Work

The state and fate of noble metals in bornite and digenite

Year: 2025

Type: article

Abstract: 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 Cu Au 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 2 a , digenite 1 a , 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 1 a into lower temperature polymorphs. Among these phases is an enigmatic Cu 2 Te phase, which may be a catalyst for precious metal enrichment in bornite-digenite ores. Merenskyite is also intergrown with the Cu 2 Te 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 Cu Te melts formed during phase transition in Cu-Fe-sulphide minerals with wide application in ore petrology.

Source: Lithos

DOI: 10.1016/j.lithos.2025.108198