Abstract

The sedimentary textures of the Sokoman Iron Formation are similar to those of limestones; therefore the classification of textural elements in limestone (Folk) can be applied to the iron formation. The authors recognized the following textural elements: (a) femicrite (a matrix of iron silicate and carbonate) and matrix chert, both analogous to micrite; (b) cement chert and carbonatic cements; (c) aggregated particles, comparable to Folk's allochems: pellets, intraclasts, oöliths, and pisolites. Shard textures are derived from oöliths and intraclasts by compaction. Rock types are defined by the combination of textural elements they contain.

The iron formation suffered extensive epigenetic alteration. Dessiccation, shrinkage of silica-gel, compaction, and cementation are early diagenetic. Primocrystallization of quartz concludes the early diagenetic stage. It leads through a cryptocrystalline stage to the end phase of micropolygonal quartz. Quartz re-crystallized further during late diagenesis (burial metamorphism?) and again during a synkinematic to postkinematic regional metamorphism.

Hematite dust is the oldest iron oxide. Much of the microscopic magnetite and specularite formed during early diagenesis. Migration of iron occurred; iron has been enriched in magnetite- or hematite-rich layers (“metallic” layers) during early diagenesis. Renewed crystallization of iron oxides occurred during the regional metamorphism.

Microgranular siderite is perhaps primary. Porphyroblasts and glomero-porphyroblastic concretions of siderite, ankerite, and ferriferous calcite are early diagenetic. Minnesotaite and stilpnomelane are late diagenetic minerals. Riebeckite (and perhaps talc) formed during the regional metamorphism.

First Page Preview

First page PDF preview
You do not currently have access to this article.