Geology of the volcanic-hosted Brockman rare-metals deposit, Halls Creek Mobile Zone, northwest Australia. II Geochemistry and petrogenesis of the Brockman volcanics

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