Ore minerals down to the nanoscale: Cu-(Fe)-sulphides from the iron oxide copper gold deposit at Olympic Dam,South Australia

Cu-Fe-sulphide mineral assemblages from the Olympic Dam (OD) Fe-oxide Cu-U-Au-Ag deposit, South Australia, are studied down to the nanoscale to explore the potential these minerals have for understanding genetic processes such as primary deposit zonation. Cu-Fe-sulphide pairs: ‘brown’ bornite associated with chalcopyrite (bornite-chalcopyrite zone); and symplectites of ‘purple’ bornite with species from the chalcocite group, Cu_2 − xS (bornite-chalcocite zone), co-define an upwards and inwards deposit-scale zonation at OD. In the bornite-chalcocite zone, there is also an increase in the proportion of chalcocite relative to bornite within the symplectites towards upper levels. In this case, two-phase Cu_2 − xS assemblages are also present, as anisotropic, hexagonal chalcocite (CcH) with lamellar exsolutions of digenite, distinguishable at the μm-scale. Using compositional data (electron microprobe) combined with Transmission Electron Microscopy (TEM) study of foils prepared in–situ via Focused Ion Beam (FIB)-SEM, we show that Cu-Fe-sulphides from different ore zones feature nanoscale intergrowths, lattice defects, superstructure domains (na) and antiphase boundary domains (APBs) that can be interpreted as due to exsolution, coarsening and phase transformation during cooling from high-T solid solutions in the system Cu-Fe-S and sub-systems according to published phase diagrams. ‘Brown’ bornite [(Cu + Fe)/S > 5] contains pervasive lamellae of chalcopyrite which extend down to the nanoscale; such specimens appear homogeneous at the μm-scale. ‘Purple bornite’ [(Cu + Fe)/S < 5] in high-bornite symplectites is associated with chalcocite that shows APBs with 6a digenite and low-T chalcocite. Comparable APBs are also found in the ‘chalcocite’ zone with apparent homogeneity at the μm-scale. Both bornites contain exsolutions of djurleite. Systematic variation of Me/S and Cu/Fe in the two types of bornite points, however, to distinct origins from different bornite solid-solutions in the system Cu-Fe-S. Both show 2a and 4a intermediate superstructures. High-order superstructures (6a and incommensurate na) are restricted to the ‘purple’ bornite whereas the 2a4a low-T superstructure is found in both cases. Me/S ratios in the chalcocite group are variable; lower ratios (down to 1.8; digenite) are more common in chalcocite from symplectites with ‘purple’ bornite. Me/S can be as low as 1.4 where associated with ‘blue’ varieties (‘blaubleibender covellin’) of replacement origin. The two-phase Cu_2 − xS associations contain hexagonal chalcocite (Me/S = 1.95), lamellae of Cu-rich digenite (Me/S = 1.92), and anilite (Cu₇S₄) as nm-scale lamellae. Digenite shows 3a and 6a superstructures and CcH shows transition to pseudo-orthorhombic chalcocite. The presence of superstructures, high-T species and APBs is evidence for Cu-(Fe)-sulphide formation from high-T solid solutions at T > 300 °C (high-T phases, Cu-poor digenite), followed by cooling along distinct paths down to < 120 °C (APBs). The scenario of ‘exsolution from primary solid-solution’, corroborated by the consistency in phase relations within each zone across different scales of observation from deposit scale to nanoscale, backs up a model of primary hypogene ore precipitation rather than replacement, and accounts for the observed vertical zoning at OD. The FIB-TEM approach here is readily applicable to other deposits and shows that nanoscale observations are a valuable, although often overlooked, source of information to constrain ore genesis.