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Author Topic: placer and paleoplacer gold?  (Read 8919 times)

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Offline swdawg

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placer and paleoplacer gold?
« on: February 05, 2008, 09:24:52 PM »
 Hi,

 I'm so new that I don't know anything.lol.Sorry, hope you'll bear with me.

 I was doing a little reading and came across a reference to placer and paleoplacer gold.I guess I know what placer gold is(but haven't had the thrill of finding any)but what is paleoplacer gold?

 I'm in an area supposedly without gold.Need to go on a long roadtrip maybe this summer.lol.But I have recently heard of some kind of recent survey that might suggest the possibility of this placer or paleoplacer gold.

 I don't imagine it is a big find,only trace amounts.Not much info,just heard about it.It would be fun to check it out sometime as it is only a couple hours or so from home.Could do some fishing and maybe try to play with a goldpan.

 Don't know where this crazy interest came from.Just saw a magazine late last fall and bought it for fun.Probably never find any gold,but I guess the fun is in the hunt.

 Cheers,

 Mark

Offline shiver

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Re: placer and paleoplacer gold?
« Reply #1 on: February 07, 2008, 02:22:11 AM »
Paleoplacer gold deposits
by Derek Wilton
professor of geology at Memorial University
St. John's, Nfld.



Paleoplacer deposits consist of placer concentrations of minerals in which the host material is a consolidated rock (sediment comprising weathered detritus that was subsequently cemented together). The prefix "paleo" simply means "ancient."

Gold and uranium are the only commodities mined from paleoplacer deposits. Both commodities can be mined from a single deposit, but often only one or the other is present.

Paleoplacer gold deposits have been mined in the Witwatersrand district of South Africa, the Tarkwaian system of Ghana and in the Jacobina mine of east-central Brazil.

Paleoplacer uranium has been mined at Elliot Lake, Ont., and extracted from the gold deposits of the Witwatersrand district. There is a strong temporal control on paleoplacer uranium occurrences, as these occur only in rocks more than 2.5 billion years old. Gold-bearing paleoplacers are predominantly Archean-aged, but have been mined in rocks as young as 2.1 billion years. Gold in paleoplacer deposits is present as discrete grains. Uranium occurs as uraninite (UO2-U3O8). Like gold, uraninite is a dense mineral with a high specific gravity (6.5-10 grams per cubic centimeter) compared with common detrital minerals. Uraninite is unstable in oxygen-bearing surface waters, and its presence as detrital grains suggests that the earth's early atmosphere was oxygen poor. Some researchers, however, suggest that gold and uranium may be at least partly composed of hydrothermal fluid introduced along faults that bound depositional basins.

The host rock in paleoplacer deposits is quartz pebble conglomorate, a rock containing rounded grains of pure quartz up to 32 mm in diameter. The well-rounded nature and relatively equivalent size of the pebbles defines the host sediment as mature. As such, the particles have been subjected to prolonged agitation in an erosional environment.

This type of sediment forms in a regime of intense weathering and corrosion, wherein quartz is the only common rock fragment to survive, owing to its hardness and resistivity to chemical weathering.

Other minerals are locally concentrated with gold and uraninite. As is the case with a placer deposit, these minerals are dense, hard and/or resistant to chemical alteration. Such minerals include pyrite (in paleoplacer deposits fewer than 2.5 billion years old), platinum group metals, chromite, zircon and arsenopyrite. These minerals are intergranular to the quartz pebbles.

The host rock of a paleoplacer deposit can be composed of up to 3% pyrite. Such rocks are often referred to as pyritic quartz pebble conglomorates. Owing to the differences in their ages, the host rocks of

Witwatersrand-Brazil and Ghana gold ores have subtle compositional differences. The oldest rocks, those found in Witwatersrand and Brazil, are pyritic. The younger rocks of Ghana are hematitic, further reflecting the presence of oxygen in the atmosphere. Uranium does not occur in these younger rocks.

Coal-like layers of organic matter (kerogen) are closely associated with some ore-bearing conglomorate horizons. Gold and uranium are locally concentrated in these organic layers, which are either the remnants of algal mats or the products of later hydrocarbon migration. According to some authors, this organic matter could represent paleo-angal mats. Should that analyses prove accurate, then the mats trapped gold and uranium in either of two ways: physically (from gold and uranium detritus) or chemically (from gold and uranium dissolved in stream waters).

