Pyrite-chlorite and pyrite-biotite relations in sandstones

Authigenic euhedral pyrite in feldspathic sandstones of the Visingsö Group (Upper Proterozoic, southern Sweden) is observed to be closely associated with authigenic, fine-crystalline, interstitial chlorite and/or sand-sized chlorite or biotite. Microprobe analyses revealed that chlorites which are accompanied by relatively large amounts (≤ - 3 vol.%) of pyrite are characterized by a lower Fe/(Fe + Mg) ratio than chlorites with lesser amounts (≤ - 0.5 vol.%) of associated pyrite. The lower Fe/(Fe + Mg) ratio of the former chlorites might have resulted from subsequent substitution of Fe²⁺ by Mg²⁺ in an initially Fe-rich chlorite; the released Fe²⁺ would then react with sulfide ions (in an anoxic-sulfidic environment) to form pyrite. Another alternative is that the chlorites with lower Fe/(Fe + Mg) ratio were crystallized from solutions characterized by a higher activity of Mg²⁺ than chlorites with higher Fe/(Fe+Mg) ratio. Iron from the biotite grains became available due to diagenetic illitization, kaolinization and/or chloritization.