Sedimentary organic matter, sulfur source

Geologic sources of sulfur include primary and secondary sulfide minerals as well as chemically precipitated sulfate minerals. In sedimentary rocks, sulfur is often concentrated. Examples include pyritic shales, evaporites, and limestones containing pyrite and gypsum in vugs and lining fractures. In many cases, these rocks can be both the source of organic matter as well as sulfate for the biogeochemical reduction of sulfate to occur. 

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