Petrology of the Low-Grade Rocks of the Gunflint Iron-Formation, Ontario-Minnesota

The relatively unmetamorphosed middle Precambrian Gunflint Iron-Formation of Ontario has undergone considerable post-depositional recrystallization and locally intense replacement. Although these tend to obscure primary textural-mineralogical relations, textural elements similar to those of limestone can be identified and their mineralogy defined. Two fundamentally different kinds of iron formation are recognized: (1) cherty iron formation, which consists of granules, ooliths, and interstitial cements; and (2) banded or slaty iron formation, which is composed of matrices (fine-grained internally structureless laminae). Cherty iron formation corresponds broadly to the thick-bedded taconite and algal chert facies of Goodwin; slaty iron formation encompasses the thin-banded chert-carbonate and tuffaceous shale facies.

Greenalite associated with cherty quartz and minor minnesotaite are dominant mineral constituents of granules; stilpnomelane and hematite are less common. Recrystallized calcite, ankerite, and siderite occur locally as cements and as replacement minerals. The most common cement is quartz. Iron silicate and siderite matrices are major constituents of slaty iron formation, which also contains considerable amounts of secondary calcite and ankerite. Stilpnomelane and chamosite are locally abundant in slaty rocks as apparent pseudomorphs after shards.

Microprobe analyses of greenalite reveal little compositional variation; stilpnomelane from slaty iron formation is extremely heterogeneous. Both siderite and ankerite exhibit considerable substitution of Fe by Mg (and Mn locally) whereas calcite is almost pure CaCO₃.

Comparison of the greenalite, minnesotaite, and stilpnomelane crystal structures reveals many similarities. The crystal chemistry of magnesium and nickel analogues (serpentine, talc, and garnierite) has been used to predict structural details of the iron silicate minerals. These are consistent with textural and compositional data supporting a primary origin for the iron silicates. Quartz, recrystallized carbonate cements, microcrystalline siderite, hematite, and possibly magnetite are also considered primary phases.