Magnetite and ilmenite occur throughout the Sudbury Nickel Irruptive. The magnetite almost invariably contains oriented lamellae of ilmenite and the ilmenite often contains lamellae of oxide magnetite and/or hematite. The upper and oxide-rich gabbros and the micropegmatite contain between 1 and 10 percent oxide minerals with magnetite far in excess of ilmenite. The felsic and south range norites contain 0.1 to 2 percent oxide minerals with ilmenite in excess of magnetite. The mafic norite, a unit developed intermittently around the perimeter of the north range of the intrusion, contains 0.5 to 1 percent oxide (largely magnetite) while the quartz-rich norite, developed continuously along the margin of the south range, contains the same amount of oxide, largely ilmenite.Electron microprobe data on magnetite, coupled with estimates of the proportion of ilmenite lamellae developed within grains, indicates that magnetite from the upper levels of the Irruptive was very much richer in titanium at the time of its crystallization than that from lower levels. Chromium shows the reverse trend. Magnetite from the upper part of the mafic norite contains 8 to 13 percent Cr 2 O 3 but this decreases rapidly to less than 2 percent across the 10-50 ft transition zone between the mafic and over-lying felsic norite and then falls more gradually to less than 0.1 percent in the micro-pegmatite. Magnetite from the upper part of the quartz-rich norite and lower south range norite is also rich in Cr 2 O 3 (3-5 wt %) and this decreases upwards to less than 1.5 percent halfway up this unit. These variations in magnetite composition support previous evidence (Naldrett et al., 1970) of cryptic variation in the Irruptive.The final partitioning of iron and titanium between magnetite and ilmenite occurred at high ( nearly equal 900 degrees C) temperatures on the north range and much lower (<600 degrees C) temperatures on the south range, indicating a very different cooling history for the two ranges. This is consistent with the hypothesis that the Irruptive is funnel shaped and that the south range is a section through a deeper, thicker portion of the funnel.

First Page Preview

First page PDF preview
You do not currently have access to this article.