For those of you who have traveled Highway 17 North from Sault Ste. Marie and stopped at the Alona Bay scenic lookout, there is a historical plaque at the site which commemorates the initial discovery of uranium in Canada by Dr. John Le Conte in 1847 in the Alona Bay area (Nuffield, 1955). One hundred years later, in 1948, a prospector by the name of Robert Campbell discovered the presence of pitchblende at the promontory of Theano Point. This plaque provides a glimpse on an important period of mineral exploration history, and a story which bears re-visiting now and then.

In 1949, the recently created Atomic Energy Control Board of Canada recommended that the Federal Government lift the war-time restriction on private prospecting for radioactive minerals. This was shortly followed by a large staking rush in the Lake Superior - Montreal River area which led to a substantial amount of staking, prospecting, mineral exploration, and underground development on several properties. Like the Lake Superior - Montreal River exploration play, a similar exploration history was unfolding in the Beaverlodge area of northern Saskatchewan. As will be noted later, the link between these two areas is not just in the historical timing of events, but in the similar styles of uranium mineralization - what later came to be known as ‘classical’ uranium vein deposits. Rummaging through some of the old (historic) Ontario Department of Mines Reports and assessment files on this area, it’s still possible to find a few gems of information. This is particularly true of the report by E. W. Nuffield (ODM Volume LXIV, Part 3, 1955), on the Geology of the Montreal River Area.

Montreal River Area Geology

Based on Nuffield’s report and detailed 1 inch to ½ mile map, and my own, more recent, visits to this area, the geology is dominated by massive to pegmatitic, late Archean granite transected by west-northwest trending early Keweenawan-age, north dipping diabase dikes. The granite and most of the diabase dikes are unconformably overlain by mafic volcanics and red-beds of the Mamainse Formation which are in turn unconformably overlain by siltstones and sandstones of the Mica Bay Formation. The Mica Bay Formation has been correlated with the closing stages of the Keweenawan Midcontinental Rift and represents the early part of the basin-fill clastic sequence which was later dominated by the red sandstones of the Jacobsville Group (Late Keweenawan).

Location Map: Eastern Lake Superior and Montreal River Area

It is notable that throughout the eastern shore of Lake Superior we have evidence for two major angular unconformities of mid-proterozoic age: one below the Jacobsville - Mica Bay sediments and the other below the Mamainse Formation volcanics.

Montreal River Uranium Mineralization

Nuffield notes the close structural relationship between the pitchblende-bearing veins and the tectonized contact between the northerly dipping diabase dikes and enclosing granites. The mineralized veins range anywhere from small ¼" veinlets over a foot, up to several inches thick and 2 - 5 feet in length. The veins may occur at the sheared and brecciated contact with the granite or cutting the diabase, but tend to preferentially occur at angles to the contact, within granite. The veins consist of pitchblende, pink calcite, hematite and chlorite, with the adjacent granite wallrocks altered to a brick-red colour for several inches due to pervasive hematization. From my own field visits to the Theano Point area, the granites and pegmatite segregations have an above normal radioactivity, attributed to the presence of uranium refractory minerals such as uraninite. Nuffield identified a tantalum - columbium bearing mineral within the radioactive granites (ellsworthite). We also found large outcrop exposures where the feldspar crystals in the granite pegmatite were up to one foot in length.
Granite pegmatite with large feldspar crystal (white reflections) located near Theano Point pitchblende prospect; Paul Mora and Delio Tortosa in the far ground.	Theano Point pitchblende prospect;on the left is the granitic country rock;on right is a Keweenawan diabase dike; contact is along steeply north dipping fractures.
There is, apparently, no documented radiometric age date for the pitchblende of the Montreal River area. Based on Nuffield’s geological mapping, including geological sketches of mineral deposits together with geological maps from the assessment files, the pitchblende-bearing veins and fractures transect both the diabase dikes and the granites. They also appear to occur in fractures at the contact between Keweenawan lavas and granite. From this we surmise that uranium mineralization was likely early Keweenawan in age.
Generalized Geology of the Montreal River area

Geological Comparisons with Beaverlodge, Saskatchewan

On a visit to Theano Point several years back with Mike Hailstone and Paul Mora of the Sault District Office, I noted how similar these pitchblende veins, vein minerals, and wallrock alterations were to uranium deposits and occurrences in the Beaverlodge area of northern Saskatchewan. I had spent a number of years on exploration programs in the Athabasca and Beaverlodge areas in the late 70’s, and so it was somewhat of a ‘deja vu’ to encounter what appeared to be a striking similarity both in structural style and geological setting between what I saw at Beaverlodge and that at Montreal River.

Similar to Montreal River, Beaverlodge uranium deposits and occurrences are structurally controlled near faults and occur as pitchblende veins and breccias containing calcite, hematite, chlorite and with a characteristic brick-red wallrock alteration imparted on the mylonitized basement rocks (granites, gneisses and metasediments). The basement rocks are unconformably overlain by the Martin Group, a red-bed clastic sequence intercalated with mafic to intermediate volcanics. Both the basement rocks and Martin Group are unconformably overlain by the Athabasca Group, a basin-fill sequence dominated by altered and predominantly fluviatile sandstones. Again these rocks fall into a nearly mid-proterozoic age, and reflect two major mid-proterozoic unconformities.

