Gold upgrading in metamorphosed massive sulfide ore deposits: Direct evidence from laser-ablation-inductively coupled plasma-mass spectrometry analysis of invisible gold

The metamorphosed Boliden volcanic-hosted massive sulfide (VHMS) ore deposit, northern Sweden, is characterized by an abundance of massive arsenopyrite ore and unusually high gold enrichments (up to 300-600 g/t) in crosscutting veins and fracture zones. The ore textures observed record progressive recrystallization and porphyroblast growth during metamorphism. We conducted a systematic laser-ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) study on the gold distribution in fine-grained massive arsenopyrite and pyrite ore, partially recrystallized arsenopyrite ore, and both arsenopyrite and pyrite porphyroblasts found in gold-rich veins. The gold concentrations in massive arsenopyrite are highest (30-50 ppm on average) and systematically decrease to 0-2.7 ppm with increasing degree of recrystallization. By contrast, gold concentrations in pyrite ore are much lower and do not exceed 0.2 ppm. The LA-ICP-MS data, coupled with textural data, provide the first direct conclusive evidence that gold is progressively liberated from massive sulfide ores during metamorphism and reprecipitated in veins and other low-strain sites. We suggest that such upgrading is the principal mechanism responsible for significant gold enrichment observed in many metamorphosed VHMS deposits worldwide.