Two different worlds: lode and placer
Two different things get mixed up whenever people talk about finding gold. The first is the lode, or vein: a crack or zone in rock where gold is still locked in, usually with quartz and sulphides. The second is the placer: gold that has been freed from that rock, moved by water and gravity, and concentrated somewhere else. Lode gold is the business of hard-rock mining, which means crushing, grinding and chemical processing. Placer gold is the only kind you can recognise with a pan, a screen and a shovel.
The distinction matters because every grain you pan from a stream was once in a vein. A placer is not a source in its own right; it is the eroded product of one. For a stream to carry gold, there has to be gold-bearing rock somewhere upstream, that rock has to have broken down, and the water has to have gathered the grains at particular spots. If any one of those three links is missing, the gravel holds no gold, however good the stream shape looks.
This section covers the first two links: how gold gets into rock, how it gets out again, and where it sits on its way down to the stream. The third link, where water drops the gold, is the subject of the section on where gold collects. You need both in mind at once: geology tells you which stream to go to, and trap geometry tells you where to look once you are on it.
The systems that make gold: orogenic and epithermal
Nearly all placer gold comes from veins left by hot, mineral-rich fluids moving through the Earth’s crust. As those fluids pass through cracks, faults and broken zones, they cool, lose pressure or react with the surrounding rock, and the gold they carry drops out together with quartz and sulphides. Geologists divide these systems into many types, but the two families you will meet most often when reading about placers are orogenic and epithermal deposits.
Orogenic deposits are tied to mountain-building. In belts where continents have been squeezed together and rocks have been heated and changed at depth, fluids rise along large shear zones and faults and leave thick quartz veins behind. Gold in those veins is often coarse enough to see and is commonly free. Streams draining old metamorphic belts, greenstones and the quartz veins cutting them are, for that reason, classic placer country.
Epithermal deposits form much closer to the surface, in systems linked to volcanic activity and hot springs. Here the gold is frequently very fine, scattered through banded quartz, chalcedony, or rock that has been silicified or turned to clay. Streams fed by a source like this usually carry small grains, and rich placers are rarer. Knowing the kind of source keeps your expectations honest: a few fine colours in the pan, not chunky pieces.
The lines between these families are not always sharp, and one region can hold more than one kind of system. The practical lesson for you is simpler: gold arrives with particular rocks and structures. Where there is no quartz veining, no alteration zone, no fault and no intrusion margin, there is also no mechanism that could have brought gold there in the first place.
Weathering and erosion: how gold gets out of the rock
Every rock exposed at the surface slowly falls apart. Rainwater, air and plant roots react chemically with its minerals; freeze and thaw, heating and cooling, and gravity break it physically. The sulphides in a vein rust and dissolve, often leaving a rusty, porous rock behind. Quartz resists chemical attack but shatters. Gold barely reacts at all, so while everything around it breaks down, it stays behind as free grains.
This happens on a geological timescale, not a human one. A vein has to erode for a very long time before it can feed a stream with a meaningful amount of gold. That is why, alongside today’s landscape, it matters how long and how fast a region has been wearing down. Terrain that is rising and being cut quickly delivers fresh material; terrain that has weathered in place for a long time may have built up gold within its soils.
Gold changes along the way. Grains near the source are angular, rough and sometimes still attached to quartz. As they travel they are hammered flat against rocks, their edges round off, and they become flakes. Silver slowly leaches from the grain surface, so the gold tends to become slightly purer. The shape of a grain therefore gives you a rough hint of how far you are from its source: coarse, angular gold points to a nearby source, while fine, flat flakes have come a long way.
From source to stream: residual, eluvial, colluvial and alluvial deposits
Freed gold does not drop straight into the stream; it moves down the slope in stages, and geologists give those stages separate names. In a residual deposit the gold sits in the weathered rock and soil directly over the vein, having barely moved at all; as the surrounding material dissolves and washes away, the gold becomes richer in place. In an eluvial deposit it has crept a little way into the slope soil just below the source. It is still coarse and angular, and it tells you the source is very close.
A colluvial deposit is one step further on. Soil creep, slope wash and landslides carry material to the foot of the slope and pile it up as mixed, poorly sorted debris against the valley floor. Gold in that material is sparse and patchy, because water has not yet sorted it. But when a stream cuts into the toe of the slope, it picks up the colluvial gold and adds it to its own gravel.
