Gold Refining by Bismuth Melts in the Iron Oxide-Dominated NICO Au-Co-Bi ( Cu W) Deposit, NWT, Canada

Abstract

The NICO Au-Co-Bi(±Cu±W) deposit is located in the Great Bear magmatic zone, NWT, Canada, where numerous polymetallic, iron oxide-dominated mineralized systems have been recognized. Petrographic, electron microprobe analysis (EMPA), and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) studies of host-rock alteration and ore mineralogy, together with sulfarsenide geothermometry, have been carried out to constrain the nature of alteration and/or mineralization assemblages in this deposit. Metasedimentary rocks of the Treasure Lake Group host NICO and are pervasively altered to an assemblage of ferrohornblende I + actinolite I + biotite I + magnetite I ± orthoclase, which is cut by barren veins composed of quartz ± ferrohornblende-orthoclase-calcite (Set 1). These alteration events are overprinted by metasomatic prograde and retrograde mineralized assemblages and both brittle and ductile deformation accompanied the metasomatism. The prograde assemblage (>400°C) consists of cobaltite, Co-rich loellingite, and Co-rich arsenopyrite (stage I), magnetite II, ferrohornblende II, actinolite II, biotite II, pyrite, and minor scheelite and orthoclase. The earliest retrograde mineralization consists of arsenopyrite (stages II and III), which contains variable amounts of Co, together with native Bi (±bismuthinite) and Au, with lesser magnetite, marcasite, pyrite, hastingsite, and minor quartz. The preservation of solidified native Bi droplets suggests a temperature range of 270° to <400°C for precipitation of this assemblage. The final stage of retrograde mineralization consists of a chalcopyrite-bismuthinite-hematite-chlorite assemblage, together with hastingsite ± emplectite, which formed at temperatures of less than 270°C. Textural and trace element evidence indicates that the Au and Bi present within arsenides and sulfarsenides in the NICO system resulted from the initial partitioning of structurally bound Au and/or “invisible” (nanometer-sized particles) of Au and Bi into the prograde sulfarsenide and arsenide phases, which contain up to 81 ppm Au. The Au and Bi were remobilized following retrograde alteration of those minerals to arsenopyrite II. Molten Bi droplets are interpreted to have scavenged Au insitu, resulting in the formation of the Bi-Au inclusions observed in arsenopyrite II. The second mechanism of gold refining is explained by the occurrence of contemporaneous Bi (±Te) melt and hydrothermal fluids that also could have fractionated gold during transport in solution and deposited it in fractures, interstitially to earlier mineral grains, and as disseminations within Ca-Fe-amphibole-magnetite-biotite-altered rocks. Overall, the gold upgrading at NICO is consistent with the liquid bismuth collector model, suggesting that this process was an important control on gold concentration in this and potentially other Au-Bi-Te-Fe-As-S-rich iron oxide-copper-gold (IOCG) deposits.