SMM Ameen* and SA Wilde School of Earth and Planetary Sciences, Curtin University, GPO Boc U1987, Perth, 6845 The Brilliant gold deposit lies at the northern boundary of the 2.76 Ga Lady Lydia granitoid (LLG) (Ameen and Wilde, 2018) in the northwestern part of the Yalgoo area, Murchison Domain, Western Australia. We investigated the nature of gold mineralisation, alteration assemblages, host rock, and fluid inclusions within the gold-bearing veins in the historical Brilliant open pit. The pit shows remnants of major mineralised veins running parallel to the trend of the pit (137°). The mineralised veins (~15-30 cm wide) are flat-lying, have a sigmoidal to boudinage character in places, and are rimmed by thin haloes of hydrothermal alteration within the biotite hornblende monzogranite (BHMzgrt) host rock. BHMzgrt is fine-grained and has a mylonitised contact with the 2.76 Ga porphyritic granodiorite phase of the LLG. Abundant quartz, pyrite, and epidote, with minor selvages of biotite and/or hornblende make up the ore veins. The proximal alteration zone is ~10-25 mm and comprised of an assemblage of mainly K-feldspar, biotite, and sericitized plagioclase, with subordinate epidote, and chlorite. Epidote-biotite-chlorite and pyrite constitute the alteration phases in the distal zones (15-40 mm). The ore phases in the quartz-pyrite veins are pyrite (~70%), and variably martitised magnetite (~30%), along with traces of pyrrhotite, bismuth telluride, gold telluride, gold-silver-telluride, chalcopyrite, galena, and rare native gold. Notable occurrences of pyrite are found in large (3-10 mm) subhedral crystals along the vein margins. Smaller (~0.1 to 0.4 mm) pyrite grains occur in hornblende in both the vein and wall rock. A few pyrite crystals are seen to be bordered by martitised magnetite, which might have been formed during a late oxidation reaction after primary magnetite. Minute (~2-4 μm) inclusions of native gold occur within Ag-Au-Te (petzite) and in the martitised magnetite rim over pyrite. Hornblende–plagioclase thermometry of the BHMzgrt implies the host rock post-dated the granodiorite phases of 2.76 Ga LLG and was emplaced into the LLG at ~672-644°C under a lithostatic pressure of ~2.4 kbar and at a depth of ~8.3 km as a mid-level intrusion or dyke. Microthermometry of the primary CO2(± H2O) and H2O-CO2(± NaCl) fluid inclusions in the quartz from the ore vein suggest a medium salinity (5 to 9 wt% NaCl equiv.), medium density (0.7-0.9 gm/cc), with a CO2-H2O-NaCl solution being the parental fluid for the gold and associated metals. The average temperature of gold formation was calculated at ~310°C. The intersection of the lowest and highest total homogenisation temperature (~308 and ~313°C) with the minimum and maximum isochores defines a minimum and maximum fluid pressure of 2.0 and 3.2 kbar, respectively. A deep source of the ore fluid is inferred from the abundance of CO2-rich fluid inclusions. The mutually parallel trends of the Brilliant open pit and sigmoidal to boudinaged quartz-pyrite veins implies the formation of the veins in a ductile-brittle structural regime, likely syn-kinematic with the formation of the regional Noongal shear zone. Reference(s) Ameen, S.M.M., Wilde, S.A.,2018. Multiple sources for Archean granitoids in the Yalgoo area, Yilgarn Craton, Western Australia: Geochemical and isotopic evidence. Precambrian Research, 314, p.76-110.