1. The Algoma district, north of Manitoulin Island and east of Lake Superior, marks a general northwesterly transition to progressively older rocks, dominated by late Archean rocks, 2,700-2,500 million years old, of the Superior province of the Canadian Shield. Here is the spectacular Agawa Canyon railway in early October, a popular day trip from Sault Ste. Marie. The route runs north from "The Soo" through the hills east of Superior. Most tourists bent on touring the lake follow the coast west of here, on Highway 17, the Southern Route of the Trans-Canada Highway.
2. The Hemlo Gold Camp. The highway passes Wawa -- an old iron-mining centre and since 1991 the focus of modern exploration for diamonds -- and proceeds west through White River. In 1981 the old railway halt at Hemlo lent its name to a new and exciting development in the region: a major gold discovery. Mining commenced in 1985: the deposit supports three gold mines and is currently the top gold producer in Canada. This view shows the Williams mine, at the west end of the Archean-age deposit. Numerous smaller gold mines have operated in past decades in the Wawa-Michipicoten district, and a few such as Eagle River are current producers.
3. The Coldwell Igneous Complex. West of Marathon, the highway climbs over a majestic region of rocky hills dissected by steep valleys, with rushing rivers far below. A stop at the Neyes overlook is rewarded with spectacular views of Lake Superior to the south, and the railway line which snakes around the shore. The observant will note complex igneous rocks, part of a 500 km2 sub-circular mass of igneous rocks. Here we see veins of pink syenite cutting gabbro. These are the principal rock types in the intrusion, which is of middle Proterozoic age, just over 1,100 million years old.
4. Gold Deposits in the Beardmore-Geraldton Area. The former glories of gold mines in this region, accessed along Highway 11, the Northern Route of the Trans-Canada Highway, are displayed by this headframe of the former Hardrock mine at the entrance to the town of Geraldton. The landscape on this highway is flat compared to the lakeshore route, passing a region known as the Clay Belt, until rugged cliffs of a younger igneous province are encountered near Lake Nipigon. The cliffs, like the Coldwell complex, are associated with a major volcanic episode 1,100 million years ago.
5. Banded Iron Formation. This is a spectacular example of a banded iron formation, a sedimentary rock believed to have formed as a chemical precipitate on an Archean sea floor. The constituents of the rock were driven by submarine volcanic activity, similar to the "black smokers" observed near modern mid-ocean ridges. Such rocks commonly display delicate layering, and may contain units dominated by oxides such as magnetite (iron oxide), by carbonates or by silicates. This deformed iron formation is a host rock to the gold mineralization at one of the old mines in Geraldton.
6. Shore of Lake Superior at the Neyes Park. The rugged beauty of the northern shores of Superior can be attributed to the variety of resistant rocks in the region, including Archean granitic and metamorphic rocks and Proterozoic igneous rocks underlying the western Superior region. This contributes to the attractive shores in Canadian national and provincial parks such as Superior, Pukaskwa, Neyes and Sibley.
7. Stratigraphy of the Keweenawan. Passing west from Terrace Bay, the coastal route of the Trans-Canada Highway (Hwy. 17) ventures into increasingly rugged scenery, with engineered road cuttings and cliffs of imposing scale. Here we see the distinctive red clastic sediments of the Sibley Formation, cut by a subvolcanic sill, a near-horizontal intrusion of magma injected along the bedding of the host strata, distinguished by sharp contacts at both base and top. The sediments are often overlain by a suite of Keweenawan lava flows, to which diabase sills and dykes (cross-cutting intrusions) were probably conduits, feeding magma towards the surface.
8. The Sibley Peninsula. Much of the lakeward view from the Thunder Bay area is dominated by the recumbent form of the Sleeping Giant, an imposing mass of Keweenawan diabase. Much of the peninsula is a wild and beautiful park. A rocky reef off the southeast shore of the peninsula harboured the rich silver deposit that was exploited by the Silver Islet mine, the richest of a number of vein silver and base-metal deposits in the area.
