Deposits Formed by Sedimentary and Surficial Processes - Metals and Society: An Introduction to Economic Geology - page 131

iron minerals and silica are thought to precipitate when reduced Fe-rich deep
seawater mixes with oxygenated shallow seawater. The different types of iron
minerals are ascribed to different depths and conditions of deposition. Fluctuations
in the flux of incoming fluid explain the centimeter-scale banding. As for the sub-
millimeter scale laminations, it has been suggested that these reflect a cycle of
heating, evaporation and oxygenation during the day and subdued activity during
the night. In other words, the thickness of the lamellae could perhaps reflect the
length of a day, more than two billion years ago!
Algoman-type iron formations probably result from more local mixing of
reduced and oxidized fluids in restricted basins adjacent to Archean continents.
An association with carbonate and sulfide facies suggests that the source of the iron
were exhalative hydrothermal fluids emanating from the oceanic crust. Rapitan-
type deposits are thought to form following “Snowball Earth” periods. This term is
applied to periods during the Proterozoic when the entire planet was covered
blanketed by ice sheets that covered both continents and oceans. Seawater became
reducing, rather as in Archean times and was able to dissolve ferrous iron. During
interglacial periods the reduced seawater mixed with oxidized surface waters,
leading to the deposition of iron formations.
Primary iron formations contain 20-30% Fe whereas the ores mined in most
countries contain 55-65% Fe. Enrichment processes that act on the iron formations
after they are accreted to continents and as they are exposed at or near the surface
explain the difference. Exposure in hot, humid climates to circulating groundwater
leaches silica from the rock and replaces it by iron oxides. In the Hamersley
Province in the Pilbara district of Western Australia, for example, three main
types of enrichment process are recognized: supergene enrichment; dissolution of
iron from the BIF and its redeposition as iron oxides along ancient, mainly Tertiary
river channels; erosion, transport of fragments of iron deposits and redeposition of
this detrital material in secondary deposits.
5.3.3 Other Sedimentary Deposits: Mn, Phosphate, Nitrates, Salt
Bedded deposits of Mn form in a manner very similar to iron formations. The ore
minerals, pyrolusite, an oxide (MnO 2 ) or rhodochrosite, a carbonate (MnCO 3 ),
precipitate from seawater as bedded sedimentary rocks. Controls on the solubility
of Mn are like those of Fe: the metal is soluble in acid reducing fluids and its
precipitation is caused by an increase in alkalinity or by oxidation. Manganese
deposits are commonly associated with iron deposits and in several cases it is
believed that initial precipitation of Fe leaves the water enriched in Mn that
subsequently precipitates as the degree of oxidation increases. This process can
be observed in the Black Sea where highly reduced (euxinic) deeper waters
precipitate pyrite-rich muds as more oxidized surface waters precipitate Mn oxides.
Manganese deposits occur in rocks of all ages. The largest deposits are the
Proterozoic ore bodies of the Kalahari in South Africa, the Cretaceous ores of