban Mine Series.
* The presence at Grand Calumet of the hangingwall carbonate amphibolite, the footwall magnetite-bearing amphibolite, the marble topping the stratigraphic sequence, and the hornblende-enriched country gneisses.
* The main gold horizons occur in different locations relative to the base metal zones.
* The high sulphide content of the auriferous North Gold zone at Montauban compared with the Gold Migmatite zone at Calumet.
In a general sense, the main differences between these deposits have to do with the lithologic sequence of host rocks. Any significant differences concerning the auriferous zones must be regarded as tentative, since more work is neeeded to refine the definition of the Gold Migmatite at Grand Calumet Christopher Bishop is a project geologist with Lacana Mining Corp. in Toronto and Vincent Jordain is a PhD student at the University of Quebec in Montreal.
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