Canadian Rockhound: Thunder Bay Agate Mine

Approximately 12 years ago, a remarkable discovery occurred near Thunder Bay, Ontario, while an area was being re-logged. A bulldozer was making a road and struck the corner of a shallow outcrop. This was no ordinary outcrop. As it turned out, the exposed rock was an agate vein. The agate was staked for the mineral rights, but not developed. In 1995, when the mineral rights were open, the present owners re-staked the ground and began to develop the agate. In November of 1996, trenching with backhoe outlined approximately 500 tons of agate. The Thunder Bay Agate Mine opened to the public on July 1, 1997. The mine is owned and operated by Dennis Seargeant and his partner Neil Maunula.

This article is about the agate from the Thunder Bay Agate Mine, and its importance to geologists and collectors around the world.

What is Agate?

Agate, from the Thunder Bay Agate Mine, Ontario.

Agate is a semi-precious gemstone that consists of thousands of microscopic quartz crystals. Chalcedony, which is a member of the quartz family, is the main silica ingredient. This gives the agate its colouring and hardness (7 - 7.5 on the Mohs scale, diamonds being 10). The colouring of the chalcedony while in the gel form is picked up while flowing through the surrounding minerals. Iron and hematite tend to give it the reddish brown tinges, while manganese, magnesium, and calcium blend gray to black agate. Other minerals present give the orange, blue, green, and yellow highlights. Chalcedony does not form large quartz crystals.

During rock formation, the silicas are the last to form and the last fluids that fill the cracks and cavities. Agate is formed at lower temperatures and less pressure. This results in a number of vugs or cavities that are not completely filled. The fluid in a gel form begins to form around the outside of a cavity. As more fluid accumulates, another layer is formed giving the ring effect. When there is a lot of fluid present, as was in this case, most of the cavities become solid filled causing it to become a solid mass of agate known as fortification agate. It is expected that the agate found at the mine was formed approximately 1.5 million years ago.

Where is it found?

Agate is generally found along lake shores, sea shores, or in gravel beds. The Thunder Bay Agate Mine is one of three known places in the world where agate is found in vein formation, Brazil and Mexico being the other known sources of significant quantity.

Agate is usually the last mineral formed during the formation of rock and thus tends to be more exposed to the weather and breaks off easily. Eventually, it finds its way into gravel beds and lake shores. It is thought that this formation may have been protected due to the faulting effect. This may have caused the rock to shift downwards and be preserved.

Thunder Bay Agate is unique and ranks as some of the finest agate in the world. Because of the fact that is comes in large solid pieces, it can be shaped and carved into almost any imaginable shape. This distinguishes it from Mexican Crazy Lace Agate, which is similar in colour and pattern but is not found in large pieces and generally is not suitable for shaping and carving.

The Thunder Bay Agate Mine hosts the world's largest agate. Some of the agate veins seen here can be 3 to 4 feet thick or more. The length of the vein is unknown.

Agate Stalactites

Agate stalactites are formed in a manner similar to normal stalactites, which have the same principle of formation as an icicle. Over saturated silica fluid drips into hollow cavities and like an icicle, the resulting stalactite tends to be almost round.

Agate Stalagmites

These are formed by over saturated silica fluids being forced upward into hollow cavities due to pressure. They also are round in appearance.

Thunder Bay Agate Mine Tours

As well as providing tours of the open pit mine from mid May until mid October, visitors are welcome to visit our workshop where we tumble agate and polish small flat surfaces. We also have a well stocked gift shop with a variety of items, such as jewelry (pendants, beads, earrings), small animal carvings, book ends, pen sets, and clocks.

www.agatemine.com

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