1Where does gold originally come from?
From hot mineral-rich fluids that moved through cracks in the Earth’s crust and deposited gold along with quartz and sulphides. Those filled cracks are veins — the primary source of nearly all placer gold.
Where gold comes from, how it is freed from rock, and why some valleys carry it and others do not.
From hot mineral-rich fluids that moved through cracks in the Earth’s crust and deposited gold along with quartz and sulphides. Those filled cracks are veins — the primary source of nearly all placer gold.
A crack or zone in rock filled with minerals from hydrothermal fluids, gold among them. Veins are what hard-rock mines follow. When a vein is exposed at the surface and weathers, its gold ends up in the streams below.
Because quartz is the most common mineral to crystallise from the same fluids that carried the gold. Not all quartz veins hold gold, but most gold veins are quartz.
Weathering breaks the rock down chemically and physically over long periods. Sulphides dissolve, quartz shatters, and the gold — which does not corrode — is left as free grains that gravity and water move downhill.
Gold that has been freed from its source but has barely moved — sitting in the soil on the slope right below the vein. Eluvial gold is coarse and angular, and marks that the source is right there.
Gold moved and sorted by running water and laid down in stream gravel. This is placer gold in the usual sense: the bars, bedrock cracks and terraces this guide is about.
Because the geological events that make gold veins — deep magmas, old mountain-building, fluids moving through fractured crust — happened in some places and not others. A geological map of a region shows which rocks were involved.
Old metamorphic and volcanic rocks cut by quartz veins, granite margins, and altered zones with iron staining. Young sedimentary basins with no veins are poor prospects however good their stream shapes.
A belt of ancient altered volcanic rock, often very old, that hosts many of the world’s gold deposits. Streams draining greenstone belts are classic placer country.
An ancient placer that has itself turned to rock — a gravel bed from a vanished river, now cemented or buried. Some are enormous gold sources; where they crop out, modern streams rework their gold.
Gold is chemically very unreactive. While the sulphides and iron minerals around it corrode and wash away, the gold grain survives intact, which is why it accumulates as a free metal in gravel.
Yes. It is flattened by hammering against rocks, its edges round off, and it becomes finer as pieces break and grind. Its purity also tends to rise, because silver at the surface leaches out slowly.
Large heavy pieces need very strong current to move at all. Floods shift them only a short distance per event, so over time they stay close to where they entered the stream, while fine gold is carried on.
A specific layer in a gravel deposit where the gold is concentrated — typically the base on bedrock, but sometimes a buried older bed or a false bedrock layer. Finding the horizon is what digging a test hole is for.
Ice carries everything without sorting and dumps it as mixed till, which spreads gold thin. But streams cutting through till re-sort it, and the best placers in glaciated country are often just downstream of where a stream has reworked a moraine.
Uplift makes streams cut down and leave old beds as terraces on the valley sides; subsidence buries old beds under new sediment. Both explain why gold can be found far from today’s channel.
Because they formed when the stream was carrying more freshly released gold, and they have not been worked. The modern channel may only be reworking what the older beds gave up.
An old stream course now covered by younger sediment, volcanic rock or till. Its gold is still on its old bedrock. Buried channels are found by geology and drilling more than by panning, but their edges can feed modern streams.
Faults create the fractured rock that hydrothermal fluids move through, so many veins sit along faults. Mapped fault zones cutting old rock are worth a look on a geological map of the catchment.
Rock chemically changed by hot fluids — bleached, iron-stained, clay-rich or silicified. Alteration halos surround many gold veins and are visible in outcrops and often in satellite imagery.
Frequently. The same fluids carry several metals, so gold occurs with sulphides of those metals in many deposits. Signs of one are a general sign of mineralisation.
Because a vein is a narrow thing. One catchment may cut across it and the next may miss it entirely. Gold in a stream is decided by what its own catchment drains, not by the region’s reputation.
The gold itself is as old as the veins it came from — often hundreds of millions of years. Its placer deposit may be anything from ancient to formed by last winter’s flood.
Its purity, usually expressed as parts per thousand. Natural gold is alloyed with silver and a little copper; placer gold tends to be purer than vein gold because silver leaches from the grain surface over time.
Rock types, structures and alteration that are known to host gold: old fractured rock, quartz veining, sulphide zones, greenstone. Favourable ground upstream is why a stream is worth walking; trap geometry is where to walk it.
GOLDONGO scores entire stream networks for the trap geometry this guide describes — slope breaks, inside bends, junctions and bedrock. Pick your region and see where it stands.
This guide is general education, not advice for a specific place. A score, a map or a sentence here is not a licence to dig: the rules for prospecting, land access and protected areas differ from place to place — learn the ones that apply where you are before you go.