Report on the Dominion Government Expedition to Hudson Bay and the Arctic Islands on board the D.G.S. Neptune, 1903-1904

by A. P. Low

GEOLOGY OF THE NORTHWEST SHORES OF HUDSON BAY.

The following account of the geology of the northwest shores of Hudson bay has been compiled from observations made during the trip of the launch from Winchester inlet to Chesterfield inlet in September, 1903. These are supplemented by the notes made in May, 1904, while making a track survey from Cape Fullerton to the entrance of Chesterfield inlet. The observations to the north of Fullerton were made by Mr. Caldwell, in April and May, 1904, while on his surveying trip to the head of Wager inlet; to these are added observations by the writer made on a boat trip along the coast later in that spring, on the way to and from Southampton island, when the rocks of the mainland were examined as far north as Yellow bluff.

The rocks seen along the shore between Chesterfield and Winchester inlets are largely a flesh-red to pink mica-hornblende granite-gneiss, often only slightly foliated, and varying in texture from medium to coarse-grained. These are associated with broken bands of dark-gray or red gneisses, usually very quartzose, and containing a considerable quantity of mica and hornblende, the latter often partly decomposed to chlorite. These gneisses have evidently been cut and broken by the intrusion of the granite-gneisses. Many veins of pegmatite cut all these rocks; it consists chiefly of red or violet feldspar with much quartz, and in some places large crystals of black hornblende.

The granite-gneisses are also most abundant inland, as was seen along the lower part of Chesterfield inlet and in the country forty miles inland from Winchester inlet.

The granites occupy the shores and islands between Winchester inlet and the west side of Island bay, about half way to Fullerton, when they give place abruptly to a series of dark schists. These schists are largely micaceous, but there are also frequent bands containing considerable quantities of hornblende, and these are more basic than the more common mica-schists, which always contain quartz in varying amounts. These mica-schists appear to have been clastic rocks associated with bands of trap, all having been altered and foliated by the granite intrusion. The schists are very regular, and have a constant strike of N. 10° W. Many of the basic bands contain varying quantities of pyrite, but it was never seen in sufficient abundance to constitute a mine. All the schists carry dark-red garnets, some of which are regular in their crystallization and of good size.

The islands about Fullerton harbour are formed partly of granite and partly of these dark schists. On this eastern contact of the granite with the area of schistose rocks, the latter have been greatly disturbed by the intrusion, being squeezed, contorted and broken by granite masses, as may be seen from the illustration. Areas of coarser basic rocks now occur in the rocks along the coast, usually in the form of coarse gabbro, but often in a more altered condition as coarse hornblende-gneisses. The granites are the prevailing rocks along the coast as far north as the mouth of Wager inlet, and have generally a red or pink colour. Associated with them and evidently altered by their intrusion are patches of gray quartzose gneiss, and less frequently areas of old basic intrusive rocks.

At Whale point, where the rocks were closely examined, the oldest rock was represented by a medium to fine-grained, gray and pink, very quartzose gneiss. This had been cut by a coarse diabase, and both had been foliated and broken by the intrusion of the newer granite. Dikes of a newer diabase cut all the other rocks.

Contorted Gneiss and Granite at Fullerton.

The granites prevail about Wager inlet, but there are more and larger areas of the dark basic rocks about that great bay than to the southward, making it a more promising field for economic minerals, especially as these basic rocks generally carry sulphides, and Rae reported free gold to have been found about the head of the inlet.

All the geological information concerning the western coast of Hudson bay to the northward of Wager inlet is contained in the narratives of the voyages of Parry and Dr. Rae. The explorations of Rae ended at Repulse bay. He reported only Laurentian rocks along the coast, with granite-gneisses predominating, these being accompanied by considerable areas of greenstones, showing the rocks in the northern part to be very like those along the southern shores of Roes Welcome.

