There is not really
enough information to make a determination, but a best guess would be methane from anarobic decomp. Which is pretty common in the bottom cline zone of swampy waters. Alternatively, it could be radon gas, particularly considering you already know it's a basalt depression, and basalt is often contaminated with thorium. I would recommend that you get one of those radon detector kits, and set it up under a
cardboard box (protected from rain) somewhere near your shoreline. If it's radon, it's not just where your pond is.
If you do find radon,
DO NOT build a basement in your new home. Only a vented crawlspace.