This is a little confusing people will have to sell what to miners? The miners produce the ore to sell to the bank no-one else has currency. Therefor everyone has to be a miner to have currency or perform services for the miners. It seems the miners will be kings and everyone else their peons, unless it is possible to rob the miners in which case the miners will become enslaved... my head is reeling. Why would miners pay for ore when they can just dig it for themselves Why would people sell ore to the miners when they can sell it to the bank for currency?

Perhaps you could clarify for me what you mean. I am obviously missing something.

Hrm... I think you have it even though you don't think you do.  You said:

The miners produce the ore to sell to the bank no-one else has currency. Therefor everyone has to be a miner to have currency or perform services for the miners.

And that's correct. Those would be the only two ways to get currency.  Now, looking at the second bit:

It seems the miners will be kings and everyone else their peons, unless it is possible to rob the miners in which case the miners will become enslaved....Why would people sell ore to the miners when they can sell it to the bank for currency?

Why, today, do you buy groceries when you could grow food yourself? Why pay someone to make you a house when you could build one yourself? The reason is that not everyone will want to (nor will they be able to) master every trade. It will all end up with some sort of balance: if a player wants ore, she can either learn to mine and get it for herself, or go find a miner that will trade either currency for a service (to get the ore from the bank) or directly trade for the ore outright.  It's all about relative cost in terms of effort to get the ore for yourself: if it's easier to get ore for yourself, you'll just do that. If it's easier to trade with a miner, people will do that.

Now, why will people bother with currency at all? The same reason we do today - it's easier to trade currency than it is to find someone who wants to trade what I can do for what I want. After all, farmers aren't really interested in systems engineering, so how could I get food from them without currency? It would be a pain.

As for becoming a slave (either the miners or the non-miners) it's not likely, because there will likely be no mechanism for the miners to prevent the other folks from doing what they want to get ore some other way, and there will be no mechanism for the others to withhold things from the miners.  (Unless players can manage to physically prevent others from reaching a mine or learning new skills, and I don't think that is feasible in this universe.)

I'm not sure I understand what the problem is exactly with the current economy. Granted, I just started so I haven't seen too much, but typical economic problems in MMOs are:

P1) Inflation.  Players that have been around a long time have tons of currency units (Trias in this case) so buying things from other players is expensive.  This is only a problem if things that are worthwhile are only available from other players and cannot be purchased from NPC vendors.

So the question is - is there bad inflation? If there's no inflation, then I don't see any trouble. Having "unofficial" currency isn't a big deal at all.

Now, if inflation is a problem, this has a very simple fix:  do not have currency drop off mobs or be quest rewards.  Have the only source of currency be trading commodities with NPCs - specifically, a single commodity to a central bank.  For instance, iron ore.  Have this exchange at a fixed rate, say 100 Tria per kg.  This way, the amount of currency in the game is directly related to the amount of iron ore currently on deposit in the bank.  Players can either trade tria or goods among themselves, but if they want more Tria they will have to either mine and trade with the bank or give more goods to miners to trade.

This isn't a fiat system, but it's the only way to really combat currency inflation.

It's a little odd to have currency not be a quest reward, as this is very common.  However, if you use the concept that currency reflects an actual chunk of wealth somewhere this makes sense (and is why money was NOT fiat for most of its history - fiat money is actually fairly new).  It will be "different" than what we're used to, but it can work, and it will prevent inflation.

Now, if you want to offer Trias as quest rewards, you need to make the NPCs actually do work to get money from others so they can trade it :-)