Sunday, October 25, 2009

How the Iron Ore Formed

Under the right circumstances both iron and magnesium ions are capable of acting as weak acids under the influence of alkali or alkaline mineralization as exemplified by the underlying Stockbridge marble formation, or as at the time of deposition a back arc basin coral reef. The re4actions that occurred between the CaCO3 and the proposed iron and magnesium oxides found in the overlying Walloomsac formation providing the iron to form the hydrous iron oxide (limonite) at the contact between the Stockbridge and Walloomsac formations. The Walloomsac also provided the necessary magnesium ions for converting the limestone to dolostone a mixture of calcium and magnesium carbonate. Undoubtedly there were chemical reactions occurring at the beginning of the depositation of the Walloomsac on top of the Stockbridge formation.

The real transference of ions did not really become strong until the Taconic orogeny during the late Ordovician that buried both formations underneath a nappe of older Cambrian rocks that were associated with the island arc that had formed earlier off the south coast of the North American craton. The position of older rocks on top of younger rocks was a puzzle to geologists for decades that was known as the ‘Taconic Problem.” This puzzle was finally unraveled in the 1970s with the discovery of plate tectonics.

Tuesday, October 20, 2009

The Walloomsac Formation - The Bearer of Iron

The Walloomsac Formation was formed from volcanic ash falling on top of a coral ref in the back arc basin of the volcanic arc that formed off the south coast of the North American craton in the mid-Ordovician. Since that time the North American Craton has rotated approximately ninty degrees counterclockwise making the ancient south coast today's east coast. The situation at the time was similar to today's Japan and the Sea of Japan between Mainland Asia and Japan.

The ash fall was intermittent depending on volcanic activity in the adjacent island arc. The prevailing wind would have blown the volcanic ash into the back arc basin. We must assume that the volcanics were mafic to andesitic in nature having ejecta that was enriched in iron and magnesium.

This was originally laid down as interbedded layers of ejecta and coral reefs that grew on top of each layer of ejecta after it was erupted. At the beginning of the eruptive cycle in the island arc the ejecta was more mafic then it became later as it approached andesite in composition. It was these early eruptives that supplied the iron and magnesium that later were converted into dolostone and iron ore.