According to Chris Ralph from mindlab, you should go for these signs:
Color Changes: In many districts, acidic mineral solutions have bleached the area rocks to a lighter color. This can be an indicator
of gold.
Iron Staining & Gossans: Not all veins produce much quartz – gold bearing veins can consist of calcite or mostly sulfides – which often
weather into iron stained spots when the pyrites convert to iron
oxides. Large amounts of iron oxides like hematite, magnetite and
ironstone can be favorable indicators.
Quartz Vein Matter Accumulations: At times, small accumulations of quartz vein material can indicate mineralization in the area. This is
a very common indicator.
Productive Rock Types: The concept of favorable host rocks is an important one, but the types of rock which constitute “favorable” can
vary a lot from one location to another and can be significantly
different.
Rock Contact Zones and Faults: Many quartz veins and other hard rock gold deposits occur in "zones" along faults or at the contact of
two different types of rock.
Correct Topography: As a general concept, the coarser gold does tend to hang up farther upstream. In the deserts, most of the best
residual placers form in areas with moderate to flat slopes.
Extensions of Known Mineral or Placer Areas: Other than pipe shaped bodies, most small scale gold deposits have a linear component. It is
fairly common that new deposits can be found along this linear zone of
deposition by looking for extensions along the line of deposition.
Similar Geologic Areas Nearby: If a certain rock type or geologic environment has been productive for gold in one area, and the same
rock type or environment occurs a few miles away in the same mountain
range, it may well be worthwhile to investigate.
Another interesting article regarding this topic is from "Gold Fever Prospecting":
While you can find small amounts of natural gold just about
everywhere, finding concentrated gold deposits takes a little
knowledge of just how gold gets around.
Gold is very heavy. Actually, gold is about 19 times as heavy as water
- about 3 times as heavy as iron. Knowing this makes finding it much easier. Because of its weight, gold will always sink to the lowest
level as possible. As rain, wind, freezing & thawing, and geologic
disruptions move the earth around, gold is freed up and relocates to
the lowest point.
For example, take a rain storm on the side of a hill. As the rain
falls, little rivulets form, flowing down the hill forming larger and
larger streams. As the water moves, it erodes the earth and rock
beneath it freeing the trapped gold. The gold, caught up in the fast
moving water, will cascade down the hill looking for the first crack,
undercut, or obstruction along the way to sink into. Over the years,
more and more freed-up gold will collect in these cracks, making for
some fine pickings if you are willing to look for them. Browse crevice
tools
Same thing goes in a stream bed. Look for where the water slows during
a flood. If the gold has a chance, it will sink. Sample or test where
the stream bends or widens, or where there are natural obstacles or
falls. Even a rock or boulder in a stream will disrupt the flow of
water, causing the gold to fall to the bottom and collect. Don't be
afraid to 'turn over a few stones'!
Where to look for gold
- Gravel bars usually found on the inside of the river bends. Although the gold here is mostly small flakes to very fine, there
sometimes is a lot of it.
- Where the stream levels out after a steeper part such as downstream of rapids or waterfalls.
- Newly formed gravel bars.
- Small streaks of gravel laying on the bed rock but you will need some sort of sucker to retrieve it if it is underwater.
- Down stream sides of large boulders and other obstacles which because of size or other factors appear to have been there for a long
time.
- Pot holes in the bed rock
- Cracks in the bed rock. In popular prospecting areas, the large, obvious cracks have most likely been cleaned out many times. Look for
lines of moss running along the bed rock. There is almost always a
small crack under the moss and these cracks can contain a surprising
amount of gold.
- Moss and grass roots near the river.
- The high benches. As a stream cuts deeper into a canyon, it can leave patches of gravel high on the canyon wall. These are called
benches. Look for round or rounded rocks well above the present high
water level. Round or rounded rocks have lived in a river at some time
in their lives.
Always keep in mind that these are the most likely places to find
gold. There is an old saying: "Gold is where you find it." What this
really means is, you may find a spot that looks perfect and not find
any gold at all or you may find a spot that looks like it would be
barren but you find a "bonanza." Just try to keep your mind open to
all possibilities.
