GEOLOGY OF THE VOLCANIC-HOSTED BROCKMAN RARE-METALS DEPOSIT, HALLS CREEK MOBILE ZONE, NORTHWEST AUSTRALIA .2. GEOCHEMISTRY AND PETROGENESIS OF THE BROCKMAN VOLCANICS

Lavas and subvolcanic intrusions of the 1.87 Ga Brockman volcanics comprise a cogenetic suite of alkaline, Qz-normative, metaluminous trachyandesites, trachytes and trachydacites/rhyolites. They are genetically related to the rare-metal-bearing ''Niobium Tuff'' which contains extreme enrichments in high-field-strength incompatible elements (av. 1660 ppm Y, 9700 ppm Zr, 3200 ppm Nb, 175 ppm Yb). Neodymium isotopic data indicate the Brockman parent magma was mantle-derived with epsilon(Nd)(initial) approximate to + 3, analogous to basaltic magmas generated in some modern intraplate ''hot-spot'' volcanic provinces. The geochemical evolution and incompatible element enrichments in the Brockman suite can be modelled by AFC processes involving extensive degrees of crystallization and progressive contamination of derivative magmas with granitic/ metasedimentary upper crust. The large degrees of crystallization required to derive the more differentiated members of the Brockman suite are best accommodated by a process of ''liquid fractionation'' resulting in internal compositional stratification of the magma chamber with extreme differentiates such as the Niobium Tuff forming a volatile-enriched ''cap'' in the magma chamber roof-zone. The high fluorine content of the Brockman magmas played a crucial role in enhancing rare-metal contents by increasing the efficiency of crystal-liquid separation and decreasing mineral-melt K-d's. There appears to be no special role for fluorine-rich fluids in generating the rare-metal enrichments. However, leaching of fluorocarbonate minerals by late hydrothermal solutions, rather than fractionation of a LREE-selective phase, caused marked LREE-depletion in the Niobium Tuff.