9. Silver Islet is famous for a "bonanza" silver deposit, worked briefly and with impressive results under adverse conditions in the late 19th century. This is a photomicrograph of a sample of the ore, showing a mass of silver (white, high reflectance), galena (lead sulphide, grey with small pits) and cobaltite (cobalt arsenide, somewhat yellowish here) in a matrix of calcite and minor quartz. 40X magnification, long-axis field of view 3.0 mm, in reflected, plane-polarized light.
10. The Lac des Iles area, some 85 km north of the city of Thunder Bay, is notable for unusual igneous rocks with complex brecciated textures and local concentrations of volatiles, as exhibited by this gabbroic pegmatoid, pictured in the outcrop of the Roby zone in 1986, prior to the development of the successful palladium mine on the deposit. Ironically, these striking textures were easier to see in the original weathered outcrops than in the large expanses of fresh rock exposed by mining! As with the other current palladium mine in north America, the Stillwater mine in Montana, many years elapsed between the initial recognition of sulphides and precious metals and the commissioning of a modern mine.
11. The Lac des Iles mine was developed on the mineralization of the Roby zone. This view of the mining operation dates to 1995. There are three principal ore types containing various proportions of palladium (Pd), platinum (Pt), gold (Au), copper (Cu), nickel (Ni) and other metals. In 2000, the mine produced 95,116 oz Pd, 6,074 oz Pt and 6,035 oz Au.
12. The Lac des Iles deposit contains a range of ore types, including this assemblage of sulphide minerals. Here we see pale sulphides of iron plated by chalcopyrite, the common yellow sulphide of iron and copper, in a matrix of silicate minerals. The chalcopyrite is crowded with small flakes and prisms of hydrous silicate minerals, indicative of the volatile-rich environment in which the final crystallizing portions of the magma concentrated the base and precious metals. 40X magnification, long-axis field of view 3.0 mm, in reflected, plane-polarized light.
13. Belts of Archean rocks north of Thunder Bay, west and southwest of Lake Nipigon, form bedrock to the much later Keweenawan strata of the Nipigon plate. The terrain around Lac des Iles is currently (2001) the focus of intense exploration for platinum group elements (PGE). Possible host intrusions for deposits such as Lac des Iles are a small part of the terrain, and it is first necessary to understand the regional bedrock geology and other clues imparted by the surficial deposits formed by the action of ice sheets, rivers and surficial erosion. This image is a zoned granitic vein cutting gneissic bedrock.
14. Archean bedrock and younger glacial float stand in colourful contrast in this modest outcrop. Metamorphosed and deformed Archean sedimentary rocks are cut by at least two sets of white granitic dykes. A latecomer, the red boulder of Sibley sandstone, is a glacial erratic, a block entrained in the ice and carried from an outcrop which may no longer exist to this site, nearer to the southern limit of the last glaciation.
15. Heavy mineral grains such as this garnet crystal play an important role in revealing the bedrock source of surficial materials. Sand grains may be collected from rivers, from glacial till (sandy and pebbly deposits left by retreating glaciers), or from soil. Certain minerals may be indicative of valuable mineral deposits which may be hidden below till, soil or lakes upstream, or in the direction from which the ice sheet flowed (in this region, typically from the north or N.N.E.). Such detective work, a geological form of forensic science, is known as a provenance study.
Here is an unusual garnet grain about 1 mm long, seen in quartz and feldspar of a pale granitic rock, probably related to the veins seen in Photos 13-14. Garnet sand from local streams has been matched in chemical and optical properties to this rock type, a technical success of no immediate practical application. Photomicrograph, 80X magnification, long-axis field of view 1.4 mm, in transmitted, plane-polarized light.