Gold (and uranium) is concentrated in paleoplacer deposits much as it is in placer deposits, in paystreak-like concentrations. The paystreaks are thin sheets of quartz pebble conglomerate interlayered with thicker beds of sedimentary rocks.

In the Witwatersrand deposits, paystreaks can extend for up to 10 km, but are usually less than 3 metres thick. As such, these paystreaks resemble those found in river (fluvial) sediments of modern-day placer deposits. Overall, the host sedimentary rocks were deposited in high-energy fluvial conditions, such as in modern-day braided streams that flow from mountainous regions into alluvial plains. The Witwatersrand rocks are fan delta-like sedimentary horizons deposited at the base of hills from which erosion took place.


Paleoplacer gold and uranium deposits are generally mined as underground operations, as the hard-rock host material is usually deep beneath the earth's surface.

The gold mines in South Africa's Witwatersrand region have reached depths of 4 km, and are the deepest operations in the world. The deepening of these mines increases production costs, however, and is part of the reason that gold production in South Africa dropped to 500 tonnes in 1997 from 1,000 tonnes per year in the 1970s.

Ore from paleoplacers is crushed, and sometimes leached, in order to extract gold. These techniques differ from those used at placer operations, where the gold, which is contained in unconsolidated host rock, is won through gravity processing techniques.

Average grades at the Witwatersrand deposits are about 9.2 grams gold per tonne, but have been as high as 19.4 grams. Tonnages are in the order of 4 billion tonnes.

Paleoplacers have been extremely important in terms of the world's gold and uranium resources. Before the explosion of interest in gold deposits in the 1980s, paleoplacer deposits accounted for 75% of the world's gold resources and up to 50% of its uranium resources. More than 42,500 tonnes of gold have been extracted from the Witwatersrand district since mining began there in 1886. The paleoplacers near Elliot Lake, Ont., produced in excess of 140,000 tonnes of uranium, whereas South African paleoplacers have produced more than 130,000 tonnes of uranium.

These deposits form through a subtle interplay between tectonic forces and paleoenvironmental conditions. The sedimentary host rocks form on erosional surfaces that have developed on old rocks. Paleoplacers form in a high-energy fluvial (river) system. Dense detrital gold and uranium grains are deposited when the river flow is no longer fast enough to keep them in motion. Exploration efforts for paleoplacer deposits are usually concentrated on areas that exhibit these geographical properties.

Unlike epithermal or mesothermal lode gold occurrences, paleoplacer deposits are not associated with broad alteration halos (a chemical and mineralogical change in rocks surrounding certain types of gold deposits), which can be used to map potential deposits.

Although geophysical surveys are of little use in the exploration for paleoplacer gold deposits, radiometric surveys can be useful in the search for paleoplacer uranium deposits. These surveys, which employ radioactivity, map the distribution of uraninite, the mineral from which uranium is extracted.


Offline swdawg

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Re: placer and paleoplacer gold?
« Reply #2 on: April 07, 2008, 11:40:11 PM »
 Wow, that was quite a reply,thanks.

 I haven't been on the forum for 2 months and just read your reply.

 There is radon gas in the area,so probably uranium deposits.Also coal.Both are mentioned in your reply.So, who knows, maybe there is gold somewhere.

 

Offline rockpup

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Re: placer and paleoplacer gold?
« Reply #3 on: April 08, 2008, 07:08:11 AM »
This might help shed some light on the gold deposists in the lethbridge area.There is urnium found near here there is some type of drilling project going on right now near fort macleod for urinium.They have also found radio active dinosaur bones and sands I cant remember where but near lethbridge.I have found alot of clear quartz, some clear as glass in my pan and also conglomarate stones.Are garnets another host material found in these deposists? and are black sands also found?Just curious trying to figure out a source for this deposist in lethbridge.

GoldBug

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Re: placer and paleoplacer gold?
« Reply #4 on: May 14, 2010, 08:12:13 AM »
Garnets being heavy tend to settle out with gold and black sands  in placer deposits