Canadian Rockhound: The Madoc-Bancroft Geological Corridor of Eastern Ontario

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The Madoc-Bancroft Geological Corridor
of Eastern Ontario
By Bob Bredberg

The Madoc-Bancroft geological corridor represents a highly diverse assemblage of rock types which change rapidly over short distances. This variety has given Bancroft a reputation as the Mineral Capital of Canada. Apparently there are only two locations on earth, one in Siberia and the other around Bancroft where one can find samples of most all (estimated at 94%) of all the minerals known to exist on earth. Such sites are unusually rare, and this accounts for the geological reputation of the Canadian Shield formations in the Bancroft area.

The Canadian Shield

The Canadian Shield extends across most of Northern Canada, and represents some of the oldest crustal material known to exist (more than 3 billion years). From northern Ontario toward the south, the rock becomes younger and acquires a general downhill slope at approximately six feet to the mile. From Bancroft south to Madoc, the elevation decreases nearly 200 feet before it dips beneath the sedimentary cover at Madoc. The Shield formation continues southward, and passes nearly a mile below the state of Ohio before ending somewhere deep below the Gulf of Mexico. The Shield in east central Ontario is referred to as the Grenville Province, and was formed approximately 1.35 billion years ago. At Madoc, where the shield dips below the sedimentary cover, the rocks are slightly younger, with an average age of 1.25 billion years.

Seismic Influences

The mechanical activity involved during formation of the geological structures in this region had considerable influence on the features as we know them today. The still active north-west trending Ottawa Valley Seismic fault zone, some two hundred miles to the east, appears to be forcing the shield upward toward the south-west, and is likely responsible for much of the folding and other pressure-induced distortions of this geological assemblage. Early in 1997, three shallow low intensity earthquakes were detected in an area 25 miles north east of Madoc, reminders of a the geological process that is still alive today.

Volcanic Origins

The presence of basalt and pillow lava harkens to a volcanic origin once thought to be mainly the result of extrusive venting, but recent studies have found evidence of a volcanic cone situated south of Bancroft near the town of Apsley. Owing to the age of this structure, very little direct evidence remains intact due to erosion and extended periods of metamorphic activity. Although a 23 carat diamond, found near Havelock 35 miles west of Madoc, signals a volcanic presence, this single stone, discovered amidst glacial debris, is presumed to have been transported from an as yet unidentified source.

The mineralisation found at a pyrite mine near Queensborough 6 miles north-east of Madoc is thought to have been the result of an "acid" volcanic vent which provided sulphur to the iron rich host rock to produce the pyrite. Such deposits are typical to the area, though not all are so clearly traceable to classical volcanic vent structures. Many of the smaller vents identified in the Madoc area appear to be the result of gasses and liquids given off by large rock masses as they cooled and contracted, and hence have a more shallow origin than one would expect from classic volcanism.

Near the close of an early era during which much of this area was submerged beneath a fresh water sea, many of these large plutonic rocks where forced upward through the soft crust, triggering new lava flows and exhalative venting. Although low grade metamorphic activity continued for nearly 500 million years and considerably masks an absolute determination, many of the gold mines to the north and west of Madoc are situated at the margins of plutonic intrusions and appear to have formed at late stage exhalative vent sites.

Plutonic Bodies

Immediately west of Madoc, the local geology is dominated by the Deloro Pluton. This single piece of rock covers nearly 30 square miles, and contains approximately 170 cubic miles of Perthite Granite extending to an estimated depth of six miles. The Mount Moriah Pluton, situated 15 miles north of Madoc, is about half a mile square and rises some 200 feet above the surrounding terrain. It has such a uniform unfractured composition that it was once considered as a possible repository for radioactive waste. Much of the topography from Madoc to Bancroft is dominated by plutonic bodies of this nature which were emplaced into volcanic flows and metasediments approximately 1.3 billion years ago.

Many of the sixty plus plutons in this area are thought to have originated hundreds of miles beneath the crust where magmatic segregation, pressures and temperatures were ideal for the formation of tourmaline and other crystals of similar origin. Post emplacement venting produced many of the quartz veins and the attendant mineralisation of gold and silver, while late stage fracture filling deposited calcite, fluorite, lead and zinc. Moira Lake at Madoc appears to be a shallow basin formed between three plutons whose individual motions precipitated the venting which formed the famous Canada Talc deposit. Originally discovered in 1897, Canada Talc is one of the few mines in the world to have operated continuously for more than a century. Later stage fracture faults at Moira Lake were filled with fluorite, calcite and barite, and these became the economic basis of Madoc's well known fluorite mining industry that flourished in the 1930's and 40's.

Past Metamorphic Activity

The area endured extensive periods of metamorphic activity, and as a result there was ample time for the heat softened "plastic" rocks to breed new rock types through various chemical interactions. This process can also cause areas of the rock to become locally enriched with particular chemicals or minerals, and often encourages the formation of crystals. Generally speaking, crystal size is a function of cooking time, e.g. the longer the rock stays hot, the larger the crystal. In other instances, however, holding certain types of rock at an elevated temperature for an extended period of time may encourage the chemical constituents of the rock to "unmix", resulting in a chemical segregation which markedly alters the characteristics of the rock. This process of mixing and "unmixing" in metamorphic rock can be a valuable tool in determining its history and the process by which it was formed.

Sixty miles to the east of Madoc the metamorphic activity endured for an even longer period, and this provided an ideal opportunity for the concentration and deposition of crystal forming materials. In the late 1800's numerous quarries produced feldspar, apatite and mica from these deposits, and frequently encountered enormous crystals, some of which weighed more than 20 tons. One crystal ("book") of biotite mica was reported to be 12 feet across and four feet thick.

The sedimentary deposits immediately north of Madoc which survived the turbulence of the shield's late stage activity were typically converted to marble. During the last century, more than fifty small shallow quarries produced marble chips for the terrazzo industry, several of which are still active. Marble can be found in most every colour of the rainbow with iron compounds being the most common source of pigmentation. Once considered too fractured by stress faults to be suitable for structural applications, recent operations at Tweed east of Madoc have found excellent structural marble at depths below two hundred feet where it was apparently sheltered from stress damage.

Uranium Ores

Further to the north toward Bancroft, the mineralisation changes from base metals to the radioactive uranium ores. While there were small deposits of fluorite, feldspar, graphite, magnetite, apatite and garnet as are typically associated with granitic intrusions, it was the uranium mines which brought Bancroft to the world spotlight in the early 1950's.

The Ordovician Limestone Beds

South of Madoc, the shield is hidden beneath thick beds of Ordovician limestone. At various localities and depths, the limestone contains the fossilized remains of the marine life which once flourished in the area. Although the glacial retreat buried much of the limestone beneath a heavy deposit of clay and gravel, the last hundred years have seen a myriad of quarries opened to extract ornamental building stone, crushed limestone for road construction and for the manufacture of cement and masonry powders. The limestone also plays host to small deposits of sphalerite and galena, several of which have been mined on a small scale.

In summary, the Madoc-Bancroft corridor of Eastern Ontario is of interest to geologists and collectors alike because of the great geological diversity created by plutonic intrusions, extensive periods of erosion, sedimentation, volcanic and metamorphic activity, exhalative venting and late stage fissure filling. Geologically, it is easy to visualize this area as a cluster of plutons adrift in a river of volcanic and metasedimentary rock flows occasionally disturbed by volcanic eruptions, and so it should not be difficult to understand how an area 50 miles long and 15 miles wide could host more than 4,000 mineral occurrences and 500 mines and quarries.

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