Gold Deposits Hosted in Sedimentary Rocks
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Gold Deposits Hosted in Sedimentary Rocks

Gold is often hosted as disseminated deposits in sedimentray rocks. The lodes found in these environments are distributed worldwide.

The largest gold deposit in the world is found in the sedimentary rock called the Witwaters Rand in South Africa. This is a thick conglomerate laid down in the early Precambrian Eon. Some of the mines found in this deposit are the deepest in the world going down over 11,000 feet. It is assumed by many geologists and mining engineers that this deposit and others like it were enriched by the gold washed down from ancient mountains that no longer exist.

According to Albert H. Hofstra at the US Geological Survey office in Denver Colorado these are often called Carlin type deposits named after the Carlin trend in northern Nevada. They even have a special name sedimentary rock hosted disseminated gold (SRHDG) deposits that are capable of being formed in several different tectonic settings or physical environments adding different characteristics and endowments of the gold the gold contained in them.

The latest research on these kind of deposits indicate that they form in various kinds of sedimentary basins such as organic belts where mountains are being built, magnetic arcs similar to Japan or the other islands in Indonesia. They can also form in Rift Valley's that are associated with the rifting of Tectonic plates. An example of this is the Great Rift Valley in eastern Africa that contains some of the largest lakes in the world.

The gold is carried to them as placer deposits from the mountains surrounding the basin from the higher mountains surrounding the basin. The gold is usually quite disseminated throughout these older mountains and is concentrated by the action of running water in rivers and streams until it finally finds a resting place in the basin.

As the depth of the sediments increases with time there are various kinds of low sulfur brine systems that develop within the basin. Many of these brines are still insufficiently studied to allow their classification as to whether they are from metamorphic rock, magmatic or volcanic rock, from precipitation or whether they are hybrids. There are even a few brines that don't fit any classification and all. They all have one thing in common though that is heated brines are all capable of dissolving gold.

The school very heated brines are then carried to the surface by the action of the sedimentary rock being dewatered. As this gold bearing water is forced closer to the surface of the earth it will eventually reach a place where it is either cooled to a point that the gold will no longer remain in solution. It can also reach a place in the upper sediments where it encounters chemicals that will also make it fall out solution.

One of the most common chemical traps is carbonaceous material such as coal or petroleum although neither of them are considered as gold ore if they are analyzed for their metal content the best thing that can be said is, if you want to find it just go looking for it, its there!

References:

Diverse Origins of Sedimentary Rock-Hosted Disseminated Gold Deposits Worldwide: Overview, Albert H. Hofstra, USGS, 2002, http://gsa.confex.com/gsa/2002AM/finalprogram/abstract_42250.htm

What Happens to Create the Lode?, Stephen Miller, http://pulse.pharmacy.arizona.edu/9th_grade/culture_cycles/science/create_lode.html

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