Vanadium in magnetite gabbros and its behaviour during lateritic weathering, Windimurra Complex, Western Australia

Shephards Discordant Zone is a 500-600 m thick interlayered sequence of deformed, altered and metamorphosed magnetite metagabbro and about 50 layers or lenses of magnetitite (> 80-90% magnetite). The sequence shows progressive magmatic fractionation upwards: Ti and Ti/Fe increase, and V, V/Ti and Cr decrease upwards in magnetite and in whole-rock compositions. The main magnetite-rich sequence (about 400 m thick) is deeply weathered, with 40 m of saprolite showing vertical zonation of weathering minerals due to progressive weathering. Magnetitites (average 1% V2O3) are resistant to weathering and show little chemical change, but magnetite gabbros (average 0.27% V2O3) are extensively weathered and show progressive loss of Ca, Na, Mg and S. Plagioclase, magnetite (1.37% V2O3), chlorite (up to 0.35% V2O3), actinolite, epidote and minor sulfides in unweathered rocks weather to kaolinite, hematite, goethite and minor vermiculite, ilmenite remaining largely unaffected. Vanadium is essentially immobile during weathering and is unaffected during weathering of magnetitites (1% V2O3), but is slightly depleted during weathering of magnetite gabbros (0.23% V2O3).