Middle Devonian, low-K, rhyolitic volcanic rocks in the Mt Morgan district of central Queensland hosted the 50 million tonne Mt Morgan Au-Cu deposit. At Mt Morgan, rhyolitic volcanic rocks form a belt ~6 km long and <2 km wide, known as the Mine Corridor, that is surrounded by the Mt Morgan Tonalite. Correlative volcanic rocks are exposed east of Mt Morgan over a strike length of approximately 30 km extending SSE. These are conformably overlain by dacitic volcaniclastic rocks of the Raspberry Creek beds. Together, both volcanic units and the Mt Morgan Tonalite form a geochemically coherent suite known as the Mt Morgan volcano-plutonic suite. The rhyolitic volcanics record a period of voluminous pyroclastic eruptions that resulted in the deposition of thickly-bedded, volcaniclastic mass flows into a submarine basin, below storm wave base. In addition, passively emplaced, syn-sedimentary cryptodomes and probable effusive, sub-aqueous, lava domes can be correlated chemically and mineralogically with the volcaniclastic deposits. Fallowing the cessation of rhyolitic volcanism, a group of predominantly dacitic dykes and irregular intrusions were emplaced and probably fed the overlying Raspberry Creek beds. Trondhjemitic plutons eventually stoped into the volcanic pile with associated magmatic and/or phreatomagmatic eruptions resulting in the development, in places, of extensive brecciation and hydrothermal alteration. Tonalite and mafic plutons are relatively unaltered and were emplaced subsequently, following the termination of large scale hydrothermal fluid circulation.
The Mt Morgan volcano-plutonic suite has mineralogical and geochemical affinities with low-K island arcs such as the Tonga-Kermadec arc, i.e. high K/Rb, Rb/Zr and Sr/Nd, but low Rb/Sr, La/Yb and TiN. Trace element abundances of basaltic and gabbroic rocks are below those of average mid-ocean ridge basalt (MORB), being similar to the Kermadec section of the Tonga-Kermadec island arc. All rocks have low overall abundance of large ion lithophile (LIL) and high field strength (HFS) elements, and show LIL/HFS decoupling. The strong negative Nb anomaly in all rock types, and complimentary negative Ti anomaly in the felsic rock group, are features typically observed in modem subduction-related magmatic rocks. Strontium, neodymium and oxygen isotope compositions straddle the mantle array and overlie the fields for modem, SW Pacific island arcs with no evidence for the involvement of an evolved crustal source, i.e. ISr = 0.70362 to 0.70431, ԐNd(381Ma) = 6.3 to 9.0, and δ18O= 6.1‰ to 8.1 ‰. lsotope and trace element geochemical data are most consistent with derivation of the felsic plutons, and their volcanic equivalents, by low pressure partial melting of metamorphosed, basaltic andesites, grossly similar to the Mt Morgan basaltic andesites, but with slight light REE (LREE) and moderate Zr-enrichment. This interpretation is consistent with the presence of ubiquitous, fine-grained mafic inclusions in the tonalite plutons. These source rocks are likely to have been fractionated components of a juvenile island arc, initially derived from a depleted mantle source. A fractional crystallisation model for trace element data indicates that 40% of this melt subsequently crystallised, probably after ascending to a higher crustal level, thereby producing a more silicic, trondhjemitic melt. Isotope and trace element data, together with the predominance of felsic magmatism, evidence for the involvement of crustal melts, rapid lateral thickness variations within the volcanic stratigraphy, and the steep, normal nature of early faults suggest that magmatism occurred during an extensional phase of arc volcanism at or near the margin of Gondwana.
Field relationships and alteration paragenesis indicate that sub-seafloor ore formation at Mt Morgan predated tonalite emplacement and occurred during or soon after rhyolitic volcanism. Previous fluid inclusion and stable isotope studies indicate that ore fluids comprised a mixture of seawater and magmatic fluids. Modelling the behaviour of Cu, Pb and Zn during magmatic differentiation of the Mt Morgan volcano-plutonic suite predicts the formation of a late-stage, Cu-rich (~314 ppm), Zn and Pb-poor (~60 ppm Zn and 9 ppm Pb) residual magma with a trondhjemite/low-K rhyolite composition. The limited available data for the behaviour of Au during differentiation of island arc magmas suggests that Au closely follows Cu. If the rhyolitic magma contained 2 - 4 % magmatic H20, then subsequent degassing could yield a vapour phase containing up to 7,850 - 15,700 ppm Cu, but with substantially less Zn and Pb. During explosive, pyroclastic eruptions this vapour phase would be lost as aerosols in the atmosphere; however, during passive degassing in response to crystallisation in high level cryptodomes, it could condense into a hypersaline liquid and be available for mixing with deeply circulating seawater. The intersection of major long-lived faults at the core of the Mt Morgan Au-Cu deposit suggests that massive and stringer sulphide ore formed as a result of structural focussing of this hybrid hydrothermal fluid following interaction with cold, seawater-saturated volcaniclastic detritus, and/or in response to boiling. Similarities in the lithological, volcanological and geochemical association of large tonnage, Cu-Au volcanic-hosted massive sulphide (VHMS) deposits world-wide suggest that this magmatic-hydrothermal model is generally applicable to global exploration strategies in Archean and Phanerozoic terranes alike.