Placer gold is gold which has collected in ancient stream and river gravels through physical and chemical processes.
The gold is originally emplaced as vein gold deposits in ultrarmafic host rocks through a hydrothermal process. Mineral rich hot fluids carrying gold move under high pressure from deep within the earth up towards the surface. These fluids invade the host rock through cracks and fissures and cool as they rise. As the fluids cool the minerals
crystallize into veins which contain quartz, hard rock gold deposits and other minerals.
Over time, the country rock and gold bearing quartz veins are eroded through the abrasive action of streams. Freezing in winter, high water flows in spring and chemical variations in the water add to the erosional process. Gold is more dense or heavier than the host rocks in which gold deposits form. As the gravels collect in the stream channel, gold's high density forces the gold particles to the bottom of the stream bed. Gold particles also have a tendency to aggregate together so placer gold is concentrated on stream beds and gold nuggets can form here.
Over thousands of years of erosion and deposition, placer gold deposits of economic value can develop. Placer miners excavate the gravels and, using the high density property of the gold, are able to collect the gold nuggets in a sluice box by washing the gravel with high flows of water. The gravels are washed through the sluice-box while the gold is
trapped in the riffles.
Each gold nugget is unique in character and shape. Gold nuggets suitable for use in jewelry are difficult to find and are highly valued.
Subscribe to this comment's feed