Abstract

This paper is essentially a progress report on the geological study being made in the Southeast Missouri District by the St. Joseph Lead Company. The problems encountered are described and the progress toward their solution is assessed. These problems have consisted of the subdivision of the Bonneterre formation into units that are mappable underground, the recognition of the types of rock alteration accompanying or preceding ore deposition, the recognition of the structural controls of the ore trends, and the collection of data that may indicate the method of ore genesis.

The Bonneterre formation is divided into eight principal zones which are numbered from top to bottom. Contacts between zones are mapped, and structural features are indicated by contouring. Ore in each zone has distinctive characteristics.

Buried Precambrian igneous knobs have localized some ore bodies in the Lead Belt area, but sedimentary arch structures of depositional origin are the outstanding features controlling ore trends. Fracture zones exerted a secondary influence where arches are present and, where arches are absent, were, in some places the dominant ore-controlling structures.

Major alteration effects are dolomitization of limestone, “fingering” and spotting, recrystallization, and removal by solution. Glauconite and adularia have been introduced or redistributed.

Lateral migration of the ore solution was important, and the vertical component of motion almost surely was up rather than down. The marginal position of sphalerite in the galena ore bodies is in accord with the relative solubilities of the two sulfides.

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