Ehrig, Kathy1,2, Ciobanu, Cristiana3, Verdugo-Ihl, Max3, Dmitrijeva, Marija3, Cook, Nigel2, Slattery, Ashley4
1BHP Olympic Dam, Adelaide, Australia, 2School of Civil, Environmental and Mining Engineering, University of Adelaide, Adelaide, Australia, 3School of Chemical Engineering and Advanced Materials, University of Adelaide, Adelaide, Australia, 4Adelaide Microscopy, University of Adelaide, Adelaide, Australia
’Invisible gold’ in pyrite refers to gold either present within the sulfide lattice or as discrete nanoparticle (<100 nm-diameter) inclusions (NPs), making it undetectable by conventional optical and scanning electron microscopy. Investigation of “invisible gold” in chalcopyrite-pyrite ores from the Olympic Dam Cu-U-Au-Ag deposit (one of the world’s largest Au deposits) confirms the presence of Au in some arsenic-bearing pyrites at concentrations measurable by laser ablation inductively coupled-plasma mass spectrometry (LA-ICP-MS). Arsenic-bearing pyrite in the studied sample shows As-Co-Ni-oscillatory zoning patterns with variable complexity suggesting grain re-crystallization during replacement by chalcopyrite. LA-ICP-MS data obtained from 164 pyrite grains plot below the Au and As solubility limit empirically defined from studies of epithermal and Carlin-type deposits.
Several As-rich pyrite grains were analyzed using Scanning Transmission Electron Microscopy (STEM) and EDX-STEM analysis of foils obtained by Focused Ion Beam methods. Micron-scale, oscillatory zoning patterns observed on back-scattered electron (BSE) images and LA-ICP-MS element maps extend down to the nanoscale. Decoupling between trace elements is common, for example Ni depletion wherever As and Co are enhanced, with nucleation of discrete Co-As-bearing NPs (cobaltite/safflorite?)
Importantly, Au-bearing NPs are identified in all cases, in intimate association with other (sulpho)tellurides. In addition, abundant cassiterite and rare chalcopyrite NPs are also identified. Some of the largest Bi-Ag-telluride NPs contain electrum as tiny pore-attached NPs within the larger telluride. Nanometer-size electrum NPs were also identified in association with chalcopyrite. Silver-Au-telluride NPs form mono- or bi-component NPs. These NPs occur along trails displaying As-Co-enrichment, or formation of nm-wide lamellae of Bi-Pb-sulphotellurides marking pyrite twin boundaries. One wider lamella was identified from the layer stacking as a strongly disordered member of the aleksite series. Coarser tellurobismuthite (Bi2Te3), a few μm-wide, is associated with altaite (PbTe) at pyrite-chalcopyrite boundaries.
Pyrite displays kink- and screw-dislocations associated with trace element remobilization or NPs nucleation. These defects can be associated with either ‘marcasitization’ or loss of Fe (formation of pyrrhotite), within nanoscale domains affected by fluid percolation and pyrite recrystallisation. Twin planes in pyrite enriched in heavy elements (Bi-Pb-Te) represent zones of weakness and assist element exchange between host mineral and percolating fluids during coupled dissolution reprecipitation reactions (CDRR), analogous to those known for hematite from Olympic Dam.
Nanoscale textures in pyrite allow for interpretation of Au-NPs as Au released from solid solution in pyrite during CDRR associated with marginal chalcopyrite replacement. Nanoscale analysis lifts the cloak of invisibility for Au in pyrite at Olympic Dam. These results show that confirmation of whether gold occurs as NPs or in solid solution based solely on position above or below the solubility limit of Au in pyrite on a plot of Au vs. As is impossible without corroborative studies at the nanoscale.
Kathy completed a PhD from the University of California- Berkeley in 1991 and left San Francisco in 1992 to join the former WMC as a research geologist to work on the genesis of the Olympic Dam deposit. She currently leads the team who built the geometallurgical model of Olympic Dam.