Alkaline and peralkaline igneous rocks are important components of several mineral systems, including those that yield critical metals such as REEs and other HFSEs. The GSQ are investigating the presence of these rock types and the potential for critical metal enrichment in eastern Queensland through the Peralkaline Magmatic Systems Project. This work has identified Cretaceous and Cenozoic igneous rocks, emplaced during continental rifting or mantle plume activity, as having the highest potential. As an extension of this work, we examined rocks associated with the Cenozoic Nebo Central Volcano in central Queensland in more detail and present the results in this report.
The Nebo ‘Central Volcano’ is the second central volcano province in Queensland that lies along the well-known Cosgrove hotspot trail extending through eastern Australia and was emplaced at ca. 35.1 - 16.7 Ma. It is not a single edifice, rather a cluster of geochemically diverse lavas, intrusions and domes emplaced into the Permian sequences of the northern Bowen Basin. The field area investigated is in the eastern part of the province centred around the town of Nebo (Figure 1). Felsic rocks in this area were formerly mapped on a regional scale as unnamed unit Tv. Here, we propose the name Britton Range Volcanics and describe the rock types and relationships as well as geochemistry in detail.
Figure 1. Simplified geology of the Nebo area
Volcanic and sub-volcanic facies present in the Britton Range Volcanics include intensely welded to rheomorphic ignimbrite, coherent lavas and associated autoclastic breccias (Figure 2), domes, extensive dykes and volcanogenic sedimentary sequences. These overly on or intrude an extensive series of vesicular to scoriaceous olivine basalt lavas. The felsic rocks range from trachyte to rhyolite in composition and along with the associated mafic rocks define a strongly bimodal suite. Peralkalinity values are high, but only two samples are truly peralkaline and overall HFSE contents are modest relative to the enrichment observed in similar suites (e.g., Peak Range Volcanics). The mineralogy of the felsic rocks is dominated by alkali feldspars, but locally also includes riebeckite, aegirine and other pyroxenes.
Figure 2. Landforms and rocks associated with the Marling Spikes and Sydney Heads
Petrographic observations and geochemical data reveal evidence for a complex magmatic plumbing system and petrogenesis associated with the Nebo Central Volcano. Many of the individual felsic bodies have unique geochemical characteristics (Figure 3) and probably represent a distinct magma batch with different petrogenetic origins (i.e., different degrees of fractional crystallisation or partial melting, different pressure/temperature/volatile conditions or different degrees of assimilation of different materials). Basic trace element modelling shows that the majority of the felsic rocks in the Britton Range Volcanics cannot be produced by simple fractional crystallisation of a basaltic parent (i.e., one of the main models for critical metal enrichment in peralkaline systems). Additionally, the lack of correlation between HFSE content and either peralkalinity or indices of differentiation, suggests that any unexposed or unsampled bodies that are more evolved or more highly peralkaline are unlikely to exhibit strong HFSE enrichment.
Figure 3. Variation in Chondrite-normalised REE patterns between bodies of felsic rocks
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