The characteristic gangue of the zinc ores of the Tri-State district and northern Arkansas, locally termed jasperoid, is believed by some geologists to be older than the sulphides and to have been extensively replaced by them. Others believe there has been no replacement of the jasperoid and that jasperoid and sulphides were deposited contemporaneously. New microscopic evidences of replacement of jasperoid by sphalerite and other sulphides are presented.

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