New Mexico Geological Society Annual Spring Meeting — Abstracts

Two examples of red bed copper mineralization were studied at the Scholle and Pastura districts in New Mexico. Each district represents a differing style of copper mineralization. At the Pastura district, the sandstone units are mineralized and the bracketing shales are barren or only slightly mineralized. In contrast, at the Scholle district, chalcocite mineralization is found in conglomerate, sandstone, siltstone, and shale. The relative lack of organic matter and diagenetic (pre-copper) pyrite at these deposits suggests redox interface precipitation models may be inadequate to explain mineralization. Both of these deposits can be instead attributed to the geologic membrane model for sedimentary copper genesis. The differences in mineralization style between these deposits can be attributed to variations in geologic membrane efficiency at each location.

Mineralization at the Pastura district is, primarily confined to permeable sandstone units bounded above and below by shales. The shales are of low permeability and consequently operate as effective membranes. As a result, the groundwater (ore solution) reaches supersaturation with respect to chalcocite within the sandstone before significant solute penetration of the confining shale occurs. The entire sandstone unit is mineralized and the bracketing shale units are left barren. Any organic matter in the sandstones enhanced precipitation of copper sulfides but was not requisite for mineralization. Mineralization is also confined by stratigraphic pinchouts (at the Stauber Mine) and synclinal structures that serve to further localize ore deposition.

In the Scholle district, mineralization is present in a large variety of lithologies ranging from arkosic sandstones and conglomerates to siltstones and shales. The siltstones and shales that occur above and below the ore-bearing conglomerates and sandstones are more permeable than the shales at Pastura. As a result, solute separation occurs less efficiently and solute penetrates farther into the shale before encountering a shale layer sufficiently impermeable to have a significant membrane effect. Consequently, ore mineralization occurs primarily within the more permeable portions ofthe shale and silt and does not extend completely throughout the sandstone. The lack ofgeologic structures and stratigraphic "traps" that act to enhance membrane processes results in scattered mineralization in the Scholle District.