Abstract

Discussion of a paper on this deposit, Ontonagon County, Michigan, by W. S. White and J. C. Wright (Econ. Geology, 1954, v. 49, p. 675-716). Four general theories of origin may satisfy the observed geological conditions: 1) the Cu was deposited with the original mud; 2) the Cu, diffusing downward into the uppermost few inches of mud, replaced some primary constituent; 3) the Cu was introduced into the shales from below, prior to final compaction or deformation; 4) the Cu was introduced into the rock after lithification and deformation. The fourth theory is rejected on several counts. The third is favored. The theory of a pretectonic origin for Cu finds support in the distribution and disposition of Cu in shale and sandstone and in the fact that fractures show little relation to shale-ore zones. The typical rhyolites of the region appear to be older than the Nonesuch shale.

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