GEOLOGY OF THE VOLCANIC-HOSTED BROCKMAN RARE-METALS DEPOSIT, HALLS CREEK MOBILE ZONE, NORTHWEST AUSTRALIA .1. VOLCANIC ENVIRONMENT, GEOCHRONOLOGY AND PETROGRAPHY OF THE BROCKMAN VOLCANICS

Rare-metals mineralization of the Brockman deposit (Halls Creek Mobile Zone, NW Australia) is hosted in a fluorite-bearing, rhyolitic volcan iclastic unit informally termed the ''Niobium Tuff''. The Tuff, more c orrectly described as a tuffaceous volcaniclastic deposit, is the lowe rmost unit of a sequence of trachyte-to-rhyolite lavas, trachyandesite subvolcanic rocks, and volcaniclastic units of the Brockman volcanics located within the Halls Creek Group, a thick, early Proterozoic volc anosedimentary sequence. High precision SHRIMP ion-microprobe zircon d ating of the Niobium Tuff gives an eruption age of 1870 +/- 4 Ma. Regi onal geochronological constraints indicate the Niobium Tuff was deposi ted about 15 Ma before major orogenic activity affected the area. Desp ite folding, faulting and low-grade metamorphism, the Brockman volcani cs show excellent preservation of primary volcanological features, inc luding pillow-lavas and pillow-breccias, that suggest a dominantly sub aqueous, below-wave-base emplacement environment. The style of eruptio n products and magma volume constraints suggest the trachyte-dominated volcanics were erupted from a small shield volcanic complex probably in a rift-related basin in a shallow-marine setting. The tectonic sett ing was intraplate but differs from most Tertiary to recent trachyte v olcanic complexes which are largely subaerial, are built on relatively thick continental crust, and show no post-eruptive orogenic history. Brockman-style rare-metal deposits are characterized by preservation o f subaqueous volcanics beneath a thick sedimentary sequence, eruption of early incompatible-element enriched products followed by less diffe rentiated magmas, and fine-grained mineralogy influenced by alteration processes. Prospects exist for discovery of analogous deposits, parti cularly in early Proterozoic mobile belts and Tertiary intraplate shie ld volcanic provinces.