The most common natural method of concentration of gold is through the ancient action of hot fluid inside the Earth's crust. (Fluids deep in the crust are heated by the Earth's internal heat. As they move towards the surface they cool down.)
The fluids moved through the rocks over a large area and "dissolved" the gold. When these fluids cooled or reacted with other rocks the dissolved gold precipitated (came out of the fluid) in cracks or fractures forming veins.
If the fluids move over a large enough area, and dissolve the gold for a long enough period of time, gold can be concentrated in amounts in the parts per thousand or even greater.
As well as gold, the fluids carried other dissolved minerals, such as quartz. This is why gold is often found with quartz. These are known as primary gold deposits and to extract the gold the rock containing the veins of gold has to be dug up (mined), crushed and processed.
In Australia this concentration of gold took place in the Earth hundreds of millions of years ago in the eastern states, and thousands of millions of years ago in Western Australia.
The rocks containing the gold veins have now been exposed on the surface and are eroding away. The gold that these rocks contained has been washed down into creeks to form alluvial gold deposits. Here, the gold is further concentrated by the action of water.
Because gold is heavier than most of the material moved by a creek or river, it can become concentrated in hollows and trapped in the bed of the river. These are known as secondary gold deposits and they can be worked using a gold pan, cradle.