Precambrian Iron Formations Research Articles

Origin of Precambrian iron formations

Bulk composition of Precambrian iron-formations is remarkably similar, despite differences in mineral facies. They differ from post-Precambrian iron formations in their low contents of alumina, titania, manganese, phosphorus, and lime. Uniformity of iron content, despite likelihood of variation due to sedimentary processes and depositional environment, suggests fundamental control at sources of materials. A model is proposed by which lateritic weathering in a reducing atmosphere yields solutions with Fe and Ca in approximately their 1:1 ratio of crustal abundance. These elements are deposited as banded calcite-siderite rocks. Different mineral facies are largely due to differing amounts of silica replacement, oxidation, etc., during diagenesis and metamorphism.

Geology of the Raposos gold mine, Minas Gerais, Brazil

The gold ore bodies are mineralized segments of folded Precambrian iron formations separated by shear zones, diabase dikes, and barren iron formation. The type of iron formation that acts as principal host is banded siderite-quartz rock. Belts of this rock are enclosed in chlorite schist, some of which is chromium-rich. Gold-bearing hydrothermal solutions, rising along the axes of plunging folds, preferentially replaced bands of siderite in the iron formation. Early solutions altered the schist wall rock to green chromian sericite schist and deposited pyrite. Arsenopyrite, pyrrhotite, chalcopyrite, quartz, ankerite, and most of the gold were introduced during the later, principal stage of mineralization, from an unknown source or sources.

Oolitic iron formations in Northern Australia

Recent exploration in northern Australia has discovered three shallow water oolitic iron formations, one of Permian age, and two of Upper Proterozoic age. The Permian formation resembles the oxidized oolitic chamositic formations of south-eastern England, but contains more detrital quartz and felspar. The two Precambrian iron formations, consist, below the zone of weathering, of oolites of hematite and of a chamosite-like substance, in a siderite cement, together with well-rounded quartz grains, and are interlayered with sandstones with a chamosite-like cement. Within the zone of weathering the siderite and chamosite are more or less completely replaced by quartz, but with preservation in detail of original textures.