Bulletin 49: Geology of the Duncan Lake Area, Lardeau District, British Columbia
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Bulletin 49 covers the Duncan Lake area, which includes about 200 square miles in southeastern British Columbia containing Duncan Lake and the north end of Kootenay Lake. The area straddles the Purcell trench, the western part lying along the eastern slope of the Selkirk Mountains and the eastern part lying in the Purcell Mountains. Rocks of the area form part of the Kootenay arc, a curving belt of complexly deformed sedimentary, volcanic, and metamorphic rocks extending from Revelstoke southeast, south, and southwest across the International Boundary. This report is primarily a study of the structure and the structural setting of lead-zinc deposits developed recently at the Duncan mine.
Rocks in the area belong to the Hamill and Lardeau Groups and are highly deformed sedimentary and volcanic rocks. The lithology and stratigraphy are summarized in the table on page 32. They are very similar to rocks of the Ferguson area 50 miles to the northwest, with which they have been correlated. Many of the formations can also be readily correlated with rocks in the Salmo area, 150 miles to the south, and the Rogers Pass area, 100 miles to the northwest. Though no fossils have been found in the Duncan Lake area, the rocks are known to be pre-Mississippian, and the Badshot Limestone, a prominent marker in the area, elsewhere contains Lower Cambrian fossils.
Sills of felsite, which are common in the southwestern part of the area, dykes of lamprophyre, and small sill-like bodies of amphibolite constitute the only intrusive rocks.
Within the area the grade of regional metamorphism increases from low grades characteristic of the northwesterly trending part of the arc northwest of Duncan Lake to garnet and higher grades characteristic of rocks along Kootenay Lake. The garnet isograd trends north from near the northwest corner of Kootenay Lake to the northern end of Duncan Lake.
Complex folds dominate the structure of the area. Several stages of folding are recognized, which probably all belong to one orogenic period, thought to be Mesozoic. The oldest folds recognized, called Phase I folds, are isoclinal and plunge at low angles to the north. Most of these folds cannot be seen, and are reconstructed from studies of the distribution of rock sequences and the mapping of formations. The limbs and axial planes of these folds are curved and have been folded by Phase II folds.
The principal Phase I folds are, from east to west, the Howser syncline, the Duncan anticline, the St. Patrick syncline, and the Meadow Creek anticline. Phase II folds are more open than Phase I folds. The folds plunge mainly to the north and northwest at angles as great as 30 degrees, but most plunge between north 15 and 25 degrees west at 5 to 10 degrees. Phase II folds are clearly visible in many outcrops and are defined by the layering of the rocks and by the attitudes of the formations.
Strike faults, many of which are parallel to the cleavage planes in Phase II folds, are common. Some faults are related and subordinate to the folding; others are superimposed on it.
The most important mineral deposits in the area are relatively low-grade zones of lead-zinc mineralization that have been developed recently but not mined. They are referred to as the Duncan type of deposit, from the Duncan mine. Essentially all the deposits of the Duncan type are in the Badshot Formation on the Duncan anticline. Some 15 to 20 mineralized zones of this type are known within the map area.
They consist of pyrite, sphalerite, galena, and minor pyrrhotite disseminated in dolomite and siliceous dolomite. They are lenticular zones with gradational but, in general, well-defined margins. The attitude is essentially parallel to that of the enclosing formations; the largest dimension is parallel to the strike, and the intermediate dimension is parallel to the dip. The longest axes of the mineralized zones plunge at low angles to the north, parallel to the axes of Phase II folds. The greatest plunge length found so far is more than 3,000 feet . The height may be as great as 500 feet, and the thickness is generally a few tens of feet but may be as much as 100 feet. The average grade is less than 10 per cent combined lead and zinc, and the grade of the zinc is greater than that of the lead.
Dolomite and siliceous dolomite in which The Duncan type of deposits are found are dark-grey rocks with mottled, flecked, and banded textures resulting from deformation. The mineralized zones appear to be structurally controlled replacements of the dolomite. Relatively thick and continuous dolomite layers on the Duncan anticline have localized mineralization.
The only production from mines in the area has been from a number of silver-rich lead-zinc deposits of various types. Total production from the area amounts to 2,100 tons from five properties. Four of these properties are veins and replacements in limestone and dolomite. The fifth property contains quartz veins carrying tetrahedrite.
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