Evolution of pyrite trace element compositions from porphyry-style and epithermal conditions at the Lihir gold deposit: implications for ore genesis and mineral processing

The Lihir (also known as Ladolam) Au deposit in Papua New Guinea is a telescoped ore deposit, in which volcanic sector collapse(s) led to superimposition of shallow-level Au-rich epithermal mineralization upon preexisting but genetically related, porphyry-style alteration. This superimposition created a giant 56 Moz Au resource but also created complications for ore processing, specifically with regard to the difficulties in processing the refractory Au-rich pyritic ore.

We have analyzed trace element zonation and composition of pyrite grains using laser ablation-inductively coupled plasma-mass spectrometry imaging coupled with NaOCl etching from a subset of spatially and paragenetically constrained pyrite-bearing samples from the Lienetz orebody. Pyrite grains belong to either porphyry or epithermal stages or are composite pyrite grains with a multistage history. Trace element zonation and metal contents of pyrite are unique for each paragenetic event, providing insights into the nature of the mineralizing fluids. Early generations of coarse-grained pyrites that formed under higher-temperature porphyry-style conditions have low trace element contents compared with epithermal-stage pyrites, except for Co, Ni, and Se. Later generations of oscillatory zoned pyrites that formed under lower-temperature epithermal conditions are enriched in trace elements such as As, Mo, Ag, Sb, Au, and Tl. The composite pyrites are relatively coarse grained and display textural and geochemical evidence of modification (i.e., dissolution and reprecipitation). They are interpreted to be porphyry-stage pyrite grains that have been overgrown by rims of delicate banded epithermal-style pyrite enriched in Au, As, and other trace elements.

The composite pyrite grains are volumetrically dominant in the deep-seated anhydrite zone at Lienetz. Because Au is concentrated only along the rims of these pyrite grains, the pyrite can be subjected to a shorter period of oxidation and leaching to liberate most of the Au. This is in contrast to areas dominated by high-grade epithermal-stage mineralization where pyrite grains are As- and Au-rich throughout and thus require longer oxidation and processing time. Understanding Au deportment in telescoped deposits is therefore essential for optimizing mineral processing and can significantly impact the economics of mining complex hybrid ore deposits.