The majority of the gold mines that have been discovered along the 60 to 70 miles betweeen Noranda and Val d’Or, Que., are located close to, or within a few miles north of, the major break known as the Cadillac-Bouzan Fault. The Camflo orebody is two miles north of this fault and is associated with a porphyritic monzonite stock. The oldest rocks in the area are the Malartic Group, consisting of andesite, basalt and dacite flows. This belt also contains tuffs, agglomerates, some rhyolite flows and many sill-like bodies of diorite and peridotite. Regionally, they strike east-west and have been folded into a major syncline. In the vicinity of the mine, the Malartic Group consists mainly of dark green andesite flows and tuffs which range from massive to very finely bedded. These rocks are intruded by sills of dark green medium-grained diorite which contain up to 10% disseminated magnetite. Locally, the rocks strike north 55 west and dip northeast at 50 . They represent the overturned north limb of the regional syncline and have contorted into a major drag fold around the stock. South of the volcanics is the Kewagama Group of sediments, composed mainly of dark grey fine-grained greywackes. Immediately adjacent to the volcanics is an altered 25-ft thick bed of conglomerate, followed by a 10-ft to 40-ft band of magnetic iron formation containing 35% magnetite. A second narrow band of iron formation is separated from the first by about 300 ft of greywacke.
Intruded into the sedimentary rocks is the porphyritic monzonite, locally termed feldspar porphyry. Two-thirds of this rock consists of quarter-inch feldspar crystals which occur in a matrix of fine-grained feldspar and biotite. The feldspar crystals are rounded and their outlines are blurred. The porphyry is salmon to pale pink to grey in color and contains about 4% biotite.
The porphyry forms an oval, pipe-like stock which in plan is roughly 300×600 ft. It rakes downward at 55 along a strike of north 55 east, following the nose and plunge of a large drag fold, more or less maintaining its position between the two bands of iron formation in the sediments. The outlines of the pipe are irregular. Much of the porphyry is jointed and fractured. The fracture patterns tend to be irregular, except for one persistent set normal to the long dimension of the pipe.
There is a series of parallel faults in the mine area which strike north 55 west and have an average dip to the northeast of 43 . The faults are from 10 ft to 150 ft apart, and little displacement is evident. They intersect the formations at an acute angle. The faults appear to have been the channelways for fluids which chemically altered the host rocks and, also, provided channelways for the migration of gold.
In the greywacke, porphyry and tuffs, the faults are in most cases much weaker. They are usually represented by 2-inch to 12-inch cherty quartz veins with an alteration zone of a few inches. In a few locations, 5% to 10% pyrite has been developed in the greywacke, and ore zones exist in these pyrite-rich areas.
In the diorite, the faults are surrounded by a large zone of silicification from 10 ft to 50 ft wide. The color of the diorite has been changed from dark green to a light blueish grey and the original disseminated magnetite has been altered to pyrite. The fault zones contain about 50% quartz, are from one ft to 10 ft wide, and contain from 1% to 30% pyrite. The amount of gold present varies roughly with the amount of pyrite and ranges from 0.01 to 0.40 oz gold per ton.
The ore along the fault zones makes up only about 10% of the ore discovered to date. Most of the ore has been found within the stock of porphyritic monzonite in both large and small irregular zones. The structural control of the ore zones has not been ascertained, but they make up about 25% of the porphyry. The rest of it is barren. The ore may be as wide as 130 ft and over 400 ft long.
Visually, the ore appears to be identical to the barren porphyry, except where it is quite rich. The richer orebodies contain 3% to 5% white-grey quartz stringers and 1% to 3% disseminated pyrite. The quartz stringers are -1/8 inch to 2 inches wide and contain -1/8 inch to inch subhedral pyrite crystals. Small specks of visible gold are common in the quartz stringers, and gold is often found on the face of pyrite crystals. Minor amounts of scheelite and occasional tellurides are sometimes found with the higher-grade ore.
Porphyry containing no quartz stringers has very little pyrite and is uniformly barren. Calcite and fluorite are common in stringers in both ore and barren porphyry.
The following sequence of events is postulated for the formation of the orebody: deposition of the tuffs and lavas and near-surface injection of diorite sills; deposition of the sediments; deep regional folding; formation of a batholith; differentiation within the batholith; injection of the porphyry pipe; cooling; movement along fault zones causing fracturing of the brittle porphyry and diorite; introduction of quartz along the fault and fracture zone; refracturing of the quartz; introduction of sulphur and gold; and, finally, erosion. Alex Makila, Chief Geologist, Camflo mine
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