Ore Mineralogy
Abstract
Subeconomic concentrations of copper sulfide minerals are irregularly distributed throughout the Lower Permian Wellington Formation and the Ninnescah Shale of south-central Kansas. The host rocks consist principally of gray shales and siltstones with lesser argillaceous dolomites and limestones in which the sulfides occur principally as replacements of earlier diagenetic pyrite or as irregular stringers.
Although two spatially distinct and separate sulfide assemblages are identified on the basis of drill hole information, the sequence of ore minerals indicates that a descending copper-rich solution percolating downward was slowly depleted of its copper content
In the northern portion of the area, pyrite is replaced by chalcopyrite and bornite at shallower depths, while in the southern part of the area pyrite is replaced by "chalcocite-like" minerals. It is suggested that pH and oxygen fugacity are controlling factors responsible for the different assemblages.
Electron microprobe studies indicate that no true chalcocite (Cu2S) is present; instead two phases having the compositions Cu1.78±0.04S (similar to anilite) and Cu1.91±0.03S (similar to djurleite) were identified. The mineral anilite has not been reported before associated with the Midcontinent copper occurrences.
We propose that the mafic rocks associated with the Central North American Rift System were the source rocks of the copper that was subsequently introduced into the Permian basin and adsorbed onto the sediments. Oxidizing ground waters were responsible for redissolving the copper and secondarily concentrating it in a reduced environment.
