Nick,

Thanks for the clarification.

In fact, apologies to my old chemistry teachers that I needed the clarification. If I'd thought about it for a minute, I might have remembered that acids and limestone don't mix! Even if you had a lot of CO2 sitting on top of a solution of sulfuric acid and minerals like calcium, you wouldnt get carbonates precipitating from it (indeed, dump calcium carbonate into the solution and you get gypsum + yet more CO2).

Also, by surface deposit, do you mean you think it was originally deposited in water on the surface, or just that it was on the surface whether subaqueous or subareal (like sand dunes or ash fall)?