here is a pic of some iron pyrite. Seeing as i deal exclusively with hydrothermal systems (as opposed to petroleum), secondary mineralization are indicators of the presence of hot water and pressure (primary is during the rock formation, secondary is emplaced at a later date), fools gold is the second most abundant hydrothermal alteration we find (calcite is first), it is a good indicator of hot water being there at least for a while, it(pyrite) forms in association with buried organic material (in this case plant fragments transported in from the inundations of the Colorado River, iron and sulfur are not stable in proteins in these conditions, and bond to form pyrite in the presence of water (which enables the ion mobility), we see a lot of pyrite in sand stone here, occasionally in shale bits, sometimes disseminated, often aggregate like in this case. the pic was actually hard to take, this is pic number 8, its the only one to turn out with the right exposure and focus, have to hold the camera about an inch from the eye piece, and get the focus of the light on the ccd, instead of reflecting off the lens of the camera, and time it right so the auto exposure adjust to the amount of light coming up through the microscope, challenging, but interesting.
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