Ahha! I now see why couldn't I build a mine, there was a temple in a forest nearby. Where can I get info on what they do? Their pedia descriptions are blank.
I recommend a look at the
MoM Quick Start Guide posted by Kingknarf over on the offical MoM site's forums.
From that, and playing some, it looks like a mana shrine gives +2 mana/turn (same as an improved mana node, or double that of an unimproved mana node location, or the same as two mage guild buildings), a witch hut gives +3 spell research/turn (same as one mage guild), and a lost temple gives +4 faith/turn (base, there is a time related adjustment that starts out at -100% penalty on the turn you switch to a religion, increasing each turn until it hits a +100% bonus somewhat later) if you have a state religion (the same as two cities with your state religion). You don't need to be working the plots to get these benefits, they just need to be within your borders. Not mentioned in the guide, but from some hints in the game text file, it also looks like a mana node may provide some spell research each turn (or maybe not - the text may be out of date) and the witch's hut provides some of the new herbs accumulating resource (I think it is 1/turn). You may need to work the plot to get the herbs.
The mouse-over text for the plot with the thing on it should give info about it, like that you can't improve the adjacent plots and what extra yields it gives. Tags for this exist in the mod's game text files. Unfortunately that text is not actually displayed anywhere in the game that I have noticed.
The total benefit from lost temples is clearly listed on the mouse-over help pop-up for the faith resource, but the mana bonus is grouped in with other things and the research bonus is not listed separately anywhere I have noticed - by adding up everything else you could deduce the added research since there is a total given on the Overview page of the Magic screen.
Sadly, even if you had looked in the XML you won't be able to tell what these 3 improvements do since they are apparently hardcoded in the DLL (I expected to find new tags in the XML so that what they provide can be set there, but that is not the case and I find no mention of them in the Python therefore it must be in the DLL, the source for which is not in the mod download so I couldn't check to see what it says). The XML does reveal that they have the bPermanent tag set, which is why you can't raze them (and you can't settle a city on top of one to wipe it out either - on my cramped and resource poor section of continent in the game I mentioned above I ended up settling a city right nest to a lost temple because of this, which greatly hampers its development).
One more note. They don't block water plot improvements (fishing boats or fishing villages/pirate coves), which also don't count in the main improvement related upkeep calculations. Any time an improvement is blocked by one of these there should be a line in the displayed help text indicating why you can't do it, like "Your people refuse to work at this mystical place. They say Nature itself would smite upon them if they dare to construct an Improvement next to a Mana Shrine."