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Monday, April 18, 2011

Detrital Gold Deposits

Maybe you have heard of the Witwatersrand in South Africa if you haven't yet is the world's largest deposits of gold that since its discovery in 1886 has produced over 40% of the total gold supply ever mined in the world. This makes it the most glaring example of a detrital gold deposit in the world. The type of rock hosting this vast deposit is conglomerate that until about a year ago the source of its gold was unknown. By working with an isotope of iridium a team of geologists was able to pinpoint the source of this gold In eroded highlands delayed to the north.

Conglomerate the gold in this rock is usually found in the cement holding the pebbles together.
Photo by Hannes Grobe

The Witwatersrand is not the only deposit of conglomerate in the world that contains gold. In the area around Timmins Ontario there are also mines that produce gold from ancient conglomerates of Archean age between 2.8 M2 .6 billion years old. It is not only gold that these ancient conglomerates are apt to contain a common Association in rocks of this type are uranium mineralization. This is been taken advantage of at Elliot Lake, Ontario where some of the largest uranium mines in the world are found. The same deposits also contain significant amounts of rare earth elements to the point that the mining company Pele Mountain Resources (TSX-V ~ GEM) headquartered in Toronto, Ontario. At their Eco Ridge Mine in Elliot Lake, Ontario they are actively searching for both uranium minerals and rare earth minerals. The conglomerate at the Eco Ridge Mine also contains some gold.

Sandstone in a red bed that has been cemented and stained by iron oxide.
Photo by Anne Burgess

Ancient conglomerates are not the only place where gold can be found in sedimentary rocks as detrital gold is also found in many types of sandstone as small grains, and can even be found in fine-grained shale as microscopic particles that are too small to be seen with the naked eye. This kind of gold was never recovered until recently when Newmont Mining developed the gold deposits of the Carlin Trend in northern Nevada.

A closeup of the Marcellus Shale found in the eastern United States.  Aside from holding a vast amount of natural gas the Marcellus Shale sometimes contains gold in very fine specs that are invisible to the naked eye.
Photo by Lvklock

Most of those miners that are working with placer gold deposits in the rivers and streams of the world or along the beaches are also dealing with a type of detrital gold deposit in the form of specs of gold to have been eroded from hard rock and have been swept down river by the action of running water to where we can find them in the gravel deposits of the stream bed.

Conglomerate is nothing other than stream gravel that has become fossilized over time; just about any time you find conglomerate when you are prospecting it is a good idea to have a very careful look at the deposit. This also applies to sandstone and shale since they are also sedimentary rocks that have been produced by the action of running water.

3 comments:

  1. Very informative and helpful, thanks, John!

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