Abstract The structural history of part of the Purcell Mountains of British Columbia is interpreted in the light of the discovery of the Precambrian age of the Hellroaring Creek stock. This stock, the first Precambrian granitic intrusion recognized in situ in the Canadian Cordillera, cuts Moyie (Purcell) Intrusions and verifies their Precambrian age. The Sullivan ore body, one of the world’s largest deposits of lead and zinc, is younger than a low-grade regional metamorphism of its host metasedimentary rocks but, on the basis of potassium-argon dating of a lamprophyre, is probably no younger than 765 million years. The lead-isotope content of the Sullivan deposit is homogeneous and indicates a single mineralization. Deposits of the East Kootenay district contain two main types of lead; a relatively nonradiogenic variety and a markedly radiogenic type. The irdistribution is unrelated to geography and to major structural trends, out there is an apparent relation between isotopic composition and specific combinations of structure and host rock. Deposits that are structurally conformable with strata of the Aldridge Formation probably belong to the relatively nonradiogenic lead group, although there is no relation between isotopic compositions and occurrences in the Aldridge Formation i n general. Deposits believed to be genetically related to the Moyie Intrusions in which they occur are probably characterized also by lead of the relatively nonradiogenic group, a correlation that is in harmony with their proposed origin. Another correlation is between isotopic composition and size: small deposits, with the significant exception of those probably related to Moyie Intrusions, are likely to be of the markedly radiogenic type, whereas the commercial lead production of the district has come from deposits of the relatively nonradiogenic type. If any single lead was common to a ll the deposits of the dist rict its presence in many of them, especially the smaller ones, has been masked by additions of more radiogenic lead.

Abstract Lead-isotope data from a number of mining districts in the Northern Rocky Mountains in Idaho, Montana, and Washington are examined to help evaluate the lead-isotope evidence that the major Coeur d’Alene lead-zinc deposits were formed in Precambrian time rather than at the time of intrusion of the Idaho batholith. This regional reconnaissance is extended by detailed isotope studies that the Geological Survey of Canada has made on ore deposits in British Columbia, as described by Leech and Wanless in this volume. The new evidence does not clearly prove or disprove the apparent Precambrian age of Coeur d’Alene ore lead. Such ore lead with model ages of 1.2 to 1.4 billion years has so far been found in abundance in only two districts of the region: the East Kootenay district, British Columbia, and the Coeur d’Alene district, Idaho. Elsewhere in the Northern Rockies ore lead in so-called “Coeur d’Alene-type” ore deposits in Belt rocks is more evolute and ranges widely in isotopic composition.

