What black sand is and why it follows gold
Black sand is the dark, heavy residue left after the light material has been panned away. Its main component is usually magnetite, a magnetic iron oxide, together with other heavy minerals: ilmenite, hematite, garnet, and often pale zircon or honey-coloured monazite. What these grains share is not colour but density: all of them are markedly heavier than the quartz and feldspar that make up most river sand.
That density is why black sand and gold end up together. Moving water sorts sediment by weight, size and shape. Where the current slows — behind a boulder, on the inside of a bend, in a bedrock crack — it drops its heaviest load first, and every heavy mineral in the catchment falls out in the same place. Gold is denser still, so where there is gold it lies in the same layer as the black sand or slightly below it.
In practice black sand is a marker you can see long before you see gold. When you dig a bar and hit a dark band of heavy sand, often near the base of the gravel or on bedrock, you have found the layer where the stream’s sorting put its heaviest grains. That is the layer to pan first and most carefully.
What black sand does and does not tell you
Black sand proves one thing: at that spot, the stream sorts by weight. That is a necessary condition for a gold trap, not a sufficient one. Many streams carry plenty of black sand and no gold at all, because their catchments hold no gold source. Black sand tells you where to look in a stream that might carry gold; it cannot tell you whether this stream does.
The amount is still useful. A pan that finishes with a thick layer of black sand came from a spot where heavies concentrate strongly; a pan with only a few dark grains came from weak sorting. Comparing test pans along a bar by their black sand is a good way to find its best part before any gold shows. If a trap that looks good on the map barely produces black sand in the pan, its geometry may not be working at the scale that matters.
Occasionally a stream has little or no black sand because the rocks in its catchment contain almost no magnetite. The sorting still happens; the marker is just less obvious. In ground like that the heavies may be garnet or hematite, or simply the coarsest gravel resting on bedrock. Look for the densest material, not only the black one.
Reading a concentrate: a worked example
Say a day’s concentrate from a single bar is sitting in a jar. Before doing anything else, spread it thinly in a finishing pan and look. Note how much there is, and whether you can see colours, red or pink garnets, pale glassy grains, or yellow flakes that flash in the light. Then separate it step by step and note what each fraction holds, because what comes out describes the rocks upstream as well as the gold.
Run the bagged magnet through the concentrate a few times. Magnetite jumps to it; weakly magnetic ilmenite may partly follow a strong magnet, but hematite, garnet and zircon largely stay behind. Pan the magnetic fraction once before throwing it away, because fine gold gets trapped in the clumps. What is left is a smaller, lighter-coloured concentrate in which gold is easier to see and to pick up.
The residue is information too. Plenty of garnet suggests metamorphic rocks upstream; silvery or dark heavy grains that are not magnetite may, in some regions, be tin, tungsten or platinum-group minerals and are worth noting. Rough, angular gold alongside them suggests a source that may be close. Write it down with the location; over several spots the pattern starts to describe the catchment.
- Look at the whole concentrate first and note how much there is
- Run a bagged magnet through it several times
- Pan the magnetic fraction once before discarding it
- Finish the non-magnetic part with very little water
- Record gold, garnet and any unusual grains with the location
Gold or not: pyrite, mica and other look-alikes
Three things are mistaken for gold more often than anything else. Pyrite, the classic fool’s gold, is a brassy iron sulphide that forms cubes and sharp angular grains. Chalcopyrite, a copper-iron sulphide, is yellower and often tarnishes. Weathered mica, especially biotite, produces thin golden flakes. None of them behaves like gold, and a few simple checks tell them apart in the field.
Watch the colour as you tilt the pan. Gold keeps a steady, warm yellow at every angle and in shade; pyrite turns dull and brassy out of the sun, and mica flashes brightly and then vanishes as the angle changes. Watch how it moves: gold stays put at the top of the black sand fan, while mica drifts and floats with a little water. Press it with a needle: gold is soft and dents or flattens without breaking, pyrite crumbles, and mica splits into thinner sheets.
For larger grains, a streak test on the back of an unglazed white tile helps: gold leaves a golden-yellow streak, pyrite and chalcopyrite a greenish-black one, and hematite a red-brown one. In practice the needle and a hand lens settle almost every question. If you are unsure, keep the grain in a separate vial and look again at home in good light.
- Tilt the pan: gold keeps its colour from every angle
- Look in shade: gold stays warm yellow, pyrite goes dull
- Move it with a little water: gold stays, mica drifts
- Press with a needle: gold dents, pyrite crumbles, mica splits
- Streak larger grains on an unglazed tile
What the shape of gold says about distance
Once you have real gold, its shape is an indicator in its own right. Gold freed from a vein close to its source is rough and angular, sometimes with crystal faces or still attached to bits of quartz. As it travels, the gravel hammers it: edges fold over, grains flatten into flakes and become rounded. Gold is soft enough to be reshaped rather than broken, so every kilometre of transport leaves its mark.
Angular or crystalline gold therefore suggests that the source is close — within this catchment, perhaps a short way upstream. It is the best reason to walk upstream and sample the tributaries one by one, noting where the gold stops. Smooth, flat, rounded flakes suggest a long journey or recycling from older gravels; their source may be far away or already eroded.
Very fine gold, called flour gold or dust, is too small to pick up as single flakes. It is common far from sources and in slow, sandy reaches, and it is the gold most easily lost in the pan. Its presence says the stream carries gold; its shape says little about where it came from. The tool for all of these observations is a hand lens.
Weighing signs together: walking a catchment
A single sign rarely decides anything; the way signs support each other does. Say a trap low on the main stream gives you a few flat, rounded flakes along with plenty of black sand. That tells you the stream carries gold and that the spot collects heavies, but the shape of the flakes points to a long journey. The next question is which tributary the gold came down.
You walk upstream and take test pans from similar traps just above each tributary mouth. One tributary has black sand but no gold; the other gives smaller but rougher flakes, and the bed holds rusty, pitted quartz pebbles. Going up that tributary, the gold becomes more angular, and then above a certain point it stops altogether. The reach between the last pan with colours and the first empty one narrows down where a source might be.
No single step of that walk was proof on its own: the black sand spoke about sorting, the shape about distance, the quartz about the type of source, and the pans about whether gold was really there. Read together, they build a story. It is still a hypothesis; if you record every step with its location, you know where to pick up on the next trip, and you can see where you were wrong.
Signs in the landscape, and the best indicator of all
Some indicators are in the rocks along the stream rather than in the pan. White quartz pebbles show that the catchment contains quartz veins, and much gold occurs in quartz veins. Quartz stained brown or red, or pitted with small box-shaped cavities, is more interesting: those stains and holes are left when pyrite and other sulphides weather out, and vein gold is often associated with sulphides.
Strongly iron-stained outcrops, called gossans, mark zones where sulphide minerals have weathered and are worth noting on your map. Green staining can point to copper minerals, which occur in some mineralised zones that also carry gold — a general sign of mineralisation, not of gold. Old workings — tailings piles, hand-stacked stones, water races, shafts — are the strongest sign that someone found gold there, and sampling around them is usually worthwhile.
GOLDONGO scores stream segments on trap geometry; indicators suggest whether the catchment might feed those traps. Neither replaces the pan. Every sign in this section says “maybe”. A few colours in a test pan from a good trap are worth more than any amount of promising quartz, and an empty pan from a well-placed trap is a clear answer too.