Lignite and pyrite

While out drilling not so long ago, I happened upon a unexpected layer of lignite and pyrite in the overburden.

The lignite chips were from ~160m below the current surface and came up as small, black spongy rodlike shapes with a clear woody texture. A number of the chips had positive buoyancy and would float in my rinse water. Samples above and below the lignite, appear to be a form of brown or red dolomitic mudstone as it fizzed on contact with acid. I'm leaning away from a siderite classification, as i also recovered the following:

Imbedded in the mudstone, and proximial to minor lignite, were small grains of pyrite. They sparkled like gold in the sun and had a dark streak. Siderite is prized as it is low in sulfur however the presence of pyrite indicates sulfer enrichment.

I hypothesize these rocks are the result of a significantly more humid climate where water was able to pool at the surface. In this particular location, a swamp has formed. Dead vegetation was covered by water and sediment before significant decay could occur. The anoxic environment of the swamp has preserved some of the vegetation prior to burial. The water chemistry probably has also contributed to the precipitation of sulfur as pyrite.