Comparison between Red Sea Deposits and Older Ironstone and Iron-Formation

Abstract

The iron-rich sedimentary rocks of the geologic record are divided into two major types: ironstone, typically as oolitic limonite and chamosite in beds a few meters to a few tens of meters thick; iron-formation, which consists of alternating silica-rich and iron-rich layers and which occurs in units commonly hundreds of meters thick. Most, but not all, ironstone is post-Precambrian in age, and most, but again not all, iron-formation is Precambrian. The great majority of ironstone and iron-formation deposits are the products of normal erosional and sedimentary processes, but a few are believed to be exhalative-sedimentary in origin, genetically related to seabottom hot springs and fumaroles. These latter deposits are marked by temporal and spatial association with volcanic rocks, rapid facies changes, and higher content of metals than that of “normal” ironstone and iron-formation.

Physically the Red Sea sediment is utterly unlike either ironstone or iron-formation, but chemically it has affinities to both except for its remarkable content of base and precious metals. If the Red Sea sediment were being deposited in an environment of strong current and wave action, the resulting product probably would be an ironstone of reasonably orthodox character. If it were to be buried in its present form, a remote possibility exists that it could become chemically differentiated through dewatering and diagenetic processes to yield an end product comparable to iron-formation, but more likely a substantial part of the iron and much or all of the base metals would migrate in hot brine solutions to be reprecipitated in laterally continuous carbonate host rocks.

Publication authorized by the Director, U.S. Geological Survey.