Alluvial deposits are very tricky, and volumes never big, I think all you need is a scrubber plant followed by fine screening at say 150 micron.

Rotaspiral screens are economic and efficient, you will need a water cleaning system, and the new Itomacs are pretty economic and efficient separating at low micron sizes using clean water.

If you want to design a small mobile plant, then I can suggest some design companies with real good affordable experienced engineering skills.

106 micron gold (140 mesh), depending upon shape, other mineral SGs, slurry viscosity and equipment operation, is readily recoverable in all gravity devices including jigs. Indeed, jigs are good down to 53 microns (270 mesh) and lower however recovery does fall away.

It is the finer size fractions (<53 microns) that can be more troublesome to recover and spirals, tables, etc. have been used with success. Even flotation, if the gold surfaces are clean, can be useful.

As an extreme example, in the recovery of alluvial tin (cassiterite), which is much lighter than gold (SG of -7.1 cf. 19.3), using a flowsheet with primary sluices/secondary and tertiary jigs/ quaternary tables, recovering 106 micron particles is not a problem and 'good' recoveries down to 37 microns (400 mesh) are routinely achieved. 

Agreed, I guess the issue and the main trade off are between handling the copious volumes and feed rates versus trying to maximise recovery of the lower fractions. Your ore should tell you what is worthwhile and practically no project is the same as another, making this whole business that much more convoluted.

Flotation, in its traditional form, will be rather difficult to work with when you are dealing with low tenors of free gold, and of course you would pass the ore through a traditional gravity circuit to recover a very high percentage of up to 140 mesh, so the rest of the fractions should either have to be a material percentage of the overall grades otherwise you are in a worse situation than needles in massive haystacks.

Can anyone list all possible ways of recovering alluvial gold known to have been used?

Can't agree that alluvial projects do have large volumes! Our Kapuas project has a resource of some 25 million m3, approximate dimensions 7kms (length) x 350m (average width), and 12-13m (average thickness). All lying beneath some 10m of slowly flowing water! (No "overburden"). We could have another 2 or 3 similar prospects in our second exploration area a little further downstream, where we have 70 kms of river under exploration permit! Huge potential!

There are well documented methods to recover fine particle alluvial gold, but you have to consider more factors than just recovering all the gold.

For example, in large scale refining of catalytic materials such as car converters, the most cost effective and processing time method is to use a submersible arc furnace. However you only recover about 70% of the values using this process. You could use leaching techniques to recover in upwards of 90% but it is time consuming. As the processes you use become more complex, more labour is required to recover higher percentages.

When dreaming new processes to recover more precious metals, we often don't consider the cost involved because usually chemists and metallurgists are not considering the business, profit end of the equation. With that being said you could utilize electrolytic processes and/or material scavengers to recover really fine gold recovery for example. But these processes are not often used in large scale operations for the reasons stated above.

The reason why old mining tailings do contain fine gold is because it was never cost effective to attempt to recover those values. It wasn't until fairly recently in history that precious metals were worth enough to even bother.

What are the most efficient jigs for recovering fine and very fine gold, in the knowledge that gold particles do not exceed 0.25mm? Under what arrangement would they be able to handle 50 m3 of classified material at -3/8''.

Would it instead pay off to classify further before reporting to the jigs, say down to 1/32'', and if so are multi-deck vibrating screens the best method to do so?

If you are serious about altering or starting any type was plant, you should really pay to have a consultant/engineer to draw up the plans. There are so many variables with source material, per processing perhaps, even flow rates and angles of wash plants needs to be considered. If you really want to design the best process for your material at your location you really should hire a professional that has experience in this field.

We are indeed doing that. Beyond the knowledge and experience you get by hiring one professional, forum like this may provide different ideas to consider, debate and try in practice. I think starting this forum was probably the best thing you can do. Many people ask the same question, and leveraging their input is valuable. There is a lot of common sense in the above comments and some great rules of thumb.

Remember what you are trying to do - separate a particle that is very small and consider the variables of: shape factor, settling rate, residence time within the concentrator, slurry viscosity, size distribution of gangue minerals, SG of gangue, and throughput rate versus efficiency. There MAY or MAY NOT be an economic solution to the problem with existing technology. It’s your job to use the experts in the industry (through forms discussions, network buddies, or paid consultants) to find out the solution. Look up the work on air sparged hydrocyclone for the recovery of fine gold by flotation. There was a paper written in the 90's on recovery of gold from placer deposits on the Colorado River.