For both areas, there is an apparent close proximity between the occurrence of pitchblende veins and the unconformity between basement rocks and the younger basins. In both areas we see the development of red-bed fanglomerates and sandstones with mafic to intermediate volcanics forming as a consequence of large-scale tectonic fracturing, faulting and basin development.

Uranium Deposits and Unconformities

The close spatial proximity of uranium deposits to unconformities is nothing new. It had been suggested by J. Robertson (1975) for the pitchblende mineralization in the Montreal River Area, and by F. Joubin (1955) for Beaverlodge . The most significant difference between Beaverlodge and Montreal River is the widespread faulting, fracturing, and brecciation in the Beaverlodge area. There, brittle structures hosting uranium veins developed along wide zones of regional-scale mylonitization. The lack of well developed and pervasive structures in the Montreal River area resulted in veins with very limited extent and therefore uranium deposits with sub-economic potential.

But the story doesn’t stop here.

The Athabasca Group: Uranium Elephant Country

In Beaverlodge, the classical vein deposits represent the earliest period of unconformity associated uranium mineralization (ca. 1780 My), which is also roughly the age of the Martin Group. This was followed by a much more prolific period of uranium mineralization (ca. 1100 My) associated with the younger series of continental clastic sediments - the Athabasca Group. The Athabasca Group hosts some of the largest and highest grade uranium deposits in the world.

What about our neck of the woods?

Back in our own neck of the woods, we’ve yet to discover anything comparable to the deposits of the Athabasca. But once again, the geological setting conducive to their development is not that dramatically different here. The Jacobsville Group bears some resemblance in its depositional environment to that of the Athabasca Group; both represent a craton-derived, predominantly fluvial clastic sequence variably affected by diagenetic alteration. In the few places where the Jacobsville unconformity is exposed, the underlying basement is characterized by a well-developed weathering profile - with a hydrothermal overprint - similar to the sub-Athabasca basement weathering (Kalliokoski, 1982).

The Jacobsville Group: a potential analogue to the Athabasca?

Standing on sandstone outcrop from left to right: Gerry Bennett, Paul Mora, Delio Tortosa; Mike Hailstone photographer.

The Jacobsville/basement unconformity is not well exposed or explored - that’s primarily because it lies under water (Lake Superior). In recent years a few of us had an opportunity to explore the unconformity for a short distance on the south shore of Goulais Bay due to the extremely low lake water levels. We were struck by the extensive area of pseudomorphic alteration in the Archean gneisses immediately below the shallow north dipping unconformity and basal conglomerates of the Jacobsville.

J. Kalliokoski (1976), had already noted the Athabasca - Jacobsville similarities and suggested that meteoric waters and deep weathering could be responsible for the potential development of unconformity-associated uranium deposits. He pointed to the presence of some known radioactive occurrences found within soft iron ores in Michigan which he suggests formed due to deep weathering.	Shallow dipping Jacobsville sandstone overlies basal conglomerate; unconformity outcrops parallel to the shoreline.

Exploration Guides for Elephant Country

One should keep in mind that the key to finding many of the ‘blind’ uranium deposits at the base of the Athabasca is the presence of graphitic conductors and associated post-Athabasca fault structures. The general view is that graphite provides the source of reductants which mixed with uranium-bearing, oxidizing, formational brines and resulted in the deposition of pitchblende-rich plumes above the unconformity. With erosion of the cover rocks the deposits for the most part disappear. Those that still remain are protected due to the cover rocks and lie deep below the current surface. At surface there is very little indication of what lies below.

Final Thoughts

Much of the previous discussion stretches geological speculation, but it does serve to remind us that it might still be worthwhile to dust off that old geiger counter, scintillometer, or spectrometer and carry it around in the field now and then - and stay open-minded when it comes to prospecting and mineral exploration. I find that sometimes it's better to focus on an understanding of geologic history and geological processes that were likely to have been active at various periods of time to lead me to a better appreciation of the mineralizing events that might lead to the development of economic mineral deposits.

Delio Tortosa, Sault Ste. Marie, Ontario
email: eliris@soonet.ca
web: www.soonet.ca/eliris

Credits:
Photographs for locations were kindly provided by Mike Hailstone, District Geologist, Sault Ste. Marie Resident Geologist District, Ministry of Northern Development and Mines, Ontario.

References:

Joubin, F.R., 1982, Some Economic Uranium Deposits in Canada; Precambrian, Vol 28, No. 1, pp. 6-8.

Kalliokoski, J., 1976, Uranium and Thorium Occurrences in Precambrian rocks, Upper Peninsula of Michigan and Northern Wisconsin, with Thoughts on other Possible Settings; Department of Geology and Geological Engineering, Michigan Technological University, Houghton, Michigan, p. 259 p.; prepared for the Grand Junction Office, Energy Research and Development Administration, Grand Junction, Colorado;

Kalliokoski, J., 1982, Jacobsville Sandstone, in Geology and Tectonics of the Lake Superior Basin, edited by R.J. Wold and W.J. Hinze, Geological Society of America, Memoir 156, pp 147-155.

Nuffield, E. W., 1955, Geology of the Montreal River Area; Ontario Department of Mines, Volume LXIV, Part 3, Sixty-Fourth Annual Report.

Robertson, J. A., 1978, Uranium Deposits in Ontario; Short Course in Uranium Deposits: Their Mineralogy and Origin, edited by M.M. Kimberly Mineralogical Association of Canada, 1978, Toronto,  pp 229-280.