FALCONBRIDGE AT TIMMINS: THE GEOLOGY OF KIDD CREEK

Falconbridge’s Kidd Creek copper-zinc-silver massive sulphide deposit is at the west end of the Archean-age Abitibi greenstone belt, 27 km north of Timmins, Ont. The orebody was discovered by diamond drilling in November, 1963, by Texas Gulf Sulphur Co. It was following up by a regional geological reonnaissance and helicopter-borne EM surveys.

Mining began in the autumn of 1966. Since that time, 89 million tonnes averaging a grade of 2.2% copper, 7.1% zinc, 0.27% lead and 100 grams silver per tonne have been mined by open pit and underground methods. The 1990 year-end reserves, down to the 5,600-ft. (1.7-km) level, were 41.2 million tonnes containing 3.3% copper, 5% zinc, 0.17% lead and 65 grams silver per tonne. Reserves and production combined show that, at the very least, the deposits amount to 130 million tonnes, placing the Kidd Creek volcanogenic orebodies among the largest in the world.

The ore occurs in two main deposits, the South and North orebodies. These orebodies and the enclosing formations are overturned, dip steeply east and plunge northeast. Post-ore deformation resulted in brittle and ductile faults, folds, and cleavage and lineation fabrics. The two deposits are separated by a major, near-vertical, east-striking fault with a major downward displacement on the north side. Other steeply south-dipping reverse faults displace the ore downward by about 150 metres. These faults structurally extend the orebody in the down-dip dimension.

The orebodies and the rocks immediately surrounding them strike north-northeast in marked contrast to the regional easterly strike. The present geometry of faulted segments of steeply dipping lithological units in the mine area is inferred to be the result of thrust faulting followed by isoclinal folding.

Greywacke occurs directly to the south in apparently thrust-faulted relationship to the mine volcanic sequence. Variably carbonatized serpentinites stratigraphically underlie the rhyolite volcaniclastic ore host rocks, which at surface have a thickness of 365 metres (1,200 ft.). The chalcopyrite stringer ore and the succeeding sphalerite-pyrite-rich massive sulphide ores occur near the top of the rhyolites and together attain a maximum thickness of 150 metres (500 ft.) and a maximum strike length of 750 metres (2,500 ft.) in the open pit.

Rhyolites, stratigraphically below the ore, exhibit strong pervasive silicification and fracture-controlled chloritization and sericitization of variable intensity. These rhyolites, in proximity to mineralization, are enriched in iron and magnesium oxides and depleted in sodium, calcium and aluminum oxides. Carbonaceous argillite, containing lenses of epiclastic sulphide breccia, is interpreted to locally overlie the North orebody and underlie part of the South orebody. This interpretation implies that the South orebody is slightly younger than the North orebody. The ore host rocks are, in turn, overlain by a thick basaltic sequence.

Chalcopyrite-bornite Zone

The South orebody, which terminates at the 3400 level, exhibits poor metal zonation. Within the copper stringer ore there is a linear chalcopyrite-bornite zone that follows down the plunge along the stratigraphic keel of the orebody from the 1200 to 2600 level. This zone and the surrounding high-grade copper mineralization are rich in silver, with subordinate selenium, bismuth, arsenic and cobalt. Up section, toward the mining footwall, there is a rapid change to massive pyrite-sphalerite mineralization interspersed with erratic chalcopyrite-rich lenses. Within the zinc-rich areas, toward the stratigraphic top of the orebody, are patches where the galena content may reach several percentage points. It is noteworthy that selenium and lead are mutually exclusive in their areas of occurrence. Toward the top of the orebody, some repetition of copper stringer mineralization indicates fluctuating depositional conditions.

By contrast, it would appear that the North orebody developed steadily and progressively and, in so doing, developed a very large copper stringer zone. Increasingly large chalcopyrite lenses occur toward the top of the copper stringer zone, which in turn grades upward into a large body of zinc-rich, weakly-banded pyritic massive sulphides with appreciable lead and economically important silver.

The overlying discontinuous argillite and epiclastic breccias, prevalent in the upper levels of the mine, mark a time of volcanic quiescence followed by an erosional episode during which sulphides, formed earlier, were re-deposited. There is some evidence to show that when the hydrothermal system reactivated, it vented adjacent to the silicified mass underlying the North orebody and thus gave rise to the South orebody.

The North orebody attains its greatest known areal dimensions at surface and diminishes in size gradually with depth. The copper stringer mineralization is continuous to depth but the massive sulphide body fades out at the 3200 level and reappears below the 3600 level. Relatively narrow ore-grade massive sulphide lenses have intersected as deep as 6,800 ft. below surface by drilling from the 4600 level. In all, sufficient material of high enough grade has been indicated by exploratory drilling to warrant sinking the internal No. 3 shaft from just above the 4600 level to a projected 6950-level depth. At the end of March, 1991, the shaft-sinking program had progressed below the 5600 level.