Abstract

The majority of the zinc-lead orebodies near Shullsburg, Wisconsin are of the pitch-flat type first described by T. C. Chamberlin. A study of about 1,000 exploratory churn drill holes shows conclusively that solution of two limestone members (Oilrock and Glassrock) in an otherwise dolomitic section has been largely responsible for the formation of the pitch-flat structure in which the ore occurs. It has been possible to correlate the degree of solutional thinning of the Oilrock and Glassrock with the thickness of ore mineralization. Thus, the greater the thinning by solution, the more extensive the collapse of the overlying competent beds, forming the pitch structure. Correspondingly, incipient to poorly developed ore deposits show much lesser degrees of solutional thinning of the limy members.Successful exploration techniques are discussed with emphasis on structure, degree of solutional thinning, and halo mineralization as the chief criteria to be used in finding an ore deposit.It is hypothecated that minor adjustments of the Precambrian basement rocks have been responsible for the complex minor folding in the overlying Paleozoic sediments. It is further hypothecated that a large body of magma was intruded into the Precambrian rocks underlying the district, but did not rise high enough to send out any apophyses into the overlying sediments. This hypothetical body of magma is also suggested as the probable source of the mineralizing agencies which were able to rise into the overlying sediments through faults in the roof of the pluton.

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