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Petrography and Diagenesis of Sedimentary Ironstones in Silurian of Appalachian Mountains

Thomas W. Hudson, J. Barry Maynard

Sedimentary ironstones (> 15% metallic iron) occur in the Silurian Red Mountain Formation of northeastern Alabama-northwestern Georgia and in the Rockwood Formation of eastern Tennessee. Clinton-type ironstones, composed of oolitic hematite with bioclast and quartz-sand nuclei, are the most common and best developed in the Birmingham iron-ore district. Less ferruginous lithologies commonly contain superficially coated grains. Oolitic chamosite (14 A iron-chlorite), siderite, and pyrite are associated with the more typical hematitic ore at Rockwood, Tennessee. Glauconitic limestones occur in western facies of the Rockwood Formation near Chattanooga, Tennessee.

Preliminary conclusions include the following. (1) Oolitic hematite and chamosite (or their possible precursors, goethite and berthierine, respectively) are primary minerals, forming near the site of deposition. (2) Small amounts of iron were added during preburial replacement of mud-infilled bioclast cavities by hematite or chamosite. (3) Glauconite replacement of bioclasts probably occurred syngenetically, but the timing of similar replacement by hematite and chamosite is unclear. (4) Siderite beds and pyrite nodules observed at Rockwood, Tennessee, also formed during shallow burial conditions. (5) A minor, early, hematite cement occurs in most oolitic ironstones, followed by sparry ferroan calcite as the primary cement. (6) Ferroan dolomite and ankerite replacement of calcite was t e dominant secondary process in thin ironstones from Cumberland Gap and White Oak Mountain, Tennessee.

Syndepositional processes are responsible for most of the ironstones in this study. Later deposition of secondary cements and replacement by ferroan minerals is relatively unimportant in oolitic beds but may be significant in some associated lithologies.

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