SUBOXIC DEEP SEAWATER AT CA. 1.74 GA: EVIDENCE FROM SEAFLOOR-HYDROTHERMAL JASPER AND IRON-FORMATION IN THE JEROME DISTRICT, ARIZONA

Fine-grained quartz-hematite rock (jasper) in the Jerome district forms bedded units <1 m thick below, within, and above rhyolite tuff of the ca. 1.74 Ga Cleopatra Rhyolite. Interlayered jasper and hematite-facies iron-formation up to 3 m thick occur in overlying turbidites of the Grapevine Gulch Formation. The rhyolite-hosted jasper beds are both proximal and distal to Cu-rich volcanogenic massive sulfide (VMS) deposits at the Jerome and Copper Chief mines. Hematitic filaments 1-3 µm in diameter and 30-50 µm long in some jasper are morphologically similar to remains of iron-oxidizing bacteria in Ordovician and younger VMS-related jasper. Water depth during mineralization was >1500 m as constrained by inferred temperatures of ≥350°C in non-boiling vent fluids, which were needed to carry appreciable Cu in solution to form the seafloor VMS deposits. Whole-rock analyses of the jasper and iron-formation show low MnO concentrations (<0.1 wt %) and REE patterns with generally small negative Eu anomalies and no or small positive Ce anomalies (Ce/Ce* up to 1.48). Analogy with REE systematics in modern stratified water columns suggests that the positive Ce anomalies developed by redox cycling at an oxic-suboxic boundary. The resultant positive Ce anomalies formed in underlying suboxic waters through scavenging of Ce(III) by ferric oxyhydroxide particles that precipited in VMS-related hydrothermal plumes.

A recent model for Proterozoic seawater chemistry (Poulton et al., 2004) involves a fundamental change at 1.8 Ga when previously anoxic deep ocean waters evolved to euxinic (sulfidic) conditions with this redox state continuing until about 1.0 Ga when the deep oceans became dominantly oxic. Occurrence in the Jerome district of abundant primary ferric iron in jasper and iron-formation, together with remains of Fe-oxidizing bacteria, imply suboxic conditions in the deep ocean at ca. 1.74 Ga. The redox state of deep seawater in the Jerome region was above that required for Fe-oxidation but below the redox states of Ce- and Mn-oxidation. Our data suggest a significant lag in oxidation of the deep oceans after the rise of atmospheric oxygen at ca. 2.3 Ga, and raise the possibility that 1.8-1.0 Ga deep seawater was neither euxinic nor fully oxygenated.