Genesis and characteristics of contact-metamorphic flake graphite from the Leliyn deposit, Pine Creek Orogen (Australia)

Abstract

The Leliyn deposit in the Pine Creek Orogen (Northern Territory, Australia) hosts abundant flake graphite within Paleoproterozoic schists of the Mundogie Sandstone (Woodcutters Supergroup). Graphite crystallized during contact metamorphism (≥600 °C) adjacent to granitoid intrusions (Cullen Supersuite), following earlier regional greenschist-facies metamorphism of carbonaceous sedimentary units, which imparted schistosity. Subsequent retrograde hydrothermal alteration at temperatures as low as ∼ 300–350 °C produced phyllic assemblages (muscovite and kaolinite-group minerals). Graphite occurs mainly as flakes ≤ 100 µm (often < 20 µm) within feldspar-muscovite-biotite matrices, foliation planes, and fine-scale deformation zones. Locally, coarser flakes (> 100 µm) formed in less schistose, alkali-feldspar-rich rocks, placing the resource in the fine- to medium-flake category. X-ray diffraction (XRD) indicates high crystallinity (d₀₀₂ ∼ 3.357 Å; L_c ∼ 800–1300 Å), with systematic variations reflecting thermal gradients from adjacent intrusions. Raman spectroscopy and electron-microscopy imaging reveal pronounced intra-flake heterogeneity in defect density and crystallite size. These data support a multi-stage model for deposit formation: 1) deposition of carbonaceous sediments (maximum age ∼ 2284 Ma); 2) regional greenschist-facies metamorphism (∼ 1855 Ma; Litchfield Event), concentrating carbon within schistosity; 3) contact metamorphism (∼ 1835–1820 Ma; Cullen Supersuite), producing well-ordered graphite, with locally coarser flakes in less schistose domains; and 4) subsequent retrograde hydrothermal alteration, which enhanced microstructural heterogeneity and crystallite mosaicking within flakes. Grain-scale Raman thermometry records apparent temperatures of ∼ 390–620 °C, reflecting contact metamorphism and hydrothermal overprinting, while bulk spectra yield narrower deposit-scale estimates of ∼ 500–530 °C. Overall, the Leliyn flake graphite is highly crystalline and shows strong potential for resource development. This study highlights the predictive value of multi-scale characterization and underscores the Pine Creek Orogen—and similar terrains worldwide—as promising targets for flake graphite exploration.