Insights into the Liquid Bismuth Collector Model Through Analysis of the Bi-Au Stormont Skarn Prospect, Northwest Tasmania

摘要

The liquid bismuth collector model has previously been proposed to explain the common associationbetween gold and bismuth seen in many ore deposits, and involves the scavenging of gold from hydrothermalfluid by liquid bismuth. Here, textural relationships and temperature and chemical conditions of mineralizationat the Stormont Bi-Au skarn prospect in northwestern Tasmania are analyzed to investigate whether theliquid bismuth collector model can explain gold accumulation at this prospect. The calc-silicate skarn assemblagecontaining grandite, hedenbergite, actinolite, magnetite, epidote, quartz, and calcite suggests that initialmetasomatism involved temperatures >400° to 500℃. Visible gold is associated with native bismuth, bismuthinite,maldonite, galena, and an unknown Bi-Te-S mineral. Native bismuth is directly associated withgrandite, which formed during initial stages of metasomatism. This implies that bismuth would have precipitatedfrom a hydrothermal fluid as a liquid. Heating experiments showed that bismuth alloy inclusions in garnetmelt between temperatures of 250° and 288℃. The mineralization setting at Stormont and textural relationshipssuggest that gold was scavenged from hydrothermal fluids by liquid bismuth. Rhythmically zonedgrandite crystals provide evidence for fluctuating hydrothermal fluid conditions, which may have contributedto dissolution and reprecipitation of gold, thus facilitating an ore zone refining process within the prospect. Thisrefining process is made possible by the tendency of liquid bismuth to scavenge gold from significantly undersaturatedfluids. Infiltrating gold-undersaturated fluids can dissolve gold not in contact with bismuth, which canthen be preferentially redeposited upon encountering a bismuth droplet, enhancing the correlation betweenthe two elements. A long-lived and focused fluid flow system would maximize the efficiency of this ore zonerefining process, which is likely to be relevant to gold deposition at a range of deposit types.