Learn English – always argue or are always arguing

The first example is incorrect, and the second is correct.

In the second clause, the subject is "known deposits of bauxite" and the predicate is "can provide the world with aluminum...".

An alternate wording would be

At present production levels, there are known deposits of bauxite that can provide the world with aluminum for years to come.

There can be used as a kind of "dummy" subject here, when there is no actual subject to the clause.

They can not be substituted here. To use they you would have to have some context that establishes what they refers to:

Jamaica has many bauxite deposits. They are enough to supply to world for many years to come.

Here they refers to the specific bauxite deposits in Jamaica that were previously mentioned.

The main differences between are and are being, or is and is being, are volition and duration; are being means roughly, "are currently choosing to be (and could easily choose to stop being)".

So, "why are you being so unreasonable?" means roughly, "why are you choosing to be so unreasonable right now"; and ^?"he is being ill" would mean roughly "he's choosing to be ill right now".

What's more, even if you really do mean that someone is (somehow) choosing to be ill, this is a very odd thing to say, so you would want to be explicit; you wouldn't just suggest it by saying "he is being ill".