ABSTRACT

A transition between limestone and dolomite in the MacGregor Member of the Platteville Formation ranges up to a mile in width and is marked by irregular and patchy dolomitization. Locally the dolomitized patches roughly parallel the dominant joint set. There is some evidence of permeability control of dolomitization.

Small quantities of epigenetic pyrite appear to have been introduced before dolomitization but traces of galena postdate the dolomite. Sphalerite is also present in very small quantity. The source of the epigenetic sulfides may be related to mineralization in the nearby Southwestern Wisconsin Zinc and Lead District.

The dolomite contains more ferrous iron than the limestone. Some dolomite crystals show compositional zoning with respect to ferrous iron.

Dolomitization resulted from the introduction of magnesium and took place after lithification. Sea water, ground water or hydrothermal solutions may have been the source of magnesium and the diagenetic history of the rock suggests either ground water or hydrothermal solutions.