VANADIUM IN MAGNETITE GABBROS AND ITS BEHAVIOR DURING LATERITIC WEATHERING, WINDIMURRA COMPLEX, WESTERN-AUSTRALIA

Shephards Discordant Zone is a 500-600 m thick interlayered sequence o f deformed, altered and metamorphosed magnetite metagabbro and about 5 0 layers or lenses of magnetitite (> 80-90% magnetite). The sequence s hows progressive magmatic fractionation upwards: Ti and Ti/Fe increase , and V, V/Ti and Cr decrease upwards in magnetite and in whole-rock c ompositions. The main magnetite-rich sequence (about 400 m thick) is d eeply weathered, with 40 m of saprolite showing vertical zonation of w eathering minerals due to progressive weathering. Magnetitites (averag e 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 mi nor sulfides in unweathered rocks weather to kaolinite, hematite, goet hite and minor vermiculite, ilmenite remaining largely unaffected. Van adium is essentially immobile during weathering and is unaffected duri ng weathering of magnetitites (1% V2O3), but is slightly depleted duri ng weathering of magnetite gabbros (0.23% V2O3).