Analytical work on samples from the Runruno alkalic epithermal gold deposit, Philippines, was conducted during the year as part of a collaboration with Rosana Balangue and PhD student Cleodette Lagata of the University of the Philippines. These analyses aim to understand the timing and nature of gold mineralisation at Runruno to inform understanding of ore genesis and mineral processing. A manuscript on the geology and mineralisation of Lihir was accepted for publication in the Society of Economic Geologists Special Publication on gold deposits. Erin Lawlis, as part of her PhD study, has interpreted results from additional pyrite samples from Lihir. These will form the basis of a new manuscript to be drafted in 2020.

Results from Stephanie Sykora’s PhD study of the Lihir gold deposit in Papua New Guinea (PNG) were published in two articles in Economic Geology, which documented the structural evolution of mineralisation, and the gold deportment in pyrite. Stephanie’s gold deportment study was followed up later in 2018 with a pilot study by Angela Escolme and Leonid Danyushevsky (Quantification of gold-bearing pyrite in processing samples from Lihir gold mine). A pilot study of the pyrite footprint of the Runruno alkalic epithermal gold deposit, Philippines, has been initiated by Angela Escolme and David Cooke in collaboration with Rosana Balangue and PhD student Cleodette Lagata at the University of the Philippines. Samples are currently en-route with analytical work to be conducted in 2019. Jian Ma published the results of the pyrite footprints study at Fengtai, China, in Ore Geology Reviews in 2018.

Stephanie Sykora’s PhD study of the origin, evolution and significance of anhydrite-bearing vein arrays and breccias, Lienetz orebody, Lihir gold deposit, Papua New Guinea, was completed in 2017. Two manuscripts that summarise key findings of the PhD study were finalised for publication in the Economic Geology special issue in early 2018.

Stephanie’s research findings provide significant modifications for the genetic model for Lihir, and for our understanding of mechanisms for telescoping epithermal mineralisation onto early-formed porphyry alteration. In previous models, sector collapse was inferred to have led instantaneously to catastrophic explosive brecciation and accompanying epithermal gold mineralisation at Lihir. Through detailed field mapping and a comprehensive structural and paragenetic analysis, Stephanie showed that the porphyry and epithermal systems at Lihir were in fact separated by a period of widespread anhydrite dissolution caused by infiltration of cool groundwaters, facilitating reactivation of low angle faults and contributing to mass wasting of the volcanic edifice, which all occurred prior to the formation of the giant epithermal resource. The identification of anhydrite dissolution, rather than explosive brecciation, as a key process associated with sector collapse is a major revision of the genetic model for Lihir and has profound implications for models of telescoping of porphyry and epithermal systems in similar volcanic terrains.

Stephanie applied the learnings from her detailed structural analysis at the open pit scale to unravel the complex paragenetic relationships of pyrite at the microscopic scale through a combination of petrographic and LA-ICP-MS techniques. This work provides compelling evidence that porphyry-stage alteration and veins at Lihir produced low-grade gold mineralisation, in the form of refractory gold in early pyrite (up to 0.5 ppm Au in the pyrite cores and in early porphyry-stage veins). This weak porphyry gold mineralisation was followed by, and contrasts markedly with, the formation of epithermal pyrite, which typically contains in excess of 10 ppm refractory Au, and which constitutes the bulk of Lihir’s gold resource. Stephanie has demonstrated a complicated history of pyrite dissolution and overgrowths in the evolution from porphyry to epithermal conditions. The resultant composite pyrite grains have only weak porphyry-stage gold mineralisation in their cores, with most of the gold in thin epithermal rinds, somewhat akin to Carlin-style refractory gold. This finding has profound metallurgical implications for mineral processing at Lihir.

Stephanie Sykora graduated during the year.

Additional case studies were conducted during the year with visiting Chinese PhD students Xue Gao (Yidan), Bin Lin (Tiegelongnan), Chao Wu (Halasu), Xin Xiao (Dongguashan) and Jian Ma (Fengtai). Results from several of these studies are expected to be published in 2018.

Stephanie Sykora’s PhD project at the Lihir gold deposit in Papua New Guinea (PNG) was incorporated into the TMVC early in 2016. Her research aims to resolve the genesis of an anhydrite-rich zone at the deposit, and was converted from an MSc to a PhD in 2015.

Stephanie analysed pyrite grains from structurally and paragenetically constrained vein samples that span from the early porphyry-style to late epithermal system at the Lihir gold deposit, PNG. Statistical analyses of LA-ICP-MS data from trace element images of pyrite grains has revealed the compositional signatures of various hydrothermal stages, and provides evidence for gold remobilisation. Her results have implications for ore genesis, mineral exploration and mineral processing at Lihir. A manuscript summarising the pyrite geochemistry results was submitted to a Special Issue of Economic Geology in late 2016. Stephanie delivered an invited oral presentation on her results at the MGEI conference in Bandung, Indonesia, in October, and submitted her PhD thesis in December.

David Cooke, Lejun Zhang and Leonid Danyushevsky co-authored an article on pyrite chemistry from the Haopinggou Au-Ag-Pb-Zn vein deposit in China, which was published online in Contributions to Mineralogy and Petrology. This study demonstrated that single veins formed by two temporally distinct mineralising events at Haopinggou, a Triassic metamorphic gold-forming event, and a Cretaceous magmatic-hydrothermal event that produced silver-lead-zinc mineralisation. The temporally distinct events produced pyrite grains with markedly different trace element compositions within composite veins.