KINGSTON PEAK IRON FORMATION (NEOPROTEROZOIC, MOJAVE DESERT): TECTONICS NOT SNOWBALLS

Neoproterozoic iron formations should not exist because abiotic and biological oxidation rates should have prevented Fe²⁺ from accumulating in the global oceans after Proterozoic atmospheric oxygenation. Yet Cryogenian iron formations are known from several locations. Two competing models have been invoked to explain iron formation in the Neoproterozoic: 1) the “Snowball Earth” theory whereby ice sheet isolation inhibited ocean-atmosphere mixing and 2) increased flux of iron into a stratified ocean as the result of tectonic events.

We examined iron formation in the upper Kingston Peak Formation (KPF) in the Silurian Hills and nearby ranges of the northern Mojave Desert, California. The iron formation formed in isolated, proximal settings compared to other KPF deposits in the Death Valley area. 60-65 meters of thin, hematite-bearing (reduced to magnetite by greenschist metamorphism) siltstone beds occur near the top of the formation, above a hundred meters of stacked diamictite. A few meter-scale lenses of diamictite occur within the siltstone.

REE concentrations and Fe isotope analyses were performed on individual magnetite grains by laser ablation HR- and MC-ICP-MS. δ⁵⁶Fe isotope compositions (relative to IRMM-14) range from -0.76‰ to 0.26‰, similar to other Neoproterozoic iron formations, confirming aqueous redox processing of Fe during deposition or early diagenesis. REE patterns for the KPF are generally similar to other Cryogenian formations with minimal Eu anomalies and smooth LREE–to–HREE enrichment. However, the KPF is distinct from other Cryogenian formations in its pronounced negative Ce anomaly (Ce/Ce*_PAAS as low as 0.4) that reflects important oxidation of Ce(III) to Ce(IV). This strongly suggests highly oxygenated surface waters at the time of deposition, in contrast to other Neoproterozoic iron formations studied thus far.

Mapping and stratigraphic analysis elsewhere yielded magnetic units only in the southern Salt Spring Hills. Other localities either showed secondary iron mineralization or were stratigraphically lower in the KPF. The combined field and geochemical evidence suggests that the iron formation in the upper Kingston Peak is distinct from other Neoproterozoic occurrences and more likely related to enhanced iron input into rift basins than to Snowball Earth glaciations.