Bulletin 39: Geology of Lower Jervis Inlet, British Columbia
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Bulletin 39 notes that the flanks of the Coast Mountains have long been recognized as favourable prospecting ground and the eastern flank, which is entirely in Canada, has proved to be particularly favourable, with the discovery of the Tulsequah, Granduc, Premier, Alice Arm, Bridge River, and B.C. Nickel deposits. In contrast, the core has failed to receive comparable attention.
It is difficult to estimate the possibilities of finding new ore deposits in the core of the Coast Mountains because large areas are completely unmapped and the regional geology is relatively unknown. Nevertheless, the available information provides some basis for cautious optimism.
Within the area under appraisal important mineral deposits have been found in four localities: Howe Sound (Britannia mine), Observatory Inlet (Hidden Creek mine), Ecstall River, and Princess Royal Island (Surf Inlet mine).
The Surf Inlet deposits were gold-bearing quartz veins that averaged 0.39 ounce of gold per ton and 0.3 per cent copper for 1,012,067 tons mined. They proved sufficiently productive to sustain a profitable operation for many years. The veins occur largely in quartz diorite, but it is probable that the shear zone in which the veins occur was localized by a narrow septum of older rock.
Gold-bearing quartz veins have been found elsewhere in the granitic rocks. In addition, minor amounts of chalcopyrite, magnetite, molybdenite, and scheelite have been found but, generally speaking, developments to date have yielded little to encourage exploration of the granitic rocks, except along contacts with rocks that they intrude.
The copper-bearing sulphide deposits of Howe Sound, Observatory Inlet, and Ecstall River occur in roof remnants of layered rocks that are surrounded in plan by younger granitic rocks. The deposits of Observatory Inlet and Howe Sound have each yielded more than 100 million dollars in mineral wealth, and the Ecstall River deposits are known to contain at least 8,000,000 tons of similar ore but of lower grade. It is thus apparent that the rocks in which these large sulphide deposits occur are worthy of some scrutiny, and it is therefore important to consider what proportion of the core of the Coast Mountain is composed of them.
From existing maps it would appear that there is definitely a higher proportion of granitic rocks in the Vancouver-Skeena River interval of the Coast Mountains than in the Skeena River-Portland Canal interval. A recent estimate (Bostock, 1948, p. 82), however, that more than nine-tenths of the Coast Mountains core south of Skeena River is composed of granitic rocks would appear to be too high. An interpretation of the latest geological map of British Columbia (Geol. Surv., Canada, Map 932A, 1948) is that, at shoreline, at least 15 per cent of the region south of Skeena River is composed of non-granitic rocks. Moreover, it is perhaps reasonable to expect that, at higher altitudes, a greater percentage of these rocks will be found. Furthermore, the work in Jervis Inlet has shown that small areas of older rock have been overlooked in the rapid coastal survey, and that stretches of coast barren of outcrop are more likely to be under-lain by sedimentary or volcanic rocks than by the more durable granitic rocks. Thus, although a great deal more work is required before an accurate estimate can be made, it is probable that as much as 20 per cent of the Coast Mountains core south of Skeena River is composed of non-granitic rocks.
It has been shown that in Lower Jervis Inlet the non-granitic rocks occur largely in narrow, steeply dipping belts that persist over a vertical range of several thousand feet with little change in width. This generalization, applicable to an area chosen at random, demonstrates that the septum or deep remnant is not unique to Britannia or Ecstall River. It should not be assumed, however, that all the roof remnants persist to depths of several thousand feet. The Caren Range body of the Sechelt Peninsula is regarded as a true roof pendant, and other remnants probably occur with essentially the same shallow irregular form. The point, however, is that certain of the remnants do persist to depths of several thousand feet with little evidence of diminution in size and, because they do, there is no justification for a generalization that most deposits found in the older rocks will necessarily bottom in granitic rocks at depths of a few hundred feet.
The remnants of older rock found in the core of the Coast Mountains are small compared to their very productive counterparts in the Canadian Shield. Nevertheless, they are of sufficient size to accommodate very large ore deposits. For example, at Britannia the remnant in which eight very productive orebodies have been found is much smaller in area than some of the other known remnants in the Coast Mountains.
In summary, several points have been presented as a basis for appraisal of the economic potential of the interior of the Coast Mountains. They are:-
(1) Three localities are known in which large sulphide deposits occur.
(2) The sulphide deposits are of similar type; in all, copper is an important constituent.
(3) The sulphide deposits occur within remnants of the eroded roof.
(4) Probably 20 per cent of the core of the Coast Mountains south of Skeena River consists of roof remnants; the proportion of roof rocks in
the Skeena River-Portland Canal interval is considerably higher.
(5) Some of the remnants persist to depths of several thousand feet and should not be regarded as roof pendants in the usually accepted
sense of that term.
(6) Some of the remnants are of sufficient size to accommodate large deposits.
The logical conclusion is that, pending evidence to the contrary; roof remnants in the Coast Mountains are definitely worthy of consideration in the search for new deposits of mineral, particularly copper.
Prospecting in the area has shown that rocks of the Jervis group contain small deposits of copper, zinc, lead and gold. Very small amounts of copper and gold have been found in the batholithic rocks; in addition, occurrences of molybdenite were observed southeast of Sakinaw Lake. Nothing has been found, however, to suggest that the granitic rocks of the map-area warrant further attention by prospectors.
Readily accessible dolomitic limestone, in sufficient quantity to be of potential economic importance occurs on the Cambrian Chieftain property.
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