The state and fate of noble metals in bornite and digenite

The role of bornite and digenite as carriers of noble metals (Au, Ag, Pd) is assessed based on study of skarn ores from the Ertsberg East Skarn System, Ertsberg-Grasberg district, Papua, Indonesia. Characterisation of petrography, mineralogy, and geochemistry at the micron- and nanoscale answer the long-standing question of whether noble metals occur in solid solution in Cu-(Fe)-sulphides, or as nanoparticles (NPs), and how these distributions change as the ore cools. Endo- and exoskarn CuAu ores display a wide range of lamellar bornite-digenite intergrowths that are heterogeneous in high-magnification images. These intergrowths are interpreted to form by coarsening of nanoscale basket-weave textures composed of four species: bornite 2a, digenite 1a, anilite, and a phase we interpret as a variety of rhombohedral digenite. Monoclinic chalcocite is not present. These basket-weave textures are considered to result from bornite-digenite solid solutions in the system Cu-Fe-S. The diverse range of subtly different bornite-digenite intergrowth types, grain sizes, and morphologies are attributed to multiple generations of precipitates from bornite-digenite solid solution. Various Au-Ag-Pb-Te-(Se) phases and merenskyite occur as (sub)-micron inclusions throughout the lamellar intergrowths, indicating that the bornite-digenite precipitates are significant carriers of noble metals and accompanying elements. Bornite from forsterite skarn contains tens of ppm Au measured by LA-ICP-MS. Nanoscale imaging reveals the presence of Au-Ag-Te nanoparticles. Lattice-bound Au is estimated at <10 ppm; the reworking of lamellar bornite-digenite intergrowths assists formation of nanoparticles and even micron-sized gold from initially lattice-bound Au and Ag. The nucleation of NPs containing Au, Ag, and Te alongside Cu, but not S, appears to be promoted during transformation of parent digenite 1a into lower temperature polymorphs. Among these phases is an enigmatic Cu₂Te phase, which may be a catalyst for precious metal enrichment in bornite-digenite ores. Merenskyite is also intergrown with the Cu₂Te phase, and contains trace Au, further emphasising the role of bornite-digenite solid solutions in scavenging noble metals from fluids in deposits like skarns formed at relatively high temperatures. The new evidence underpins a new model of noble metal entrapment via CuTe melts formed during phase transition in Cu-Fe-sulphide minerals with wide application in ore petrology.