"Rock of the Month # 27, posted September 2003" --- four images (in plane-polarized, reflected light) of sample "SB2003", provided by Scott Breaton of Ivanhoe, Ontario. Polished thin section by George Taylor and Shawn McConachy of the Department of Geology, University of Toronto, from a chip sawn by Chris Papertzian at the Ministry of Northern Development and Mines, Tweed.

The slag is composed of roughly 66 volume percent olivine (and perhaps other silicates), as elongate prisms with aspect ratios 25:1 to 100:1, in a dappled silicate glass (30%) which may well be inhomogeneous. The remainder of the sample includes iron-bearing oxides (3%) and sulphides (1%). Qualitative electron microprobe examination of representative grains provides additional information. Small oxide grains (<0.05 mm wide) appear to be wustite, FeO, with appreciable manganese content. The coarse oxides (see Photos 1-2) are chrome spinels, rich in Cr with magnesium and lesser iron and aluminium.

The observed sulphides are tentatively identified as tawny chalcopyrite, mauve bornite and blue-grey chalcocite, a Cu-rich suite of the Cu-Fe-S ternary phase diagram. The material has been quenched by rapid cooling, and so a range of transient compositions, unstable in the longer term, may have been preserved, including Cu-rich "iss" (intermediate solid solution; Cabri, 1969; Fleet and Pan, 1994). A few grains, red and translucent, are probably the zinc sulphide, sphalerite.

Metallurgical slag, a waste product of the processes of recovery of metals from the host rock (ore), would ideally contain minimal traces of the sought-after metals. In practice, and especially in historical smelting sites, the recovery is never 100 percent, and slags contain chemical, microscopic and occasionally naked-eye evidence of their origins. This particular sample, collected from the northern margins of the town of Belleville in southeastern Ontario, may have been transported a long distance from the refinery site, perhaps as a construction material for one of the local railways. Nonetheless, it is possible that the original ore was mined within a few counties to the east and north, in the Grenville province of the Canadian shield (see the catalogue of mines by Udd, 1999). The slag may well be relatively old (1920s or earlier).

The slag shows evidence of relatively slow cooling in the large size of the abundant prisms of silicate (apparently the magnesium-iron silicate olivine, up to 3 mm long) in the glassy matrix. This cooling was nonetheless quick by geological standards (hours or days) and there is abundant evidence that the components of the slag did not have time to react to a chemical equilibrium. Specifically, the silicate crystals show skeletal form, the larger oxide grains are zoned, and the sulphide blebs contain a wide and intimate mixture of compositions not seen in typical copper ores.

Slags are typically porous and chemically reactive, and form secondary salts by reaction with air and surface waters. Detailed microprobe studies often reveal compositions rare or unknown in nature, including Zn-rich fayalite (iron-rich olivine, e.g., Parsons et al., 2001). Sulphide droplets in fayalite slags may include a range of Cu, Cu-Fe and Ni sulphide species. Modern smelter slags from Australia, Canada and Finland may contain bulk values of some 0.5-0.8 wt.% Cu and 0.1-1.5 wt.% Ni (Itoh et al., 1995), the metals occurring largely as sulphide which tends to remain segregated from its glassy silicate host, like oil in water, and indeed like sulphide in some well-documented mineralized intrusions.

References

CABRI,LJ (1969) New data on phase relations in the Cu-Fe-S system. Econ.Geol. 64, 443-454.

FLEET,ME and PAN,Y (1994) Fractional crystallization of anhydrous sulfide liquid in the system Fe-Ni-Cu-S, with application to magmatic sulfide deposits. Geochim.Cosmochim.Acta 58, 3369-3377.

ITOH,S, CHOO,RTC and TOGURI,JM (1995) Electrocapillary motion of copper and nickel matte droplets on fayalite-based slag surfaces. Can.Metall.Q. 34, 319-330.

PARSONS,MB, BIRD,DK, EINAUDI,MT and ALPERS,CN (2001) Geochemical and mineralogical controls on trace element release from the Penn mine base-metal slag dump, California. Applied Geochemistry 16, 1567-1593.

UDD,JE (1999) The Mines of Ottawa: A Guide to the Mineral Deposits of Southeastern Ontario and Southwestern Quebec. CJ Multi-Media Inc., 89 Ridgefield Crescent, Nepean, Ontario, 1st edition, 375pp.