Black smokers and density currents: A uniformitarian model for the genesis of banded iron-formations

Abstract

Banded iron-formations (BIFs) were comparatively abundant and widespread marine sedimentary rocks in the Archean and Lower Proterozoic eras, but thereafter they appear to be restricted to the Neoproterozoic and Paleozoic eras, although there are indications of similar rocks forming at present. BIFs are important as the major source of iron ore for industry and have also been used to support hypotheses regarding the evolution of life, oceans, and the atmosphere in the Archean and Proterozoic. They apparently formed in deep water and consisted of a semi-regular alternation of quartz (chert) and iron-rich minerals with little or no terrigenous sediment and low (< 1 wt.%) alumina content, in contrast to Proterozoic to Phanerozoic oolitic ironstones (Clinton–Minette style) that formed in shelf environments, were comparatively small and rare and contain abundant iron aluminosilicates and hydroxides but little or no chert. Although ooliths may be locally abundant in granular iron-formations (GIFs) they differ from the Clinton–Minette style oolitic ironstones that formed in shelf seas during periods of slow sedimentation. A common mode of origin for marine deep-water iron-formations is proposed, in which hot fluids, consisting of marine and connate water leaching iron, silica and other elements from mafic and ultramafic rocks associated with mantle plumes or mid-oceanic ridges and active spreading centres are released into the ocean at underwater hot springs (black smokers). On contact with cold marine water, the least soluble elements are precipitated in the form of colloidal hydrous silicates (clay minerals) and hydroxides close to the hydrothermal vent. The hydrothermal fluids are high in silica and low in alumina causing the precipitation of alumina-poor iron silicates (nontronite) that dissociate into iron hydroxide and amorphous silica during diagenesis. The amorphous silica is typically entrapped by iron oxide laminae to form bands of chert. Breaches of the iron oxide laminae permitted the escape of the gelatinous amorphous silica during compaction and dewatering leaving a chert-free residue as the protore of non-hydrothermal sedimentary high-grade iron ore. The rapid deposition and abundant included water formed unstable mounds and chimneys around the vents. Slumping of the mounds caused by compaction, dewatering, gravity sliding, and seismic events produced turbidity currents forming proximal fans of GIF, but colloidal particles remained suspended in longer-lived density flows to deposit the ultra fine-grained BIF over vast areas of the ocean floor. Episodes of density current deposition were separated by intervals of slow pelagic sedimentation and silicification of the sea floor. The density currents commonly caused minor erosion of the sea floor with rip-out clasts incorporated in the new layers. The iron-rich hydrothermal fluids triggered the precipitation of dissolved ferrous iron accumulated by anoxic weathering to produce the huge deposits of BIF in the Archean and Paleoproterozoic until the rise in atmospheric oxygen stopped the accumulation of ferrous iron in the oceans leaving only the hydrothermal source for later deposits.