Parry explored the west side of Fox channel from Frozen strait to Fury and Hecla strait. The result of his observations has been summarized by Dr. Dawson as follows:

‘The geological specimens brought back were examined by Prof. Jameson, and the detailed maps of the expedition include indications of the character of the rock at so many places, as to afford the means of tracing the geological outlines with very considerable accuracy. Granitic and gneissic rocks occupy the whole of the southern part of the east shore of Melville peninsula, and are continued northward behind a low track of limestone country, forming a range of mountains in the centre of the peninsula to Hecla and Fury strait. They also form the south shore of this strait, and most of the islands in it, and apparently the whole eastern shore of the adjacent south part of Cockburn “island.”

‘The rocks referred above, in a general way, to the Archæan, probably include areas of Huronian. Jameson mentions as among the prominent varieties of rocks derived from this region, “Granite, gneiss, mica-slate, clay-slate, chlorite-slate, primitive-trap, serpentine, limestone and porphyry.” In association with these the following minerals occur: “Zircon and beryl, also precious garnet, actinolite, tremolite, diallage, coccolite, rock crystal, calc-spar, rhomb-spar, asbestos, graphite or black lead, specular iron ore, magnetic iron ore, chromic ore or chromate of iron, titanic iron, common and magnetic iron pyrites.” Some of the “transition rocks” noticed by Jameson should probably also be classed with the Archæan, and in addition to several of the minerals above mentioned, in these were found tourmaline (schorl) and molybdenite.’

The coast between the mouth of Chesterfield inlet and Churchill, was examined by Tyrrell, and the following summary of the geology is taken from his report:—

‘On the low flat shores of Hudson bay between Seal river and Cape Esquimaux few rock exposures occur, but those seen consisted of granites and gneisses of typical Laurentian aspect. For forty miles north of Cape Esquimaux no rock in place was seen, and thence northward to Baird bay some of the points consisted of granite and gneiss, though the shore generally consisted of Huronian rocks.’

‘The largest area of Huronian rocks found in this district extends more or less continuously for 120 miles along the west coast of Hudson bay, from near Baker’s foreland to a point 45 miles north of Cape Esquimaux.’

‘The rocks constituting this system may be divided into three more or less distinct groups, viz.: The Marble island quartzites, the greenish quartzites and graywackes, and the more or less highly altered, and often schistose diabases and gabbros.’

‘The Marble island quartzites are composed of hard white quartzite, consisting of more or less rounded grains of quartz, of moderately regular size, cemented together by interstitial silica. They are very distinctly bedded in thick and thin beds, and the surfaces of the beds are often covered with beautiful ripple-markings. The heavier beds also often show distinct false bedding. They are usually in a more or less inclined attitude, but they were nowhere seen to be very much crumpled or squeezed into minute folds. Their total thickness was not determined.’

‘These quartzites were first noted by Dr. Bell from Marble island, and although this island was not examined by the writer, rocks of undoubtedly similar character to those described by Dr. Bell, were seen in many places along the shore, and consequently the name is here retained.’

‘In one place near the cache on the west side of Hudson bay, a thickness of sixty feet of this quartzite, in a nearly vertical attitude, was seen almost in contact with the Laurentian gneiss, there being but a narrow, drift-filled gap between the two. This would indicate either the existence of a fault, or that here the quartzites are the base of the Huronian, or that the gneiss represents an eruptive rock which has risen through or into the Huronian subsequent to the deposition of the quartzite.’

‘Dark-green eruptive rocks, chiefly diabases, often very much squeezed and altered, are largely developed in the Huronian, composing a considerable proportion of the rocks of this system. On the west coast of Hudson bay these rocks are cut by many veins of white quartz, highly charged with iron and copper-pyrite.’

‘Associated with the massive diabases, and often indistinguishable from them except on close examination, are many beds of fine-grained, often schistose graywacke, or greenish quartzite, which appear to have been caught up in, or surrounded by the eruptive rocks.’