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We describe a vision for a national-scale heavy mineral (HM) map generated through automated mineralogical identification and quantification of HMs contained in floodplain sediments from large catchments covering most of Australia. The composition of the sediments reflects the dominant rock types in each catchment, with the generally resistant HMs largely preserving the mineralogical fingerprint of their host protoliths through the weathering-transport-deposition cycle. Heavy mineral presence/absence, absolute and relative abundance, and co-occurrence are metrics useful to map, discover and interpret catchment lithotype(s), geodynamic setting, magmatism, metamorphic grade, alteration or mineralization. Underpinning this vision is a pilot project, focusing on a subset of the national sediment sample archive, which is used to demonstrate the feasibility of the larger, national-scale project. We preview a bespoke, cloud-based mineral network analysis (MNA) tool to visualize, explore and discover relationships between HMs as well as between them and geological setting or mineral deposits. We envisage that the Heavy Mineral Map of Australia and MNA tool will contribute significantly to mineral prospectivity analysis and modeling here, particularly for technology critical elements and their host minerals, which are central to the global economy transitioning to a more sustainable, decarbonized paradigm.

Granulite facies cordierite–garnet–biotite gneisses from the southeastern Reynolds Range, central Australia, contain both orthopyroxene-bearing and orthopyroxene-free quartzofeldspathic leucosomes. Mineral reaction microstructures at the interface of gneiss and leucosome observed in outcrop and petrographically, reflect melt-rock interaction during crystallization. Accessory monazite, susceptible to fluid alteration, dissolution and recrystallization at high temperature, is tested for its applicability to constrain the chemical and P–T–time evolution of melt-rock reactions during crystallization upon cooling. Bulk rock geochemistry and phase equilibria modelling constrain peak pressure and temperature conditions to 6.5–7.5 kbar and ~850°C, and U–Pb geochronology constrains the timing of monazite crystallization to 1.55 Ga, coeval with the Chewings Orogeny. Modelling predicts the presence of up to 15 vol.% melt at peak metamorphic conditions. Upon cooling below 800°C, melt extraction and in situ crystallization of melt decrease the melt volume to less than 7%, at which time it becomes entrapped and melt pockets induce replacement reactions in the adjacent host rock. Replacement reactions of garnet, orthopyroxene and K-feldspar liberate Y, REE, Eu and U in addition to Mg, Fe, Al, Si and K. We demonstrate that distinguishing between monazite varieties solely on the basis of U–Pb ages cannot solve the chronological order of events in this study, nor does it tie monazite to the evolution of melt or stability of rock-forming minerals. Rather, we argue that analyses of various internal monazite textures, their composition and overprinting relations allow us to identify the chronology of events following the metamorphic peak. We infer that retrograde reactions involving garnet, orthopyroxene and K-feldspar can be attributed to melt-rock interaction subsequent to partial melting, which is reflected in the development of compositionally distinct monazite textural domains. Internal monazite textures and their composition are consistent with dissolution and precipitation reactions induced by a high-T melt. Monazite rims enriched in Y, HREE, Eu and U indicate an increased availability of these elements, consistent with the breakdown of orthopyroxene, garnet and K-feldspar observed petrographically. Our study indicates that compositional and textural analysis of monazite in relation to major rock-forming minerals can be used to infer the post-peak chemical evolution of partial melts during high- to ultrahigh-temperature metamorphism Refereed/Peer-reviewed

Mylonitic shear zones crosscutting homogenous granitoids can retain evidence of fluid-driven metasomatic retrogression and reactivation. However, the relationships between fluid-rock interaction, retrogression, deformation and mylonitisation, and the timing thereof, are often cryptically recorded. This study focuses on the granulite-facies Boothby Orthogneiss from the Reynolds Range, central Australia, which contains a large scalemylonitic shear zone with an apparent record of structural inheritance, fluid infiltration and reactivation. The chosen site provides an ideal natural laboratory in which to investigate the timing of deformation, associated fluid flow and mass transport. U—Pb isotope analyses of monazite indicate an average Pb recrystallization age of c. 1560 Ma, demonstrating that the orthogneiss fabric developed during the Mesoproterozoic Chewings Orogeny (1590–1550 Ma). Structural mapping suggests that this shear zone represents a Riedel branch of larger structures that were subsequently reactivated during the Paleozoic Alice Springs Orogeny (450–300 Ma). The timingof reactivation and fluid flow is constrained by U—Pb dating of apatite, which is present as a stable U-bearing mineral in both orthogneiss and mylonite. Modelling of apatite radiogenic-Pb retention ages, accounting for a wide potential range in common Pb compositions, demonstrates at least some growth and/or recrystallization at c. 1500 Ma and c. 400 Ma, confirming apatite precipitation during Alice Springs shearing and the reactivation of Chewings-age structures. In addition, Alice Springs-aged apatite is found along pre-existing fabrics in the orthogneiss in the vicinity of the shear zone, indicating pre-kinematic fluid flow across the shear zone boundary and into country rock that was otherwise largely unaffected. The combined datasets demonstrates that integrated apatite and monazite U—Pb geochronology is an effective method to unravel the record of superimposed fluid-flow and deformation events. This includes the detection of an ‘inverse younging relationship’, where younger ages are preferentially recorded in the wall rock as rather than in the reactivated shear zone. Such effects are potentially common where deformation is driven by pre-kinematic fluid-rock interaction, with subsequent deformation enhancing the removal of replacement assemblages in more deformed rocks and favouring their preservation in less deformed rocks. Refereed/Peer-reviewed