(14

ECONOMIC GEOLOGY

22 per cent. of Ni); millerite or capillary pyrites (NiS) is
well known from its hair-like crystals. Nickel also occurs as
an arsenide, and as a hydrous nickel-magnesium silicate,
the garnierite of New Caledonia.
SupBury — THE GeNEsis oF ITs Ores — The most important
 nickel mining field is around Sudbury in Ontario,
35 miles N. of Lake Huron. The field consists of pre-Paleozoic
 rocks, including a basin of sediments, 36 miles by
16, surrounded by a ring of igneous rocks, outside which are
steeply tilted sedimentary rocks with greenstones and still
older igneous rocks and gneisses. The formation may be
tabulated as follows :—
Keewenawan Intrusives—
ith Quartz - diorite; 3rd Granite; 2nd Micropegmatite ;
ist Gabbro (Norite).1

F16. 35 —NIckeL SuLpHIDE ORE
OF SUDBURY,
Nickel sulphide ore of Sudbury
(after T. C. Phemister), The
sulphides, black, are replacing
 the silicates in * norite,”
The replacement often follows
the cleavage planes in the
felspars.

Animikie. Sandstone, slate, and tuff with the Trout Lake
Conglomerate at the base.
Greenstone including pillow-lavas like those of the Keewatin,
and steeply tilted sediments, conglomerate, and slates
which are in places altered to schist.
Granitic gneiss and older schists.

The nickel ores occur chiefly with pyrrhotite and chalcopyrite.
 Some mines contain sperrylite, the arsenide of
platinum (Pt As,). The veinstones are quartz, secondary
biotite, a felspar-quartz intergrowth, and fragments of the
countrv rocks. The sulphides occur (Fig. 38) as veins

I The traditional name is norite ; Prof. Coleman remarked that much
of it has no rhombic pyroxene, and according to T. C, Phemister the
oulk is gabbro.