Hey guys.
In NZ we have quite a few volcanoes, and luckily for me living in christchurch, the most recent eruption in the south island was about 20 mya.
On the "vanished world" trail, there is a preserved basaltic volcanic dyke/sill that has been injected into the strata beneath an ancient lagoon.
While I'm aware that these magmatic intrusions don't break the surface, there is a broad fossil horizon not far from the top most termination of the sill, with beautiful small spirulla (?) (Some kind of elongate fossil snail) preserved in the rock.
I was wondering, that as I mentioned before that this fossil horizon is not too far from the intrusion, just how much heat or mineral replacement does it take for igneous rocks the the associated hydrothermal minerals to completely destroy/deform/disfigure fossils in these kinds of geologic environments?
I have included a rough sketch. Like all great geologic exposures, it occurs at a public road cutting, to provide context.