Core Shack THE WINSTON LAKE DEPOSIT

Winston Lake, 32 km north- northwest of Schreiber, Ont., is a base- and precious – metals – bearing volcanogenic massive sulphide deposit with drill-indicated ore reserves of 3.1 million tonnes grading 1% copper, 16% zinc, one gram gold and 30 g silver per tonne. The north-striking and 50 degrees eastwardly dipping deposit has a strike length of 750 m and width of 350 m. It has an average true thickness of 6 m and is open to depth. Winston Lake is owned by Minnova Inc. with an interest held by Zenmac Zinc.

Exploration of the Winston Lake area dates back to the 1870s. Initially, production was from the Zenith mine which was operated from two 25-ft shafts from 1885 to 1903. In 1951, the Zenith claims were re-staked by Patrick Harrison of Zenmac Metal Mines, and between 1966 and 1970, some 164,503 tonnes of ore, grading 16.5% zinc, were recovered from a 425-ft shaft.

In 1978, Corporation Falconbridge Copper mapped the area and recognized a predominantly volcanic environment; and in 1979 a hydrothermally altered “pipe” was discovered on claims staked to the west of the Zenmac claims. An agreement was signed with Zenmac in 1980, and in the fall of 1981 four diamond drill holes tested the down-dip extension of the alteration pipe. One of these intersected 4.3 m of sphalerite-bearing cherty exhalite.

In the spring of 1982, downhole pulse electro-magnetic (pem) surveys were conducted on three holes. Strong off-hole anomalies in two of these holes suggested a down-dip conductor. In June, 1982, drill testing of the pem anomaly intersected 2.1 m of massive sulphide grading 1.1% copper, 19.1% zinc, 0.73 g gold and 22.2 g silver per tonne. This marked the discovery of the Winston Lake deposit.

Both surface and underground exploration is continuing with some potential for the down-dip extension of the Winston Lake deposit. The Pick Lake zone, 1.5 km southwest of the Winston Lake deposit and stratigraphically below it, is a maximum 1.5-m-thick sheet of massive sulphide that runs 16% to 28% zinc.

The Winston Lake deposit occurs at the southwest part of the west- trending Big Duck Lake greenstone belt of north-facing mafic-to-felsic volcanic rock stratigraphically overlain by sedimentary rock and intruded by gabbroic plutons and bordered by granitic batholiths. All rocks are regionally metamorphosed to upper greenschist or upper amphibolite facies. The greenstone belt consists of the Winston Lake sequence of predominantly calc-alkalic rocks and the stratigraphically overlying Big Duck Lake sequence, which is tholeiitic in composition. The contact between the Big Duck Lake and Winston Lake sequences is marked by a large mafic- to-ultramafic composite sill-like intrusion which hosts the Zenith deposit.

The Winston Lake deposit lies at the top of the Winston Lake sequence within cherty exhalite and altered felsic-to-intermediate laminated ash tuff. In places, gabbro forms the hangingwall for the deposit. The footwall consists of altered mafic flow rocks and felsic-to-intermediate volcaniclastic rocks which are underlain by altered quartz and feldspar porphyritic rhyolite and fledspar pyritic basalt with intercalated sulphide-rich, bedded, tuffaceous rocks. In turn, are underlain by the “Main” quartz- fledspar porphyry which is intruded by gabbro and pyroxenite.

Hydrothermal alteration, confined to the Winston Lake sequence, and later metamorphism of altered rock have resulted in spectacular assemblages of cordierite, anthophyllite, biotite, garnet, sillimanite, staurolite, muscovite and quartz coincident with an increase in iron, magnesium, and potassium and a decrease in sodium and calcium. Field classifications such as “unaltered, fringe, patchy and pervasive” are used to define the intensity of alteration with pervasively altered rock forming a crosscutting, pipe-shaped zone below the deposit. Zinc content is directly proportional to the intensity of alteration. However, it reaches only a mean of 73 parts per million zinc in the pervasively altered rock. High copper values occur at the flanks and top of the alteration “pipe” with the core of the pipe containing relatively depleted copper values.

The most common forms of ore are finely banded sphalerite and pyrrhotite and massive-to-coarsely- banded sphalerite and pyrrhotite with minor pyrite and chalcopyrite and up to 45% of sub-angular mafic and felsic fragments averaging 3 cm in diameter. Locally, zones of massive, coarse- grained sphalerite with minor pyrrhotite, pyrite and chalcopyrite grade up to 50% zinc.

Present mine development has created a stockpile of 24,000 tonnes which will be treated when the mill starts up in January 1988. The proposed mining methods are cut-and-fill and long- hole stoping at a mining rate of 1,400 tonnes per day, five days a week. The mill will treat 1,000 tonnes per day, seven days a week. Pamela Phillips is a Toronto-based consulting geologist. She would like to thank Robert Sim, Minnova Inc.’s mine geologist at Winston Lake, for supplying information.