Abstract

It is suggested that the ores at Franklin and Sterling are pyrometasomatic deposits formed in the Franklin Limestone, at some distance from the unknown igneous source of the ore-forming fluid. The general classes of skarn minerals developed by contact metamorphism (contact metasomatism in Barrell's sense of the term)--spinels, garnets, pyroxenes, epidotes, micas, olivines, and melilites--all are present in the deposits. These minerals are, however, unique in their high content of Mn (super +2) and Zn (super +2) and their relatively small amounts of Fe (super +2) and Mg (super +2) . The explanation advanced for this peculiar situation is that, in the source magma of the ore-forming fluid involved, an unusual abundance of Mn (super +3) resulted in the oxidation of most of the Fe (super +2) , which normally would have entered the ore fluid in that form, to Fe (super +3) and the concomitant reduction of much Mn (super +3) to Mn (super +2) . This oxidation of ferrous to ferric ion resulted in an uncommonly high acceptance of magnesium ion in the dark silicates of the crystallizing source magma and almost entirely eliminated Mg (super +2) from the ore fluid. The unusual scarcity of ferrous and magnesium ions in the ore fluid forced Zn (super +2) and Mn (super +2) to neutralize in large part those various contact mineral anions which are found in normal contact deposits combined with Fe (super +2) and Mg (super +2) . The lack of Cu (super +2) is thought to have been caused by its proxying for ferrous iron in the minerals of the original source magma and the paucity of Pb (super +2) by the difficulty of the large lead ion in entering and neutralizing any of the variety of anions available.

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