Petrography of sulphide-coated grains from the Ordovician Winnipeg Formation, Saskatchewan, Canada
Binda, Pier L.; Simpson, Evanna L.
European Journal of Mineralogy Volume 1 Number 3 (1989), p. 439 - 454
published: Jul 27, 1989
manuscript accepted: Mar 17, 1989
manuscript received: Oct 30, 1988
ArtNo. ESP147050103013, Price: 29.00 €
Abstract Clay-rich sandstones of the Ordovician Winnipeg Formation from the subsurface of Saskatchewan contain abundant sulphide-coated grains. Similar sulphide-coated grains are peculiarly common in Ordovician siliciclastic rocks of Canada. The grains occur either scattered throughout the rock or in discrete sulphidite beds. Eight types have been distinguished in polished thin-section: (1) marcasite-pyrite ooids and pisoids, (2) quartz-replacement ooids, (3) coated fossil fragments, (4) ooids with titanium oxide nucleus, (5) chalcopyritepyrite ooids, (6) ooids with pyrite nucleus, (7) reworked ooids, (8) ooids without recognizable nucleus. These grains, 0.3 to 4.0 mm in diameter, form diagenetically by the accretion of an Fe-sulphide cortex around spherulitic marcasite aggregates, around chalcopyrite-pyrite intergrowths, or around detrital ilmenite or sulphide grains. No replacement of previously-formed ooids has been observed. Accretion of the cortex took place during early diagenesis, probably within 2 metres of the water/sediment interface. The diagenetic chemical environment was anoxic sulphidic. There is also evidence of minor mechanical reworking of coated grains. The nucleation of sulphide around Ti-oxide and chalcopyrite is similar to that which has been observed in a Proterozoic arenite from the Belt of southern Alberta. The process may have a bearing on metallogenic models for stratiform sulphide deposits.