Alteration out of sight: a petrographic and HyLogger study of the McPhillamys gold deposit, east Lachlan Orogen, New South Wales

Lithology, alteration and Au mineralisation characteristics of the McPhillamys gold deposit in central NSW have proved difficult to interpret owing to structural overprinting. Previous models for the genesis of Au mineralisation include syngenetic (late Silurian Au-rich volcanic massive sulfide; VMS) or epigenetic (Carboniferous orogenic Au). We combine extensive new HyLogger spectral data (SWIR and TIR), petrographic analysis, geochronology and isotopic data to determine the alteration/mineralisation paragenesis, alteration zonation and effects of lithological control on mineralisation by using both downhole and broad-scale 3D mineralogical characterisation. The mineralisation at McPhillamys is stratabound within a coarse andesitic/dacitic volcaniclastic package. Alteration involved the development of an initial potassic core (microcline (ex-adularia)–quartz–pyrite) associated with the mineralised volcaniclastic package at the centre of the orebody and extending into the overlying non-mineralised andesitic volcanic/volcaniclastic package north and south of the mineralised zone. Propylitic-style alteration (albite–pyrite–chlorite–carbonate ± epidote) forms an outer shell to the mineralisation, except at the northern and southern extents of the mineralised trend. White mica, chlorite, feldspar and carbonate all display consistent and mappable compositional variation between mineralised and non-mineralised sequences. It is difficult to decipher the early history of the deposit and geochronology (U–Pb and Ar–Ar), presented in this study, is unable to distinguish if mineralisation and alteration developed just after deposition of the host sequences during the latest Silurian (<i>ca</i> 423 Ma) with extensive recrystallisation during both Tabberabberan (<i>ca</i> 390 Ma) and Kanimblan (<i>ca</i> 350 Ma) orogenic events, or if mineralisation developed solely during the Tabberabberan Orogeny (<i>ca</i> 390 Ma) with extensive recrystallisation during the Kanimblan Orogeny (<i>ca</i> 350 Ma). We suggest the mineralogy, broad spatial distribution (700 m long, 250 m width, and up to 700 m depth) and paragenesis of alteration and mineralisation at McPhillamys are consistent with a deformed late Silurian lithologically controlled low-sulfidation epithermal, or potentially a shallow water (subsea-floor) Au-rich VMS system.