The Blackbird cobalt-copper (Co-Cu) district in the Salmon River Mountains of east-central Idaho occupies the central part of the Idaho cobalt belt—a northwest-elongate, 55-km-long belt of Co-Cu occurrences, hosted in grayish siliciclastic metasedimentary strata of the Lemhi subbasin (of the Mesoproterozoic Belt-Purcell Basin). The Blackbird district contains at least eight stratabound ore zones and many discordant lodes, mostly in the upper part of the banded siltite unit of the Apple Creek Formation of Yellow Lake, which generally consists of interbedded siltite and argillite. In the Blackbird mine area, argillite beds in six stratigraphic intervals are altered to biotitite containing over 75 vol% of greenish hydrothermal biotite, which is preferentially mineralized. Past production and currently estimated resources of the Blackbird district total ~17 Mt of ore, averaging 0.74% Co, 1.4% Cu, and 1.0 ppm Au (not including downdip projections of ore zones that are open downward). A compilation of relative-age relationships and isotopic age determinations indicates that most cobalt mineralization occurred in Mesoproterozoic time, whereas most copper mineralization occurred in Cretaceous time. Mesoproterozoic cobaltite mineralization accompanied and followed dynamothermal metamorphism and bimodal plutonism during the Middle Mesoproterozoic East Kootenay orogeny (ca. 1379–1325 Ma), and also accompanied Grenvilleage (Late Mesoproterozoic) thermal metamorphism (ca. 1200–1000 Ma). Stratabound cobaltite-biotite ore zones typically contain cobaltite 1 in a matrix of biotitite ± tourmaline ± minor xenotime (ca. 1370–1320 Ma) ± minor chalcopyrite ± sparse allanite ± sparse microscopic native gold in cobaltite. Such cobaltite-biotite lodes are locally folded into tight F 2 folds with axial-planar S 2 cleavage and schistosity. Discordant replacement-style lodes of cobaltite 2 -biotite ore ± xenotime 2 (ca. 1320–1270 Ma) commonly follow S 2 fractures and fabrics. Discordant quartz-biotite and quartz-tourmaline breccias, and veins contain cobaltite 3 ± xenotime 3 (ca. 1058–990 Ma). Mesoproterozoic cobaltite deposition was followed by: (1) within-plate plutonism (530–485 Ma) and emplacement of mafic dikes (which cut cobaltite lodes but are cut by quartz-Fe-Cu-sulfide veins); (2) garnet-grade metamorphism (ca. 151–93 Ma); (3) Fe-Cu-sulfide mineralization (ca. 110–92 Ma); and (4) minor quartz ± Au-Ag ± Bi mineralization (ca. 92–83 Ma). Cretaceous Fe-Cu-sulfide vein, breccia, and replacement-style deposits contain various combinations of chalcopyrite ± pyrrhotite ± pyrite ± cobaltian arsenopyrite (not cobaltite) ± arsenopyrite ± quartz ± siderite ± monazite (ca. 144–88 Ma but mostly 110–92 Ma) ± xenotime (104–93 Ma). Highly radiogenic Pb (in these sulfides) and Sr (in siderite) indicate that these elements resided in Mesoproterozoic source rocks until they were mobilized after ca. 100 Ma. Fe-Cu-sulfide veins, breccias, and replacement deposits appear relatively undeformed and generally lack metamorphic fabrics. Composite Co-Cu-Au ore contains early cobaltite-biotite lodes, cut by Fe-Cu-sulfide veins and breccias, or overprinted by Fe-Cu-sulfide replacement-style deposits, and locally cut by quartz veinlets ± Au-Ag ± Bi minerals.

Abstract The Jurassic of the Canadian Rocky Mountains and foothills comprising the Fernie group and the lower part of the Kootenay and Nikanassin formations is subdivided on a paleontological basis into a number of units which correspond to zones and stages of the Northwest European standard section. The following stages are shown to be present by their index fossils—part of the Sinemurian, the Toarcian, the middle Bajocian, the lower Callovian, at least part of the Oxfordian and Kimmeridgian, and the upper Portlandian. A most significant feature of the Fernie group is its incompleteness, only n or 12 of the 59 Northwest European ammonite zones being proved to be present. Ammonite zones belonging to the Hettangian, parts of the Sinemurian, and the Pliensbachian are definitely absent. Stratal equivalents of the lower and upper Bajocian, Bathonian, upper Callovian and parts of the Oxfordian, Kimmeridgian, and Portlandian are not indicated by index megafossils and may be entirely absent or represented by sediments without megafauna. A variety of different facies is developed which can only be correlated by index fossils common to different facies districts. Some of the most prominent facies units are the Nordegg member, the Paper or Poker Chip shale of the lower Fernie, the middle Bajocian Rock Creek member, the Callovian Corbula munda beds and Gryphaea bed which are equivalent to the Grey beds, the Oxfordian Green beds, and the Passage beds which are mainly of Kimmeridgian age. Correlations of the Rocky Mountains and foothills Jurassic with the Jurassic deposits in British Columbia, Yukon, Prince Patrick Island, and Manitoba are established. Absence of index megafossils in the Jurassic of the Alberta, Saskatchewan, and Manitoba plains renders correlation of these areas with the Fernie group difficult. A tentative attempt has been made to correlate these areas on the basis of existing literature and through the co-operative efforts of the Jurassic symposium committee. Interpretation of the data obtained from field studies has made possible a synthesis of the present Canadian Rocky Mountains and foothills region during Jurassic time, a region which is considered as a non-geosynclinal border zone between the Cordilleran geosyncline in the west and the land Laurentia in the east.