16. The city of Thunder Bay is an important transportation hub on the upper Great Lakes, famed for rail and ship communications, transporting prairie grain, iron ores and other products. The "Iron Ranges" around Lake Superior saw rapid development in the second half of the 19th century with the opening of the American Marquette Range in 1854, the Menominee Range in 1877, and the Vermilion and Gogebic Ranges in 1884. The impressive dockside structure shown here was used to load large vessels ("lakers") with iron ores from the Steep Rock mine at Atikokan. Trains would be shunted into the upper level of the building, and a series of hoppers would funnel the crushed iron ore from railway cars to the holds of the waiting laker.
17. Iron Formation at Kakabeka Falls. The cratons (ancient nuclei) of the Precambrian shield contain suites of rocks believed to share broadly similar origins and depositional settings. These include voluminous granitic batholiths and elongate greenstone belts. The latter are the traces of ancient volcanic arcs, often containing miscellaneous metal deposits, such as gold, copper and zinc, iron and manganese. Some belts are dominated by volcanic rocks, while others display major proportions of sediments. Some belts contain important banded iron formations (BIF), introduced in Photo 5. Some of these sediments, the major components of which were deposited as chemical precipitates, contain major deposits of iron or manganese. Others, fractured by later earth movements, became favoured sites for gold and other metals, as at Geraldton. The BIF at Kakabeka Falls is known as the Gunflint Formation, and it is famous for the preservation of traces of early Proterozoic life forms.
18. A Gravel Pit in the Shebandowan area, west of Thunder Bay, provides an impressive example of the clues to recent earth history preserved in the surficial deposits left by the retreating ice sheets. The prominent, steeply-inclined sand and gravel beds are thought to have been deposited in a delta, building outwards into an ice-dammed lake during an hiatus in the advance of the ice sheet. The sediments were deposited into the transient glacial lake Kaministikwia. The ice lobe readvanced, truncating the top of the deltaic sequence and depositing a layer of subglacial till, seen at the top of the pit wall.
19. The Duluth Igneous Complex in northeast Minnesota is a major suite of intrusions at the western end of Lake Superior. The magmas of the complex solidified under a cover of lavas during the opening of the Midcontinent Rift, some 1100 Ma. Much of it is covered today by glacial deposits, but it is very large, with a total area variously estimated at 4,700-6,500 km2. Instructive outcrops exhibit a range of structures and textures formed in large magma chambers. Associated mineralization includes copper and nickel sulphides with platinum group elements, plus iron- titanium- vanadium oxides.
In this photograph of an outcrop in the Duluth city area, a block of iron-rich, early-crystallized rock (gabbro) is enveloped by a later, lower-temperature melt of more granitic composition (granophyre).
20. Pigeonite from the Duluth complex is a familiar mineral to igneous petrologists. Named for the Pigeon Point locality, it is a member of the important mineral group known as the pyroxenes. Common pyroxenes are silicates of iron, magnesium and calcium and are greenish-black in hand specimen, with faces of well-formed crystals meeting at roughly 90 degrees. Combined with grey to white feldspar and pale green olivine they comprise common igneous rocks such as gabbro (often erroneously termed "black granite").
The colourful photomicrograph displays just two minerals: banded grey plagioclase feldspar and the pigeonite with a distinctive chevron texture. The sample is from the gabbro zone of the layered series of the complex in the Duluth area 40X magnification, long-axis field of view 2.8 mm, in transmitted, cross-polarized light.
21. Dresser Trap Rock Quarries, Wisconsin. The 1100 Ma Midcontinent Rift system, ascribed to the action of a mantle plume analogous to magmatism in the modern Hawaiian islands, may have involved the eruption of over 1 million km3 of lavas, and intrusion of a similar volume of magma at depth. This large quarry operates in the Chengwatana basalts near Dresser, Wisconsin, southeast of Taylors Falls. Basalt lavas and gabbro sills yield tough aggregate. The lava flows have been mined for "trap rock" since 1855.
22. The White Pine copper mine, which closed recently, worked a large sediment -hosted Cu sulphide and native Cu deposit, in contrast to the many old mines of the Keweenaw peninsula to the east, which targeted native Cu deposits in basalt flows and interflow sediments. This photomicrograph displays bright native copper forming a cement between rounded grains of quartz and feldspar in the host sediment. Copper also occurs as the sulphide chalcocite. 80X magnification, long-axis field of view 1.4 mm, in reflected, plane-polarized light.
