Facies architecture of a silicic intrusion-dominated volcanic centre at Highway-Reward, Queensland, Australia

The Highway-Reward massive sulphide deposit is hosted by a silicic volcanic succession in the Cambro-Ordovician Seventy Mile Range Group, northeastern Australia. Three principal lithofacies associations have bean identified i n the host succession: the volcanogenic sedimentary facies association, the primary volcanic facies association and the resedimented syn-eruptive faci es association. The volcanogenic sedimentary facies association comprises v olcanic and non-volcanic siltstone and sandstone turbidites that indicate s ubmarine settings below storm wave base. Lithofacies of the primary volcani c facies association include coherent rhyolite, rhyodacite and dacite, and associated non-stratified breccia facies (autoclastic breccia and peperite) . The resedimented volcaniclastic facies association contains clasts that w ere initially formed and deposited by volcanic processes, but then redeposi ted by mass-flow processes. Resedimentation was more or less syn-eruptive s o that the deposits are essentially monomictic and clast shapes are unmodif ied. This facies association includes monomictic rhyolitic to dacitic brecc ia (resedimented autoclastic facies), siltstone-matrix rhyolitic to dacitic breccia (resedimented intrusive hyaloclastite or resedimented peperite) an d graded lithic-crystal-pumice breccia and sandstone (pumiceous and crystal -rich turbidites). The graded lithic-crystal-pumice breccia and sandstone f acies is the submarine record of a volcanic centre(s) that is not preserved or is located outside the study area. Pumice, shards, and crystals are pyr oclasts that reflect the importance of explosive magmatic and/or phreatomag matic eruptions and suggest that the source vents were in shallow water or subaerial settings. The lithofacies associations at Highway-Reward collectively define a submar ine, shallow-intrusion-dominated volcanic centre. Contact relationships and phenocryst populations indicate the presence of more than 13 distinct porp hyritic units with a collective volume of 0.5 km(3). Single porphyritic uni ts vary from <10 to 350 m in thickness and some are less than 200 m in diam eter. Ten of the porphyritic units studied in the immediate host sequence t o the Highway-Reward deposit are entirely intrusive. Two of the units lack features diagnostic of their emplacement mechanism and could be either lava s and intrusions. Direct evidence for eruption at the seafloor is limited t o a single partly extrusive cryptodome. However, distinctive units of resed imented autoclastic breccia indicate the presence nearby of additional lava s and domes. The size and shape of the lavas and intrusions reflect a restricted supply of magma during eruption/intrusion, the style of emplacement, and the subaq ueous emplacement environment. Due to rapid quenching and mixing with uncon solidated elastic facies, the sills and cryptodomes did not spread far from their conduits. The shape and distribution of the lavas and intrusions wer e further influenced by the positions of previously or concurrently emplace d units. Magma preferentially invaded the sediment, avoiding the older unit s or conforming to their margins. Large intrusions and their dewatered enve lope may have formed a barrier to the lateral progression and ascent of sub sequent batches of magma. (C) 2000 Elsevier Science B.V. All rights reserve d.