Ninety-five shale samples from drill holes that intersected the base of the Davis shale as well as a drill hole that cored the entire section of the formation were collected and analyzed for 26 elements by neutron activation. It was found that ore-forming and related solutions had produced a broad zone of enrichment at the base of the Davis shale that, for some elements, extends up to 0.5 and 1.0 km west and east of the ore zones, respectively. The zone of enrichment is defined, in various degrees, by Mn, Fe, Zn, and to a lesser extent, by Na, K, and As. These elements, with the exception of As, have been depleted (to back-ground levels) from the base of the shale that immediately overlies the ore zone. Within the shale over the ore zone, the central third of the Davis Formation is enriched with Na, Fe, and Zn, whereas most of the section is enriched in K. The Davis shale is also enriched in As at the upper and lower contacts of the formation and sporadically through the section.These data suggest that the shale of the Davis Formation may have acted as a sink and subsequently a source for Na, K, Mn, Fe, and Zn, as well as a sink for As. Episodic mineralization and leaching are believed to be responsible for the initial enrichment and subsequent removal of these elements in the Davis shale. Arsenic was probably precipitated and/or absorbed in a form that was more resistant to leaching. On the basis of the distribution of these selected elements, it is apparent that the alteration of the Davis shale in the area of the Buick mine was controlled by fracture permeability. Ore-forming and related solutions migrated along faults and fractures formed as a result of the collapse of the underlying dolomite. Thus, in the Buick mine area, the Davis Formation probably acted as a leaky aquitard during and after ore mineralization thereby allowing ore-forming and related solutions to migrate through, and probably along the top of, the Davis Formation.The enrichment zone at the base of the Davis shale, best delineated by Fe and Zn, is laterally extensive and could prove to be a useful guide to ore mineralization in this and other Mississippi Valley-type districts. Arsenic may be of use as an indicator of pathways taken by ore-forming and/or related solutions.

First Page Preview

First page PDF preview
You do not currently have access to this article.