The Strathcona iron-nickel-copper sulfide ore deposit lies at the base of the Sudbury Nickel Irruptive along the north rim of the Sudbury basin. In the vicinity of the deposit the main body of the Nickel Irruptive consists of an upper unit of 3700 ft (1200 m) of granophyre (the ‘micropegmatite’) and a lower unit of 1500 ft (500 m) of augite norite (the ‘felsic norite’) separated by 300 ft (100 m) of transitional rock (the ‘transition zone’). Two augite norite intrusions (the ‘mafic norite’ and the ‘xenolithic norite’) that are younger than the felsic norite occur along its lower contact. The xenolithic norite is relatively rich in xenoliths and grades downwards into a unit known as the ‘hanging-wall breccia’. The breccia resembles the xenolithic norite but contains a higher proportion of xenoliths. A quartz-plagioclase-augite gneiss (the ‘footwall gneiss’) underlies all units of the Nickel Irruptive. A cataclastic breccia (the ‘footwall breccia’) which formed as a result of comminution of both gneiss and overlying Irruptive rocks is present in most areas between the gneiss and the Nickel Irruptive. The ore body occurs partly as a dissemination of sulfides in the matrix of the hanging-wall breccia (‘hanging-wall ore’), partly as a fine dissemination and massive stringers of sulfide in the footwall breccia matrix (‘main-zone ore’), and partly as massive stringers of sulfide in the footwall gneiss (‘deep-zone ore’). Xenoliths in the xenolithic norite and hanging-wall breccia range from dunite to olivine gabbro. Olivine in the xenoliths (composition estimated by an X-ray method) varies from Fo73 to Fo85, and hypersthene and augite (composition estimated by electron microprobe analysis) vary from Fs25 to Fsi3, and Fsi3 to Fs5, respectively. The iron content of the mafic minerals shows a positive correlation with the proportion of felsic minerals in the xenoliths, suggesting that the xenoliths have been derived from a cryptically layered body of mafic and ultramafic rock. The wide distribution of xenoliths around the margin of the Nickel Irruptive coupled with the absence of any obvious external source is strong evidence that the xenoliths are cognate, supporting Wilson's (1956) proposal that the Irruptive is a funnel-shaped intrusion with a zone of ultramafic rocks towards its base. Hypersthene ranges from Fs33 to Fs28 in the felsic norite, from Fs28 to Fs22 in the mafic norite, and from Fs28 to Fs20 in the xenolithic norite. Augite ranges from Fsl6 to Fs14 in the felsic norite and from Fs14 to Fsn in both the mafic and xenolithic norites. The distribution coefficient for iron and magnesium between coexisting augite and hypersthene ranges from 1-0 in some of the xenoliths to 1-5 in some samples of felsic norite, indicating that the two pyroxenes equilibrated at, or near, magmatic temperature. The composition of plagioclase in the felsic norite, mafic norite, and xenolithic norite is around An65-70 but decreases to An44 in those Irruptive rocks closest to the footwall breccia. The composition of plagioclase within the breccia varies between An32 and An43. Sodium metasomatism appears to have affected the breccia and to have spread out to affect adjacent rocks. The concentration of nickel and copper in the sulfides varies systematically across the ore deposit. The nickel content of iron-nickel sulfides varies between 2-5 and 3 per cent in the hanging-wall ore but increases regularly from 3 per cent to 5 or 5-5 per cent from hanging wall to footwall across the main-zone ore. Copper concentration shows a similar but more erratic variation. The variation is attributed to thermal diffusion of nickel and copper within the main-zone ore along a gradient induced by the overlying, hot, Nickel Irruptive. The principal opaque minerals in the ore body are, in the order of their abundance, pyrrho-tite of at least two types, magnetite, pentlandite, chalcopyrite, and pyrite. All of the sulfides in the hanging-wall ore are the result of exsolution from a high-temperature, pyrrhotite solid solution. Pyrite started to exsolve below 700° C, chalcopyrite below 450° C, and pentlandite below 300° C. Monoclinic pyrrhotite formed from the host hexagonal pyrrhotite probably between 300° and 250° C. The temperature of formation of the sulfides in the main-zone ore has been obscured by reworking of the ore after its first emplacement. The principal ore sulfides, pyrrhotite and pentlandite, are common throughout the mafic norite, xenolithic norite, and hanging-wall breccia, occurring in amounts around 5 per cent in most samples. Pyrrhotite and pentlandite are extremely rare in the overlying felsic norite where pyrite is the most common sulfide. It occurs in amounts between 01 and 0-5 per cent, commonly together with secondary amphibole after pyroxene. The sulfides in the mafic and xenolithic norites and in the hanging-wall breccia occupy spaces interstitial to the silicates, and little or no replacement of silicates by sulfides has occurred. In the main-zone ore, evidence of small-scale replacement of silicates by sulfides is common. The high percentage of pyrrhotite and pentlandite in the mafic and xenolithic norites in contrast to the felsic norite, textural relations between sulfides and silicates, and the high temperatures indicated by the pyroxene distribution coefficients lead to the conclusion that the hanging-wall sulfides (including the hanging-wall ore) at Strathcona were introduced with these younger noritic intrusions. Data on the solubility of sulfides in silicate magmas rule out the possibility that the bulk of the sulfides were in solution in the noritic magmas; the data support the hypothesis that during intrusion the sulfides were held in suspension in the in the magmas as droplets of immiscible sulfide-oxide liquid. Calculations on the rate of...