New Mexico Geological Society Annual Spring Meeting — Abstracts

Sedimentary copper deposits (also termed sediment-hosted stratiform copper deposits) account for 20-25% of world production and reserves of copper, yet no mechanism presented to date has adequately described the genesis of such deposits. Sedimentary copper deposits occur in both sands and shales, generally follow stratigraphic horizons, and occupy original porosity.

Since 1948, many studies have experimentally confirmed the ability of clays, which are a major component of shales, to act as semi-permeable membranes and ftlter out solutes. A portion of groundwater flow is through the low-permeability materials which surround an aquifer. Since these materials filter a significant portion of the solutes out of the groundwater which passes through them, the groundwater remaining in the aquifer is therefore enriched in solutes. As a result of this membrane process, as well as water-rock interaction, groundwater tends to increase in total dissolved solids along the flow path.

The bulk of the increase in solute concentration in the aquifer occurs near the aquifer/membrane interface in a zone called the concentration polarization layer (CPL). Experimental studies suggest that solute concentrations at the aquifer/membrane interface can be as much as 10 to 100 times that in the rest of the aquifer. As a result of solute concentration due to membrane processes, it is easily possible for the groundwater to quickly achieve supersaturation with respect to mineral species with relatively low solubilities, such as bornite, chalcopyrite, and pyrite. The membrane concentration process continues even though supersaturation of these species is reached. As a result, copper mineral precipitation occurs, often forming ore bodies.

We propose that the deposition of sedimentary copper deposits is controlled by geologic membrane processes and that the CPL is a primary control on the distribution of the copper ore. The CPL can be meters or tens of meters in extent. If the sand is thin, then the CPL may extend completely across it and ore deposition will be primarily located in the sand. If the sand is thick, then ore deposition will be localized near the aquifer/membrane interface. Because membrane efficiency decreases as solute concentration increases in the CPL, as the CPL grows, ever increasing amounts of solute can enter the membrane. As a result, mineral precipitation will sometimes extend into the shale membrane.