GEOCHEMISTRY AND ISOTOPIC STUDIES OF THE NEOPROTEROZOIC SAWAWIN BANDED IRON FORMATION, SAUDI ARABIA

Banded iron formation (BIF) provides important insights on Neoproterozoic climate and hydrosphere. The Sawawin BIF in northern Saudi Arabia is an outstanding example of such BIF. The jaspilitic iron formation is approximately 60 meter thick and is interbedded with tuffs, tuffaceous sedimentary rocks and shales. Sawawin BIF structurally overlies the Ghawjah metavolcanics and its upper part is an intrusive contact with metadiorite. It is oxide facies composed of hematite, magnetite with titanium rich core, cryptocrystalline quartz, jasper, apatite, barite and pyrite. A detailed geochemical study was conducted by selecting 69 stratigraphically-controlled samples from the succession. Primary sedimentation features are preserved, including delicate layerings of silica rich layers were also observed with minor evidence of soft sediment deformation. Pyrite is concentrated towards the base of the succession indicating more reducing conditions. The iron rich layers show evidence of recrystallization (mainly hematite), and magnetite grains overgrow the primary layering. NASC-normalized plots of Sawawin BIF REEs reveal negative Ce anomalies (Ce/Ce* =0.76 to 0.95, mean = 0.87) similar to modern seawater. Sawawin BIFs also are enriched in HREE over LREE, characteristic of modern seawater ([La/Yb]_NASC= 0.09 to 0.60). In contrast, a significant hydrothermal contribution to Sawawin BIF is indicated by positive Eu anomalies (Eu/Eu* = 0.84 to 1.54, mean = 1.09) and strongly positive Nd isotopic compositions (epsilon-Nd =+2.4 to +5.4) also indicates a strong hydrothermal input. These chemical indices indicate mixing between seawater and hydrothermal solutions, suggesting a distal setting in a hydrothermally-active basin for deposition of the Sawawin iron formation.