Question

how do placer deposites form?

Answer 1

Placer deposits originate in an alluvial environment, and are the result of natural gravity separation of material according to its density.
Gold (say), originally entrained in the rock mass, is gradually eroded by weathering and makes its way as small particles down to a stream bed. Since this metal does not easily form chemical compounds, it remains there until disturbed by seasonal flooding, which disturbs the river bed. Thus it is transported downstream, and where a low velocity water regime is encountered, it will settle out, along with other heavy minerals.

But, a few decades ago, a gold sovereign was found lying in the drainage water of a 100 +/- year old mine. And upon inspection, it was found that its surface was covered by tiny gold crystals, precipitated out of the mine water.

Recent work has shown that this process occurs in many small streams, where the size of the gold particles increases as one proceeds downstream! This provides an explanation for many of the gold nuggets found. They did not evolve at their current size, they had accumulated mass merely by lying in a stream where they were able to garner an accumulation of tiny gold particles.

This is not to deny that gold nuggets are found in-situ associated with quartz veins. This is a natural hydro-thermal precipitation process.

Answer 2

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