Evolution of Pyrite Trace Element Compositions from Porphyry-Style and Epithermal Conditions at the Lihir Gold Deposit: Implications for Ore Genesis and Mineral Processing

Abstract

Gold commonly occurs in pyrite (FeS2) as "invisible" or refractory gold, as is the case at the giant Lihir (i.e., Ladolam) hybrid alkali-type gold deposit in Papua New Guinea. The Lihir gold deposit is also unique as it the exemplar of a telescoped ore deposit, whereby volcanic sector collapse led to superimposition of shallow-level gold-rich epithermal mineralization upon preexisting, but genetically related, porphyry-style alteration. While this superimposition led to a giant 56 Moz gold resource, it also created complications with regards to ore processing, specifically with regards to the difficulties in mineral processing of the refractory gold-rich pyritic ore.We have analyzed trace element zonation and composition of pyrite grains, using LA-ICP-MS 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 multi-stage history. Trace element zonation and metal contents are unique to pyrite from 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 to epithermal-stage pyrites, except for Co, Ni and Se. Later generations of oscillatory zoned pyrites that formed under lower temperature epithermal conditions are comparatively 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 re-precipitation). They are interpreted to be porphyry-stage pyrite grains that have been overgrown by rims of delicate banded epithermal-style pyrite enriched in gold, arsenic and other trace elements.The composite pyrite grains are volumetrically dominant in the deep-seated anhydrite zone at Lienetz. Because gold is concentrated only along the rims of these pyrite grains, they can be subjected to a shorter period of oxidation and leaching to liberate most of their gold. This is in contrast for areas dominated by high-grade epithermal-stage mineralization where pyrite grains are arsenic-and gold-rich throughout, and thus require longer oxidation and processing time.Understanding gold deportment in telescoped deposits is therefore essential for optimising mineral processing and can impact significantly on the economics of mining these complex, hybrid ore deposits.

The Importance of Mineralogical Knowledge in the Sustainability of Artisanal Gold Mining: A Mid-South Peru Case

“…Knowledge of mineralogy is essential to optimize the mineral processing of gold ores [26][27][28][29][30]. The ore mineralogy will determine the characteristics of the physical and chemical processes that result in the best recovery grade [31].…”

Rhenium Enrichment in the Muratdere Cu-Mo (Au-Re) Porphyry Deposit, Turkey: Evidence from Stable Isotope Analyses (δ34S, δ18O, δD) and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry Analysis of Sulfides

“…Pyrite samples analyzed by EPMA from porphyry deposits in the Urumieh-Dokhtar arc, Iran (the Meiduk, Iju, Sarkuh, Dalli, Chahfiruzeh, and Keder deposits), contain similar amounts of Ni, significantly more As (mean 478 ppm), slightly more Co (mean 967 ppm), and significantly less Se (mean 9 ppm) than Muratdere (Zarasvandi et al, 2018). Pyrite samples analyzed by EPMA from porphyry deposits in the Metaliferi Mountains have Co and Se contents similar to those of the Muratdere deposit, with more Ni (mean 36 ppm, n = 148) and As (mean 575 ppm, Cioacǎ et al, 2014), while pyrite samples from the Lihir telescoped Au porphyry-epithermal deposit contain more Se than Muratdere (mean 125 ppm) but otherwise have very similar Ni, Co, and As concentrations (Sykora et al, 2018).…”

“…Muratdere has similar Ag, Sb, Zn, and Bi concentrations as pyrite samples from other porphyry deposits that have undergone sulfide trace element analysis (Reich et al, 2013;Cioacǎ et al, 2014;Zhang et al, 2016;Sykora et al, 2018;Zarasvandi et al, 2018). Pyrite samples from the Dexing and Jinchang porphyry deposits, China, the Lihir porphyry-epithermal deposit, Papau New Guinea, and the Muratdere deposit contain relatively similar Au and Te concentrations (<1 ppm), in contrast to pyrite samples from porphyry deposits in the Metaliferi Mountains, Romania, and the Urumieh-Dokhtar arc, Iran, which contain significantly more Te (mean 89 and 47 ppm) and Au (mean 168 and 145 ppm) than pyrite from other porphyry deposits (Cioacǎ et al, 2014;Zarasvandi et al, 2018).…”

“…V-CDT = Vienna-Canyon Diablo Troilite. Reich et al, 2013;Cioacǎ et al, 2014;Zhang et al, 2016;Sykora et al, 2018;Zarasvandi et al, 2018). A) Au against As in global porphyry pyrite samples (where both are above detection limit).…”

Efflorescent Sulfate Crystallization on Fractured and Polished Colloform Pyrite Surfaces: A Migration Pathway of Trace Elements

“…Pyrite is the most worldwide abundant sulfide mineral, occurring as a major constituent in ore mineralizations of various types and origin (from magmatic and hydrothermal to sedimentary geologic environments). Pyrite (FeS 2 ) incorporates metals, metalloids, and non-metals in the structure through isovalent (Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Hg 2+ , Pb 2+ for Fe 2+ ; Se and Te for S) and heterovalent (Cu + , Ag + , Au + , Bi + , Tl + , As 3+ , for Fe 2+ ; As − and Sb − for S) substitutions to an extent of percents [1][2][3][4][5][6][7][8]. Recent studies of its trace element geochemistry show trends of element associations in different deposit types [2][3][4][5][6][7][8].…”