Tol Barad and it's Peninsula is the only place to spawn Rich Pyrite Deposit. I'll show where to find the Pyrite. (The main island only. You'll be dragged into too much mob fighting for farming on the peninsula to be efficient.)
Note 1 If you realize that there is already someone farming, leave immediately. It will be a huge waste of time. So therefore the best time to farm is between 01:00 AM and 06:00 AM. This has the lowest chance of someone else mining.
Note 2 Do not try to farm right after a battle took place. Let the folks do their dailies. Let about 20 minutes go by and then start.
There are exactly 21 nodes in TB. No More; No Less! Allow me to explain the below picture.
The 21 nodes divide into 7 "packs" of 3 nodes each. Those "packs" are marked by the triangles. There will NEVER be more than one node "active" at the same time per pack. So you will NEVER find 2 nodes in the same triangle ... at the same time. If you gather ores from a triangle, the node will eventually respawn at one spot within the same triangle (might even be the same place it was gathered).
Run from triangle to triangle. Check if one of the 3 possible nodes are there (If it's there get it. ) and continue to the next "pack". I prefer going counter-clockwise.
I have experienced that every few times that I go around, 2-3 of the nodes respawn. I suppose the respawn timer is between 2 and 5 minutes. (Not 100% sure)
The Rich Pyrite Deposits are not as rare as one might think. (There are no normal Pyrite Deposits)
The following is only an estimate, based on my time farming.
Elementium Vein: 60%
Rich Elementium Vein: 35%
Rich Pyrite Deposit: 5%
Last thing to mention is the node marked by a red circle. This one spawns on a rock you cannot reach from the ground. It is only reachable though two manners. You either have to be in the attacking faction, die during the battle and near Ironclad Garrison. You will be resurrected on the Garrison graveyard, from where you can reach the mentioned node. But as we won't be farming during battles there only stays one way. Have a priest buff you levitation. You will be lifted enough to gather the ores from the ground.
Side Note: I just realized that one of my precious triangles is actually not a triangle.