It is best to think of metal detectors made for prospecting as "nugget detectors" as that is the truth of the matter.
Metal detectors are electromagnetic devices, and as such can detect items that are conductive and non-magnetic, like gold, or non-conductive but magnetic, like magnetite. Or both, like metallic iron.
When dealing with gold you are dealing only with conductivity. The more conductive the mass, the easier it is to detect. In general what this means is bigger is better. Any detector has a limit to how small an item it can detect.
Here is the kicker. Multiple undetectable targets do not add up to create a detectable target. I do not know how many times I've seen or been told of people throwing a vial of small gold on the ground and running a detector over it and declaring the detector will not find gold because it does not pick up the vial of gold.
Let us say that on a scale of 0-10 zero represents an undetectable piece of gold, and 10 one that really beeps. 0 + 0 + 0 + 0 + 0 still equals zero. So lots of tiny gold is just as undetectable as a single piece of tiny gold. You need a single conductive mass.
Fine gold usually has a coating, and putting a bunch of fine gold in a vial still results in little or no signal. If the gold is super clean and packed tightly you will get a weak signal. Melt it all together, and now it goes beep.
Another way to look at it is take some fine gold and pour it in a pile. Get a multimeter and test your little pile of gold for conductivity. It is hard to get much current if any through a loose pile of gold.
So bottom line is you might have 5 ounces of fine gold right under your feet, and you will walk right over it with your metal detector.
Jewelry hunters run into this when trying to detect lost necklaces. A fine chain is very hard to detect as each link is undetectable and the connection between the links is poor enough the signals does not add up to much. Often all you can detect is the clasp.
Now once an item is detectable, it does add up. Two large nuggets in the same spot are easier to detect than each by itself. If each link in the gold chain can be detected, then it will add up into a more detectable target.
