Geochemical characteristics and structural setting of lithium–caesium–tantalum pegmatites of the Dorchap Dyke Swarm, northeast Victoria, Australia

Abstract The Dorchap Dyke Swarm hosts the first recorded occurrence of lithium–caesium–tantalum (LCT) pegmatites in Victoria, Australia. Syn-orogenic emplacement of pegmatite dykes occurred along a northwest-trending shear system during the Benambran Orogeny. Pegmatites are derived from fractionated melt associated with the Mount Wills Granite, which is an S-type, peraluminous granite originating from supracrustal melting of Ordovician sedimentary sequences. A distinct, eastward-oriented fractionation trend across the Dorchap Dyke Swarm has highlighted a 20 × 8 km highly fractionated zone in the northeastern Dorchap Range, which includes spodumene- and petalite-bearing pegmatites. A distinct pattern of elemental enrichment (P > Cs > Be > Nb ≥ Ta > Li > Sn) is observed across the strongly fractionated zone of the Dorchap Dyke Swarm. Subsequent metasomatic fluid movements and hydrothermal overprinting have resulted in redistribution of mobile elements in the Dorchap Range, either as hydrothermal alteration species or in some instances as the development of exomorphic halos. Additionally, a regional alteration overprint, likely associated with subsequent metamorphism of pegmatite dykes has resulted in alteration of primary petalite to spodumene plus quartz, and primary spodumene to cookeite. Bulk rock geochemical data from the Dorchap Dyke Swarm suggest a syn-collisional setting for dyke intrusion, consistent with the inferred tectonic setting of the central Lachlan Fold Belt at the time of pegmatite emplacement. Pegmatite dykes locally contain an overprinted structural foliation, which is consistent with the primary structural trend of deformed metasediments and may indicate that dyke emplacement was syngenetic with regional folding and compression of the surrounding Omeo Metamorphic Complex during the Benambran Orogeny. Subsequent hydrothermal alteration of some dykes likely occurred immediately following the Bindian Orogeny. KEY POINTS Dorchap Dyke Swarm are the first recorded Li–Cs–Ta pegmatites in Victoria, Australia. Pegmatites were emplaced synchronous with or immediately following the Benambran Orogeny. Dorchap Dyke Swarm pegmatites are geochemically correlated with the Mount Wills Granite.