An alluvial deposit is what people usually mean by a placer: gold carried by running water, sorted by weight, and laid down in stream gravel. Because gold is roughly six to seven times denser than quartz, water separates it from the light material and drops it where the flow loses energy. Coarse, rounded grains travel a short distance; fine, flat flakes can be carried much further. The boundaries between these names are soft, and different sources use them a little differently, but the sequence holds: the further from the source, the finer, rounder and better sorted the gold.
Terraces, old channels and buried beds
Streams do not stay put. As the land rises, or the base level falls, a stream cuts down into its bed and leaves its old floodplain as a step on the valley side. Those steps are terraces. A terrace is what is left of a bed the stream once ran along, and its gravel can still hold the gold that was moving at the time. Terraces are sometimes richer than the modern channel, because they formed while fresh gold was arriving from the source and nobody has reworked them since.
To recognise a terrace in the field, look for rounded stream gravel on the slope well above the present water level. Flat benches that hold the same height along the valley, gravel bands showing in cut banks and road cuts, and old workings on the hillside are all good clues. There may be several terrace levels, each one a different chapter in the stream’s history.
Some old channels are completely buried. Younger sediment, volcanic flows or glacial debris may have covered the old stream course, and its gold still sits on its own old bedrock. These paleochannels are found through geology and drilling far more than by panning. Old gravel beds that have turned to rock are called paleoplacers. What matters to you is this: where a paleochannel or paleoplacer crops out and a modern stream cuts through it, that stream can rework the old gold, which may explain why gold turns up somewhere you would not otherwise expect it.
Why one basin carries gold and its neighbour does not
Two streams coming off the same mountain can differ remarkably. The reason is simple: a vein is a narrow structure. One catchment happens to cross it; the one next door misses it entirely. Gold only moves downstream, so a stream can only pick up gold from sources inside its own catchment. A region’s reputation does not decide what an individual stream carries; the rocks its own catchment drains do.
The level of erosion plays a part too. In some places the gold-bearing part of a system has not yet reached the surface; in others it has already been worn away, leaving only the roots of the vein. Even where two catchments cross the same rock, one stream may be cutting the rich level and the other not. Add climate, time and the stream’s own power: gold only concentrates in a stream that has been supplied for long enough and has the geometry to hold on to it.
That is why placer richness depends more on supply and concentration working together than on the grade of the source. A good source drained by a steep gorge with no traps cannot hold its gold. A stream full of perfect traps with no gold-bearing rock in its catchment comes up empty. GOLDONGO today measures only the second half: it scores every stream segment for trap geometry from 30 m elevation data. It does not tell you whether the catchment holds a gold source; that question belongs to geology, and to your own research.
Reading a geological map at a general level
A geological map shows the rock units at the surface, or just under a thin cover, as colours. The legend ties each colour to an age and a rock type: old metamorphic rocks, volcanics, intrusions, young sediments. Lines mark faults and contacts, and special symbols may mark veins, mineral occurrences or old workings. It looks complicated at first, but the question a prospector brings to it is narrow: does this stream’s catchment contain rock that could have made gold?
Start by sketching the catchment on the map: follow the ridges where water divides and enclose everything that drains into the stream. Then list the units inside that outline. Old, fractured rock, faults cutting it, the margin of a granite or other intrusion, and notes of alteration or mineralisation are all positive signs. If the whole catchment is young sediment with no veins, it is a weak prospect however good the stream shape.
Know the map’s limits as well. The smaller the scale, the more narrow veins and small alteration zones are left off; a map tells you a rock is there, not that it holds gold. Young alluvium along valley floors, and older terrace deposits where they are mapped separately, show you where gravel has accumulated. Use the map as a filter rather than a verdict: it narrows down which catchments are worth walking, and the field tells you the rest.
- Outline the stream’s catchment from the ridges and look only at the rock inside it.
- Mark old metamorphic and volcanic units, intrusion margins and the faults that cut them.
- Check the legend for vein, mineralisation, alteration and old-working symbols.
- Note young valley-floor alluvium and older terrace deposits on the slopes separately.
- If the whole catchment is young sediment, move it to the bottom of your list.