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Geochemistry of the Phanerozoic Oolitic Ironstones--Genetic Implication

Deba P. Bhattacharyya

A set of 12 Phanerozoic oolitic ironstones, ranging from Cambrian to Miocene, have been studied mineralogically and chemically. Ten of these are of oxide facies, ooid-rich (> 75% ooid), containing 60-90% hematite and/or goethite, and the remainder is mainly a mixture of berthierine, kaolinite, and quartz in the ooids. Matrix/cement contains the same minerals with a maximum of about 10% identifiable detrital quartz grains. Two of the samples belong to the silicate facies; one contains nontronite, and the other berthierine as the dominant silicate mineral.

Major and trace elements of the oxide facies ironstones indicate that these are enriched in Fe, Al, Ti, V, Cr, As, Ta, Hf, Sc, Nb, Th, U, and REE compared to the oxide facies cherty ironstones of the Precambrian. The silicate facies samples do not show a chemical character that is distinctive from those of the oxide facies. Positive interelement correlation is shown by two groups of elements: (1) Fe-V-As-HREE, and (2) Al-Ti-Cr-Hf-Ta-Sc-Nb. Chondrite normalized REE patterns are similar to those of the upper crustal rocks and NASC, except the oolitic ironstones are relatively enriched in Ce and HREE.

The results conform with the genetic model proposed for a Late Cretaceous occurrence in Egypt. Enrichment of these ironstones in elements listed above are constrained by a surficial weathering source for the components, contributed both as dissolved ions and as detrital colloids. The detrital colloids contributed Al-Ti and related elements of group 2, whereas scavenging by a slowly precipitating ferric hydroxide probably caused the enrichment of the rocks in V, As, Ce, and HREE.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.