ORIGIN OF THE ORES.

Molybdenite is generally regarded as a primary mineral deposited by magmatic waters, but it has been described from deposits that have been formed at vastly different depths and at very different temperatures. In the R and S mine its association with pyrite, fluorite, sericite, biotite, and quartz in a mineralized shear zone indicates that the ore has been formed by ascending magmatic waters, probably at a moderate temperature.

CONCLUSION.

As yet molybdenum ore has been found in only a comparatively small area in Sulphur Gulch. The veins are in a zone of shearing in an alaskite and there appears to be a number of veins that promise to be of sufficient width for economical mining, and many smaller stringers. The veins are of a kind that not uncommonly lack persistence and it is not safe to assume a continuation of the ore bodies much beyond the developed areas. However, the 300-foot tunnel exposes several thousand tons of ore and the chance of considerable more ore in the vein prospected by the tunnel and in some of the other veins seems good. The ore exposed in the main tunnel is of very good and of rather uniform grade.

U. S. GEOLOGICAL SURVEY,

WASHINGTON, D. C.

SOME CANADIAN OCCURRENCES OF PYRITIC DEPOSITS IN METAMORPHIC ROCKS.

This paper is the outcome of an attempt to discover what relation the form and structure of pyritic deposits bear to the nature of the enclosing rocks, and whether similarities in form and structure are related in origin. Further, it was hoped that the data obtained during the study would be of some value in helping to clarify some of the general problems connected with the origin of these ore bodies.

A few examples of ore bodies of this general type in Canada were studied in the field and in the laboratory. In trying to deduce explanations for the phenomena observed, specimens from other ore bodies which show many points of similarity were studied. This has brought out the fact that many of the similarities observed, particularly in regard to form and structure, may be produced by different causes.

The laboratory study was carried out in the laboratories of the Massachusetts Institute of Technology. The writer is especially indebted to Dr. Lindgren for helpful criticism and to Dr. Bruce of Queens University for the loan of ore specimens.

THE SULPHIDE DEPOSITS OF NORTHERN MANITOBA.

Location.

The sulphide ore bodies of northern Manitoba to be discussed in this paper are in the vicinity of Schist Lake, about 400 miles northwest of Winnipeg and 70 miles north of The Pas, Manitoba. Schist Lake is reached most conveniently from The Pas, where the Hudson Bay Railway crosses the Saskatchewan River.

FIG. 61. Index map showing location of the Mandy and Northpine Mines.

From The Pas, during the summer months, travel is by steamboat up the Saskatchewan River to Cumberland House, across Cumberland Lake and Sturgeon Lake to Sturgeon Landing at

the mouth of the Sturgeon River. From here the route is either by wagon road to Schist Lake or by canoe via Sturgeon River, Goose River, Goose Lake, and Athapapuskow Lake.

Geology.

The Schist Lake area is of slight relief and is characterized by the low rock ridges and intervening lakes and marshes so typical of the Canadian Shield. Rocky prominences rarely rise to elevations of 100 feet above the level of the lakes. The geology of the district is complex. The area is underlain by metamorphosed igneous and sedimentary rocks of early Pre-Cambrian age and by later Pre-Combrian granite. To the south, the Pre-Cambrian complex is overlain by gently dipping Ordovician dolomite. Pleistocene and Recent rocks are practically lacking.

The following table of formations has recently been published by Bruce.1

The Amisk volcanics are of widespread distribution in the district and form the country rock of all the important sulphide deposits. The rocks are chiefly metamorphosed lavas, agglomerates, and tuffs, of intermediate composition. The flows are represented by massive greenstones, which in some places are ellipsoidal and in a few places, amygdaloidal. The agglomerates contain angular rock fragments, light green in color, embedded in a matrix of darker green material. The composition of the fragments is practically the same as that of the matrix, but the latter is more sheared and contains more chlorite. The greenstones and schists are composed almost entirely of secondary minerals, and it is impossible, in many instances, to determine the exact nature of the pre-metamorphic rock. Thin sections of greenstone show altered phenocrysts of plagioclase. Other minerals present are abundant chlorite and often abundant calcite; epidote and zoisite in variable quantity; always minor quantities of quartz and magnetite; and sometimes small amounts of pyrite. The matrix of the agglomerate shows a similar min1 Bruce, E. L., "The Amisk-Athapapuskow Lake District," Geol. Surv., Can. Mem. 105, p. 9, 1918.

eral composition. It is probable that the fissile chlorite schists represent fine-grained and less competent beds such as tuffs.

The Kaminis granite occurs as extensive batholithic intrusions. All gradations from granite to granodiorite are found. A thin section of the granite from the west side of Phantom Lake contained the following minerals: quartz, orthoclase, microcline, albite-oligoclase, hornblende, biotite, apatite, titanite, magnetite, zircon, chlorite, zoisite, epidote, and sericite. The alteration products are rare.