A geologic membrane-microbial metabolism mechanism for the origin of the sedimentary copper deposits in the Pastura district, Guadalupe County, New Mexico
V. W. Lueth and T. M. Whitworth
Examination of copper deposits in Pastura district shows that mineralization occurred at the sand/shale interface. Mineralization was greatest adjacent to the thinnest shales and decreased with increasing shale thickness, becoming non existent adjacent to the thickest shales. This pattern of mineralization is consistent with geologic membrane effects that can occur in shales and clays in the subsurface. We propose a conceptual model invoking solute-sieving effects to help explain the observed mineralization in the copper deposits in the Pastura district. We suspect that the role of the membrane functioning shales was to concentrate the copper and the dissolved nutrients necessary for metabolic-functioning of sulfate reducing bacteria in "layers of increased solute concentration," usually called the concentration polarization layer (CPL), adjacent to the shales. The sulfate-reducing bacteria, as a part of their metabolic process, produce H2S, which precipitated the copper sulfides. Experimental studies have demonstrated that the CPL can range from several to more than 100 times that of background concentrations within the aquifer. Flux magnitude through shale is inversely proportional to its thickness and strongly affects CPL development. For any given solute, there is a threshold flux value that must be exceeded before rejected solute becomes unable to effectively back diffuse into the reservoir. Therefore, thick shales (with correspondingly low flux rates) typically do not form a CPL. This physical fact corresponds with the field observation that the sands adjacent to the thicker shales in the Pastura district are not mineralized. Our model explains why copper mineralization is localized in proximity to thin shale units and is not ubiquitous in the host rock.
- Lueth, V. W.; Whitworth, T. M., 2001, A geologic membrane-microbial metabolism mechanism for the origin of the sedimentary copper deposits in the Pastura district, Guadalupe County, New Mexico, in: Geology of the Llano Estacado, Lucas, Spencer G.; Ulmer-Scholle, Dana S., New Mexico Geological Society, Guidebook, 52nd Field Conference, pp. 333-340.