Abstract

Developments not previously published in geological studies of the Michigan copper district by the Calumet and Hecla Geological Department are presented. The earlier estimates of the thickness of the basic lava and sedimentary parts of the Keweenawan section, recently disputed, have been substantially confirmed in many explorations. From time to time the question has been raised as to whether the Greenstone may not be a sill rather than a flow. Recent evidence confirms its identification as a flow. Some of the features of the known lodes as revealed by underground studies are discussed.Numerous attempts have been made to relate the known deposits to broad geological features and structures and to group them into various sorts of patterns. The results of some of these studies are given.Arguments are presented which indicate that the deep channels along which the copper depositing solutions ascended to form the known conglomerate and amygdaloid ore bodies were in most if not all cases, conglomerates interbedded in the flow series.Calumet and Hecla geologists still believe that the copper deposits are epigenetic, having been formed by the activity of ascending hydrothermal solutions which brought in the copper from some source in depth. The suggestion that the source of the copper is the basic trap flows themselves is discussed and discarded as unlikely. The possibility that the deposits are of syngenetic origin is likewise examined and discarded as unlikely.The various incentives from a geological point of view for continued exploration in the district are discussed. While there can be no positive assurance that undiscovered deposits do exist, the chances are in favor of that conclusion. The prevailing cover of glacial drift could effectually conceal such deposits. The tendency for conglomerate ore shoots to have short lengths along the strike near surface and the erratic distribution of copper in the amygdaloid deposits would make it possible or even likely that the widely spaced drill holes over much of the district would have failed to reveal them.

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