EARLY DIAGENETIC SEDIMENT-HOSTED STRATIFORM COPPER DEPOSITS: SOURCING AND TRANSPORTING COPPER VIA A ROLL FRONT-LIKE FOOTWALL LEACHING MODEL

The traditional genetic model for early diagenetic sediment-hosted stratiform copper (eSSC) mineralization derives from the well-documented deposition of copper from oxidized, low-temperature cupriferous brines infiltrating basal sulfide-rich graybeds from coarse-grained footwall aquifers. Copper is thought to have been leached from the immature clastics of the footwall unit, associated volcanics, and/or deeper basement rocks. Oxidation of remnant mafic minerals in the aquifers should reduce the Eh of pore solutions to moderately oxidizing levels suitable for copper transport; continued oxidation could produce reduced brine incapable of carrying significant copper. The basin-scale context is that of intracontinental rift basins, with basin brines driven through footwall aquifers by meteoric recharge in rift marginal highlands and with meteoric water evolving to brines by assimilation of evaporites. Note: late diagenetic SSC-type mineralization is not considered here.

Attaining and maintaining moderately oxidized cupriferous brines in footwall aquifers may be explained by analogy to a roll front in which initially oxygen-rich footwall brine upstream of the front slowly oxidizes and leaches the aquifer of its traces amounts of copper. During the overall leaching process, the brine becomes progressively less oxidized and more capable of transporting copper via alteration of labile ferrous minerals (e.g., mafic silicates, magnetite). Copper solubilities may reach as high as 100 ppm under moderately oxidized conditions. Downstream, at the mineralizing front within sulfide-rich graybeds, the cupriferous brine precipitates copper sulfides under oxygen-poor conditions. To infiltrate and mineralize downstream basal graybeds over an extended period of time (possibly millions of years), the upstream leaching process should continue over an equally long period of early diagenetic alteration, a concept consistent with long-term meteoric recharge and with estimated large volumes of cupriferous brine needed to form significant eSSC deposits.