23. The Minesota copper mine is one of many old workings in the Keweenaw district, centred on Houghton. The site lies on the outskirts of the small town of Rockland, near the Ontonagon river. The mine was developed in the 1840s, and was famous for large, metre-scale masses of native copper. Cornish miners were involved in the early days. On 07 March 1857, the largest copper mass ever reported was discovered: its estimated mass was over 500 tonnes and the mass was 14 m long, 5.6 m wide and roughly 2.5 m thick. Because copper is a malleable metal, it had to be removed by cutting instead of blasting.
24. Giant masses of native copper such as this exhibit on the Keweenaw peninsula were encountered in underground mining, as described above. Others were found on surface as glacially transported "float" boulders, and some were used by native Americans thousands of years ago to construct a range of weapons, tools and ornaments.
25. Nuggets of Native Copper. A number of elements occur in the "native" or uncombined state, including sulphur, carbon (as graphite and diamond) and metals such as copper, silver, gold and bismuth. Samples of native copper are popular with mineral collectors. These 6-cm examples are from the Houghton district.
26. Archaeological Artefact made of Native Copper. The provenance of copper artefacts made by the native Americans can be tested by various means, including bulk chemistry and metallography. Copper tools from the Minnesota-Ontario border have been dated back to 7,000 years ago! Copper was a valuable trade item, either as a raw material or worked into arrowheads, knife or axe blades or beads. This photomicrograph is a native copper item from the McCollum site in Ontario. Note the visual purity of the metal, cut by a ragged veinlet of cuprite (copper oxide). 160X magnification, long-axis field of view 0.7 mm, in reflected, plane-polarized light.
27. Archaeological Artefact made of Smelted Copper. Smelted copper can be distinguished chemically by elevated levels of gold and (usually) silver. Rare copper samples from the Keweenaw district may however have extreme compositions, with physical admixtures of silver and copper metals or with occurrence of unusual copper-arsenic alloys. This sample, from the Ball site in Ontario, displays cuprite as tiny blebs of slag, a diagnostic feature of smelted copper. The copper would have been traded to the native peoples by European traders, and was brought to North America in the form of copper kettles, valuable trade goods. 160X magnification, long-axis field of view 0.7 mm, in reflected, plane-polarized light.
28. The Lake Ellen Kimberlite is an example of a class of unusual rocks which may host commercial diamond deposits. Named for Kimberley in South Africa, these rocks are generally olivine-rich and susceptible to deep weathering at the Earth's surface. The Lake Ellen kimberlite, exposed in a small pit, was a 1971 discovery in eastern Iron County. The body is a diatreme-facies kimberlite roughly 20 acres (8 ha) in area.
29. Heavy Minerals in Kimberlite are frequently used in exploration for these rocks, and for the diamonds contained in a minority (roughly 10 percent) of kimberlite occurrences. While the olivine and alteration products such as serpentine may disintegrate quite close to the source, more resistant minerals may be transported long distances by water or ice (see Photo 15 for an introduction to provenance studies). These resistant heavy minerals include chromite and ilmenite (black oxides of iron with chromium and titanium), diamond itself, and silicates of the pyroxene and garnet families.
Because diamond is so rare, distinctive compositions of the other, relatively common minerals provide valuable exploration clues. In this pan, a refinement of the classic gold prospector's tool, we can see highlights of abundant red and orange garnets amongst the dull green material of the kimberlite matrix.
30. Cliffs of the "U.P." (Upper Peninsula) of Michigan east of the Keweenaw peninsula are mostly low, sandy affairs, with the exception of the colourful strata at Pictured Rocks near Munising. This stretch of shoreline displays the erosion of unconsolidated sediments that can be observed at many sites around both upper and lower Great Lakes.