Seafloor-deposited polymetallic massive sulfide rock from the Precambrian of Ontario, Canada.

This is a copper-zinc ore sample from the Potter Mine of Ontario. The mine's bedrock geology consists of very old, Late Archean-aged Kidd-Munro Assemblage rocks (2.711 to 2.719 billion years old). This unit consists of ultramafic and mafic volcanic rocks intruded by mafic to ultramafic dikes and sill-like bodies. Minor felsic volcanic rocks are also present. Polymetallic volcanogenic massive sulfide deposits occur in the Kidd-Munro Assemblage - these are the mining targets at the Potter Mine. Five to six million tons of orebodies are available here. The orebodies consist of seafloor-deposited massive sulfides and sulfide-mineralized sub-seafloor volcanic rocks.

Rocks in the Potter Mine area include a lower komatiite unit, a middle tholeiite unit, and an upper komatiite unit. The latter is well exposed at nearby Pyke Hill, a world-class komatiite locality. The volcanic succession was originally deposited more or less horizontally, but has since been structurally tilted. Volcanic beds are now nearly vertically oriented.

The Potter Mine has 8 levels that go down to 1,100 feet (~300 meters). It was shut down in 1972. The mine made copper concentrate. Recovered metals include copper (Cu) and a little silver (Ag). A drill hole study was done after the mine's shutdown and the subsurface geology has now been explored down to over 3000 feet. Massive sulfide mineralization occurs down to that depth, at least. Sulfides were encountered in the drill cores that contained cobalt (Co). The main mineralization is now recognized to be copper, zinc, and cobalt. The zinc is in the form of sphalerite (ZnS). There is also some silver and low-grade gold.

A volcanogenic massive sulfide (VMS) deposit should not be thought of as one ancient, shut-off, seafloor black smoker. The black smokers had to be long-lived, with multiple inputs of material. The Potter Mine area has stacked, stratiform VMS deposits. They were originally horizontal but are now nearly vertical. The deposits were originally on the seafloor, in an island arc basin environment. Eleven zones of sulfide mineralization have now been defined and 3-D visualized, within the limits of current exploration data (= drill holes).

Ore samples consist of sulfidic hyaloclastites, plus some massive sulfide samples. No pyrite is present - the ore rocks have sphalerite + chalcopyrite + pyrrhotite. The ores are dominated by pyrrhotite (Fe1-xS - imperfect iron monosulfide), which is grayish-brown to brownish-silver in color - it is not an ore mineral. Triclinic pyrrhotite and monoclinic pyrrhotite varieties occur at the Potter Mine - one of them is magnetic and the other is not. The sphalerite is brownish-colored to slightly reddish-brown. The chalcopyrite (CuFeS2) is brassy gold-colored and contains copper - it is the principal ore mineral at the mine.

Description from Houlé et al. (2010):

Two main types of base metal mineralization have been recognized: 1) subseafloor-style and 2) seafloor-style mineralization. A third type of base metal mineralization occurs as fragments or veins within mafic and ultramafic intrusions. All the mineralization styles are characterized by an assemblage of sulfides dominated by pyrrhotite with minor sphalerite and chalcopyrite. Subseafloor-style mineralization appears to be the most volumetrically significant ore type at the Potter Mine deposit. It occurs within the matrix of the volcaniclastic rocks as disseminated to semi-massive sulfides that may replace an earlier carbonate matrix. In intensely mineralized areas, the matrix consists entirely of sulfides and the basaltic lapilli are partially replaced by sulfides. The seafloor-style mineralization consists mainly of massive sulfide lenses intercalated with graphitic argillite and carbonaceous mudstone within the volcaniclastic rocks. This style of minearlization was interpreted by Gibson (1998) to have formed by exhalative activity on the sea floor during volcanic hiatuses marked by the deposition of argillaceous sediments. Several of the ore lenses contain both styles of mineralization and their distribution and relationships are still under investigation.

The Potter Mine area was affected by low-grade regional metamorphism (lower greenschist facies) that is most likely responsible for widespread chlorite found within the basaltic and komatiitic volcanic rocks. Hydrothermal alteration associated with the Potter Mine deposit occurs in the Middle Tholeiitic Unit, where it is best developed and displayed within the volcaniclastic rocks that host the VMS deposit. Preliminary observations indicate that chlorite, albite, and carbonate are the main alteration minerals developed within the matrix of the volcaniclastic rocks that envelopes the deposit. East of the deposit, the alteration is dominated by wispy chlorite, whereas the deposit itself is marked by an increase in albite, carbonate, chlorite, with minor pyroxene and epidote. Proximal to the mineralization, Fe-rich chlorite and minor talc was observed in the fragments, which results in the wispy nature of the fragments, in a matrix of sulfides. These wispy chloritic fragments are interpreted to be the remnants of former basaltic lapilli. Local veins of carbonate are also associated with basaltic and ultramafic intrusions.

Stratigraphy: Middle Tholeiitic Unit, Kidd-Munro Assemblage, Abitibi Greenstone Belt, lower Neoarchean, 2.711-2.719 Ga

Locality: Potter Mine (= Centre Hill Mine), north of Route 101, east-northeast of Matheson & south of the western end of Lake Abitibi & ~83 kilometers east of the city of Timmins, Munro Township, southern Cochrane District, eastern Ontario, southeastern Canada (48° 35' 57.39" North latitude, 80° 12' 39.81" West longitude)

Reference cited: