THE FRANKLIN MINING DISTRICT
The minerals certainly to the regarded as primary are the four—franklinite, willemite, zincite, and calcite—that make up nearly the whole mass of the two or bodies, at Franklin and Sterling Hill. To these may be added tephroite as a rare, probably primary associate of willemite. The average percentage of each of the four principal minerals in the ore is given on page 17. There are, however, several wide departures from the average. Franklinite, alone or with calcite, forms some large masses of ore, and zincite is segregated in some masses almost to the exclusion of the other minerals. Willemite is in a few places the only ore mineral in the calcite gangue. The typical ore, however, is a layered mass of all four minerals in rather coarse grains.
The relative age of the four ore minerals was determined by Ries and Bowen (223) through the study of thin sections of the ore. These show that tephroite and willemite are the earlier minerals and that their formation was followed by that of the franklinite and zincite but with some overlap in time.
Of the four ore minerals, only tephroite and willemite have been found elsewhere, and only as subordinate constituents of ores of manganese and zinc. The exceptional mineral character of the Franklin ores is convincing evidence of unusual conditions controlling their deposition.
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© by Herb Yeates 1997-2006.
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