CH4-N-2 in the Maldon gold deposit, central Victoria, Australia

The Maldon gold deposit in central Victoria has geological, geochronological and fluid chemistry characteristics that distinguish it from typical vein-hosted, 'orogenic' gold deposits in this region. The deposit lies within the thermal aureole of the Late Devonian Harcourt Granite and associated granitic dykes that postdate regional metamorphism (similar to 445 Ma) and large gold deposits such as Bendigo. The fluid inclusions are characterised by the presence of non-aqueous (i.e. carbonic) fluids, which exhibit complex freezing and heating behaviour, as well as mixed CO2-low-salinity aqueous fluids (mostly <= 10 wt.% NaCl eq.). Raman analysis indicates that carbonic inclusions can vary from CO2-rich to CH4 + N-2-rich. Furthermore, higher-salinity fluid inclusions, containing 20-22 wt.% NaCl eq., occur locally. Overall, fluid inclusions in the K-feldspar zone are much less abundant by volume than those in the cordierite zone probably due to recrystallisation, suggesting limited magmatic fluid input. The Harcourt Granite is a moderately reduced, I-type granite and it is suggested that the 'retrograde', reduced fluids (e.g. CH4 + N-2-rich), formed within the thermal aureole of the granite and associated dykes during contact metamorphism, are not part of the regional mineralising fluid system, which was dominated by deeply derived CO2-low-salinity aqueous fluids of metamorphic origin. Thus, the Maldon deposit is an 'orogenic' gold deposit that was metamorphosed and/or remobilised during the emplacement of post-orogenic intrusions