MINERALOGICAL AND CHEMICAL COMPOSITION OF OOLITIC IRONSTONES FROM THE TYPE LOCALITY, CLINTON, NEW YORK

Oolitic ironstones were once an important source of iron, providing much of our country’s iron from Colonial times up through the early 20^th century. While no longer economically important, these unique rocks provide a record of an unusual, and still poorly understood, depositional marine environment. Previous studies documented the defining characteristics of these rocks: fine sand grains (usually quartz) surrounded by concentric rings of hematite, chamosite, and silica embedded in a carbonate matrix (Smyth, 1892; Schoen, 1962). The purpose of this study was to re-examine samples of this unusual rock collected from the type locality using modern analytical tools (SEM/EDS, WD-XRF).

We collected samples of the two major hematite beds (the Westmoreland and Kirkland) along with samples of the surrounding lithologies of the Silurian Clinton Group from exposures along Sherman Brook in the town of Kirkland. Our studies confirmed that most of the ooids are small rounded quartz grains surrounded by rings of hematite, silica, and chamosite cemented together by a matrix of dolomite. In some samples, however, many of the ooids contain bands of calcium phosphate. While the presence of phosphorous had been previously noted in the bulk chemical analysis of the ore (up to 2.5 wt. % P₂O₅)(Newland & Hartnagel, 1908), and in the presence of small angular clasts of apatite (presumably bone or shell fragments) (Schoen, 1962), it had not previously been reported in the ooids themselves.

Our studies also revealed that the matrix consists of two compositionally distinct varieties of ferroan dolomite, along with hematite, chamosite, and small (< 50 um) grains of detrital zircon, rutile, and chromite. Some ironstone beds contain relatively high concentrations of zircon, making them ideal candidates for sediment provenance studies. Calcite is a minor cement, and is commonly found in thin, cross-cutting veins.

The ironstones appear to have formed in a shallow marine environment with very limited input of mature clastic sediment. Ooids with alternating concentric bands of silica, apatite, hematite and chamosite (or their precursors, goethite and berthierine), require a very unusual marine environment with rapidly shifting conditions for the precipitation / formation of these four phases.